Eli5: Why do offroad bicycles have fatter tires while offroad motorcycles have skinnier tires?


Eli5: Why do offroad bicycles have fatter tires while offroad motorcycles have skinnier tires?

In: 17

Gas bikes don’t need as much to cross harsh terrain like sand. But if you’re pedaling, more surface area will get you down the beach easier due to less resistance.

Thin, light-weight tyres are more efficient because there’s less weight to carry around and they cause less wind resistance. On hard surfaces they have less rolling resistance too. There’s an advantage to having the thinnest practical tyres and so road racing tyres are pretty extreme. Everyday cyclists want a little more comfort and less maintenance so their tyres are larger.

Off-road the problem is often that thin tyres will sink into soft terrain and so be actually less efficient. Also, thin tyres with the necessary very-high tyre pressures can’t absorb bumps in the surface and, not only can this make for a very rough ride, it will lead to failures of the tyres, wheels and other parts of the bike.

Road motorbikes have plenty of power to go fast and can thus handle some lost efficiency from beefy tires. Wide tires give more stability, traction and offer more space for a gradual curvature for leaning into a turn. Offroad bikes need to be nimble and maneuverable and have to sacrifice less width for extreme leaning angles. Touring offroad bikes are kind of a middle ground, which reflects in the tires being not so studded, but also not so wide.

Road bicycles need to make the most out of muscular power and the thin, stiff and lightweight tires are very efficient, allowing cyclists to hold higher speeds for a longer time compared to a mountain bike. Mountain bikes need wheels and tires to be able to take a beating and be more stable on uneven underground, as well as have studs to bite into loose underground. This all is at the expense of speed on paved roads.

Off-road bicycle: i want to keep the tire pressure as low as possible to gain grip on surfaces. This provides also a smoother ride which increases the visibility: high frequency vibrations are absorbed/prevented by the tires. The higher the tire pressure the more your eyes shake on off-road. If you reduce tire pressure, you need to increase the contact surface to compensate. This is true for downhill riding. It really feels better and grip better. As soon as you are not propelled by gravity, you want to get higher pressures and smaller tires, like on gravel bikes. More pressure, smaller tire= less friction and more speed for the same muscle power. And this is why road bikes have very small tires.

Motorbikes off road: I’m not using one since a long time… small tires makes tight cornering easier. They are still twice as wide as bicycle ones. I’m not sure they are trying to make them any smaller. You can see a pattern between size and power. Due to tire wear, the more power you have the more tire you need.

Road big tires allow you to carry more tire, which means it takes more to wear it down, which allows to use softer rubber that grips more. Conversely to offroad tires, this wide tires handle like a drunk pig at tight corners and low speeds. But that extra stability comes handy as soon as you step up the game and ride at 100-300 speeds. Again there’s a gain into carrying enough tire for the power. A lot of power on a small tire will inevitably wear it too quickly.

In all cases too much is too much. All of these are trade offs. There has been a time someone tried to make things way bigger or smaller and failed. There’s a compromise between all the effects of a bigger or smaller tire.

Off-road bicycles, road and off-road bikes: don’t need anything bigger than what they use.

And road bicycles don’t need tires smaller than what they use.

First of all your premise is incorrect.
Typical mountain bike (bicycle) tires are around 2.5” (~60mm) wide. A typical motorcycle tire is 4-5” wide (120-140mm). There are some variations like fat tire bicycles, but generally these are pretty close — with motorcycles having wider tires than bicycles.

Motorcycles need the wider tires for traction since the rear tire is driven by an engine, not a human. The front tires are not as wide as the rear in order to maintain steering feel and agility, but they’re still wider than most typical mountain bike tires.