Does a heavier bicycle go faster downhill?

Question

Does a heavier bicycle go faster?

My guess is not, but some of my friends tell me I'm wrong, and that in fact a heavier bicycle does go faster downhill.

My theory is that weight doesn't matter if going downhill, because when going downhill the only force acting on the bike is gravity, which is 9.8 m/s^2. And that doesn't depend on weight. Am I wrong?

Answer 1

The easier to be handle by rider is the one to be faster going downhill.

Is this valid for downhill in smooth road?

Answer 2

im not a scientist to get into all that stuff, but in practice, i weigh more than my friends.  my bike weighs more, and they have to pedal to keep up with me coasting.  hevier bikes also maintain momentum better over really rough surfaces as well.  like i said, i have no idea of the science, but i know in practice that is how it is.

Answer 3

Here's a link:

http://gravitybike.com

All the racers add fifty pounds of lead "buckshot" to their bike.

Heavier does make the bike go faster.

Answer 4

Nice job MtBikr.

That is the exact answer.

While I was aware of this I could not explain it half as well as you did.

Answer 5

We just discussed that in science class.  The weight has no effect on the speed, only the steepness of the downhill slope.  Heavier bikes are better for downhill riding because they are stronger and have beefier shocks, but they are not better simply because they are heavier.

Answer 6

I was going to give the same answer you did. I don't think weight can cause a bike to go downhill faster. However, you will definitely have more momentum, which might matter.

Answer 7

a heavier object will have more inertia going down hill. It's not only gravity that you have to factor in. The ground absorbs some of the inertia. Another big factor is how wide are the tires. If you have a 200 pound guy and a 100 pound guy on the exact same bike, the 200 pound guy will accelerate faster.

Answer 8

If you take the same ride on 30lb bike and go downhill the rider will achieve a given speed with no pedaling.

Now take that same rider and bike and have him put an additional 30lbs to the total weight and he will go faster down the same hill, his top speed will be higher. Here is the reason, while all factors such as hill angle and gravity are constant the total weight of the rider/bike has increased. So far so good, but now the key factor that is normally not payed attention to is the aerodynamic resistance. Two objects of the same weight but with different surface areas will fall at two different rates, gravity didn't change. What we have is with the bike & rider is the same surface area with increased mass, therefor the speed of decent will have to increase due to the higher mass, the difference in speed is due solely because of the higher wind resistance needed to overcome the higher mass. Now if this test was performed in a total vaccum there would be no speed differance.



The Math;

Vt =  sq root of  (2 m g / p A Cd)

where

Vt = terminal velocity,

m = mass of the falling object,

g = gravitational acceleration,

Cd = drag coefficient,

ρ = density of the fluid through which the object is falling, and

A = projected area of the object.

EDIT:

For a sky diver he is in a direct freefall, The direction of the sky diver and gravity are working in the same direction.

A biker going down a slope must have the angle of the grade calculated in.

If you and your bike are on a flat surface you won't go anywhere due to the pull of gravity. (you and gravity are at a 90deg angle to each other.)

90deg= 0% pull

45deg=70% pull    

0deg = 100%pull

(you'll need to calculate the vector angle for the above formula for the correct effect of gravity)  

However if you miss a corner on a mountain top and go off a cliff you will achive the sky divers speed.(and you probably won't be wearing a parachute at the time.)

EDIT2:

Thanks for the compliment MR

Answer 9

Yes, all other factors being equal, heavier bikes go downhill faster.  In practice, even ten pounds of rider will make a noticeable difference in downhill speed. Acceleration due to gravity is the same no matter what, but a heavier bike has higher cross-sectional density with no increase in the coefficient of drag. Aero drag may be the same, but when it's acting on more mass, it has less effect. Or, to put it another way, you can stop a baseball with one hand. Don't try to stop a car with one hand, though!