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It's the zombie apocalypse. A group of survivors on the east coast learn about a safe settlement in the west. However, they must travel through the zombie-infested heart of the continent. The inner regions are inaccessible due to extensive zombie infestation. The roads from civilization's past are in shambles. The survivors must brave the zombie-infested country, carrying all the food and supplies they can.

Zombies are extremely lethal and can infect at close range, but fortunately, they move relatively slowly, with a maximum speed of about 2 miles per hour. However, this is not true for the deeper parts of the continent's zombie population, a fact unknown to the survivors who are in for a nasty surprise.

A group of approximately 25 survivors set out on their journey through the continent in vehicles resembling wagons but with engines instead of horses. These slow-moving motorized wagons travel at a maximum speed of around five miles per hour (this is crucial to the story because of the nasty surprise ahead).

The question remains, why would the resourceful group of survivors, consisting of MacGyvers, engineers, and tinkerers, construct slow-moving motorized wagons from salvaged parts of older civilization instead of faster cars?

EDIT: The wagons are not necessarily like the ones in the old days with tarp, the survivors do indeed add armor plating and weapon mount. It's just that they resemble the old ones and similar speed.

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    $\begingroup$ Logically in a zombie apocalypse you would use existing vehicles rather than build new ones. If these people are capable of building new ones it's hard to see why they would make them so slow. Is the number of people really important? I think there may be a way to make it work but there would have to be a lot more survivors, maybe at least 100+. $\endgroup$ Commented May 29 at 11:41
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    $\begingroup$ The roads are filled with burned out cars so they are limited to traveling offroad at a much slower rate. Any faster and the wagon might hit a bump and topple over. $\endgroup$ Commented May 29 at 16:07
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    $\begingroup$ Don't have time for a proper answer, but an interesting bit of tension you could add would be that not all of the vehicles are slow. Perhaps they bring along a piece of relatively heavy machinery for clearing the road (or making it) and are forced to wait for it, or even abandon then recover it. Imagine being in a convoy with a backhoe, more or less. $\endgroup$ Commented May 29 at 19:45
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    $\begingroup$ Ever played Project Zomboid? Going 'normal' fast is a fast way of getting dead. Running in crashed vehicles, or hordes of zombies is going to be a big problem, as any collision is dangerous to you, and your car is unlikely to survive the collision even if you do. If your car does survive the collision, it will be of worse than before. And there is no shortage of zombies waiting to scramble in front of your car. $\endgroup$
    – vinzzz001
    Commented May 30 at 8:39
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    $\begingroup$ I feel like a shoddily-contstructed DIY motorized wagon built from spare parts would probably have a high likelihood of breaking apart if driven much faster than 5mph; that might be sufficient. $\endgroup$ Commented May 30 at 18:35

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Some thoughts that come to mind:

Slower vehicles are usually more fuel-efficient. It would be fair to suppose that fuel is limited and precious.

Maybe they have no gasoline at all and are forced to use electric vehicles. There are presumably no charging stations so they use vehicles with electrical motors run by solar power. Fast solar power cars have been built but there are a lot of trade offs, not to mention technical expertise. Just suppose they need to carry a bunch of cargo and that there are no genius solar engineers in the group.

As you say, one would expect the roads to be in bad shape. A formula 1 racer does not do well on dirt roads filled with pot holes. I've owned cars that had just a few inches of clearance from the ground, which would be totally unsuitable to such roads. You might posit that to travel on these roads vehicles need balloon tires or treads. Such vehicles tend to be relatively slow.

They are using old vehicles left over from pre-zombie days. These vehicles are in bad shape. They break down often and need to be repaired, and so the whole convoy has to slow down or come to a halt while one vehicle is repaired. And they have to run them slow because if they try to ran them fast they explode.

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    $\begingroup$ A lot of the answers here focus on why it might be advantageous to drive very slow, but this still doesn't really explain why the vehicle would be incapable of exceeding 5mph. You generally do not want to drive a vehicle while constantly redlining it and pushing its physical limits, it would generally make more sense to have a vehicle capable of higher speeds which is only operated at low speeds. I agree it might make sense to operate the vehicle at only 5mph, but it would strike me as odd if the vehicle were physically unable to go faster in a life-or-death emergency... $\endgroup$ Commented May 29 at 20:05
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    $\begingroup$ I love this answer, but the first assertion, "slower vehicles are usually more fuel efficient" is false. What you meant to say is, "fast vehicles that are driven at lower speeds are usually more fuel efficient." Vehicles designed for torque move more slowly than vehicles designed for speed and they are most certainly not more fuel efficient. Notably without a load they're horrible. And no matter how slowly you drive them the efficiency doesn't increase perceptibly. $\endgroup$
    – JBH
    Commented May 30 at 19:36
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    $\begingroup$ For what it's worth, gasoline will be one of the first things that run out - it doesn't store well/last. If it's more than 5-10 years after collapse gasoline could be virtually non-existant. $\endgroup$
    – aslum
    Commented May 30 at 20:33
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    $\begingroup$ That's why The Road Warrior got everything right (they extracted oil from the ground, cracked it in a tower into gasoline, and stored it in a tanker)... except, where are all the tires coming from. +1 $\endgroup$
    – Mazura
    Commented May 31 at 1:39
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    $\begingroup$ @aslum Gasoline stores fine. It's the precisely-tuned octane boosters necessary for it to run in a modern, high-compression engine that decay and/or settle out. Over time it reverts to its natural octane rating of about 45. Put it in a low compression engine designed for low octane and it still burns fine. But diesel engines will probably be more common since they can run on a variety of fuel oils available from myriad sources. $\endgroup$
    – Perkins
    Commented May 31 at 17:50
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Slow moving vehicles are silent and less probable to be detected by zombies. Zombies can hear loud sounds and a loud engine will surely be noticed.

