I know, not all bicycles have thin tires nowadays, but generally why bicycles have much thinner tires than the motorcycles?

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    Motorbikes tend to go a lot faster, and have a much greater power applied when accelerating and braking. Plus motorbikes weigh 8-12 times as much as a real bicycle. – Criggie May 19 '17 at 6:51
  • @Criggie There is a "motorized bicycle" motor cycle vehicle class where I live. Those weigh maybe 40kg and go up to 45km/h. Considering total weight, the relative difference is pretty small, they have low acceleration and don't go much faster than a fast cyclist. They still have significantly wider tires than electric bicycles (same vehicle class/top speed). So I don't think your comment/existing answer is the whole picture. – Nobody Aug 28 '18 at 11:55
  • @Nobody Could add "fat-bikes" in there too for even more stuff between Bicycles and Motorbikes. And, never discount the effect of "looks" on how something is made. – Criggie Aug 29 '18 at 0:27

With the exception of fatbikes, the widest bike tyres are narrower than the narrowest motorbike tyres. I'll ignore fatbikes as somewhere between too specialised and a novelty.

You'll also notice that all motorbikes have significant amounts of suspension. Fat tyres and suspension both absorb energy from vertical motion. This can be a good thing, for example if you ride over a pothole, or a bad thing, if the vertical motion is from pedalling hard. Motorbikes aren't propelled by pedalling so don't have to worry about this. They also go much faster so the consequences of hitting a pothole are more severe, or would be without something to absorb the hit.

Motorbike tyres are likely to have higher rolling resistance. With the rims to support them they also have much higher weight and rotational inertia, meaning bigger brakes -- but you need those anyway given the speed. The rolling resistance is significant in terms of human power output, but given that even a 125cc motorbike can put out 11kW continuously and a pro cyclist more like 500W it's not significant in motorbike power terms. This higher power (and higher braking force) has to be coupled to the road somehow, and that's through the contact patch. Typical bike tyres wouldn't last long until even modest motorbike acceleration, or give enough grip for hard acceleration/braking.

In summary: motorbikes can have bigger tyres because they've got more power, and they need to because they're faster and more powerful.

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    The size of the contact surface to the road determines the amount of driving and braking power that can economically be transmitted considering life expectancy of the tyre. – Carel May 18 '17 at 11:34
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    @Carel good point. I'm expecting to need to edit having finished the answer in a hurry, so I'll incorporate that (unless of course you answer first) – Chris H May 18 '17 at 12:08
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    @Chris H: It is OK to incorporate the comment in your edit! – Carel May 18 '17 at 15:51
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    Because they're faster and more powerful and much, much heavier. – David Richerby May 18 '17 at 18:00
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    @DavidRicherby big bikes are. But a Honda 125 only weighs about as much as a stoker+tandem (128kg). It still has a top speed of 65mph and 4" tyres. That's not the smallest bike I could think of, just something globally very common. – Chris H May 18 '17 at 18:10

Power and Weight.

Power: The engine of a motorbike puts immense forces on the road. Guess what would happen if you turned the wheel of a racing bike with that kind of force. It would just spin, burn rubber, and likely wouldn't last a second. The motorbike needs a big contact area with the road to be able to transfer its power. And its tires need to be made of thick, stiff rubber to be able to transfer these forces from the rim to the road contact area.

Weight: The lesser of the two factors, but certainly not negligible. I don't know about you. but my bike does not weigh a quarter ton. It's more in the 20kg range, and that's quite heavy for a bike. Of course, with a heavy biker, even a bike can have over a hundred kilo total weight, but that's still nothing compared to the weight of a motorbikes' engine and the strong frame that carries it. This weight needs to be carried, and that means either wider tires, or more pressure. Since there is nothing to be gained for a motorbike by increasing pressure, they go the easier route of increasing size.

Also consider this: Bike tires are optimized for efficiency. The amount of power that's available on a bike is rather limited, any inefficiency means less speed. That is the reason why bikes use such insanely high tire pressures. Cars use less than 3 bar, bikes can go up to 10 bar. Afaik, only the tires of airplanes use similar pressures (and they optimize for weight as well). Motorbikes, on the other hand, don't worry about efficiency at all (at least in comparison). They have more power available than they can make good use of. Their tires are optimized for road safety and fat looks only.

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