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Why do tires have the width and diameter they do? (sopuli.xyz)

submitted 4 hours ago* (last edited 4 hours ago) by rain_enjoyer@sopuli.xyz to c/nostupidquestions@lemmy.world

25 comments fedilink hide all child comments

Why are bike tires so narrow and large diameter compared to car tires? What tradeoffs are here exactly? Motorcycle and some ebike tires are more similar to car tires than to bike tires, so i guess it has something to do with braking length at maximum expected speed, and probably also with weight of vehicle, as to not exceed some specified pressure on road. There has to be so many more reasons (weight? air resistance? some other things affecting efficiency or safety? ???)

update: apparently friction involving things that are bendy is monstrously complicated subject, and also there are material limits like maximum allowed shear stress

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[–] Triumph@fedia.io 10 points 2 hours ago

I can speak to motorcycle tires.

Moto tires are rounded, not flat like car tires. They're designed to provide an optimal contact patch no matter the lean angle. Often, Moto tires won't be perfectly rounded, but come to a bit of a point in the middle. This makes for a smaller contact patch going straight, for better fuel economy, and a bigger one while turning for more traction. Finally, moto tires are often made of multiple kinds of rubber: harder in the center for lower rolling resistance, and longer tread life; softer on the sides for more traction while turning.

I'd bet that small bicycle tires share some of these characteristics, being rounded for sure, possibly others.

[–] grue@lemmy.world 15 points 3 hours ago (1 children)

The pressure the tire exerts on the road is always equal to the pressure it's inflated to. When the vehicle weight increases while tire PSI stays the same, the contact patch (area squished flat against the pavement) increases in size.

Bike tires are narrower than car tires because bikes are much lighter (so the contact patch doesn't need to be as wide), and also because they lean into turns (so the contact patch can't be wide). Bike tires are often larger diameter than car tires because they have more gyroscopic effect and thus make the bike easier to balance. They also make it easier to ride over bumps, but on a road bike (as opposed to a mountain bike) that's probably a relatively minor reason.

I think motorcycle and ebike tires are a little wider (but still round in cross-section, so not like a car tire) for durability reasons because all the forces they're subjected to are larger.

[–] NeatNit@discuss.tchncs.de 2 points 2 hours ago (1 children)

When the vehicle weight increases while tire PSI stays the same, the contact patch (area squished flat against the pavement) increases in size.

If the vehicle gets heavier, doesn't the tire pressure increase?

[–] Elting@piefed.social 2 points 1 hour ago

Yeah I think in that case they had meant to say that the amount of air in the tire stays the same and the PSI increases when the tire deforms. They are right that the PSI in the tire has to match the pressure on the road, that can only happen if the PSI increases when you increase the load.

[–] Alsjemenou@lemy.nl 11 points 4 hours ago (1 children)

More rubber on the road means more grip. As the weight and corner speeds increase, you need more of it.

In adverse conditions, wet, mud, you need profile on your tires to channel the water while keeping rubber on the road and maintaining grip. Wider tires make up for the rubber lost in grooves.

More grip also means more resistance. So there is always a balancing act between the grip you need and the resistance you can put up with.

[–] untorquer@quokk.au 2 points 3 hours ago (1 children)

There is no pressure nor area in the friction equation. 'Fu=u*Fn'

Fu - friction force

u - friction coefficient

Fn - normal force

Pencil thin tires of the same material have the same grip as extra wide ones in dry conditions. Tread geometry can change for wet but only to prevent hydroplaning, friction is still the same between the tires fire a given condition.

More contact area means less stress on the rubber which means the rubber wears less for the same load. A car weighs 2 tons, a bike+rider is less than 100kg/200lb. 1/20 the contact area for similar wear. If a car had bike tires they would probably melt after a few km

[–] Azzu@leminal.space 3 points 2 hours ago* (last edited 2 hours ago) (2 children)

While there is no area in the friction equation, in the real world it is nonetheless a factor. You got to remember that a larger area makes it so small patches of lower friction (sand patch or water or trash or whatever) have less of an impact, with more area you have a higher chance to still be in contact with the asphalt. More area also gives more opportunity for the ridges in tires to displace water or grip onto gravel/dirt. While simple contact friction is the same, total friction is not necessarily depending on the conditions.

[–] untorquer@quokk.au 1 points 23 minutes ago

Responding to:

More rubber on the road means more grip. As the weight and corner speeds increase, you need more of it.

However,

a larger area makes it so small patches of lower friction (sand patch or water or trash or whatever) have less of an impact, with more area you have a higher chance to still be in contact with the asphalt.

Unless we're talking about liters of sand/gravel then this fails to explain how motorcycles have as much grip as cars do, as is evidenced by cornering ability.

More area also gives more opportunity for the ridges in tires to displace water or grip onto gravel/dirt.

Tread depth and pattern handle these.

...

Racing tires are wider because they need to handle higher loads. Racing slicks also maximize contact area to extend tire life and reduce wall thickness. There's thermal conduction as well. Rubber is an insulator. Rubber friction changes with temperature. So does modulus, leading to more deformation and thus more heating. Too thick rubber makes a hot tire that loses friction coefficient. Too thin and it wears too fast, you're on steel 3 laps in. so you make it wide enough to distribute the load, reduce stress, and control heating while trading off mass. This is also controlled by chemical composition. After that you're designing for weather conditions.

Passenger vehicle tires focus more on climate and adverse road conditions and long life. Because they are much lower performance, they have much lower loads and use tires with less contact area. The same is the case for motorcycles and bicycles.

[–] AmidFuror@fedia.io 1 points 57 minutes ago

This is perhaps an easier to understand explanation:

https://www.physlink.com/Education/Askexperts/ae140.cfm

Although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together.

