What would happen if ultralights had off road treaded tires?

Mountain bike tires are made for going off road through mud, etc. They have voids between the tire lugs to clear mud etc.

Why not off road going airplanes?

I assume the reason is, the lugs would catch air and start the tire spinning during flight?

Is this correct?

Or is it just a drag thing?

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    $\begingroup$ For posterity, lugs are the "knobs" or "bumps" sticking out from a tire, which make up the part of the tire which contacts the ground. (When I did a Google search for 'tire lugs,' initially Google just told me about lug nuts, which are unrelated to the lugs mentioned in the question.) $\endgroup$ – Terran Swett Aug 8 '19 at 15:41
  • $\begingroup$ Not sure what type of tires are currently used on an ultralight, but my hunch is that your typical MTB tire is larger in diameter and heavier than what you'd want for your ultralight. Even very light competition MTB tires/rims. $\endgroup$ – FreeMan Aug 8 '19 at 15:41
  • $\begingroup$ "Ultralight" is an ambiguous term. It's a very different thing in the US compared to the rest of the world. I doubt mountain bike tires would handle the forces of a half ton aircraft landing at 50-ish knots, so that's probably not what you're referring to, but you might still want to clarify what variation of "ultralight" you're referring to. $\endgroup$ – a CVn Aug 8 '19 at 21:03
  • $\begingroup$ Fair enough. Actually my question was just about aggressive treads vs smooth tires $\endgroup$ – Fred Aug 10 '19 at 13:32
  • $\begingroup$ Consider the difference between a tire used as part of a propulsion system and a tire not used as part of a propulsion system. Consider the difference between a tire used as a part of a steering system and a tire not used as part of a steering system. $\endgroup$ – quiet flyer Aug 10 '19 at 16:28

Mountain bike tread is meant to sink into the mud in an attempt to gain traction on firmer ground beneath the mud. A plane generally doesn't want to sink into the ground (even during braking), if it sinks too far during landing or take-off, the airplane is more likely to catch the forward gear and flip/strike the prop.

Bush plane tires are designed to land on soft fields. The difference between mountain bike tires and bush plane tires is that the high surface area of the bush plane tires attempt to "float" on top of the soft ground rather than dig into it:

Bush plane tires

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    $\begingroup$ Excellent point. $\endgroup$ – John K Aug 8 '19 at 15:58
  • $\begingroup$ So....If you did use a mountain bike treaded tire, would it start spinning during flight? $\endgroup$ – Fred Aug 8 '19 at 16:07
  • $\begingroup$ If the tread is non-reversible, I'd imagine they could due to the small amount of asymmetric drag. I've got Continental der Barons on my MTB, the one side of the tread is chamfered while the other isn't, the top of the tire would produce more drag than the bottom. Would also depend on the wheel bearing. The larger the aircraft, the heavier-duty the bearings and the higher the bearing rolling resistance $\endgroup$ – LStrom Aug 8 '19 at 16:46
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    $\begingroup$ @Fred: It's quite normal for light aircraft tires to spin during flight. Doesn't hurt anything. WRT treaded tires, you want the least amount of friction practicable for takeoffs, and as much flotation as possible for soft fields. AFAIK, the only reasons for tread grooves in airplane tires are 1) to prevent hydroplaning on wet runways; and 2) as a wear indicator. $\endgroup$ – jamesqf Aug 8 '19 at 16:55

It's mostly the tendency of the lugs to throw up dirt and mud, and the fact that most of the time you aren't doing the hard braking in soft ground that would make tread lugs worth having, so it's really no point unless you really want soft ground traction for braking and are willing to live with your tires coating the bottom of the wings with crud all time. Most ultralights stop pretty quickly when they land anyway and don't really need heavy braking ability on soft terrain.

Big airplanes in the 30s and 40s commonly used diamond tread tires because they still operated from grass a lot, but that practice faded as soon as paved runways became the norm.

If your ultralight is a taildragger with good brakes and lugged tires, you'll end up with too much braking traction and nose it over, so it needs to be a trigear if you want to be able to do that. I think you'd find in the end that 90% of the time, having lugged tires or regular ones makes very little difference overall if you are only landing at 25 mph in the first place.

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  • $\begingroup$ Hmmm-- related question-- what was the tread on the tires of the Spirit of St Louis (Ryan NYP) -- Lindbergh carried mud from New York on the bottom of his wings all the way to Paris. $\endgroup$ – quiet flyer Aug 10 '19 at 16:30
  • $\begingroup$ Tires of the time typically used a diamond tread pattern. $\endgroup$ – John K Aug 10 '19 at 17:46

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