# Advantages of “Spider Mechs” over Tanks?

I've seen the discussion on how bipedal mechs are inferior to treaded and wheeled vehicles in almost every way. However, all of these questions appear to be related to bipedal mechs.

I'm wondering if spider mechs have any advantages over the aforementioned normal vehicles. For those of you wondering, a spider mecha is one that is very low to the ground and has four or more legs.

I'd imagine these would be far more practical than bipedal mecha, as these are lower, probably faster, and more agile than bipeds.

But what about advantages over normal wheeled and treaded vehicles that fill the same combat role (armor, specifically tanks). Do they have any advantages, or once again are boring normal tanks better than mechs?

• Spiders have eight legs, not four. – RonJohn May 14 '18 at 1:52
• @RonJohn I know they have eight, but the term "Spider Mecha" often refers to ones that have at least 4 legs. – Sydney Sleeper May 14 '18 at 3:35
• You'd think the Japanese would know that... :) – RonJohn May 14 '18 at 3:43
• @Jesse I doubt that's true for any vaguely realistic setting, at least not on the scale of main battle tanks. We might see smaller autonomous units meant to traverse urban or otherwise complex terrain. OP, have you considered a setting with an alternate technology base that would somehow justify a different tech path than our own? – Harabeck May 14 '18 at 20:21
• @sphennings, since when does the context matter? What about any of what you mention precludes a storyline and the need for a consistent set of world rules? Frankly, a fair number of questions on this site are merely thought experiements (some by our senior participants) that have nothing to do with writing a story or building a complete world at all. There needs to be a better reason to complain than, "you didn't say the magic words: fictional world." – JBH May 14 '18 at 23:05

Okay, so a did a bunch of research into legged robots for my thesis and they aren't going to replace armored vehicles any time soon. The most basic reason, they aren't as efficient in doing what armored or wheeled vehicles are specialized in. They are also very complicated to program, since you have 4/6/8 or more legs which all need to interact versus 2 treads, or some wheels which only really move in 2/4 directions.

Legged Vehicles are all terrain vehicles. They can walk over rocks, climb mountains, jump up buildings, jump over gaps, lower and raise their bodies, kick stuff and anything else you could imagine a robot doing. This is 100% more flexible than any wheeled vehicle, because a legged robot could be designed to do all this with just their legs. Your not going to see a tank ducking down or jumping over a gap, it's just going to steam roll over whatever there is in its way. So legged robots are extremely flexible in their movements.

The problem is, that if you want to compete with tanks you need to be able to do something a tank can't do, and do it more efficiently and cheaper. Fundamentally, a 4 legged robot is going to need roughly 12 motors as well as a generator and fuel to run it and that is a lot of weight to carry. It can't move as efficiently as a tank over flat ground because it has to lift its legs and body weight, while a tank just rolls around and has to defeat friction, not gravity.

Next if you want more firepower, you need to carry it around with you, so you run into the same problem as before. More weight, means larger motors, which decreases efficiency.

So in the end, a legged mech won't actually replace the role of a tank. Even bipedal robots won't replace the role of a tank, because a tank is good at its role. What you will see, is the use of smaller sized legged robots like boston dynamics Big Dog which will assist soldiers. Its small enough that it's motors won't over burden it. It maintains its flexibility of movement because now its human size, so it can avoid debris and jump around if it needs to without knocking down buildings. It can have a gun mounted on it to fight infantry or carry supplies for soldiers, or scout ahead over rough terrain.

There are even smaller ones, like remote controlled car small, which gives you even greater flexibility. They can search through small gaps, cling onto piping and climb around, stalk people, or perform search and rescue. Like having an actual remote control spider.

So back to your question. Legged mechs advantage is their flexible movements. They won't replace a tank, because a tank can already cover 80-90% of the situations a legged mech would be better in, but a tank would still end up cheaper, more efficient and less prone to breaking (There are a lot more moving parts in a legged mech).

• I could imagine environments where a mech would be more suited than a tank, but we're talking specific environments here - imagine your country is covered with fields of terrain like: lemarsh.files.wordpress.com/2012/09/p1130403.jpg and a mech capable of climbing/jumping around would be able to traverse the terrain better than a tank, but we're really getting into unlikely scenarios here. – Baldrickk May 14 '18 at 16:18
• Hmmmmm... tiny spider mechs (~1-2 ft leg span) with explosive payloads... anti-tank suicide missions, i.e. get under tank and detonate against vulnerable sections? – Doktor J May 14 '18 at 17:26
• @Baldrickk Terrain like that is called "helicopter country". :-) – StephenG May 15 '18 at 11:43
• I disagree with the 12+ motors - they are likely to have 1 large motor power a hydraulics system which is then used for moving all the limbs – user2813274 May 15 '18 at 14:35
• What hypothetical power source is allowing a 50+ ton vehicle to jump up buildings, and why no mention of what a conventional vehicle could do with that kind of on-demand torque? These comparative analyses always seem to give the legged robots incredible science-fictional capabilities, and then compare them to modern-day tanks. Level the playing field and those advantages disappear. – Catgut May 16 '18 at 13:43

Do they have any advantages, or once again are boring normal tanks better than mechs?

Existing vehicles are still better than spider mechs.

The reason is ground pressure.

Note how much contact this tank has with the ground, dispersing the weight so that it doesn't sink. Compare that with the GITS spider tank. One's going to sink in the mud, and the other isn't.

Of course, you might say that the spider tank would work better on streets or roads, but wait until the road gets wet. That think has the traction of a peeled banana...

• Okay but what if the mech had like a whole bunch of legs? Like so many legs that it's basically just solid legs? To help fit all the legs under the mech, we could put them on a spinning wheel. Then, to help give the legs more traction, we could wrap the spinning wheels in... oh wait I just invented treads. – Pink Sweetener May 14 '18 at 2:18
• If you look at the bottom of your shoes they're not designed to give you the maximum surface area, indeed when a shoe's tread is worn flat that's when you're most likely to slip over. The reason being that a larger surface reduces ground pressure and ground pressure (as counter intuitive as this may seem) contributes more to traction than surface area. This is an essential aspect of tank treads, their low ground pressure enables them to slip when the taking is turning, otherwise the tank wouldn't be able to turn or would risk breaking its tracks. – Cognisant May 14 '18 at 2:19
• @Cognisant: flat soled shoes (or slick tyres) are only slippier in damp or shifting conditions. If the ground is dry and hard then a flat soled shoe or treadless tyre will give considerably more traction than it’s bumpy companion. That’s kind of irrelevant in the case of a tank, though, as the operating conditions are generally wet and shifty. – Joe Bloggs May 14 '18 at 7:53
• To illustrate JoeBloggs' point: Formula 1 tyres have to have great traction to make sure that every last bit of their engine torque is applied, with no waste due to wheelspin - and F1 tyres are entirely slick. When the track is wet, however, they switch to tyres with patterned tread. The grooves give the water somewhere to go that isn't between the grip surface and the track, while the raised portions can push through the water and grip the track beneath. On slick tyres, the water has nowhere to go that doesn't interfere with grip. – anaximander May 14 '18 at 12:51
• @RonJohn It depends on how the spider legs work. If they are mobile and long, they can work their way out of the mud somewhat easily. Tanks don't have that option - they can try to bruteforce their way out, but they can't adjust their leg joints for a better position. I don't like mechs for other reasons, but mud isn't one of them. – T. Sar - Reinstate Monica May 14 '18 at 15:00

They would fulfill a role, but only in extreme terrain.