If you were to increase the force as you increased the area to keep PRESSURE the same, then increasing the area WOULD increase the frictional force between the two surfaces. Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

[–] Miller@lemmy.world 2 points 2 hours ago (1 children)

How could they not have the width and diameter they do?

[–] Azzu@leminal.space 1 points 2 hours ago

Are you joking? Car wheels could theoretically have a diameter of 1 inch/3 cm and a width of 2 meters. Why do they not is the question.

[–] OhNoMoreLemmy@lemmy.ml 6 points 4 hours ago* (last edited 4 hours ago) (2 children)

The answer to width is rolling resistance.

Tires stick to road which is great for accelerating and breaking, but it makes maintaining your top speed harder. This doesn't matter if you have a big engine, but it makes a difference if you're cycling.

For a road bike, you want to minimize the contact area of the tire with the road so you have very narrow tires and inflated a lot so that they don't deform much under your weight.

The type and width of tires changes depending on what you're riding on. For off road you have wide knobbly tires that will catch in mud and push you forward. For riding on a beach you have very wide smooth tires for traveling over sand.

The diameter is about stability. If you have small wheels turning fast it effectively lowers your center of gravity due to gyroscopic effects (this is a massive over simplification of the physics) big wheels turning more slowly results in a higher center of gravity, which is more stable.

Cars have four wheels and don't need to care about this.

[–] Successful_Try543@feddit.org 2 points 3 hours ago* (last edited 2 hours ago)

For a road bike, you want to minimize the contact area of the tire with the road so you have very narrow tires and inflated a lot so that they don't deform much under your weight.

It's slightly different:

As the top speed of a road bike makes air resistance (drag) an important factor, road bikes use narrow tyres, resulting in a smaller silhouette area than a wide tyre (on a rim of the same diametre) would have.
As the rolling resistance increases with the length of the contact area, with the same internal pressure (inflation), i.e. same area of contact, narrow tyres have a higher rolling resistance than wide tyres. Thus, to (over-)compensate and decrease the length of the area of contact the internal pressure of road bike tyres is much larger than of normal, wider tyres.

As a result, narrow tyres of road bikes have smaller drag and due to over-compensation by inflation an even lower rolling resistance than standard bike tyres.

Edit: This over-compensation is possible for road bikes, as hard surfaces (asphalt, concrete, pavement) allow a high surface pressure.

For tyres of dirt bikes not sinking deeply into soft ground (gravel, soil), they need to have a low ground pressure, i.e. a large area of contact and low inflation and thus, cannot be narrow, but have to be wide.

[–] rain_enjoyer@sopuli.xyz 2 points 4 hours ago (1 children)

higher center of gravity, which is more stable.

you sure about it chief? bigger wheels also have larger rotational inertia

[–] OhNoMoreLemmy@lemmy.ml 4 points 4 hours ago* (last edited 4 hours ago) (2 children)

Yeah, try riding a folding bike with small wheels, you wobble a lot more.

It's not about the magnitude of the rotational inertia (which is roughly the same or maybe a bit higher on larger wheels that are probably heavier), it's about the center of the gyroscopic effect. It's at a greater height if you have big wheels, and closer to the ground if you have small ones.

[–] floquant@lemmy.dbzer0.com 3 points 2 hours ago

A low center of gravity is more stable than a high one.

Gyroscopic effects do exist on bikes but they are not the main source of stability - it's the fork geometry, which tends to straighten the handlebar

[–] Successful_Try543@feddit.org 2 points 2 hours ago

the magnitude of the rotational inertia […] is roughly the same or maybe a bit higher on larger wheels that are probably heavier

The rotational moment of inertia increases a lot due to the larger diameter J ~ m r^2. Even if the masses were the same, the relatively heavy tyre and the rim are further away from the wheel hub.

[–] AeroNaut@lemmy.world -1 points 4 hours ago (2 children)

Hello! Probably a lot simpler than you think. Here's a couple of things to consider:

The larger the tyre (diameter), the higher the top speed achievable practically. This inversely scales with acceleration, so smaller tyres make acceleration of the vehicle easier. An example you might see is in a city car (small tyres, think a smart car or a honda jazz) versus a sedan (larger car optimised more for motorways).
The thicker (wider) the tyre, the more it has contact with the ground improving grip. The tradeoff is that the tyre becomes quite heavy, which is worthwhile for, say, a drag car, but not for a bicycle where weight really matters.

Hopefully that helps, there are other reasons like you say and there is more to it, but the above are the primary considerations that would be made for the design conditions of the vehicle to my knowledge.

[–] rain_enjoyer@sopuli.xyz 1 points 4 hours ago (2 children)

The larger the tyre (diameter), the higher the top speed achievable practically

this only holds when it's using the same transmission?

as of width, here i think tradeoff is that with wider tire you can use lower pressure, and if it's wide enough, also lower diameter. with lower pressure i think there might be less wear (?) but also bigger width means there's more rubber to flex and that means energy losses by this mechanism. this is why it makes sense to use wider tires where all the power is not supplied by user

[–] Successful_Try543@feddit.org 2 points 2 hours ago* (last edited 2 hours ago)

The larger the tyre (diameter), the higher the top speed achievable practically

A larger wheel allows a smoother ride as a smaller wheel and thus allows for higher speeds on uneven ground. They also have a lower rolling resistance as their curvature ("roundness") is smaller and thus, for identical inflation, need to be deformed less to obtain the same area of contact (which on hard surfaces, is defined by the load onto the tyre and its inflation).

Wider tyres have a lower rolling resistance than narrow tyres when the internal pressure is identical. Thus, wider tyres can be driven with less inflation.