With identical bicycles, will a heavier rider coast down a hill faster than a lighter rider.?

Question

It seems to me that considering just friction and gravity, the heavier rider would actually coast slower.
Please straighten me out on this because several of my bar room buddies seem to think the opposite.

Answer 1

Good answers, I'll dive deeper. There are multiple forces at play in the 'coasting downhill' problem - gravity, the person, the air.

Gravity provides the acceleration therefore producing speed. This is constant for each rider, no matter what the weight. In a vacuum, each rider would reach the bottom at the exact same moment provided exact same rolling resistance...

Rolling resistance - gravity also slows the riders down. Combined with the weight of the person, it provides a downward force that causes the friction with the ground. The heavier rider does indeed have more rolling resistance than the lighter rider. In a vacuum, the lighter rider reaches the bottom first.

But on to real life - the greatest force in cycling, and other sports where motion is at high speeds, is the medium through which the subject travels. For a cyclist it's the air, (like for a swimmer it's the water). This is a much bigger influence on the speed than rolling resistance. Force is a factor of mass and acceleration, and therefore it is also a factor of momentum. Heavier masses can produce more momentum.

The air provides a great 'slowing down' force against the riders. As their speed increases, the heavier rider gains more momentum and is able to counter the force of wind resistance stronger than his lighter counterpart. The bowling ball vs. ping pong ball example was extreme but illustrates the point well. On this planet, provided each rider has same aerodynamics, the heavier rider will gradually pull away from his lighter counterpart on a long descent.

But then you have aerodynamics, etc. Could go on forever on this topic. The heavy rider could sit straight up, wearing a baggy windbreaker and cargo shorts and the lightest rider in the world wearing a TT outfit would crush him down the hill.

Answer 2

The heavier rider will coast downhill faster all other things being equal. There is also wind resistance to contend with, but let's assume that the two are of equal size. Force = mass* acceleration. We'll acceleration is always constant (gravity). The heavier rider is more massive and therefore produces more force.

Let's get away from the physics here and just talk practically. Think about a bowling ball and a ping pong ball rolling down a hill. Which one will have more speed at the bottom?

Answer 3

You would think that and it would make sense. But the reality is that the two riders are going to roll down the hill at the same speed. Assuming that the two riders start from the same point, using the same type of bicycle, and all the control elements are identical--meaning there is no outside variant in temperature, wind speed, or any sudden movement from an outside force that would interfere with the experiment on any level for either person--Galileo proved that the weight does not necessarily impact the speed (velocity) of the movement of the object. Because of the density (the "stuff") of the object moving in space (where it is in relation to the movement in the planet Earth's rotation around the sun), the objects will move at the same rate (9.81 m/s2) of gravity. Where your friends are arguing--effectively--is that the larger of the two participants will move with more force ("punch," "kick" "hy-yah!") The larger of the two participants, while having the same mass, will have more weight (the force of gravity binding him to the earth while the earth is in orbit around the sun). This is an important difference that will effect HOW he moves within the space but not THAT he moves. Put another way: if a ferrari and a hummer down the local interstate gun it for the same amount of time for the same amount of speed, you would expect the ferrari to beat the hummer, but that's not necessarily true. Both vehicles are going to go the same amount of time and are effectively going to make it at the same rate. What's going to be different at the finish line is how the ferrari vs the hummer is going to come out at the end: hummers are not built to go for high speed and ferrari's are, so the hummer's engine is going to be more under stress than the ferrari's. Same thing with your fat man/skinny man question: at the end of the hill ride, the fat man will be more stressed than the skinny man, but will still have finished the race and will still be in one piece, having reached the destination in the exact same amount of time with the exact same distance and gear as the skinny man.

Answer 4

heavier people normally have more frontal area.

if the frontal area is the same, the heavier person wins.

make sure the barroom bet ONLY includes weight and and rolling resistance.. You will win.

Load a bike with identical riders and behind one load 30lbs of books. make sure the tires are jacked up pretty high on pressure. you will win.

load identical bikes with a guy 40lbs heavier, and moderate pressure wheels, you may loose.

make sure you win the bet.

Answer 5

generally speaking, yes... ever heard of gravity?
if you can get into a tucked/aerodynamic position, a heavier rider will coast down a hill faster (if both bikes are equal)

Answer 6

Not neccessarily. However, if the person is round and the bike is a time trial bike, then yes.