People have built vehicles with very small engines that can be quite silent but their drawback is that they move really slowly. With the appropriate camouflage, people can pass through large zombie groups, practically unnoticed.

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    $\begingroup$ This seems like the best kind of realistic idea so far. Its implausible to tune existing gas car engines to run so slow. But if you have to use scavenged ebike motors (forget electric cars, there is no way you can get enough electricity on the move to power them) because they are very quiet, und use them to to power something substantially heavier than an bike, that kind of speed sounds plausible. $\endgroup$
    – LazyLizard
    Commented May 30 at 11:45
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They're Using Zombie-Power.

Remember hamsters in wheels?

When all the petroleum and derv ran out, an alternative needed to be found. Harnessing zombie-power is as simple as making a big frame with harnesses for the zombie's feet and bodies, think walking exercise-machines, but with zombie's feet nailed to planks - this needs to be fronted by the sort of things zombies like - a "carrot for zombies" to attract them to attempt to get to it. A couple of pully-wheels, a simple gearing system and a brake.

Now with a team of dozen fresh zombies, the friction of the gears and the load being pulled (the driver/a few passengers), the maximum speed of a team is around the 5 Mph mark. Gearing (and a slip-clutch) allows climbing hills, stopping and reversing.

When a zombie or two burns out, and fails, they can be replaced at the nearest opportunity with "wild" ones. Failure to replace one or two can be compensated for by downshifting to allow the remaining ones to take the load with a lower top-speed.

The ultimate in environmentally friendly motoring.

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    $\begingroup$ In zombie movies zombies inexplicably don't die of starvation, so zombies should be very efficient energy wise. When times are good the survivors feed zombies extra food as a type of biological battery. The zombies also tend to get less aggressive the more time they spend near non infected. $\endgroup$
    – philn
    Commented May 29 at 20:30
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    $\begingroup$ michonne of TWD used declawed zombies as pack animals. i could really see a wagon set up, or for snowing climes, a swift sleigh. onward dasher! onward prancer! $\endgroup$
    – Dor1000
    Commented May 30 at 17:48
  • $\begingroup$ The problem I see with this is that the free zombies wandering around who aren't part of your pull team will quickly dogpile whatever/whoever you use as a "carrot" to get the team pulling in the right direction. $\endgroup$
    – T.E.D.
    Commented May 31 at 15:36
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Because when you are in hostile territory with no decent roads, a slow moving vehicle is still better than a toppled up or broken AND standing still vehicle.

If you are crossing a hostile territory you are not going to protect yourself just with a tarp, you will likely use as much as you can. And a heavily protected vehicle looks nothing close to a F1 car. Add to that the lack of properly paved roads, and it's a no brainer realizing that going fast means looking for troubles.

And don't forget that, again because of being in a hostile territory, you will want to reduce as much as possible the need to leave the vehicles, meaning you will be carrying with you all the supplies you need for the trip. Again something which you don't see on a race car: moving a large mass on an unsteady road is better down at low speed.

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  • $\begingroup$ The wagons are not necessarily like the ones in the old days with tarp, the survivors do indeed add armor plating and weapon mount. It's just that they resemble the old ones and similar speed. $\endgroup$
    – sigsegv
    Commented May 29 at 11:55
  • $\begingroup$ Ride comfort could be another consideration. The terrain can get rough, and if you are basically living in one of these things, rough terrain + higher speeds = a pretty sore back. $\endgroup$ Commented May 30 at 17:28
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Robustness vs Structural Weakness

Since (per your comment) all of the modern vehicles are broken down/otherwise inoperable, these people essentially had to design engines from the ground up, using whatever they happen to have to hand. That will make their engines wildly more prone to inefficiencies/breakdowns than current modern engines due to a variety of factors related to not having top-end manufacturing. They may well only be able to create engines similar to those used in the 1910s/20s, which had all sorts of limitations on maximum horsepower because of the materials they had to work with. Naturally your MacGyvers decide that, given the limitations on their manufacturing capabilities, a robust, easily repairable design is paramount. The only thing that trumps "we don't have to stop to fix anything between here and Los Angeles" is the machine's ability to maintain a better-than-2mph speed. So they decide on two engines with parallel motive systems so one can be off-line (for repairs/maintenance) and the vehicle still can maintain a speed greater than 2mph.

Their engine designs are going to made with less effective materials, but designed specifically for continual running and low requirements for things like oil and grease, as well as being as fuel efficient as possible. So while their designs might be more 1918 than 2018, they'll be able to use enough modern stuff like titanium etc. to avoid some of the bigger early-heavy-engine reliability problems.

All this redundancy and material inefficiency means weight. The vehicles themselves can't exceed 20ish tons fully laden or they'll have problems with crossing bridges. Depending on what they expect to find they might even want a lower weight threshold. Eventually you get something akin to the late WWI/interwar tank, which has a top speed of.... 5mph!

Why don't they just refurbish modern engines and go? Well electronics play a big part in modern machines, and without them the insanely tight tolerances go to pieces and the things break. Permanently. Your band is MacGyvers, not electricians or programmers. They can't figure out the underlying programming to turn a 2024 Camry engine into something that'd power a mobile fort. If it's near-future and everything (or at least the big tractor trailers) are hybrids or full electric you have even bigger problems. I've known car guys that have rebuilt engines etc. for 50+ years and they just can't work on hybrids or even some petrol-engine vehicles because of the amount of electronics involved with a modern car.

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  • $\begingroup$ The electronics in modern vehicles replaced mechanical timing systems. Reverting a modern engine to mechanical timing wouldn't be that physically difficult. Hybrids would be more challenging, but you'd just have the driver decide how much power to shunt where rather than the computer. Driving would take more skill, but the machine itself wouldn't be that complex. The old car guys can't work on new cars because the computers are locked down and you have to lease a $20,000 programmer system to mess with them. And replacing the computer with an open-firmware one is illegal in most countries. $\endgroup$
    – Perkins
    Commented May 31 at 18:00
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Torque vs. Velocity

Your problem isn't actually unique. Why would anybody today buy a slow-moving vehicle? Why buy a 1992 Ford F-150 when you could have bought a Ford Mustang for the same price?

The answer is very practical for your purposes: some vehicles are designed for speed.

Others are designed for torque.

If you need to rush across a flat surface, you want your car built for speed or velocity. But if you need to do that while hauling two tons of horse manure to a field you're intending to plant... you must have a vehicle designed for torque (and "rush" takes on a whole new meaning).

Vehicles designed for speed are limited in the load they can carry. They're fast! But they can only hold 1-6 people and some luggage. But if you want The A-Team or maybe MacGyver to bolt inch-thick steel plate to something to protect you from the ravening horde, you absolutely must have torque. And what you gain in the ability to deflect a bullet (or nasty chompy teeth) you lose in max velocity.

Because that's physics.

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    $\begingroup$ @Bubbles It's not the armor slowing down the vehicle. It's the design (primarily the gearing) of the vehicle that allows it to move with the load. Nevertheless, you have a point. If you can outrun the zombies, why the armor? Blood on the streets! That's what I say! Blood and brains! Braaaaiiiiiiinnnnsss! Just try stomping on the accelerator of a fast car with all that goo in the street! You won't go anywhere, but it'll smell like BBQ until the horde starts climbing through the windows! Blood and brains I tell you! (*Ahem...*) We'll now return you to your regularly scheduled program. $\endgroup$
    – JBH
    Commented May 30 at 7:32
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    $\begingroup$ @Bubbles It doesn't work that way, thank goodness for the OP. The mechanics of a load-carrying vehicle are, "you gear it to carry the expected maximum load and live with what that does to velocity." For your average vehicle (sedans and pickup trucks) the loads are related enough that maximum speeds aren't below speed limits. But when you shift to, e.g., a big mining truck the difference becomes obvious. Frankly, if we're designing the vehicle than yup, you're right... but that's not what the OP asked for. $\endgroup$
    – JBH
    Commented May 30 at 15:31
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    $\begingroup$ @Bubbles Keep in mind the goal of worldbuilding. The OP is trying to rationalize a choice he/she needs in their story by understanding what world rule would affect that choice. This isn't a debate about engineering, it's nothing more than "what Real World concept can I use to rationalize my choice?" and nothing more. $\endgroup$
    – JBH
    Commented May 30 at 15:32
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    $\begingroup$ @Bubbles Keep in mind that the engineers did design these trucks to be fast enough to outrun zombies; so, this is not a design flaw from a logical perspective. The problem is that they are encountering NEW zombies that are faster than expected. If they expected to have to face faster zombies, they may have opted for less torque/armor/cargo and more speed, but that was not a thing they knew they needed to consider going into the design of these vehicles. $\endgroup$
    – Nosajimiki
    Commented May 30 at 16:34
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    $\begingroup$ @Bubbles Answers should only ever be evaluated in the context of the question. If you think a "yeah, but..." statement has value to the OP's question, then you shouldn't be discussing it in comments (because it's irrelevant to the answer...), you should be posting a Frame Challenge. $\endgroup$
    – JBH
    Commented May 30 at 19:39
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The people aren't in the wagons Looking at the Oregon Trail, which also featured slow-moving wagons, the people weren't in the wagons. The wagons were loaded with supplies, so they had to walk alongside the wagons while taking occasional breaks sitting on them. For your situation, it could be that the wagons are loaded with critical supplies, and they don't have the resources to make enough wagons for everyone to ride inside the whole time, forcing most of them to rotate between walking alongside the wagons and taking short breaks every now and then on them. This also restricts the speed of the whole convoy to a (very) brisk walk, which is about 4-4.5 mph. The wagons may be able to go faster in theory, also raising the question of whether the people in the wagons leave the others behind. Or it could just be that no one bothered to make the wagons faster since they are restricted to walking speed anyways. TL;DR The convoy is restricted at walking speed since not everyone can ride in the wagons. The wagons themselves may or may not be able to go faster, but the people who are walking set the pace.

EDIT: To clarify, the reason the people aren't in the wagons is because space reserved for people to sit is space not being used to hall crucial supplies. Also, the weight builds up over 100 people, and parts may be limited, so there is no space reserved for carrying people.

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  • $\begingroup$ It's a good point, but the fact of carrying supplies back then was dictated by horsepower. If a pair of horses couldn't pull it all day, you couldn't do it. That's much less of an issue with an engine and a gearbox. Even a very small engine can pull huge loads slowly with appropriate gearing. $\endgroup$
    – Graham
    Commented May 30 at 8:33
  • $\begingroup$ @Graham a gear box will let you carry more weight by going slower $\endgroup$
    – Rad80
    Commented May 30 at 13:08
  • $\begingroup$ @Graham, you need torque to pull heavy loads. If you gear down the small motor for enough torque to pul the load, you have to make it slower. Torque of speed, pick one. $\endgroup$
    – Bubbles
    Commented May 30 at 14:11
  • $\begingroup$ @Bubbles That's true, but a reasonable-sized engine in a reasonable-sized vehicle isn't going to be affected by adding a few people to it. And it'll still be plenty fast enough (maybe 10mph) to outrun a regular zombie. $\endgroup$
    – Graham
    Commented May 31 at 7:17
  • $\begingroup$ @ Graham the problem is more of a space one than a mass one. Any room reserved for seating people is too valuable carrying supplies instead. I'll clarify this in my answer. $\endgroup$
    – Bubbles
    Commented May 31 at 13:54
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They will need to use an existing vehicle

In a post apocalyptic world, your ability to fabricate parts to suit your needs will be very limited. Attaching a bunch of armor plates and gun mounts to an existing vehicle is easy, but vehicles are not reliable when you start mixing and matching things like engines, transmissions, electrical systems, and chassis. You swap out one of these things for something heftier, and it usually causes one of the others to fail pretty quickly. Maybe not a big deal if they are designing short range vehicles that they can regularly service, but if you want to go on a cross-country trip, you don't want to compromise the reliability of your parts. Modern vehicles are whole engineering solutions where one part is meant to work with, and only with, all of the other parts that go with it.

Since your survivors won't have access to an entire vehicle fabrication facility and a full team of world class automotive engineers, it stands to reason that the best vehicles they will probably have access to will by far be the ones that already exist, not any that they fabricate or frankenstein together. So to begin answering this question we should first establish that whatever vehicles they use will almost certainly already exist in today's world, and that any fabrication they do will only be cosmetic additions and reductions to said vehicle, not a full mechanical redesign.

Tractors

Image by Geoffrey Wiseman: https://www.flickr.com/photos/diathesis/4043777622

With the conditions of the roads, you need to assume that large stretches of your trip must be done off-road in the worst possible conditions. Add to this the requirement to haul a lot of cargo, and the fact that your survivors probably don't have access to military grade ATV trucks like HEMTTs or MTVRs and you don't have a lot of options. There are plenty of non-military grade ATVs that your survivors may have access to that can go faster, but these tend to have little to no cargo capacity. They will probably also have access to a lot of large civilian trucks, but these are not ATVs and will get stuck in the mud or flip if you try to take them off road.

This leaves you with tractors. They are plentiful, civilian owned vehicles that can haul many tons of cargo off-road. Thier only downside is that they are slow. To achieve thier impressive hauling capabilities, they are geared very differently than cars and trucks to maximize thier torque. Although a modern tractor can typically max out at 25-30mph, this is thier maximum speed on a smooth road when not hauling a trailer. However, when they are off-road and carrying a heavy load, trying to go anywhere near this fast will just spin out your tires. According to multiple sources on this Agricultural Forum, tractors max out at about 4.5mph when you really push them hard with heavy loads and/or bad terrain; so, if the zombies start to chase you through muddy, overgrown, and/or uneven terrain, you realistically may not be able to physically exceed 5mph without ditching your trailers... and if you are being actively chased, you can't actually ditch a trailer without stopping because the hitch binds under the force of the tractor pulling against the resistance of the trailer.

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  • $\begingroup$ See also The Straight Story: in 1994, Alvin Straight drove a lawn mower ~250 miles, at 5mph, over about 6 weeks, to visit his brother. $\endgroup$
    – Tom
    Commented May 31 at 6:19
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Zombies have motion-based vision. There’s no reason to assume an undead brain has all the faculties of the living brain, so we can posit zombies only seeing things that move faster than a specific frame rate. This is real in various animals. 5mph would be an unusually high threshold, but, hey, zombie brains aren’t the fastest processors on the planet!

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Jerry Rigged Electric Vehicles that use Solar Power

There are plenty of EVs to go around once the apocalypses happened. After all, without the power grid functioning EVs became almost worthless to the few survivors. Internal Combustion Engine (ICE) vehicles require fuel that got used up over time by survivors or went bad.

Your survivors scavenged solar panels and spare EV vehicle parts and slapped them on whatever vehicles they deemed most practical. Think putting a bunch of solar panels on an 18 wheeler and powering the entire thing with a single Tesla motor with a very low gear ratio. Since lithium ion batteries go bad with time and add a bunch of weight (and can catch fire) to something that's solar powered the survivors don't bother powering the Jerry rigged EVs with batteries. Rather, if it's a sunny day and they know the route they just go faster.

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  • $\begingroup$ +1, alternatively they might be using RTGs or sterling engines or other energy sources which have low power output per mass/volume. $\endgroup$
    – Michael
    Commented May 30 at 5:07
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    $\begingroup$ really good answer. This would allow the author to cut down the total travel time by just saying there are a few sunny days and then have the cars go slow when its crucial for the plot and say it's due to bad weather. One could refine this and add a small diesel powered generator they scavenged as a back-up to also be able to drive at night/bad weather. And this generator is just not very powerful and can only get the vehicles to around 5mph. not enough to seriously travel but enough to keep ahead of the zombies that tried to close in at night $\endgroup$
    – datacube
    Commented May 30 at 10:39
  • $\begingroup$ Combined with the other answer that it also needs to be silent to not attract too much attention, this is good. But think more of ebike motors, not cars. Those are just too power hungry for a few solar panels. Electric motors have a wider power band than gas engines, but using them so far below their spec will not work. $\endgroup$
    – LazyLizard
    Commented May 30 at 11:48
  • $\begingroup$ @LazyLizard maybe, but I think it could work when you factor in an entire 18 wheeler roof (46m^2) worth of solar panels. Back of the hand calculations determined a Tesla going 30 mph uses 10 KWh. A "typical" solar roof in a sunny US state generates 5 KWh. So I think an 18 wheeler with a full roof of solar panels should be able to put out power equivalent to a Tesla cruising at medium speeds. After gear ratios / accounting for the larger mass this would translate to a slow crawl for the truck. E-Bike motors would be too small. $\endgroup$
    – philn
    Commented May 30 at 18:02
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    $\begingroup$ I am not sure about Tesla but Nissan Leaf can crawl at any speed. You can reliably and constantly move the car so slow that it would move slower than a leisurely walking person. Leaf has a 100kW motor, I suspect it would be using less than 1% of the power when going at those speeds. You can get to 1.1kW of power with 3 standard solar panels you can fix one to bonnet two to top. So noon time you will be at a brisk walk speed while at earlier or later hours of the day, your car will be moving even slower. Bonus: DC voltage supply of solar could work as a replacement of battery with little change $\endgroup$ Commented Jun 1 at 7:49
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With petroleum fuels almost impossible to obtain, having been used or evaporated, and nigh on impossible to refine, they have to rely on solid fuel: wood, coal, coke, etc. Consequently they have built steam engines, traction engines, armoured traction engines!

The Fowler B5 Armoured Road Locomotive

http://www.landships.info/landships/tank_articles/fowler_B5.html

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  • $\begingroup$ Interesting take! This would explain the armor on the vehicles, and perhaps their slower speed. It would also require the survivors to stop and refuel, perhaps at the worst possible time.. $\endgroup$
    – Alendyias
    Commented May 31 at 20:12
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These people are walking

You have a very large group of survivors (at least 100, possibly hundreds) of zombie apocalypse trying to cross from the east coast of US to the west coast. A small number of noteworthy characters are leading the group and making important decisions. That is an extremely long journey. This large group of people will need a lot of food and water for this epic trek. Some people have also taken their personal posessions with them, which they insist are absolutely essential. In short, there is a lot to transport.

These people got a hold of some vehicles. I still feel it makes a lot more sense for them to have repaired existing vehicles rather than make new ones from scratch, but it's not important to this answer. Regardless, there aren't enough vehicles to transport everyone in this group and everything they're carrying. Or they managed to secure enough vehicles but don't have enough gas to drive them all.

So what they had to do, was to load the food and water and personal posessions onto the vehicles and drive them at 5mph while the survivors walk alongside them. Average walking speed is about 5mph, which is why the vehicles have to drive this slow. Maybe the small children and the elderly ride on the vehicles as well, at least for portions of the journey, perhaps also those who are injured or really tired.

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    $\begingroup$ "Average" walking speed is hard to define, but for "average" adult civillians it would be about 4 km/hr, which is about 2.5 mph, or half the speed desired. Forced march pace for soldiers is 6 km/hr, though typically it works out over multiple hours to 5 km/hr with 10 min each hour spent shifting loads around/fixing blisters etc. I expect there are competitive walkers that can maintain 8 km/hr (5 mph), but not all the way across continental North America. $\endgroup$ Commented May 29 at 16:16
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    $\begingroup$ +1 even in World War 2 armies relied mostly on horses and human feet to get around. $\endgroup$
    – Michael
    Commented May 30 at 5:06
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Speed and Power adds stress and strain to the Vehicle.

Think of the old 1980s work truck that has 400,000 miles on it and is still trucking. against a Race car that needs to be rebuilt and worked on every 200 miles.

The faster you go, the stronger you need to make the parts or the more fragile the vehicle becomes - this in turn requires either more weight (which requires more horsepower) or decreases reliability. Lots of people mentioned F1 - and although the modern cars are pretty reliable - historically we used to see all manner of weird and wonderful failures.

In addition - they know that to outrun the threat that they face, they don't need to go very fast - so they are prioritizing Reliability over speed.

It is better to get there in one piece, than never get there at all.

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Speed is not as important as other factors.

In general, you build for speed if you need speed. You wouldn't build something that goes fast unless you have a need to go fast. If you don't, then all the other, more important, factors will be designed for first, and whatever speed you get is what you take.

