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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:

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    $\begingroup$ Closely related aviation.stackexchange.com/q/91559/60886 $\endgroup$ Oct 15, 2022 at 12:35
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    $\begingroup$ 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 $\endgroup$
    – Jim
    Oct 15, 2022 at 16:41
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    $\begingroup$ 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. $\endgroup$
    – Jim
    Oct 15, 2022 at 17:52
  • $\begingroup$ 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). $\endgroup$
    – Michael
    Oct 15, 2022 at 20:39
  • $\begingroup$ It depends on their purpose. Aircraft which have to land on rough terrain happen to have huge wheels: pinterest.com/pin/595601119461493175 $\endgroup$
    – vsz
    Oct 16, 2022 at 12:13

2 Answers 2

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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.

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    $\begingroup$ 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 $\endgroup$
    – Haukinger
    Oct 15, 2022 at 12:40
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    $\begingroup$ 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. $\endgroup$
    – Frog
    Oct 15, 2022 at 19:46
  • $\begingroup$ @Frog tell that to the Wright brothers. $\endgroup$ Oct 15, 2022 at 23:40
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    $\begingroup$ @MarkRansom All of the Wright Flyers used skids. $\endgroup$
    – Davidw
    Oct 16, 2022 at 16:14
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    $\begingroup$ @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. $\endgroup$ Oct 16, 2022 at 16:40
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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.

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  • $\begingroup$ 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. $\endgroup$
    – Mark
    Oct 18, 2022 at 0:42

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