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Old 06-27-09, 12:51 AM   #1
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Does weight affect downhill speed?

I've seen a number of posters mention that they are faster downhill (coasting) because of their heaviness.

Gravity is the same for any mass falling in a vacuum (right?), but there are many other factors and more physics going on in a downhill bike coast than gravity in a vacuum... acceleration, momentum, air resistance, road friction, etc...

Anyone with the physics knowledge can sort this one out? Basically two riders on exactly the same rigs, all conditions being equal coasting down a hill, the only difference being that one weighs twice as much as the other with comparable extra girth.

Starting from zero mph who will get off to the best start? Will the other one catch up or overtake somewhere down the hill? Who will get further up the next hill by just coasting?

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Old 06-27-09, 01:14 AM   #2
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Assuming similar aerodynamics (a bad assumption, but go with it for a second), a heavier rider will have an easier time overcoming air resistance because he will have greater momentum. Think of dropping a baseball vs. dropping a wiffle ball. So that's what gives the heavier rider an advantage.

Of course, a heavier rider is likely to present a larger cross-section than a lighter rider, so some of this advantage will be lost but not all. Put the heavier rider on a recumbent and it's no contest, even with the extra air resistance from his beard.
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Old 06-27-09, 01:25 AM   #3
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uh, assuming same cross section the wiffle ball and the baseball would fall at the same rate, and hit the ground simultaneously if dropped from a high building. From Wiki:

"In physics, gravitational acceleration is the acceleration of an object caused by the force of gravity from another object. In the absence of any other forces, any object will accelerate in a gravitational field at the same rate, regardless of the mass of the object. On the surface of the Earth, all objects fall with an acceleration of somewhere between 9.78 and 9.82 m/s"
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Old 06-27-09, 01:28 AM   #4
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uh, assuming same cross section the wiffle ball and the baseball would fall at the same rate, and hit the ground simultaneously if dropped from a high building. From Wiki:

"In physics, gravitational acceleration is the acceleration of an object caused by the force of gravity from another object. In the absence of any other forces, any object will accelerate in a gravitational field at the same rate, regardless of the mass of the object. On the surface of the Earth, all objects fall with an acceleration of somewhere between 9.78 and 9.82 m/s"
Note the bold part here. Try it. Wiffle balls aren't very good at overcoming air resistance.
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Old 06-27-09, 01:41 AM   #5
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ah right! thanks Andy K. in a vacuum this would be true... so we have the heavier dude having an advantage due to his mass, and the lighter guy having an advantage as he is more aerodynamic (right?)... please note I started this post due to my ignorance of the physics involved, not trying to prove a personal assumption

The heavier guy would cause the tires to have a wider contact with the road thus increasing friction and slowing him down (right?)
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Old 06-27-09, 01:56 AM   #6
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I think that's right.

If both riders are coasting (assumptions like this make physics much easier), the forces acting on them will be gravity pushing them down (which is proportional to their mass) and air resistance pushing them back (which is independent of their mass but increases with velocity). At some point, the force from air resistance will equal the force from gravity, and the rider will no longer accelerate without pedaling (terminal velocity). The heavier rider will require a greater force from air resistance to reach terminal velocity, which means if they had similar aerodynamics, he'd be going faster when he reached terminal velocity.

I think the difference in aerodynamics is less significant. It's also more complicated. The heavier rider, while having a larger cross section, may also be more round/less flat. I'm not sure how it would work out.
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Old 06-27-09, 06:23 AM   #7
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Assuming a good wheelset and properly inflated tires, I believe the rolling resistance differences are negligible.

Weight is a different story. Twice the weight means twice the force.

Aerodynamics I think are a bit of a factor, but, I don't think they come close to the weight factor.

So, yes, fat dude will pwn skinny guy on the downhills, assuming he is a fit fat dude.

Of course, he will give it back, plus interest when it's time to climb, so, you ain't gonna see many XXL yellow jerseys.
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Old 06-27-09, 06:25 AM   #8
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The heavier rider, while having a larger cross section, may also be more round/less flat. I'm not sure how it would work out.
It's called the aerobelly. Comes with the beard and technical degree as standard equipment on a recumbent.
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Old 06-27-09, 06:36 AM   #9
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so, you ain't gonna see many XXL yellow jerseys.
Except on ebay, were I got mine! :-)
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Old 06-27-09, 09:37 AM   #10
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All I know is that at 360lbs I manage to coast downhill faster than some people pedaling downhill.
(This drives my buddy nuts....of course , he kick my butt when it coms to climbing)
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Old 06-27-09, 09:59 AM   #11
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It's called the aerobelly. Comes with the beard and technical degree as standard equipment on a recumbent.
Sure it does



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Old 06-27-09, 10:48 AM   #12
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Anyone care to do an experiment to test it out? Get two people of significantly different weights. Have them go from a stop down a slight decline so they don't get going too fast, on the same bike of course. This should eliminate wind resistance from the equation. Use a stopwatch and time them from A to B. Try it again down a steep hill where wind resistance will have more of an effect.