"but ground pressure!"

It's called "designing things". You don't design legs like animators and model creators do, you design them to specifications. For example you could design legs that have large surface area's extending below the center of the spider Mech, this already gives you larger surface area's than current tanks can have. (imagine an "L" shape for a leg with the L pointing to the Mech center so it doesn't take extra space).

Another method which I'm favoring is to use tent-peg like idea's. Similar to a tent-peg it greatly increases the grip on the ground allowing it to climb across extreme terrain a tank wouldn't dream off. UNLIKE A TENT-PEG because somehow someone is going to envision a spider-mech with tent-pegs for legs, the legs would use conical shaped ends. The legs compact the earth as they enter the ground, the conical shape quickly increases the surface-area and also sends forces sideways besides downwards to relieve the amount of pressure the ground below needs. The legs could even have a series of cones at the ends extending towards the center of the spider-mech (again with an L shape) to spread out the surface area it stands on even more without losing its grip ability.

I did a back-of-the-envelope calculation for an 8-legged spider-mech using excavator arms for legs. If the excavator arms are 1/4rth the size of current excavator arms and 1/4rth the strength then you could build a 120 ton spider-mech (and excavator arms probably use the usual 20% safety margin for maximum forces it can take so it can handle even more under stress). If you keep it at around 80 tons it can lose 4 legs and still keep going.

"but legs are easily destroyed!"

Actually a lot harder than most people expect. Are you really going to fire at a target that constantly accelerates and decelerates and presents a small target? Of course not! You'll fire at where the legs meet the chassis, which puts it in between a tank and a wheeled vehicle in terms of vulnerability. A wheeled vehicle can lose several wheels and still be fairly operational. A tank loses a track and it's a stationary turret/artillery food. A spider-mech would take longer to disable than a tank but have worse consequences for being disabled.

"legs take up a lot of weight, you can carry less stuff!"

Tracks weigh tons more than wheels but they carry more weight. Legs would have similar properties plus extras. Legs would allow for a far higher recoil compensation than tracks or wheels. This means spider-mechs would be able to carry larger weapons for lower weight class vehicles.

"its harder to stabilize!"

Wheeled/tracked vehicles go over every bit of terrain, which makes them less easily stabilized. A walker, once the tech is there to keep it balanced, would be far more stable.

The end result: spider-mechs wouldn't replace tanks, but there's tons of terrain where tanks are far too limited. History surrounding the alpine troops show that movement through extreme terrain can give you unparalleled advantages in engaging tanks moving through limited area's, or engaging the supply routes or bypassing enemy forces and striking at construction or forward bases.

Edit: disadvantages of legs would of course include slower max speed (but probably higher than most think), more fuel consumption (although like tracks this could equalize on rough terrain), higher & longer maintenance costs and tougher battlefield repairs. This is practically the same list as between tracked vs wheeled vehicles. Tracks haven't edged out wheeled vehicles and wheels haven't edged out tracks because they both have their uses. Spider legs would also fit on that scale and have its uses, but as its own category rather than a replacement of legs or wheels.

Edit 2, limitations to tracks:

While many people focus on the disadvantages of legs, tracks also have disadvantages. People will be quick to point out that tracks have lower ground pressure even than a human standing on his feet. What they fail to realize is that this is a requirement for tracked vehicles. When turning the entire track slides across the ground. The higher the pressure, the harder it becomes to steer and the more likely you are to rip up the ground and damage the tracks. The longer you make a tank the more this problem will arise. But make the tracks too short and it becomes unstable while turning. The ratio as I've heard it is 1.5/1.8 to 1 for length vs width for the tank to remain both stable and not have trouble steering/breaking the tracks. This, much like all the disadvantages legs have, means that a tank is in actuality limited in size, weight and capabilities similar to a legged vehicle (the actual limits imposed aren't similar ofcourse).

• I like this analysis. I would also add that with a sensibly programmed walk-cycle, you could not only adapt to the loss of multiple redundant legs, you can also effectively gyro-stabilise as part of your system of mobility rather than in spite of it like a conventional vehicle does. meaning weapons accuracy goes up and the ride gets smoother the more legs you have. – Ruadhan May 14 '18 at 12:19
• just look at where legged vehicles are used today, logging on mountainous terrain. – John May 14 '18 at 20:20
• If you construct your legs right, I suspect the fuel efficiency bit is a non-issue. Walking animals tend to sway their legs if they move long distances, which reduces the energy required to move a great deal. It's probably also why humans tend to sway their arms when they walk (naturally). – Clearer May 16 '18 at 9:25
• @Clearer yes, humans use 90% of the energy of the previous step in the next step. But across any smooth terrain a wheeled/tracked vehicle will be more energy efficient... That's another reason why a legged vehicle would be used on rough terrain. – Demigan May 16 '18 at 9:46
• @Demigan more efficient sure. But if that's just 5% more efficient, does it really matter? (btw, I have no idea how much more efficient tracks and wheels are). – Clearer May 16 '18 at 11:03

I'd imagine these would be far more practical than bipedal mecha, as these are lower, probably faster, and more agile than bipeds.

No, they'd be less practical for maintenance reasons. Every leg introduces at least 3 or more complex and critical joint assemblies and every one of those is a potential point of failure.

But what about advantages over normal wheeled and treaded vehicles that fill the same combat role (armor, specifically tanks). Do they have any advantages, or once again are boring normal tanks better than mechs?

Again think in terms of vulnerability and exposure of critical components combined with maintenance and wear and tear issues. Tanks are tricky to fix in the field, especially on an active battlefield, but they're easy-peasy compared with a legged vehicle.

The legs require hydraulics to replace muscles. That adds another huge vulnerability. A hit that doesn't actually destroy anything can still be sufficient to make the operation of the locomotive system impossible.

There's a potential problem with the locomotion being an extremely uncomfortable ride.

