Suppose A motorcycle is coming down a slope. Only gravity is acting. If we put 2 wheels more in that motorcycle, will the speed increase or decrease compared to normal 2 wheels? If more wheels are helpful, why don't we attack two wheels to the back of motorcycles so that less force will be required and less accidents will occur?

• I think a way of seeing the answer to this question is to iterate it: why don't motorbikes and cars have hundreds of wheels?
– user107153
Oct 14 '17 at 10:00

Addition of two more wheels to the motorcycle that is coming down a slope would not change its speed.

If the slope makes an angle $\theta$ with the horizontal, then the net force acting on the motorcycle would be $ma = mg \sin \theta - \mu N$ where $\mu$ is the coefficient of friction, and $N = mg \cos \theta$ is the normal reaction. It is clearly evident that the mass would cancel out. So, the acceleration, and thus the velocity, does not depend on the mass. Frictional force does not depend on area of contact. So, the speed would remain the same as in the case of two wheels.

Addition of two more wheels does increase the stability. But, motorcycles are made for squeezing through small spaces. They cannot bend like at bends on race-tracks on addition of two more wheels. So, their main utility would be removed on addition of two more wheels. While practicing in cycles, two more wheels are fitted for practice and balance, but not in motorcycles.

• Some years back, you would sometimes see an extra wheel added to a motorcycle here in the UK. The reason was not speed, stability, or anything in physics. At the time, learners were limited to 250cc engines but the limit did not apply if there was a sidecar. So, minimalist sidecars were developed just so that learners could drive large bikes. I have not used one; I decided that a simpler solution was to take my test and get a full licence. Look up Sidewinder if you are interested. Oct 14 '17 at 9:05
• @badjohn Actually, I'm from Kolkata. I have never been to UK, so I don't know anything about the country. As a consequence, I was not aware of this. Oct 14 '17 at 9:10
• Not surprising; these were popular for a brief time, long ago in a faraway country due to someone finding a legal loophole. Oct 14 '17 at 15:29
• This answer basis it's argument on the resistance felt by a motorcycle being that of friction. This is not the case, a motorcycle experiences rolling resistance and air resistance, which are different.
– Eddy
Oct 17 '17 at 18:25

It would go slower, for at least two reasons: