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I was wondering why the wheels of aircraft are relatively small compared to for example cars? I could imagine that it is more efficient when traveling to have smaller wheels. The majority of commercial plane tires are only 27 inches in diameter and smaller aircraft have tires at 15 inches [1]. The wheels of standard road cars are between 14 and 19 inches in diameter [2].

Sources:

Quinten
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    Closely related https://aviation.stackexchange.com/q/91559/60886 – GremlinWranger Oct 15 '22 at 12:35
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    Hmmm. You quote cars at 14-19 and planes at 15-27. That’s a lot of overlap with plane tires being bigger. And then there’s the “40s” on some bush planes – Jim Oct 15 '22 at 16:41
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    One of the reasons for large diameter tires is for rolling over bumps and obstacles. Planes (other than bush planes) don’t have a need for that. – Jim Oct 15 '22 at 17:52
  • A more interesting question is why cars have big wheels and trucks and buses have even bigger wheels. It’s also interesting how tiny the wheels of some trailers are (which kind of shows that it’s possible to have small wheels, just not optimal). – Michael Oct 15 '22 at 20:39
  • It depends on their purpose. Aircraft which have to land on rough terrain happen to have huge wheels: https://www.pinterest.com/pin/595601119461493175/ – vsz Oct 16 '22 at 12:13

2 Answers2

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As usual in the aerospace world, it is a compromise among contrasting requirements.

Wheels must be big to pack enough disks for the (disk) brakes. But they must be as small as possible too, in order to minimise weight, drag, needed space for stowing and inertia when they suddenly start to rotate upon touchdown.

sophit
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    Same for cars, actually - the rims are as small as possible, but the brakes must fit. Some people like ridiculous-looking large rims with tiny brakes inside, though... like putting 20 inch rims on a tiny 200hp diesel – Haukinger Oct 15 '22 at 12:40
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    Also the wheels are unused for almost the entire flight. Unless they are touching the ground they are just taking up space and weight, so there are strong incentives to make them a small as possible. They also need to be quite strong, so something like bicycle wheels would be unsuitable. – Frog Oct 15 '22 at 19:46
  • @Frog tell that to the Wright brothers. – Mark Ransom Oct 15 '22 at 23:40
  • The inertia point is good for tire wear on landing. Maybe strong light spoked wheels might not be a bad idea after all. – Robert DiGiovanni Oct 16 '22 at 02:46
  • @Haukinger Note that airplanes get around the brake diameter issue by using a stack of multiple discs and friction materials... they are very similar to automatic transmission clutch packs and motorcycle clutches rather than car disc brakes. This shows the different design trade-offs: cooling is awful relative to cars, but in normal use, you have at least an hour between major uses. For the dimensioning case (rejected takeoff), the brakes may very well be destroyed in one stop, or at the very least, have to sit and cool for 3+ hours. – user71659 Oct 16 '22 at 06:45
  • @user71659 exactly, also airplanes can just add more wheels if they need more braking power. And regarding cooling, you have a fire truck ready and waiting each time you actually use the brake, if you're a plane :D – Haukinger Oct 16 '22 at 13:42
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    @MarkRansom All of the Wright Flyers used skids. – Davidw Oct 16 '22 at 16:14
  • And even the later Wright aircraft that did have wheels, also had skids in combination with the wheels. – Davidw Oct 16 '22 at 16:17
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    @Davidw I did not know that, thanks. Just assumed since they were bicycle makers they would have used bicycle wheels. Probably would have taken me 2 seconds to find an online image before I opened my mouth. – Mark Ransom Oct 16 '22 at 16:40
  • @RobertDiGiovanni: "Maybe strong light spoked wheels might not be a bad idea after all". It might be a bit difficult to put spokes on a wheels like this :) – sophit Oct 16 '22 at 19:22
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Car wheels have an entirely additional job besides supporting the chassis that aircraft don't: transfer engine power into motion.

Aircraft have only one: converting rotation into heat. Other than during deceleration the wheels spin freely. Whereas on a car larger wheels mean lower speeds in the transmission, gearbox and ultimately crankshaft. (You can try this by changing the size of the wheels on your vehicle and observing how the speedometer now needs to be recalibrated)

Brakes themselves are pretty good and don't dictate the size of wheels once you cross a certain threshold.

Another key factor in wheel size is ground clearance. The axle is often the lowest clearance on many vehicles (suspension lifts are popular in off-roading world which is slightly bizarre given it does nothing to change the minimum ground clearance unless the sump or gearbox already hangs below the axles). This isn't really a concern in aviation, in part due to the fact there aren't axles connecting e.g. left and right main gear, the wheels in each set function almost like a single wheel in terms of ground clearance. (and the clearance is really there for the engines rather than particularly rough terrain)

So it's not just a case of compromising on requirements between use cases - some of those requirements don't even exist in both cases.

Flexo
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  • On an off-road vehicle, the wheels will lift the axle over most obstacles. The body gets no such lift. Yes, a lifted suspension gives no advantage for rocks in the middle of the track, but for other obstacles such as logs or ledges, it greatly increases capabilities. – Mark Oct 18 '22 at 00:42