Likewise targeting systems. Modern tanks have targeting systems that maintain the gun stability while moving and hence allow accurate firing while moving at full speed over uneven terrain. This would be an order of magnitude harder to arrange with a walking vehicle as the motions are not as easily compensated for.

This is very important as the primary function of a tank is to plonk an armored mobile and stable gun platform on the battlefield. The capability of the gun is of primary concern. Firing first and accurately is one of the first survival rules for armored vehicles as it's almost always possible for the enemy on a modern battlefield to destroy or disable an AFV.

I'm skeptical that such a system could be as efficient as a normal tracked system. Every movement requires the lifting and adjustment of every part of the entire set of leg assemblies. That's a lot of work, most of it not doing useful work (i.e. not directed where you want it).

• Maintenance would be a pain in the posterior actuators for sure. – Joe Bloggs May 14 '18 at 9:48
• In addition humans don't like rough terain and shock/strategic points will be usually build in moreless plain terrain and most of time serviceable by roads. If the position is in high ground even your spider mech battalion will get a hard time climbing under fire. A probable cheaper and more efficient way to attack the position is by air. For a good mech scifi world see here – jean May 14 '18 at 11:35
• Think of the fun of trying to keep sand out of those joints. "All right, sarge, let's saddle up!" "Ehhh...no can do, captain. The hip joints on legs 2 and 4 are locked up again, the knees on 1, 3, and 7 are shot, the ankles on 2, 6, and 8 are wasted...and I've got this pain in the diodes all down my right side...sir..." "<grumble-grumble> ...tank! <grumble-grumble>" – Bob Jarvis - Reinstate Monica May 15 '18 at 11:35
• This reminds me of a book I once read on the battle for Stalingrad, which claimed that the beautifully-engineered German tanks were way more of a liability than the Soviet tanks, which were basically just giant tin cans with treads and guns, and therefore much easier to repair. – MissMonicaE May 16 '18 at 15:56

Since everyone's missing the question entirely and talking about the disadvantages, The Key advantages I can immediately think of:

Height

Assuming you're not talking a crawling mech (see the real-world logging mech) Then most sci-fi walkers show a much taller aspect than most real world vehicles.

That height is a point of vulnerability, it reduces opportunities to go Hull-Down under enemy fire, however at the same time, it presents a couple major advantages. First and foremost, it's always easier to kill someone down-hill from you, the height means that you can fire explosive shells down.

Against a conventional tank this translates to being able to fire directly at the top armor of the vehicle, instead of being forced to strike its sturdiest front-plating.

Secondly, an ability to flex the legs in a sort of "craning the neck" to see over obstacles without exposing the whole vehicle, being physically more flexible always presents options.

Extremes of mobility

The biggest advantage of the spider-leg configuration is that it actively can hook into rough terrain and pull the machine over obstacles rather than require ground-friction like a tank.

The spider-tank can crawl over steep hills and clamber over ruined buildings and obstacles to attack from an unexpected quarter.

Redundant components

Legs on a walker are very very segmented. The usual complaint about them is that they'll be high-maintenance, but that doesn't mean they have to result in downtime. Simply detach the leg entirely and replace with a factory-fresh replacement in half an hour or less. You could certainly never do this with a tank.

Fictional example: see the early parts of the film Chappie, where they maintain the drone robots by this exact methodology.

Additionally, beyond the basic four legs, a hexapod or even arachnid configuration would provide multiple redundancy, able to lose anywhere up to half its limbs and keep moving. Meanwhile a tank would be instantly immobilised by a single equivalent hit.

Shock and awesome

Things with too many legs, making loud servo-whines and behaving like giant animals hit a lot of Fight-or-flight buttons.

If you want to intimidate, there's nothing like the looming presence of a mech. A tank simply cannot compete on this front, and Shock & Awe should never ever be discounted.

• I can't imagine armour on the legs that would be comparable with tanks armour and coverage for wheels so I don't think that the paragraph about immobilised tank is true, because hit, which destroys a leg doesn't necessary destroys tank wheels. The truth is that legs are far more fragile so the smaller guns can easily destroys them. Smaller guns -> higher rate of fire -> more legs destroyed more quickly. Also more armour/height -> more weight -> bigger leg foot -> more expensive + easier to aim at + more problems in handling terrain. – Artholl May 14 '18 at 14:14
• That very much depends on your design I think. Multiple relatively spindly legs moving quite quickly and unpredictably are very hard to hit with any serious firepower. That and the legs themselves being essentially structural rather than mechanical, a hit would have to hit on or near a joint in order to inflict critical damage, vs hitting in the middle of the segment, where it's essentially all armour, I'm imagining that it'd be a tough skeleton with plating. so most shots would go straight through the legs without hitting anything important. – Ruadhan May 14 '18 at 15:25
• Demigan suggested the use of "Tent Peg" feet, which compensate for the ground-pressure problem by intentionally sinking deep into the ground to increase surface area and grip, The only real problem with this is the mechanical strength required to lift the feet out of the ground again and material strength to resist the forces involved. Both are fairly achievable if you can already make a mech that big :P – Ruadhan May 14 '18 at 15:28
• @Ruadhan2300 be aware though that applying significant kinetic force to an "all armor" mid-segment area could still damage the joint itself -- similar to what happens if your arm or leg suddenly gets torqued in a direction it's not supposed to: presto, dislocated joint. – Doktor J May 14 '18 at 17:32
• As for the disadvantages of height; if the legs are designed properly, the mech could actually "crouch" and minimize its profile. Speed would of course be compromised (if not entirely immobilizing the mech while crouching), but if that gives you a little extra cover or helps you dodge a shot while you return one of your own, it could make the difference (I'm imagining something on the order of "crouch, you and enemy both fire, stand/run while enemy is reloading, repeat") – Doktor J May 14 '18 at 17:34

Tanks are heavy. Really heavy. You may think that your groceries take some lifting, but that's nothing compared to a tank. The M1A2 is listed as having a weight of 65 metric tons. In order to distribute that much weight over soft ground without sinking, all except the lightest tanks are supported on tracks that spread that weight out over a very large surface area.

Take a look at a picture of a tank. Look at how much area its tracks cover - let's call it a third of its width and as long as the hull. (Yes, I'm approximating.) So for a tank 8m long by 3m wide, the tracks are distributing the weight over 8 square metres. Now imagine your spider tank of equivalent weight - in order to not sink, its weight must be distributed over "feet" that take up that much area. At all times. So let's say you have a six legged tank and two legs (one each side) are being lifted up at any moment as it "steps" forward, it means that the four remaining feet must be supporting the entire weight of the tank. This means that each "foot" must have a surface area of 2 square metres. Visualise this and you will see that your spider tank is wearing clown shoes!