Since the apocalypse already happened, there's no need to be in a rush. They're not running away from anything. Sure, they would have personally preferred faster, but everything else is causing the slowdown:

  1. All-terrain capability. That means tracks instead of wheels. That means a lot of the engine power goes towards moving half a ton of metal in a circular fashion.
  2. Armour and protection. That stuff is heavy, meaning more engine power going towards moving all the weight around.
  3. Size and connectivity. If there are zombies outside who are as lethal as you describe, you want to minimize having to move between vehicles, and probably add some simple gangways between them for when you have to, above the reach of the zombies. That means large and high vehicles. We're talking trucks here, not station wagons.
  4. Supplies. You're going to be on the road for a long time, and you need to bring all your supplies. Especially water is darn heavy.
  5. Durability. The most important thing about your engine isn't how fast it goes, but how far. You want a rugged engine that doesn't break down and if it does can be fixed on the road. Even if you have access to high-end performance engines, you'll leave them behind and rather take the one from the tractor or truck.
  6. Group movement. Travelling in a group of vehicles means everyone goes the speed of the slowest vehicle. Story-wise, this also gives you an interesting thing to write about when the nasty surprise hits: Some of them could go faster, but it would mean leaving the others behind.
  7. Tenacity. If you take shifts driving, 5 mph actually isn't that slow, if you can keep it up constantly. 24h at 5 mph is 125 mph. NYC to LA is 2700 miles. That's 22 days. It's a while, but it's not an eternity. You want an engine that can actually run constantly, without overheating or other trouble.

All that said, here is why maybe you'd want to raise those speeds just a little bit:

  • 22 days is still a lot, and it increases the amount of supplies they all need to take. If you could speed up to 8 mph you'd cut the travel time down to about 14 days. That's 2/3rd of the supplies, weight, etc.
  • 5 mph is the average jogging speed of humans. Zombies reaching that speed aren't all that much surprising. Average sprinting speed of humans starts at 10 mph for the untrained and goes up to 20+ mph for elite athletes. If you want it to be an actual nasty surprise, you should think about such speeds. If I were one of your survivors and you came up with a vehicle that I could comfortably jog along to, I'd be worried if that's not way too slow, even for zombies.
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  • $\begingroup$ You forgot a crucial point in your great answer: fuel efficiency. In a post-apocalyptic world gas stations must be far between and the engines have to move a lot of supplies, so to be eficient they are slower. $\endgroup$
    – Rad80
    Commented May 30 at 13:06
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    $\begingroup$ @Rad80 slower isn't always more efficient. A bigger engine is generally more efficient at full-throttle than a smaller engine. This is why be build supertankers with diesel engines a human can stand inside of. But, at the same time, air resistance grows exponentially with velocity. So for any given vehicle there's a crossover point. As an example, my old jeep gets the best fuel economy at about 70mph, which is hardly slow. $\endgroup$
    – Perkins
    Commented May 31 at 18:17
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They’ve built tanks

Zombies are dangerous at close ranges. Therefore, the further out they are, the less dangerous they are. Your survivors need to more without risking infection, so they built mobile fortresses on truck chasis—or whatever works (I’m not a mechanic or engineer). They put plate armor on the vehicles in order to protect for any hostile survivors—after all, there’s no guarantee that the people in the path are friendly. This results in a fairly bulky and heavy.

According to Wikipedia, M1 Abrams can hit 40 kph (25mph) during off-road condition. Assume there’s a minus 10 kph from dodgy road conditions and safe driving and another minus 10-20 kph because they had to build a new engine for each of the vessels and because they need to stay together and they have to drive as slow as the slowest one. That leaves about 10 kph (6.5 mph) as your top speed, driving without obstruction.

However, the drivers are also going to need to clear debris as needed—the vehicles they built resemble more like moving fortresses/the dream RV of a medieval weapon’s enthusiast (or gun nut, if guns kill zombies). This will further be hindered by the inconvenience of zombies on the road, which will need to be dealt with before clearing debris.

Also, they’ll need to invest in cattle-catchers on both sides of the vehicle (so they can drive forward, or back up). Assuming that entire towns/cities are infected and throwing themselves at these tanks, there’s going to be a pretty large amount of meat in the way. Cattle-catchers will alleviate this to some degree—instead of running them over, they’re pushed to the side—but every now and then, dead zombies (the twice killed?) will need to be cleared from the cattle-catchers/front of the vehicles.

Finally, there the issue of spare parts and other mechanical problems. If anything breaks, they’ll need to stop and get replacements. This includes weapons, their vehicles, and just general stuff to keep your people alive.

TL;DR:

Heavy armored vehicles, built MacGyver and friends, have all sorts of problems that limit their movement—such as terrain, mechanical and biological problems. Also, zombies will get stuck in front of the vehicles, or they need to invest in further weight to add cattle-catchers

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  • $\begingroup$ How is a person in danger of being killed by fast zombies when they are holed up in a tank? They wouldn't really be in any danger, which seems like a problem for the OP's premise. $\endgroup$
    – Bubbles
    Commented May 30 at 4:13
  • $\begingroup$ @Bubbles, things break, zombies build up in front of the vehicles, people get sick and need to raid the nearest drugstore, etc. I’m sure the OP can think of several more ideas to make his characters suffer $\endgroup$ Commented May 30 at 14:05
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    $\begingroup$ @ blue skin and glowing red eyes Yeah I guess I didn't consider them being put under siege in their vehicles. $\endgroup$
    – Bubbles
    Commented May 30 at 14:12
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Because the "salvaged parts of older civilization" do not include cars, only Vespas. At least, they don't within walking distance of where the tinkerers fitted the engines to the wagons.

Moreover, with limited skill and available parts, it is too difficult to harness one wagon with more than 1 engine. So the available power is chronically low and the engines must work at the lowest gear ratio all the time.

