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Why do heavier cyclists descend faster?

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Why do heavier cyclists descend faster?

Old 12-04-08, 01:52 PM
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ROJA
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Why do heavier cyclists descend faster?

Many of you probably remember from your physics classes that in a vacuum all objects fall at the same rate, regardless of weight. (Some background reading if you want it.) So being heavier doesn't make you fall faster.

The fact that we have air introduces the variable of wind resistance, which should work to the advantage of a smaller cyclist (which generally correlates with weight).

So why does it seem like the "big guys" always go down the hills faster???

(Title should obviously be "descend" faster. Can a mod fix? Thanks.)
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Old 12-04-08, 01:54 PM
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Thing of it as a downforce thing. They put huge spoilers on drag racers to force the tires to push down on the road as tightly as possible. Hefty guys serve the same basic function.
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Old 12-04-08, 01:58 PM
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Originally Posted by kylejack View Post
Thing of it as a downforce thing. They put huge spoilers on drag racers to force the tires to push down on the road as tightly as possible. Hefty guys serve the same basic function.
Downforce gives more grip, but it doesn't make you go faster. Or are you saying that heavier riders have more grip, which allows them to go around corners faster?
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Old 12-04-08, 02:00 PM
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I know big guys that can't descend faster than I can. Plus I've seen a lil 140 lb guy drop my 220 lb arse on a GMR (20 mile dh). He was a fast lil MOFO!
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Old 12-04-08, 02:00 PM
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This thread is heading for posterity fast. Like "heavier guy" fast.
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Old 12-04-08, 02:01 PM
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Does it have anything to do with an increased terminal velocity for the heavier rider?
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Old 12-04-08, 02:02 PM
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Originally Posted by ROJA View Post
Many of you probably remember from your physics classes that in a vacuum all objects fall at the same rate, regardless of weight. (Some background reading if you want it.) So being heavier doesn't make you fall faster.

The fact that we have air introduces the variable of wind resistance, which should work to the advantage of a smaller cyclist (which generally correlates with weight).

So why does it seem like the "big guys" always go down the hills faster???

(Title should obviously be "descend" faster. Can a mod fix? Thanks.)

There are multiple threads on this. But the short answer is we don't ride in a vacuum, and for a given surface area, heavier things do fall faster outside a vacuum.
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Old 12-04-08, 02:02 PM
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they don't.
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Old 12-04-08, 02:03 PM
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it is the stored energy of the moving mass and gravitational potential energy adding
to it for both riders

small changes in elevation and air do not affect the heavier rider as much.
a lighter rider slows down a lot for a slight decrease in grade or ripples in pavement, or air flow.

once up to speed, fatties tend to retain that speed. so they
end up faster than the lighter rider in general...downhill.

uphill though, disaster

Last edited by 127.0.0.1; 12-04-08 at 02:09 PM.
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Old 12-04-08, 02:06 PM
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Originally Posted by ridethecliche View Post
Does it have anything to do with an increased terminal velocity for the heavier rider?
Sure........if you hit 120mph.
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Old 12-04-08, 02:07 PM
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I'm no physician but I'm thinking it has more to do wind resistance than anything.

The heavier you are, the more wind resistance would be needed to hold you back. Unless you are morbidly obese, your frontal area change isn't going to be drastic enough to make you equal with the light riders.

Noone will be hitting terminal velocity...so as fast as you think you're going, you can go faster.

??? Does anyone here has a physics degree? I think we all need a refresher.

Last edited by the_drain; 12-04-08 at 02:10 PM.
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Old 12-04-08, 02:09 PM
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I had an interesting conversation with a physics professor about this. My question was: if Galileo can roll different weight boulders down a hill at the same speed, why do heavier guys on bikes go down hills faster?

The way he explained it (and what Merlin is alluding to) is that air resistance comes into play. Those heavier riders can "push harder" against the air resistance.

Last edited by sfwhiteman; 12-04-08 at 02:10 PM. Reason: for clarity
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Old 12-04-08, 02:15 PM
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I think it has to do with Cartman being on the back of the sled.
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Old 12-04-08, 02:18 PM
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inertia
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Old 12-04-08, 02:19 PM
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My guess (as a mechanical engineering student) is that it depends on the ratio of mass to drag force. The gain in drag force isn't as dramatic as that of mass. And we all know: ΣF=ma

My (quite simplified) logic is something like this:

Note: numbers are made up to show my point.