Then you need to add legs, and armour on those legs (since they are outside the hull and not protected by it) and a much more complicated drive system than a tracked tank's (which is quite complicated enough), all of which adds more weight so you need bigger feet to distribute that weight and legs and - forget it. A legged tank is not viable on earth any more than a man-sized spider is, and for much the same reasons relating to squared/cubed ratios. Low gravity worlds with very rugged terrain might make such vehicles feasible, but that's a separate question.

• Space is big. Really big.... – jsj May 14 '18 at 12:46
• +1 just for quoting Douglas Adams. – Graham May 14 '18 at 16:19
• Simple fix. Hover jets at the bottom of the feet. – MarsJarsGuitars-n-Chars May 14 '18 at 16:56
• What if each "foot" is in fact a section of tread, with an electric motor at the "heel" powering that tread (I'm thinking diesel-electric series hybrid like railroads use). When operating on relatively-flat terrain, the four "feet" on each side simply line up and become effectively one long tread (with the front "foot" on each side pitched upward a bit), but as the terrain changes, the height and pitch of each tread segment can be adjusted to keep as much as possible on the ground at any time, and to reduce the pitch and roll on the tank body as well. – Monty Harder May 15 '18 at 15:22

Quad or more legged mechs could be used to climb up terrain that is difficult for normal or heavy off road vehicles, specially if carrying a large artillery gun or cannon.

Weakness is that once a leg is shot off, speed and mobility will be reduced increasingly by the number of legs destroyed.

As another option, you could have wheels attached to the legs so they gain the same speed and mobility as wheeled tanks and these can switch out to climbing legs so that they can embed themselves much better in mountain or cliff-sides. Think along the lines of climbing axes or spikes.

Also if say a quad legged mech gets one or two legs destroyed, they can theoretically still move about with the wheels attached to their legs and if you are up to it, maybe put in some wheels on the under-carriage. This way you get the best of both worlds?

• "Weakness is that once a leg is shot off, speed and mobility will be reduced increasingly by the number of legs destroyed." Surely this is also true for wheels and treads? – Pink Sweetener May 14 '18 at 1:35
• Wheels to an extent can still function as some tanks have double wheels in case the outer one gets shot. I am not sure about tanks though how they can cope with it but as mentioned in my post, a wheel or double wheel can be installed. This is of course not a fool-proof way to avoid getting a crippled mech but it still helps in mobility and staying power as opposed to just having the legs. – Arkhaine May 14 '18 at 1:47
• @PinkSweetener You are correct. A spider with a leg shot off can still move, but a tank with a shot off tread is a "mobility kill". – RonJohn May 14 '18 at 1:49
• @PinkSweetener, a tank can give tracks and wheels some protection from armour, reducing the ability to get a critical hit on them. A leg is exposed at almost all points and can't be armoured as heavily as the main body. – Dan Clarke May 14 '18 at 4:15
• GITS has the "spider tanks with wheels in their feet" idea covered, too. The Tachikoma spider mechs use legs and tethered harpoons (think Spider Man's web-shooters) for difficult terrain or precise maneuvering and switch to wheels in their feet for raw speed on flat surfaces. – Dave Sherohman May 14 '18 at 9:40

JPL has been developing a 4-legged wheel-footed mecha called ATHLETE for lunar (or other extraterrestrial) astronaut support for some time:

And based on their brief, the wheel on side limb apporach allows for mission flexibility - as in there can be end-of-limb attachments in their scenario for sapping, and other tasks - and that applies to this OP's question also - for tanks to be useful motorized trenchers ATM, they require basically a backhoe attachment or in fact to be armoured bulldozers - the legged mecha could fulfill a large part of that role with a low weight attachment onto the existing limbs.

"...The All-Terrain Hex-Limbed Extra-Terrestrial Explorer (ATHLETE) vehicle concept is based on six 6 DoF (Degrees-of-Freedom) limbs, each with a 1 DoF wheel attached..."

Nasa & JPL are also working on a smaller unit, for very rough terrain on satellites or smaller planetoids (Europa and other targets) called LEMUR:

LEMUR article

"...The robot’s four limbs are fitted with interchangeable circular “feet,” which can be swapped out for attachments with different functions, Swiss Army knife-style, to help it traverse a variety of surfaces. Rock-climbing feet feature a series of tiny, razor-sharp steel hooks, known as microspines, to grip the rough surfaces of rocks firmly enough for one foot to hold the entire robot’s weight. For smooth surfaces, such as the outer hulls of space stations or satellites, LEMUR adheres itself with gecko-like sticky feet..."

So - to recap what we can garner from these two brief articles:

1) Multi-limbed, articulated robotic locomotion is non-problematic, and is actively being solved and approached now as a solution, not an exotic imagining.

2) Maneuvering and multi-functional advantages of said systems over wheeled locomotion on severely fractural terrain is considered a critical operational advantage - directly analogous to a strategic advantage in military applications.

3) Weight to power, power density over time, repairability and redundancy on these systems are solid enough that competent, studious, well-funded folks are seriously considering relying on them for missions without any possibility of local fallback positions, and in conditions where these systems will be light-minutes of communications away: either they have a great deal of confidence in the mechanics, or the redundancy, or the repairability and control systems.

From this we can comfortably posit that the primary solid argument advanced against this locomotion concept is that of the difficulties in armouring the drive systems against weaponsfire, explosives and so on.

My counter would be: mech carries two field-replaceable full limbs, plus two more additional "feet", as they are the most likely to be damaged in the course of regular ops, minus all the armour. Operator can then decide in situ whether operational tempo and conditions warrant field-stripping of armour from damage limb(s) during replacement, or if simple replace-and-run is called for.

Said replacement full limbs would be carried pre-combined, back to back, as linear battering ram. Similarly, the two replacement feet would be preconfigured with trenching attachment and hoist attachment, to allow for easy field use of such attachments, and easy stripping of those from replacement feet should exigencies arise.

That's my 15 cents.

Legs are good for one thing: agility

Have you ever watched two boxers fight? I mean really watched what they were doing? Credit to everything on the human body, but it could be said that boxing is about your legs. They're used to:

• Shift your weight for leverage and strength
• Move you deftly out of harm's way or into a superior position in almost any direction.

So your spider mech has an advantage over wheeled or (worse) tread tanks in that it can better keep its balance, hop out of harm's way, perhaps even right itself when knocked over (if they're designed well). Have you ever tried to tip an Abrams back on its treads? I haven't, but I bet it takes more than a couple of big Tongans to do it.

At literally everything else, legs stink

I'm assuming we're comparing apples to apples by talking about manned mechs. The arguments for drones are very, very different because you can make the mech (comparatively) very, very small. Wheels/treads are great when there's enough weight to keep them solidly in touch with the ground. They're less valuable when the object being moved is very light.

