Most people use feathers for that experiment.
To repeat: "The speed of a falling body is independent of its weight."
Please do try to get your head around that statement. Don't infer what is not implied.

It's a meaningless statement.

This is why heavier riders are faster on downhills. And it's the same reason that sprinters can be big and strong. Their extra power for a slightly larger aerodynamic resistance makes them faster in a sprint than lightweight riders.

My friend weighs 100 pounds more than I do and I own his ass on climbs, we're pretty even on the flats, but on down hills I have to pedal like a gerbil on crack just to hold his back wheel.

On my own I have noticed that my bikes that are heavier descend faster when they are coasting... when I was a kid we used to make pinewood cars and after making them as aero as possible the best way to make them go faster was to add weight that exceeded the enforced limits.

I do not ride in a vacuum... the spacesuit kills my aerodynamics.

I would think that anyone who has ridden a bike would understand that bicyclists start to react more like a feather than like a lead weight when traveling above 15 mph.

However, does twice the force equate to twice as fast down the hill?
Regardless of somethings weight, his Acceleration is what is important for the downhill race.
Force=Mass*Acceleration
Acceleration=Force/Mass
In other words, while the guy twice as big recieves twice as much force from gravity, he also has twice as much mass which cancels out the benefit.

This is true, but you have to consider all the forces acting on the rider. The net force is the gravitational force minus the force of air resistance. Air resistance increases as speed increase. At some point, the gravitational force is canceled out by the force of air resistance, at which point the rider will cease to accelerate and maintain a constant speed (assuming incline, wind, etc. remain constant). This is called terminal velocity. A heavier rider has a higher terminal velocity.

Goddamn air resistance!

RE: "The speed of a falling body is independent of its weight."

It's considered to be the discovery of a genius, brainiac!

As shown in my previous post I will always hit my top speed and that’s it, this is with no more than a 2 mph head wind more and its slower. My friend also only hits his top speed no more at the same head wind.
Sure drag differences are in effect here, I am really getting aero on these downhill tests we do, when I sit up at the bottom to slow speed drops quite fast to low 30's till we need to turn and apply brakes.

So what about at slow speeds where air resistance is negligible? Does the heavy rider accelerate faster from the very beginning or does the advantage only begin once the lighter rider has reached terminal velocity?

As I also posted in my previous post I'm not looking back at him but he tells me from the start I start to gain distance on him and as we increase speed that gap gets bigger.
This is fact, as we do these downhill runs and its always the same.

I think we need more 'bent riders in here, for they have aerobellies, beards and degrees in physics.

Sorry, no degree in Physics but I am subject to the Laws of Physics.

What a drag!

I posted the question over on physicsforums.com and the three responses I've gotten are that a heavy and light cyclist will have the same acceleration (a = g*sin(theta)), and therefore speed, when coasting down a hill. That is, until wind resistance comes into play.

Plus, with the coefficient of drag in the denominator, a higher number gives a lower velocity. (I suppose; I wasn't very good in physics).

no, drag increases at, uhhh, I think it's the square of speed.

depends on the length of the race

Not sure about the math on that part. What I am sure about was on my ride today I met up with a guy. We rode for about 15 miles together, mostly level, some short hills. We seemed a pretty good match on level ground, but on descents I ran away from him pretty badly. He said I was pretty nasty going down those hills. My response was that at least those extra 50 lbs I drag around are good for something. I am guessing I had about 30 lbs on him.

To supramax's statements concerning dropping stuff and galileo, similar shaped object weighing something close resonably close to each other will drop at approximately the same speed over a relatively short distance. The same object dropped from higher distances will certainly NOT drop at the same speed. Given similar aerodynamics, the lighter object will reach terminal velocity at a lower speed. It's not even open to debate. It is a fact. More weight=more force. More force=higher terminal velocity

Well, that's not very helpful. How fast do you need to be going before wind resistance comes into play? It's always in play, but becomes a big factor at about 15 mph.

Now it happens that while I don't ride a recumbent, I do have both an aerobelly and a degree in physics. I haven't used the physics degree lately and I'm lazy (which is why I haven't thought this through with any kind of rigor up to now), but this is the way I see it:

Let's say rider A weighs 220 and rider B weighs 110. The force due to gravity will vary depending on the slope, but it came be simplified as

F(g)(a) = 2x
F(g)(b) = x

where x is the weight of rider B times a factor to account for the incline. Now lets call the force from wind resistance y. For any given speed it will be about the same for both riders. So the net forces will be

F(net)(a) = 2x - y
F(net)(b) = x - y

Now for simplicity, let's consider the case where the gravitational force in for rider B is twice the force from wind resistance (x = 2y).

F(net)(a) = 4y - y = 3y
F(net)(b) = 2y - y = y

So at that speed (below terminal velocity), the rider with twice the weight will be experiencing three times the force, and therefore will be experiencing greater acceleration, even after you account for his greater weight.

All of the above is completely off the cuff. I may have made some really gross errors. Does it make sense to anyone else?

And wind resistance comes into play pretty quickly. Cyclists, particularly upright cyclists are absolutely horrible at being aerodynamic.

Anybody know the average Cd of a typical roadie in the drops? I suspect it is marginally better than a parachute. Hell, maybe the chutes better. It just has a lot more cross section.

The laziness probably explains the aerobelly.

How 'bout a beard?

I got about 6 years left in the navy reserve and by my calculations, I figure my lower backs got about that much time left as well. So, in 6 years I'm pretty sure I'll be on a bent since the back will be done, I won't have grooming standards to follow and I'm certain my aerobelly will still be around. If I start hitting night classes now, maybe I can have the tech degree too.

Only in the winter.

Correct. The acceleration is the derivative of the formula I gave earlier:

acceleration = g - ((Cd * rho * V^2 * A) / (2 * m))

where

V = *current* 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.

And for a vehicle coasting down a descent (ignoring friction of bearings and tires):

acceleration = (sin(theta) * g) - ((Cd * rho * v^2 * A) / (2 * m))

where theta is the angle, (measured as zero for a level surface)

I don't know what you mean by the word 'fact', but you're incorrect. A 50 lb dumbbell dropped at the same time and from the same height as a penny, will reach the ground the same time the penny does.

False.


No, it's very similar to a discovery by a genius, but actually, heavier things fall faster. Except in a vacuum, which isn't relevant because we don't ride bikes there. The whole Gallileo-Tower-of-Pisa legend is a nice story, but it never happened. Not only because there is no actual record of it, but because it would not have worked. Drop two balls of identical cross section and differenct mass, and the heavy one will hit the ground first. It took a genius (Gallileo) to figure out that acceleration due to gravity was the same for both balls anyway. Gravity pulls harder on the heavier ball (or cyclist) but the greater inertia balances that out. Air resistance (at a given speed) pushes equally on both balls (or cyclists) but the greater momentum of the heavier one overpowers it more.

Hence, after my much more hard core buddy drags my sorry butt up a mountain, I can coast down next to him while he cranks like crazy just to keep up.