General Cycling Discussion - Heavier Riders Go Down Faster

(and no, that's not the title of my latest suggestive bumper sticker)

We are having a debate at the office. People here are taking it as accepted truth that a heavier rider will roll downhill faster than a lighter one, even if both start from a dead stop. But why would this be so?

I know gravity is not supposed to pull a heavier object faster than a lighter one, because both the force of gravity and acceleration are proportional to mass. But what would cause a heavier rider to roll faster?

We have been considering friction, from increased weight on the axles and increased tire coverage on the road, but it would seem that friction would be higher for a heavier rider, and thus cause him to roll slower. Similarly, a large rider should have more wind resistance, so that should not be it (and the distances we are talking about are not all that long, anyway).

Where are we wrong? Does anyone have a link to a site explaining this issue? Or are my friends just wrong in believing that a heavier rider goes faster?

http://www.sportsci.org/encyc/cyclingupdown/cyclingupdown.html





http://www.bikeforums.net/archive/index.php/t-31567

Heavier riders do go faster (other things being equal).

The theory: Speed tops out when downward force (gravity) = resisting forces (wind, rolling resistance etc). Downward force scales linearly with rider+bike mass. Wind resistance scales (roughly) with the fourth power of the rider speed. Work this through and you'll quickly see why a heavier rider goes faster.

The practice: time yourself from a standing start freewheeling a fixed distance down a hill. Now fill you panniers/backback with a pile of bricks & repeat the exercise.

Where are we wrong? Does anyone have a link to a site explaining this issue?

Try this, get a Styrofoam ball and a marble or ball made from a denser material, but similar in size. Which one falls faster? The heavier one. In a vacuum, all objects fall at the same rate (32 ft/sec/sec), but it's pretty hard to ride in a vacuum :D

Yes, the larger rider has more wind resistance, but the increase in wind resistance is generally less than the increase in down force from weight.

-murray

I haven't had time to trace the equations in the links you sent, but I think I need to make a clarification. The way my friends are talking about it, the issue is not terminal velocity, but rather their relative speeds (or acceleration) from start to finish. I don't think they are letting themselves reach full speed. I don't know if/how that affects the answer.

Anyway, I will read carefully through the links that were posted. Thanks for the help.

Without getting scientific...my wife and I ride essentially the same bike, so friction factors related to the mechanics of the bike, such as bearing efficiency, tire size, etc. are the same. I outweigh her by about 50 pounds and of course am less aerodynamic.
I definitely out roll her coasting downhill.

The way my mind always justified heavier riders coasting downhil faster was because a heavier rider carried more inertia thru the slight imperfections in the road. I know this is not very scientific but if you are coasting downhill and you end up going over a 8" high curb on a BMX bike with 20" wheel diameter you will slow down more than if you were in a SUV with 20" wheel diameter.

It's a good thing us heavy guys roll faster downhill, it's the only way we can catch after going up a hill. :D

Mark

I'm a big guy. I almost always pass my friends going down hill... and that's with me coasting and them pedalling hard and fast in a large gear. In fact, I usually back off from most people if I am following and we are approaching a downhill. If I don't, I'll roll right up on them in a matter of seconds.

I also think we hold our speed a bit longer when coming off a downhill. When I ride in rolling terrain (lots of downhills followed up uphills), I usually coast much farther up the uphill than my friends and at a faster speed. By the time I start pedaling, I'm usually several bike lengths ahead and still going faster even though they are pedalling hard by then.

Look it up. In the case of a rolling wheel or ball, the frictional force is in the same direction as the forward travel. i.e. more friction on a tire will allow it to move forward faster. Isn't Physics fun!

Hands down. One of my cycle pals is a big guy and he comes down the hills like a rocket.

How about this for a stab in the dark? The big guy has more kinetic energy due to a larger mass, so all the little bumps in the road have less relative effect on his downward motion. Just like the small guy will be able to brake faster going downhill, all the little bumps in the road slow him down more, too.
So maybe if you were riding on the moon on a perfectly smooth downhill, both riders would go the same speed? But if you added some bumps, then the big guy would pull ahead.

Should be able to be explained fairly easily as the conversion of potention energy to kinetic energy. At the top of the hill each person has a potention energy of m*g*h where m is mass, g is the gravitational constant, and h is the height of the hill to the bottom. Assuming additional losses to friction are negligible for the heavier rider, by the bottom of the hill they have converted more energy into kinetic energy (going forward) than the lighter rider.

Yup! I'm a big guy, and I am much faster then those tiny (145lb) riders.. When they pedal like crazy down hill, I just tuck in elbows and knees and fly by them with out moving the pedals. Now, when it comes to climbing the next hill the roles are reversed... They fly by me as if they are on a leisurely ride and I am huffing and puffing... :D

I haven't had time to trace the equations in the links you sent, but I think I need to make a clarification. The way my friends are talking about it, the issue is not terminal velocity, but rather their relative speeds (or acceleration) from start to finish. I don't think they are letting themselves reach full speed. I don't know if/how that affects the answer.

Anyway, I will read carefully through the links that were posted. Thanks for the help.

it wouldn't matter, at any point along the path the heavier rider will have a higher velocity. any point except the beginning of course - that would be zero for both. not only will the terminal velocity be different, but the relative speeds will be changing as well over the distance.

... not this again..............