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black and white image of a vehicle with six legs in place of wheels, vaguely resembling an arthropod.

The commissioned mech-tank hybrid, pictured above, is a core component of a story I'm writing and I'm still not sure of some of it's specification. Things like weight and mobility, and I'm stuck trying to make it somewhat realistic for my readers.

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    $\begingroup$ Just by appearance, a lower stance would allow a greater range of movement in concert with the ground = greater speed. But being a tank. I would assume it's sop would be keep as low as possible at all times anyway. $\endgroup$
    – Gillgamesh
    Feb 8, 2023 at 13:22
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    $\begingroup$ Speaking about speed, I think the limit is how fast your tank can rotate its legs while maintaining ground contact, and whether it can use its hind pair of legs to perform actual jumps. If it can jump, as in, it has enough power to make hind legs spring the tank forward, probably it would also be able to gallop, then tall position would be faster. If not, and there are issues on how fast the legs can turn, then lower position could be faster because each step would be longer. $\endgroup$
    – Vesper
    Feb 8, 2023 at 14:47
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    $\begingroup$ Looking at the answers you have to explain the speed and weight of the vehicle. It tips over when its speed change during the corner causes the weight to project outside its support area, between its feet. At 60 tons and 30km/h it could stand very high. If it could move 120km/h it has to be low. Somewhere between 30 to 80km/h it could reach its maximum speed&turning safely while inbetween high and low stances (especially with the weight of the turret so far back). I would guess the maximum speed would be limited by leg movement rather than turn speed $\endgroup$
    – Demigan
    Feb 8, 2023 at 15:09
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    $\begingroup$ Those pointy legs do the opposite of what the threads are supposed to do, not sink into the ground. And by running, you would be jamming spikes into the ground. $\endgroup$
    – gre_gor
    Feb 10, 2023 at 11:58
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    $\begingroup$ @gre_gor thats why you dont let a game design or deviant art person design these things. These are for looking good, not practicality. A practical design of the feet would include inwards facing feet for more surface area along with small spikes on those feet for more traction, as the best use case for such vehicles is in steep terrain like hills and mountains where tanks are limited to roads. $\endgroup$
    – Demigan
    Feb 11, 2023 at 13:10

7 Answers 7

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There are a few hexapedal gaits that have been used in robotics. Wikipedia lists Alternating tripod, Quadruped, and Crawl (move just one leg at a time). Insects use the alternating tripod, which is stable because tripods are stable, and is good for covering rough ground.

The fastest (not most stable) is described here. Direct link to the video here; the summary is that it's quicker to bounce on two legs at a time, which yes brings the body up off the ground quite a bit.

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    $\begingroup$ I imagine that you run into problems with the square-cube law if you try to adapt a leaping or skipping gait to a tank-scale mech... will the materials hold up under the stress of a landing? $\endgroup$
    – Kevin
    Feb 9, 2023 at 9:07
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    $\begingroup$ The six-legged tank might actually use both modes: The faster bipod gait for fast traversal on flat surfaces and the more staple tripod gait while in rough terrain or in combat. $\endgroup$
    – Philipp
    Feb 9, 2023 at 14:45
  • $\begingroup$ Oh it definitely would Philipp, that's the whole point of a hexapod is to have flexibility. It can jump real high, use legs as arms, do all sorts of things. It's an adaptable body-shape. $\endgroup$
    – wokopa
    Feb 9, 2023 at 14:46
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Generally speaking, the lower the center of mass of a moving object, the lower the risk of it flipping when changing direction.

Translated to a vehicle, if you want to make a sharp turn, the lower the center of mass, the quicker you can turn, all the rest being the same.

This is why race cars are as low as possible and trucks sometimes flip over when turning too fast.

You can try a little experiment by yourself: put an empty plastic bottle on a tray and walk fast with the tray, then turn sharply. You will see that it is easier for the bottle to fall when it's laying on its short face than when it's laying on the long face.

In your case, the lower you can keep it, the faster it will be able to change direction. Since I assume you are not planning for having your mech going always and only straight, keep it low.

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    $\begingroup$ +1 Nice, succinct explanation. $\endgroup$
    – Qami
    Feb 8, 2023 at 14:01
  • $\begingroup$ bottles have 'long faces' and 'short faces'? $\endgroup$
    – wokopa
    Feb 8, 2023 at 20:24
  • $\begingroup$ What speed would this hexapod be going that tipping becomes a danger? Its not going to be racecar speeds right? And if you gave the water bottle legs that spread the supporting surface area even slightly, is it still as unstable or is it because the bottle is basically the center of mass without sideways support? $\endgroup$
    – Demigan
    Feb 9, 2023 at 9:16
  • $\begingroup$ As your hexapod clearly has motorized knees, why not go the ugly V22/Osprey type route and have the thing "lift up its skirts" and transition to (near) full height before it runs when it does need to go a long way in a straight line? $\endgroup$
    – Dannie
    Feb 9, 2023 at 12:14
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    $\begingroup$ Unlike a car or truck, a legged mech would be able to 'lean into' a fast turn. OTOH the fastest legged animals (e.g. cheetah, antelope, horse) all have relatively long legs projecting directly below the main body mass, rather than splayed out the sides, because it's more efficient. Cheetahs don't fall over when turning at vehicle-like speeds. $\endgroup$
    – Anentropic
    Feb 9, 2023 at 14:23
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As L.Dutch stated, for turning purposes a lower profile is needed.