Smaller Rider
Fdrag = -100N
m = 50kg

Acceleration felt from drag = -2 m/s^2

Larger Rider
Fdrag = -175N
m = 100kg

Acceleration felt from drag = -1.75 m/s^2

Last edited by Dr.L'Ling; 12-04-08 at 02:22 PM.
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Old 12-04-08, 02:21 PM
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Originally Posted by sfwhiteman View Post
I had an interesting conversation with a physics professor about this. My question was: if Galileo can roll different weight boulders down a hill at the same speed, why do heavier guys on bikes go down hills faster?

The way he explained it (and what Merlin is alluding to) is that air resistance comes into play. Those heavier riders can "push harder" against the air resistance.

Ahh, interesting. So it's only relevant when pedaling, not coasting (except for the conservation of momentum part, which is also a good answer). So say a skinny dude putting out 150 watts will go 15 mph on the flat. The big guy puts out 200 watts to go 15 mph on the flat. They are both working at 60% intensity to go that 15 mph. When you get to the downhill, the 200 watts of the big guy suddenly make him go faster than the 150 watts of the skinny dude because his heavier weight is no longer working against him. Something like that?
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Old 12-04-08, 02:22 PM
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Originally Posted by Dr.L'Ling View Post
My guess (as an mechanical engineering student) is that it depends on the ratio of mass to drag force. The gain in drag force isn't as dramatic as that of mass. And we all know: ΣF=ma

My (quite simplified) logic is something like this:

Note: numbers are made up to show my point.

Smaller Rider
Fdrag = -100N
m = 50kg

Acceleration felt from drag = -2 m/s^2

Larger Rider
Fdrag = -175N
m = 100kg

Acceleration felt from drag = -1.75 m/s^2
???
Huh?
My head hurts

So are we going to ask Cartman to be on the back of the sled or not?
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Old 12-04-08, 02:23 PM
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Think of it this way:

In a vacuum environment, if you drop a feather and a book from point A to point B, they will reach point B at the same time. Gravity is constant.

In a regular, living environment, the book will reach point B much faster than the feather. This is due to the ratio of surface area vs. mass.

Likewise, going up a hill - if both riders have the same amount of power, the lighter rider will reach the ascent considerably quicker. Imagine a golf ball and a bowling ball. The golf ball will be easier to roll up the hill, where as the bowling ball will travel down the hill faster than the golf ball.
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Old 12-04-08, 02:24 PM
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Originally Posted by ROJA View Post
The fact that we have air introduces the variable of wind resistance, which should work to the advantage of a smaller cyclist (which generally correlates with weight).
Actually it's the opposite. With wind resistance, the heavier object falls faster. Think feather vs bowling ball. Whether this has any large effect on cyclists, on a descent, it's hard to say. I would doubt it.
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Old 12-04-08, 02:24 PM
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nothing to do with pedaling vs coasting. the heavier body wants to remain in motion, in other words external forces (think air resistance) have less of an influence on the heavier body in motion.
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Old 12-04-08, 02:25 PM
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For today's lesson in physics:

"A free falling object achieves its terminal velocity when the downward force of gravity (Fg)equals the upward force of drag (Fd). This causes the net force on the object to be zero, resulting in an acceleration of zero. Mathematically an object asymptotically approaches and can never reach its terminal velocity.

As the object accelerates (usually downwards due to gravity), the drag force acting on the object increases. At a particular speed, the drag force produced will equal the object's weight (mg). Eventually, it plummets at a constant speed called terminal velocity (also called settling velocity). Terminal velocity varies directly with the ratio of drag to weight. More drag means a lower terminal velocity, while increased weight means a higher terminal velocity.

Mathematically, terminal velocity is given by

Vt = sq rt (2mg/ρACd)

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. "

I hope that answers your question for you.
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Old 12-04-08, 02:25 PM
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Originally Posted by the_drain View Post
I'm no physician
But are you a physicist?
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Old 12-04-08, 02:26 PM
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Originally Posted by the_drain View Post
I'm no physician but I'm thinking it has more to do wind resistance than anything.

The heavier you are, the more wind resistance would be needed to hold you back. Unless you are morbidly obese, your frontal area change isn't going to be drastic enough to make you equal with the light riders.

Noone will be hitting terminal velocity...so as fast as you think you're going, you can go faster.

??? Does anyone here has a physics degree? I think we all need a refresher.
LMFAO!!!

That was pretty good, even for BF.

See Phycisian Vs. Physicist
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Old 12-04-08, 02:27 PM
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Us clydesdales are more areodynamic. We are rounded where you little punks are all flat.
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Old 12-04-08, 02:29 PM
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It's quite simple: Fatties (like me) know the trolls under the bridge at the bottom of the hill have all the Twinkies.
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