Bike vs. Skateboard Acceleration (downhill with no pedaling)

[keaton]

Hi everyone,

My roommate and I were talking about which would win in a short downhill race (with no pedaling from either rider), a bike or a skateboard. Assuming the bike stays in the same gear (2, and 4), and has a wheel diameter of 27", and the skateboard has a wheel diameter of 2.4".

My thinking is that the skateboard will accelerate faster very briefly, as it has the smaller wheels, but then the bike will catch up and surpass the skateboard before the end of the hill, assuming they start at the same point, and the start of the hill.

Can anyone offer me any insight? I'll try to answer any questions for detail that I can.

Thanks!
Keaton

Disclaimer: I'm not very knowledgeable in this field, but I'm really curious about this. I also wasn't sure about which prefix, to use, it's probably more high school level, but oh well.

 

[A.T.]

(with no pedaling from either rider), a bike or a skateboard. Assuming the bike stays in the same gear

Why does the gear matter if the rider doesn't pedal?

My thinking is that the skateboard will accelerate faster very briefly, as it has the smaller wheels,

Makes sense.

but then the bike will catch up and surpass the skateboard before the end of the hill.

Could happen, depending on the surface type, body pose and the length of the race.

 

[rcgldr]

For an idealized case, assume the skateboard wheels and bicycle tires are hoops or thin cylinders, in which case the angular (spinning) energy versus ground velocity = 1/2 m v^2 (in addition to the linear kinetic energy, also 1/2 m v^2), regardless of the size (radius) of the wheel or tire. Assuming the skateboard wheels are lighter (less mass) than the bicycle tires, then the skateboard "loses" less energy to angular kinetic energy and it accelerates faster.

In a real life situation, the energy losses related to rolling are greater in a skateboard than a bicycle, a combination of rolling resistance, and bearing losses. A smaller radius wheel / tire will have more issues dealing with imperfections in a road.

 

[SirHall]

Realistically, this pretty much comes down to the friction between the wheels and the ground. Firstly, assuming friction between the ground and the wheels were identical for both the bike and the skateboard, they 'should' both reach the end at the same time(remember, heavy objects fall at the same speed as light ones). Seeing as most skateboards use plastic wheels vs rubber wheels used by bikes, and also seeing that the friction on most bike axles tend to be higher than that of skateboard axles. It would be safe to say that the skateboard would win. But of course there may be some unknown variable I'm not considering here so your best bet would be actually testing the question.

 

[rumborak]

and also seeing that the friction on most bike axles tend to be higher than that of skateboard axles. It would be safe to say that the skateboard would win.

I highly disagree. Bicycles are heavily optimized for low friction losses. For skateboards long distances is secondary. I think it's safe to assume that friction loss is less in a bicycle.

 

[SirHall]

I highly disagree. Bicycles are heavily optimized for low friction losses. For skateboards long distances is secondary. I think it's safe to assume that friction loss is less in a bicycle.

Well, I certainly could be incorrect, but firstly take a look at this. The coefficient of friction between rubber and concrete is 0.65 while the coefficient for plastic hovers somewhere between 0.2-0.55 depending on the type of plastic. So, I must say that the tires of a bicycle are great for going uphill, they are not made for speed, that is what skateboards are for(as far as I know).

 

[rumborak]

As neither the skateboard nor bicycle wheels slide across the surface, those coefficients don't apply really. The friction losses happen at the bearing level at the axle, and there, as a function of size, and rotational speed, bicycle bearings are far superior.

I also must say, it's kinda bizarre to read a statement like "bicycles aren't made for speed". Do you not have a lot of experience with bicycles? They are commonly faster than skateboards.