Your results will be an important contribution to the world cycling community.
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Old 06-27-09, 10:51 AM   #13
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I can tell you that my friend and I have done this test several times and it always comes out the same. I weight 163 lbs. and he is 115 lbs.
We climb up this hill all the time and come back down its about 1.5 mile downhill all the way down with a grade range of 3 – 10% without pedaling we start out at about 15 mph and coast all the way and I’ll hit 39 – 40 mph he gets up to about 37 – 38 mph and I’ll get to the bottom about 5 – 10 seconds before him.
He says that as soon as we start I’m pulling away and he never gains any distance the entire way down I just leave him.
Why who knows you tell me but the results are always the same.
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Old 06-27-09, 11:15 AM   #14
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If you drop a 10 pound rock and a 100 pound rock from the same height at the same time; they both reach the ground at the same time.

That being said, my fastest speed on a bike (58 miles an hour) was reached going downhill on a fully loaded bike (front and rear panniers and handlebar bag, sleeping bag and pad and tent. The load was approximately 85 pounds. If you're adept at riding with load, the bike is a tremendously stable (downhill) rocket.
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Old 06-27-09, 11:24 AM   #15
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If you drop a 10 pound rock and a 100 pound rock from the same height at the same time; they both reach the ground at the same time.
That actually isn't completely true. If you dropped them from a plane at high altitude, the heavier rock, if it was made of the same material, and shaped about the same, would hit sooner as it would reach a higher terminal velocity. If it was made of lighter material, like limestone compared to marble, it might hit later. If it was sculpted into a wide sheet with a slight bowl shape, it might drift off target or shimmy down like a sheet of paper. If one rock was shaped like a spear it would drop faster than one shaped like a snowflake.

If you dropped them from 20 feet, they'd hit pretty much at the same time.

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Old 06-27-09, 11:26 AM   #16
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If you drop a 10 pound rock and a 100 pound rock from the same height at the same time; they both reach the ground at the same time.
In a vacuum, yes. In an atmosphere, not necessarily.
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Old 06-27-09, 11:30 AM   #17
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Does the name 'Galileo' ring a bell?
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Old 06-27-09, 11:35 AM   #18
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Basically, the gravitational force is proportional to the weight of the rider while air resistance is proportional to cross sectional area. A rider who is twice as heavy as another rider - say 220 lbs vs 110 lbs, is not usually anywhere near twice as big in cross section. Weight increases roughly in proportion to the cube of height or waist circumferance, while cross section increases roughly in proportion to the square of height or waist circumferance. So in downhill coasting, weight pwns aerodynamics.

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Old 06-27-09, 11:45 AM   #19
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Just from personal observation, yes. I've gone down some long steep hills with people 20,30,30,50 pounds lighter than me and I've gone at their speed, or go faster than them, just coasting as they pedal.

I guess I should add that I'm 200 pounds and rather lean/skinny.
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Old 06-27-09, 11:45 AM   #20
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Ride around the hills and mountains of CO, and you will see examples every day;
On the climbs, it is the little people who leave the big people (like myself) behind.
Then on the descents, the little people get left behind.

As for dropping two objects of different weights;
Ask any skydiver if a heavy person and a light person fall at the same rate.
As someone else pointed out, that is true only in a vacuum.
Small skydivers on freefall teams routinely wear lead weights to bring their fall rates up to the same as their bigger teammates.

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Old 06-27-09, 11:49 AM   #21
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Get this through your heads, guys: The speed of a falling body is independent of its weight. This is a scientific fact, a law.
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Old 06-27-09, 11:57 AM   #22
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Get this through your heads, guys: The speed of a falling body is independent of its weight. This is a scientific fact, a law.
OK, Galileo.

I just dropped a book and a single sheet of paper from the same height. The book hit the ground first. Someone call the physics police! I broke the scientific law according to supramax!

In all seriousness, you need to understand that air resistance imparts a force on a falling object and that your "scientific fact" is only true of an object in a vacuum. Put a 1 lb weight on a small parachute. Put a 1 ton weight on the same parachute. Do you think that they'll fall at the same rate?
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Old 06-27-09, 11:58 AM   #23
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Get this through your heads, guys: The speed of a falling body is independent of its weight. This is a scientific fact, a law.
Vt = sqrt((2 * m * g) / (rho * A * Cd))

where

Vt = terminal velocity,
m = mass of the falling object,
g = acceleration due to gravity,
Cd = drag coefficient,
rho = density of the fluid through which the object is falling, and
A = projected area of the object.

Do you see that term "m * g"?
That is called w_e_i_g_h_t.
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Old 06-27-09, 12:05 PM   #24
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Get this through your heads, guys: The speed of a falling body is independent of its weight. This is a scientific fact, a law.
A law that only applies "in a vacuum".
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Old 06-27-09, 12:13 PM   #25
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Observational data: I routinely pass single upright bikes going downhill, both on a tandem with my wife/stoker, and on my recumbent. I weigh a bit less than 200#. Stoker weighs a lot less. The bikes are a bit heavier than average. Moderate cross-section tires running at about 90-95psi on both bikes.

The speeds are directly affected by my/our energy input and by physical forces beyond our control.
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