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Think average velocity, not top speed. (This is a frame challenge.)

There's little reason for a wheeled vehicle with no walking parts to have a top speed slower than about 20mph. Wind resistance and rolling friction, even off-road, just isn't that big of a deal at low speed.

Conversely, if 5mph really is your top speed, a slowly walking person will easily overtake your vehicle over rough terrain. Your larger, more constrained vehicle has to go around, double back, dig out when it's stuck and be hyper-cautious to avoid getting stuck in the first place (just how deep is that mud puddle?), tow or demolish obstacles, avoid enemies, go downhill without losing control, struggle uphill, rest, find fuel, etc. Pretty soon 5mph is 10 miles per day with military discipline and maps, or 5 miles per day for disorganized rabbles.

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  • $\begingroup$ Bulldozers and tractors typically go way less than 20mph because they are geared to pull/push lots of weight. With cities/roads being destroyed, expect that to be a common scenario. $\endgroup$
    – user4574
    Commented May 29 at 23:32
  • $\begingroup$ Wheeled tractors usually have a top speed of about 40mph. If you mean that they would be much slower than that over long distances because they would frequently have to slow down for reasons like pushing or pulling barriers out of the way, that is an example of what I was saying. $\endgroup$
    – g s
    Commented May 29 at 23:58
  • $\begingroup$ @g s While some may go faster... "Most farm equipment is designed to travel at speeds of 15-25 miles per hour". penndot.pa.gov/TravelInPA/Safety/TrafficSafetyAndDriverTopics/… . Looking at some specific models, the John Deere 9R390 goes 25MPh, the 9R440 goes 25MPH, the 9R490 goes 25MPH deere.com/en/tractors/4wd-track-tractors/?cid=VURL_9r_9rt . Bulldozers will go slow also. $\endgroup$
    – user4574
    Commented May 31 at 1:19
  • $\begingroup$ How about loading the walking persons with 50kg each of armour and impedimenta? Will they still run >= 5mph on rough terrain? $\endgroup$ Commented May 31 at 6:12
  • $\begingroup$ @FrançoisJurain A slowly walking person does not run. A slowly walking person slowly walks! $\endgroup$
    – g s
    Commented May 31 at 14:19
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That's actually easy and natural

Building fast-moving car is difficult. You need a gearbox and clutch in simple ICE car to go fast. Gearbox is a complex stuff to build and if you get rid of a gearbox your top speed is inversely proportional to available engine torque. And if your torque is not enough your engine will stop. Electric engine starters are also difficult components to build. Early cars used hand-crank starter instead for that reason. Starting an engine with hand-crank is a slow and dangerous process. Finally its much easier to build low-power engine than a high-power one.

Now lets combine that together. Your group managed to build an engine and a crude clutch, but they were unable to make gearbox and a starter. Engine is a poor quality and low torque and it has to be started manually with hand-crank - they can't build anything better. If engine stops its a problem that takes a while to fix with hand-crank. To make things even worse, hand-crank has to be connected to engine crankshaft and for simplicity of car construction that crankshaft is located in a place accessible only from outside of a vehicle (notice that its the case for almost all cars that do use crank-shaft). Obviously it is a big flaw for a vehicle riding on zombie-infested roads, so you want your engine to keep running no matter what happens - if it stops, your car could be dead for minutes and restarted only when someone is spending that time outside of a car (I think that could be a nice plot device). Engine stops when its torque is too small to keep moving the car, so you want torque to be as high as possible. This naturally meant that your group decided to use reduction gear for engine and without gearbox that's the only gear they got. Top speed is now slow, but reduction gear compensates poor torque of engine and makes its much less likely to stop.

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The question remains, why would the resourceful group of survivors, consisting of MacGyvers, engineers, and tinkerers, construct slow-moving motorized wagons from salvaged parts of older civilization instead of faster cars?

There is a lot of wreckage along the way. So, they are essentially constructing armored bulldozers. Regular modern bulldozers have a top speed of less than 10 miles per hour. With heavy armor added expect it to be less.

The vehicles need the lower gear ratios to plough through the wreckage, while hauling heavy armor, supplies, and climbing steep slopes.

The amount of torque at the wheels depends on gear ratio. For the same amount of engine power...

  • With a high gear ratio, you can move fast.
  • With a low gear ratio, you can pull/push lots of weight.

Sure, an M1A1 tank is 63 tons and gets up to 45mph, but it also has a high-end turbine engine.

Also worth noting that constructing transmissions with multiple gears is way harder than just using a fixed gear ratio. And if doing so you have to gear it for the worst-case load (so lowest gear).

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Zombies are dangerous. Like a minefield. You want them gone in the long run - to repopulate the earth and be done with it.

So you need- Zombie Garbage disposal. You drive a bait truck through the area, extra slow, loud and attractive, all the lights and whistles, to get even the crawlers to follow you. Yes, this can make circling necessary to give the slow ones a chance to catch up.

Then its straight towards the disposal site. Some recommend large coyote or boar pits. Coyotes are recommended, because bears take to large bites and then die from the still wriggling hands damaging the intestines. If you are high tech, you want either a burn valley (solar driven if you are environment friendly) - or a industrial sized wood chipper. They are not smart, so the best way to do it is to automate the whole process.

Who can take the flesh out? Stomp it down for you? The Garbageman can!