(1) Let's add some armor to that boxer. And a big ol' gun. Let's let him look a bit like the combat dudes from StarCraft.1 Suddenly you're having to add all kinds of mechanical enhancement to the armor — and you'd think you're doing that to enhance the combat abilities of the wearer, but you're spending most of the energy just moving the armor.

In the same way that most of your gasoline is being used to move the car... not you.

So, the more armor you add to the mech, the less valuable the legs become because adding enhancement to overcome the weight is seriously a losing battle. As armor increases, the mechanics to move the armor increases, the fuel needed to power the mechanics increases, all of which adds stress to complicated joints... and all you really wanted to do was kick the other guy's butt.2

Armor almost always succumbs to armament. You don't see plate mail anymore because rifles pack enough punch at enough distance that you might as well be standing there begging them to shoot you ... which is what you'd actually be doing.

(2) Now let's add tree roots, bushes, things that are easily squashed and driven over with wheels and treads but are an amazing hang-up for legs. The issue isn't tripping, the issue is the inability to move a hung-up leg forward, which means you're a sitting duck.3

(3) And your center of gravity, which is high for anything with legs but low for (almost anything) with wheels or treads (monster trucks violate this rule... but that's outside the scope of your question). A high center of gravity means it's easier to make the unit unbalanced, tip it over, or control it with trip wires (see my last point). It also raises the unit unecessarily high off the ground, making it an easier target.

(4) Then think about speed. This is where legs really, really stink. It doesn't matter how much you enhance joints, wheels and treads can always out run legs.

(5) Finally, add to this the increased complexity of affecting knees, ankles, hips, rotor cuffs, tendons, muscles, and a whole lot more. There's an engineering axiom that, frankly, should be considered a Universal Law.

KISS: Keep it simple, stupid!

Those cool mechs, gundam, and all other things robotic used to fight Godzilla and who knows what else from the 8th Dimension and Beyond are just that... cool... and absolutely useless. Complicated design and automation that can only be driven by an operator with a PhD in physics costing bazillions of dollars and it's all wasted by a single shot from some crazy dude with a big ol' gun that cost pennies on the dollar to build compared to your mech and that can be aimed and fired by a 10-year-old hiding behind a rock.

So, looks cool in anime, but in real life the cost-to-value ratio is way, way, way in favor of wheels and treads. If you really want mechs in your story, you need to do what all previous authors have done... you need to declare it to be so and move on with the story, because you'll never be able to justify the tech.

1Which, if you think about it, is so unbelievably unrealistic that it makes angels weep. Think about how far you'd have to dislocate your sholders to get them into the arms, etc. But, it's a heckuva game to play, so no complaints. Nosireebob.

2An astute observer might claim that the same is true for wheeled vehicles and tanks. That's true, but not to the degree of moving joints. It's relatively easy to increase engine size to turn a more heavily loaded axel compared to all that needs to be done to move 2–3 joints. If the problem increases geometrically for tanks, it increases exponentially for mecha.

3If you don't believe me, the next time you get your foot stuck in some brambles stop and really think about what your brain can do with your foot pretty much unconsiously — you twist them, rotate them, tip and tilt them... and still you occasionally get your feet stuck. What the human brain can do with a foot is almost as breathtaking as the enormous effort it takes us to simulate it artificially — and we still can't build a robot that does it as well.

The first thing that springs to mind is the likelihood of mechanical failure.

Essentially you increase the chance of mechanical problems or failure in direct relation to the number of parts involved.

While tanks are capable of traversing terrain that wheeled vehicles find difficult, when moving on roads the vibration will shake parts loose. To the point that regular vehicles carrying troops will be employed to follow up the tanks to pick up anything falling off the tanks.

WW2 tanks would often be abandoned by their crews not because they were destroyed or damaged but because of mechanical issues.

The other consideration is fragility. If you have a multipedal system it will fail if one of the legs is destroyed, therefor they need protection. One way to do this is to make them smaller to be harder to hit, the other way is to add armour, or in other words sacrifice mobility or speed

• The first thing that springs to mind is the likelihood of mechanical failure: Interestingly, that could also be a pro for the mecha-spider. A normal tank that looses one of it´s tracks is screwed. A Spider can loose one or two legs and still move... – Daniel May 14 '18 at 11:19
• I'm fairly sure if spider legs had advantages over tracked vehicles we'd be seeing them in use more than they are. You are also basing your argument on an existing vehicle with known properties fighting a fantasy vehicle which can continue to have shifting goal posts. If you base the argument on the only multipodal drone currently in existence, Big Dog, I reckon the tank would win. – Pinback May 14 '18 at 11:54
• Just saying ... also you seem to miss out on recent robotic development. See here and here and here and here just to name a few that are publicly shown. Given that a lot of the key technology is quite new or even not-quite there yet, I presume we´ll be seeing much more of those in the future... – Daniel May 14 '18 at 12:44
• GE had a prototype walking truck in the 1960s, a couple of Japanese companies sell fullsize mechs 1 2 (note the second one has an operatiing sistem that they pronounce as bushido) – Pinback May 17 '18 at 12:34
• @Daniel - I get that you're passionate about the topic but I don't really see any of currently existing the bi/multipedal walkers faring well against a tank. – Pinback May 17 '18 at 12:43

I presume you already know the works of boston dynamics, espicially the LS3? The Article list a number of disadvantages as to why the LS3 is currently not useful for field service, but most of it could potentially be mitigated with future technology - The main critique was AFAIK noise and maintenance.

In theory these could be scaled up and mounted with weaponry to fill a similar role as for example the German Wisel. They would be lacking Armour, but could be hard to hit. Imagine trying to aim at something like this with your 120 mm main gun from your conventional tank. They could also cross terrain that wheel/chain vehicle cant.

So a viable future scenario could be "spider" bots as autonomous guerrilla force against conventional tanks. Here it would be an advantage that such a unit could potentially be built much cheaper than the MBT it destroys and thus would be kind of disposable

Edit: Example calculation as thought experiment (with today's technology):

Nation A fields 10 M1 Abrams tanks, costing is \$4.3 million each

Nation B has devoped some intelligent software, and fields

• modified camera drones for ~12k
• equipped with one rpg7 equivalent ~2k
• a Pickup-truck as launch vehicle ~50k.

A whole system of one truck with 10 attack drones would cost less than 200k. Nation B could send a swarm of over 200 autonomous drones like a swarm of bees to kill off the 10 Abrams, and still save over 90% of the military budget!

Imagine what those rocket-bearing robo-dogs could do once they can be produced with economics of scale...