However for realism, I'd say speed is by far not the only thing to be concerned about! Also consider the following points:

  • Keeping a small (dense) profile to present less surface area to be hit by enemy fire.
  • Keeping low so the vehicle can take advantage of terrain variations to lower its visibility to enemies.
  • A wider stance is more stable and precise when firing heavy ordinance.
  • Spread out legs will be more stable in less ideal terrain conditions.
  • A higher profile could be faster and possibly easier on fuel.

This will all have been considered by the engineers, so the lower stance would most likely be default with different stances available for higher profile situations.

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Its faster high up, slightly bend legs. For turns you can always go into a lower stance if necessary, but considering the legs are already supports farther away from the main body it seems kind of a mute point.

With the legs bent you reduce the maximum length of a single stride. So unless you stretch the legs almost horizontally, which I don't recommend, the stride length will always be smaller if you use a lower stance. Additionally it is more likely to be slowed down by terrain if you use more horizontal legs.

So stand tall! Until your speed is less important than your visibility that is.

Also since people may bring it up: dont shoot the legs! Unlike tracks or wheels you can put armor on them entirely (even if its light armor) and the legs are the most erratic moving parts of the vehicle. If anyone tries to shoot them, have them shoot where the legs meet the chassis.

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For an alternative idea, check out the Tachikoma from Ghost in the Shell.

The core idea is they have multiple legs (only 4 in the canon but no reason to not go to six for larger models) but each leg has its own wheel, presumably driven by independent motors (just like some modern electric cars have a motor for each wheel). This means they can walk when needed, but also just drive at high speeds.

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TLDR:

If the legs are completly stretched out, no. If the legs are closer to the body and the body is closer to the ground, it would walk more efficiently and with greater force, but slower. However, you could use the back legs to move forward like a jump, like bird-like creatures do or grasshoppers.


I would suggest you to look up subjects about torque and mechanical levers.

In short, the human body (and other animals aswell) is composed of "biological levers" that work with a certain amount of mechanical disadvantage most of the time.

Illustration of lever systems found on the human body

(I couldn't find any examples with insects, unfortunately)

The same is applied to wheels and for mech legs.

If you put the legs of your mech far away from the mech's body and walking very low, the extended legs would work like giant levers, the same could be said about fully extending these vertically. However, just like Marky stated in the comments: humans walk efficiently by falling and catching up over and over again. But I don't know if such walking cycle would work for a 6 legged creature.

So, if you know about mechanical levers, the tip of the legs will move faster, but with much less force, since the speed of movement is related to the torque and RPM of a car or, in this case, a mech, then it wouldn't go very fast, but it would lower its profile on the horizon.

However, the back legs would be able to work better if these were used to step as close to the body as possible and then pushing it forward. It would work just like a grasshopper or a bird jump/walking, and the other legs could be used "just" to keep it from falling instead of actually making a lot of effort to move.

Illustration showing the working principle of a bird leg

(sorry for the big illustration, I couldn't find it in another resolution)

Unlike what people think, the bird-like leg is not actually a leg, it is the bird's ankle, the actual knee is closer to the body than to the ground. It is like they are perpetually on the tip of their toes, not particularly comfortable, but certaintly easier to run very fast.

So, the distance from the thighs to the ground could be seen as more or less a straight lever to the ground, if the ankle is centered to the axis of the thighs. It would be like crouching before jumping or running, you could put way more energy in the movement instead of your normal walk.

You could use such mechanism to walk forward, but eventually the back legs of your tank would still need to fully contract, it could be with the kness moving laterally or vertically, as long as the inital and middle movement part are closer to the body, such as the bird example.

Here is a video of a flea jumping in slow motion, its legs are similar to bird legs.

Of course, your tank wouldn't fly as high as a flea, but it could move forward more efficiently and faster.

And the same can be applied to grasshoppers.

Photo of a grasshopper

(The back legs of your hexapod tank are already pretty "grasshoppy" for this mechanical movement)

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The effect of long legs is the same as riding a bicycle: it requires generally more effort to move them if uphill but about the same or less on flat terrain, and if the legs move with the same angular velocity at the body, the speed increases with making them longer. It may require less energy to move with longer legs because this means less steps, and the step likely to include additional activity not relevant to pushing forward (lift the leg up for the step, reverse the direction, and the like). Hence a really fast robot I think should have long legs, as much as practical. There is such a term as "bicycle step", how far do you drive on one revolution of the pedals that is roughly equivalent to one step of walking. It is many meters, hence cyclists are much faster than walkers.

However, the longer the legs, the more difficult it will be uphill. Also, the creature gets wider and may not easily pass everywhere.

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