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    $\begingroup$ "Woodchipper beats everything" ~ Rufus (Supernatural) $\endgroup$
    – Nosajimiki
    Commented May 30 at 16:25
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Zombies only see faster-moving objects

The zombification altered the ability to see slow-moving objects and only the ones below 5 km/h are visible to them. Thus the upper limit of the wagon speed.

The other zombies "surprise, we see at every speed", because of the place they live in, unlocked the ability to see slow moving stuff. Except when [here you can add things that will help the story, it can be temporary, or dependent on the wind or the sun or the speed they move at or the presence of vibranium, or...]

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Because of the fun crunching sounds the zombies make when you run them over slowly!

Screenshot of a video of a car tire slowly crushing rubber toy skulls containing some sort of orange or red gel.

Credit: Crushing Things With Car Best Long Compilation | Running over stuff with a car by Crushing Experiments.

Take out as many of these dastardly things as you can while you are enroute. Their mushy decomposing bodies can help smooth the bumpy terrain.

For more inspiration, search for cars running over stuff on YouTube. Not that I've... you know... watched this sort of thing before. 😬

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Consider the physics of a vehicle traveling over uneven terrain that goes over a bump. The forces acting on the vehicle will look something like:

$$ F = m(\frac{v^2}{r}) + mg $$

where 'm' is the mass of the vehicle, 'v' is the speed, and 'r' is the radius of the bump. The 'v' being squared is the kicker. Increasing your speed by a factor of four will cause that bump to exert 16 times more force on your vehicle.

These are scavenged vehicles. You can't build more replacement parts. Keeping what you have running as long as possible is vital. Your shocks are probably already trashed from driving off-road all the time. Vertical movement is very bad with poor/no shocks because there's not much to prevent you from slamming into the ground when you come down off the bump. The vehicle lacks protection against these types of hazards. You need to reduce speed to the point where what's left of the frame can handle the forces it's experiencing.

Shocks don't just protect the car, they protect the passengers. Driving off-road at speed can be brutal for anyone in the vehicle. You get jostled around enough to end up with whiplash or to bang your head on the ceiling, and it's difficult to see the road ahead clearly when you're moving around that much. You might be able to sprint away in an emergency, but your occupants won't be able to tolerate that for very long.

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Wedding cake

These vehicles are carrying things that absolutely must not be jostled. Like a wedding cake. You move slowly so it doesn't jostle around.

Okay, it has to be something important.

So, a mobile hospital. It carries people who have been injured -- forget zombies: scavenging abandoned equipment in dangerous territory is a good way to get an old-fashioned shop injury. You're not just gonna amputate MacGuyver Jr's duct-taping arm, are you? So he and others are convalescing as they travel, and if the vehicle jostles around a bunch, he's gonna cry out in pain, and if there's one thing ol' Zed loves, it's the dinner bell of human pain. Pavlov's dogs ain't got nothing on it.

Also, in the back is an elderly lady who is the group's ham expert. She's carving the ham and it's gotta be done just right. I mean, HAM radio. She's been trying to fix that damn radio for weeks. I don't know if she can do it, but if she can, they might be able to establish contact with other survivors along the way. But it's delicate work, what with her makeshift soldering iron and big magnifying glasses, etc.

Another such vehicle has the group's astrologer. He can't read your Tarot if the cards are flying around, and everybody wants to know if the journey will be successful. And the chicken bones and tea leaves don't stay put if you're flying down the road like Ferris Bueller's garage attendant.

A couple of theses vehicles are really tall, like triple-decker busses. They can't go fast or they risk tipping over, which would be its own catastrophe. The top level of every bus is filled to the brim with kiddies, and the kiddies are all working on big jigsaw puzzles to keep them busy and distracted from the horror and devastation. Ever try to do a jigsaw puzzle in a Maserati? (In the HOV lane, obviously.)

Near the front, you've got the Crow's Nest, which is what you get when you cross a flatbed truck with a firewatch tower. Up top you've got a squad of the sharpest-eyed lookouts you ever saw, with binoculars and some rifles and all the convoy's moustache wax -- right out of the Army Field Manual. (The guy in 28 Days Later says that hot water is the foundation of civilization, but we know better.) They can't do any of their jobs -- spotting, judicious sniping, or handlebarring -- if the Nest is swaying around like a bare-faced man-boy, three sheets to the wind after his thimble of Coors Light.

These people do not feel the need for speed. Their goal is a stroll.

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Frame challenge: You should have been able to go fast.

But one of the front-most cars broke down when they tried, at a bad spot, blocking the rest.

During such an apocalypse, you can't make new car part, and even if you can, they would be sub-standard. This means any repairs done have badly fitted parts. With knowledge of how to maintain those parts properly well might also not be around anymore. Also, people don't know they can't go as fast anymore. They hardly ever try, to prevent stressing the parts.

To lower wear-and-tear, to reduce stress on substandard parts, to conserve fuel, to remain more silent, or to remain safe on a road full of blocked highways, your cars almost always drive very slowly. Normally, if they go fast they gradually accelerate, until the engines start making bad knocking sounds.

he front most cars are navigating some narrow part, like a tunnel. Now your event happens, and people panic. They floor the pedal and they try to get out of there too quickly now that danger is upon them. They give a lot of extra gas, to try and get out quicker, but ask too much of their fragile [insert part here] and the cars breaks down. The car comes to a halt, and all the ones behind are stuck.

Having no choice but to leave their cars behind and continue on foot, the untiring zombies quickly gradually catch up.

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