• Damn, I like the idea of guerrilla spider bots. Even if we can't have Spider-Tanks, Spider-Bots might just have a purpose after all. – Sydney Sleeper May 14 '18 at 8:26
• What can a legged vehicle offer over an arial or wheeled drone? Reaching in accessible places for reconnaissance? – Pinback May 14 '18 at 9:43
• @Pinback: A legged vehicle could stay operational over a long time and stay hidden in an area. A legged vehicle could hide in the woods and attack from cover. As the Marines put it for the LS3-Programme: Go where dismounts go, do what dismounts do – Daniel May 14 '18 at 11:09
• Drones are either short range, short duration - or high altitude and air-superiority is required. – Daniel May 14 '18 at 11:12
• @SydneySleeper exactly -- something with better maneuverability... and perhaps even the means to attach itself to a target and wait for a better opportunity to detonate (e.g. when personnel are boarding, etc) – Doktor J May 15 '18 at 4:33

Now what should be noted before I provide my answer is this, it matters where your fighting because every planet is different.

So on earth the simple answer is no, the dust isn't a serious problem and the only advantage a walker would have is the ability to climb up a wall or cliff-face which is a very niche application. I don't have the ability to go into the reasons it wouldn't work here but I will say that I asked questions relating to this but for the moon, they are Would walkers work on airless worlds? and Would tanks or small walkers be better for Lunar militaries?. hope they help.

Spider mechs excel in steep high-mountain terrain where tanks are completely useless.

Even if a slope is less inclined than the maximum slope (which is normally something like 60% which is still only 30 degrees), a standard tank would be stopped by the extremely rocky ground.

A spider mech has no problems with ground pressure because the terrain is solid ground. It can apply a pincer movement with two sharp legs holding the mech securely in place, so even vertical or overhanging walls with ice on it can be climbed. It can principially also jump over or traverse steep crevices. 6 or 8 legs also makes it extremely improbable that a mech will fall when 2 legs are breaking through a hidden crevice.

Still the mech should be as light and sturdy as possible to maximize climbing and traversing ability. This rules out big cannons, so a spider mech will likely only have machine guns and anti-tank rocket launchers, it will avoid to get in a direct fight. It will use surprise and its ability to retreat in impenetrable terrain as main tactic, needless to say that helicopters and ground support planes will be the archenemy of a spider mech.

Note: the question doesn't specify a "tech level", so I'm going to assume a near future setting where the required tech (power supply, actuators, stabilization) has matured a bit.

### Ability to utilize cover

Currently, tanks require relatively specific terrain conditions to make good use of cover while still being able to fire, and take time to get into those positions. Gun depression, for example, is one limiting factor.

A vehicle capable of varying its height and leaning in all directions can effectively adjust its silhouette to whatever cover is available, "peek" over or around it just enough to fire and quickly shift back.

### Modularity and active defenses

The tank evolved around the necessity to carry as much armor as possible around a gun large enough to penetrate the opponent's armor, and it's pretty well optimized for that purpose.

However, with the (foreseeable) introduction of faster and smarter drones, missiles, point defenses, directed energy weapons and sophisticated electronic warfare, having the heaviest armor might not be the ideal defensive strategy anymore.

A less compact, but more modular design that offers faster heat dissipation and greater arcs of effect for multiple, coordinated point defenses might stand a better chance against this range of threats. This does not necessarily mean legs, but a "spider-like" design would fit the requirements pretty well.

Note also that part of the reason that current tanks need to be compact is that they (still) require a crew of roughly 3-6 people. People are fragile, need room to move and air to breathe and don't react well to shock or fluctuating temperatures. If you can replace (most of) them with machines, you gain a lot of freedom in how you shape your vehicles.

### A more stable firing platform

Assuming a mature and field-tested control software, the range of motion of multiple legs and their ability to, effectively, act as springs should allow a vehicle to better compensate for recoil, the vehicle's own movement and extreme firing angles, improving accuracy in less than ideal conditions.

One advantage of "spider tanks" that hasn't been mentioned is the potential for additional angle of elevation of the main gun, which would be useful in urban warfare. All tank weapons have fairly limited max elevations (20 degrees for the Abrams according to a quick google search). In most operations, this is not a problem, but in urban warfare, it means that a tank cannot fire at anything above the second or third story of a building (depending on how far away it is).

Assuming that a "spider tank" can independently move each leg, it could raise up the front of the vehicle while lowering the rear, granting additional elevation to the gun and increasing its threat area.

"Tanks don't go in forests. Forests have trees in them." - Col. Mercier

A legged spider tank unlike a similarly sized conventional tank would be able to pivot its self on it's side or back to squeeze through gaps in trees or alleyways, edge across narrow paths on mountain sides.

I can easily see legged vehicles with this ability being dominant in forests or urban sprawl where they are the only armour available.

• that's what the french thought about the Ardennes – GlorfSf May 16 '18 at 8:54

It all depends on the circumstances, wheels are better than tracks when you're driving on concrete and asphalt as they enable you to reach higher speeds more efficiently and give you more traction when cornering. On the other hand a multi-legged design would probably be slower than a tracked vehicle but it could handle the especially rough and/or soft terrain that tracked vehicles traditionally struggle with. If your combat vehicle is being deployed to a highly developed area with reliable road quality having wheels is advantageous, if it's being deployed to a place that has been shelled or is simply undeveloped natural terrain a tracked vehicle would be more reliable. A multi-legged combat vehicle would be even more reliable and could operate in terrain that precludes wheeled and tracked vehicles, but it would only be advantageous in those circumstances.

I'm going to go ahead and base my answer around what we see in the Ghost in the Shell universe, where spider tanks are plentiful. I know this is science-based, so take this with a grain of salt.

1) They are not as big as actual tanks
I'd wager the main spider tank from GITS isn't actually as big as as a proper tank. It also seems to be equipped with less firepower - miniguns and some rockets. This leads me to believe these are designed for something else than a typical tank.

2-1) Urban warfare
Because they are smaller, spider tanks are probably more suited for urban warfare where you want to avoid damage to the infrastructure. The spider tanks in GITS can also be quite mobile, as they have wheels on each leg, allowing them to use roads at high speeds.

2-2) Combat inside buildings
Given the smaller frame and higher agility in an urban setting, spider tanks can enter and fight inside buildings (again, avoiding the potential damage driving a tank inside would cause).

3) Storage and transport
I'd imagine a spider tank can be "stowed away", making it take far less space than an actual tank. This means they can be hidden from sight in strategic places to defend critical points. Still taking GITS as an example, there are cases where spider-tanks are used as defences on ships at sea as well (albeit that one seems like a stretch).

