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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[–] 4 points 4 months ago* (3 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.

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  • [–] [S] 1 point 4 months ago (2 children)

    higher center of gravity, which is more stable.

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

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  • [–] 2 points 4 months ago* (last edited 4 months ago) (1 child)

    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.

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