4) Specialistic equipment
This is mimicking point 1 a bit, but I'd wager that given the tech needed to construct a spider tank, they aren't meant to replace actual tanks in a full out war, rather they are strategically deployed to perform certain tasks. Think sabotage, hacking, relays, strategic defences (remember these can be stowed away and / or transported with more ease).

• nice approach, especially when the GitS's author really try to explain a lot of his technical choices in the manga – GlorfSf May 16 '18 at 9:31

Note that below I use the term 'walker' instead of 'spider tank'. This is really just personal preference, since the two terms mean essentially the same thing in modern times.

### Terrain:

Tracked vehicles are very good at handling certain types of terrain. In particular, for a tracked vehicle to be effective, the terrain has to fit a couple of constraints:

• It can't be particularly steep. Most AFV's that use tracks are heavy enough proportionate to the traction provided by the tracks that they can't handle steep grades (I don't know about modern stuff like the Leopard 2 or the M1A1 Abrams, but a lot of older tanks couldn't handle more than about a 20-30% grade).
• It needs to not be jagged. Rocky beaches are bad for tracks. So are dragon's teeth.
• While it can be somewhat soft, it still has to provide decent traction. Sand is usually fine, provided you aren't going up steep dunes. Mud is usually fine, provided it's not too deep. Snow, unless it's very hard packed, is typically not (most tanks can handle snow up to a certain depth, below that, the snow will get into the gun, the engine intakes, and the cabin vents, and cause issues). Note that this is very design dependent, some old tanks (such as the Porsche Tiger) were notorious for digging into mud as often as crossing it.
• Water may or may not be an issue. Most modern tanks can ford rivers fine, provided the other constraints are met, but that was not always the case, and the vehicle needs to be designed for it.
• Certain types of fortification are problematic (the aforementioned dragon's teeth, as well as things like Czech hedgehogs).

Now, of these constraints, the first one is essentially a non-issue for a properly designed walker (if you make the feet right, you can climb vertical surfaces). The second is not as much of an issue for a walker as a treaded vehicle.

Soft ground might be an issue for a walker, depending on the design. Tanks handle it fine because they spread their weight well, there's no reason a walker can't do likewise, see for example water striders (they use other tricks in addition to spreading their weight, but the general principle is the same). However, even without spreading the weight of the vehicle, it still may not be an issue, if you can pull the legs out easily, you can just let them sink and be fine, or you can create something very light that can move very fast across soft ground (and therefore the feet don't have time to sink).

Water may or may not be an issue for a walker, it's really design dependent, though I would envision that less work needs to be done to let a walker ford a river than a tank.

As far as fortifications, a line of dragon's teeth would have to be very wide and very narrowly spaced to stop a well designed walker, and would as a result probably be impassable to infantry as well. Czech hedgehogs might be a bit more of an issue, but probably not as much as they were for tanks.

So, overall, while a walker may not do as well in certain types of terrain as a tank, it would probably do better in types of terrain tanks have more issues with.

### Maintainence

So, hydraulics are horrible for maintenance in a combat zone. One well placed bullet will render them inoperable in such a way that they can't be reliably repaired in the field. Their only real advantages are weight to power ratio (which is insanely good compared to other options), and the fact that they can be trivially made to lock into position when unpowered.

Pneumatics are a bit better for maintenance (you can patch the hose, and then just pressurize the system with a portable compressor), but are still easy to damage and trivial to cripple.

There are three alternative options to hydraulics and pneumatics:

• Rotary servos, one (or more) per joint. This is immediately out, it's too hard to replace them, and it's difficult to make them fail-safe.
• Ball screws or lead screws. They're reliable, insanely durable (most designs can tolerate a lot of damage before they completely stop working), and can be easily adapted to fail safely (that is, have the leg lock into position instead of collapsing).
• Linear servos. Not as durable as ball screws, but they're a lot easier to replace, and are also very easy to make fail safely.

Beyond that, just make sure you have more than one actuator per joint. That allows you to lock the working ones in place to safely replace a failed one.

### Defenses

A walker is harder to defend from counter-battery fire and anti-tank fire than a tank. The joints pretty much need to be at least partly exposed, and unless you're using rotary motors in each joint, so does whatever actually moves the legs.

Height is also an issue (though, unlike a tank, a walker could be made to be variable height). Being higher up makes you an easier target (though it can also make it harder for enemies to take cover from you).

On the other side of things, mines may not be as much of an issue for a walker as they are for a tank. If a tank runs over a mine, it's usually either done for, or at least completely stuck there. If a walker steps on a mine, it can just adjust it's stance to compensate for the damaged leg and foot. Of course, this won't work reliably more than once or twice, but it's still enough for the walker to relocate to somewhere better defended for the crew to repair it.

### TL;DR

Walkers would probably work, but not in the same roles that tanks do. I can easily envision them being used as armored recon vehicles in particularly difficult terrain that is uncrossable by conventional wheeled tracked vehicles, but I don't think it's reasonable to assume they would be the primary form of armored infantry.

Regardless, it depends entirely on their design. They almost certainly won't look like anything you've seen in movies or other media, considering that those are almost always designed to allow for good storytelling (classic example, the AT-AT's used by the Galactic Empire in Star Wars episode 5 are pretty much worthless from any realistic military perspective, they have no fail safes to keep them from falling over if the controls are damaged, and they're trivial to trip).

• The abrams can handle a 60 degree grade I believe. – Efialtes Apr 17 '19 at 12:41

You're posting in world-building, so I'm assuming your interest isn't replacing all tanks in your world with spider-tanks, but enough to get a good story?

As others have commented, the increased complexity of multipedal tanks will make them less efficient, due to the added weight of the mechanisms. Programming the legs should be fairly trivial. Ignore 4-legged tanks; go for 6 or 8 legs. They provide much superior balance and 8 legs provides significant redundancy.

If you're looking near-future, you could extrapolate some development of actuators like muscle wire or EAPs to provide motive force, instead of hydraulics. However, you're probably going to be looking at a lighter armoured spider-tank compared to a main battle tank. But who cares? We're fighting in places where main battle tanks just can't function.

One other potential benefit of legged tanks is that potentially they could dodge incoming attacks, by shifting their bodies, much faster than a tracked vehicle could. This would provide significant protection, as voiding attacks will always be better than relying on armour.

So you need a scenario where tracked tanks are particularly ineffective. You're likely looking at mountainous terrain, or potentially a post-urban terrain – particularly if the buildings are heavily engineered rather than light residential buildings one can drive through with a tracked tank.

The issue is that in most of these scenarios, a helicopter is likely to out-perform a tank, and is readily available and tested. So add in ground forces with guided RPGs or similar designed to take down choppers.

One other scenario springs to mind: Common spider-tank designs in manga put wheels at the end of the legs for road use. Interestingly, this could allow better on-road performance than a tracked vehicle (tracks are notoriously bad for roads, which is why the army always transports tanks on flatbeds not driving them), but better off-road performance than a wheeled combat vehicle. This could be ideal for a mixed urban environment where you don't want to cause massive damage to infrastructure by ripping up roads with tank tracks, but you need to cross rubble, climb stairs, get past bollards, etc.

• For some reason people keep bringing up the idea that legged mechs or tanks could dodge an incoming projectile. For those who suggest that, let me ask you a question: outside of comics or other fictional portrayals, just how likely are you able to dodge a bullet that someone has accurately aimed at you? A 30mm shell from a GAU-8 Avenger autocannon has a velocity of over 1000 meters per second. The Rheinmetall 120 mm gun (used in assorted NATO tanks) has a muzzle velocity of up to 1,750 m/s. If an enemy gets to within 500 meters, just how far is that legged tank dodging in 0.3 seconds? – Keith Morrison May 15 '18 at 18:30
• ...but a Rheinmetall 120 mm has an effective range of up to 8km, giving over 4.5s to detect and dodge the shot (it'll be longer as the round will drop significantly from muzzle velocity during flight). 500m is almost point-blank range for tanks. By contrast, and RPG whilst slower is much shorter range, so has a typical flight time of almost a second. Obviously, 'you', a human, can't dodge a bullet, but neither can a human shoot down an incoming missile, whereas CWIS can. – Dan W May 15 '18 at 20:01
• Finally, dodging can start before the shot is fired – in hand combat you can dodge before a thrust is released. Similarly, at closer ranges, a tank could react to seeing an enemy tank start to target it, like a soldier would start to dodge as soon as they see an enemy appear around a corner, not waiting for the shot to be fired. Compared to a tracked vehicle, a spider-tank should be able move in any direction fairly immediately. SO – I think it's not impossible. – Dan W May 15 '18 at 20:05
• and if there are are 2 enemy tanks? – Keith Morrison May 15 '18 at 21:49
• Haha, then - if they shoot at the same time - you get hit. But that’s unlikely. But what happens if your ablative armour gets hit twice? No protection is 100% effective. Realistically I would expect spider tanks to be smaller and lighter, but still vulnerable (as is almost everything else) to main tank fire. They’d not be able to go toe-to-toe with a main tank, but would excel in niche environments. – Dan W May 16 '18 at 6:27

I would imagine that spider mechs would enjoy benefits over tanks in aquatic/amphibious or zero gravity/very low gravity.

traversing liquids (eg water) could be acomplished by raising the feet above and out of the water and then propelling forward, negating the resistance of the body of water or even being more of stealth as it could pierce the water surface with minimal disturbance (especially with care to ease into the initial dip of the foot to avoid a "plunk" sound

as for space/zero gravity, the legs could double as directional thrusters or to improve grip around the surface it is traversing (like how your fingers reach around a tennis ball or inflated balloon the size of a watermellon.

Can the question be reworded to "Can spider-mecha replace conventional tanks" or "Can spider-mecha effectively destroy conventional tanks"?

There is a plenty of answers why the first option is not plausible.

On the other hand, if you want to design tank-slayer you are focusing on tank's weknesses and anticipating their strengths.

Tanks are big, heavy, slow, noisy and deal heavy damage per shot. Therefore the counter must be agile enough to escape tank's guns and deal focused damage to the critical parts. Tanks are useless in deep mud and on very steep hills. Make the spiders light enough not to get stuck and able to climb.

A small fast unarmed mecha that can resemble in a form of czech hedgehog can be effective agains moving tanks imobilising them long enough to be easy target to other weapons of your choice.

Small mecha carying a landmine and placing it at the right spot and running away can be effective too.

A swarm attack of small and agile spiders against tanks are effective because tanks cannot fire at all of them at the same time.

So. Tanks are built for a specific battlefield role. Optimized for it. While we call it a 'Spider Tank', it may not necessarily fill that precise role. As noted in other answers, a legged vehicle has certain advantaged over a tracked one: Mobility and resilience. Mobility, in the sense of being able to climb over things, like parked cars. Resilience, in the capacity lose multiple legs, without being disabled. It might be better to think of spider tanks as an 'armored car' capable of moving through a gridlocked street, than as a tank. They only need to be sufficiently armored to survive small arms fire.

1. Mobility

A walking transport's mobility is far beyond anything that normal tank can dream of. Walkers are able to move through any kind of terrain (which can cause troubles to normal vehicles) and even extremly tilted surfaces. Can a normal tank climb a 90 degrees oriented wall? The spider tank can!

1. Endurance

A car with one lost wheel becomes nearly immobile (not taking into account the art of driving cars on 2 wheels). A tank with one lost track turns into a gun with pretty thin armor for an immobile fortification. At the same time a spider tank with its one lost leg can perform without much difficulties (of course, every leg should be independent, otherwise it will totally restrict any further movement). At max it can lose all but 3 of its legs (to maintain balance) to keep moving.

1. Control

A car is driven most simply: press the pedal and turn the rudder. A tank requires a bit more advanced skill to keep both tracks coordinated. A walker requires a very complex coordination system that allows all its legs to have the job done, and with every additional leg beyond first 2 the complexity increases considerably.

1. Costs

• The more complex the vehicle is, the more money is required to build such a thing. Spider tanks should be really expensive toys
• The more complex the vehicle is, the more parts there are to break. You definetly dont want a weapon that spends more time in workshop fixing itself than on a battlefield fighting
• How much time and money does it cost to train a tank operation team? Multiply these numbers times and times to imagine what you should spend on a spider tank operation team.

These days, if you want a robot to move around and be "flexible" (this rules out bipeds because like us humans they tend to fall over), there are three options: Wheels, tracks, and spiderish things (there may be more, but these three are the most commonly seen ones). Tracks (tanks) are good for traversing rough terrain, but not very flexible. A spider-mech would excel in "tight environments", where a tank might be defeated because of turning radius or something similar.

Aside from that, there aren't that many advantages. Spider-mechs are (presumably) more lightly armored because putting too much armor on them removes their flexibility.

There is one significant disadvantage of a spider-mech or any type of mech against a tank: the joints. You can't have that much armor over a joint, or else the whole purpose of the joint is gone. You would have to have the armor on the joint. If you put too much armor on the joint, you would get these unwieldy legs.

Conclusion (TLDR): Spider-mechs would kind of work but not really.

Advantages of Spider Mechs Over Tanks

1: Look cool

That's it.

They're more expensive, more complicated and more easily damaged. When you can have a dozen tanks for the cost of one mech, tanks win every battle.

Quote by Joseph Stalin: “Quantity has a quality all its own.”