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Old 09-12-05, 05:52 PM   #1
jessica3333
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weight versus speed

Is there a general equation for the relationship between the weight of the rider and the speed of a bike as it coasts down a hill, all else being equal?
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Old 09-12-05, 06:01 PM   #2
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Quote:
Originally Posted by jessica3333
Is there a general equation for the relationship between the weight of the rider and the speed of a bike as it coasts down a hill, all else being equal?
I'm sure that somewhere in the math world is your answer but it would help to
know what you need to know for and why.
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Old 09-12-05, 06:25 PM   #3
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Yes. Speed will be the same no matter the weight.
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Old 09-12-05, 06:30 PM   #4
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Originally Posted by Xtrmyorick
Yes. Speed will be the same no matter the weight.
Not true. I'm overweight and I easily outroll anybody smaller and skinnier, since I have more mass to overcome air resistance. In very crude terms air resistance is based on your sillhouette which is two dimensional, while gravity is based on your mass which is three dimensional. An extreme example is a mouse which can fall from a great height without getting killed since his surface area is large relative to his weight, while a person can be killed by a fall of only a few metres. Or better,think about throwing a table tennis (ping-pong) ball vs a rock of the same size, or dropping a feather vs dropping a dime.
RGC
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Old 09-12-05, 06:34 PM   #5
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In perfect conditions (IE-Vacuum) then weight means nothing. Gravity acts the same on all bodies.

But in the real world, other things must be considered. Primarily, aerodynamics. It's possible for the heavier rider to have better sectional density than the lighter one. It has been observed that clydesdale-sized riders do tend to gain on their lighter brethren on steep descents.

But there's a point of vanishing returns, however. If you are so large as to present a surface area that overcomes the sectional density advantage...

Also, if the descent has a lot of turns, the lighter rider will be at advantage again, as he'll be able to drive harder through the turns without loosing traction.
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Old 09-12-05, 06:35 PM   #6
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Originally Posted by Xtrmyorick
Yes. Speed will be the same no matter the weight.
WRONG!

Pay attention to all the words when thinking of things from High School Physics.

IN A VACUUM all abjects fall at the same speed.

But on a cycle we are not in a vacuum, in fact on a road bike over 80% of the work you do at speeds of 20MPH is overcoming aerodynamic drag. Now look at an object, rider sphere whatever. The cross section goes up as the square of the height (diameter) the mass goes up as the cube. Result bigger riders decent faster. In the more real world there is more than just scale . Take a nice big beer belly. It has no impact at all on the cross section (provided it is a perfect beer belly and only sticks out forward) thus it contributes even more to the downhill speed.

You will not find formulas as position counts as does the bike. But a nationally ranked woman rider in excellent aero position will decent Mullholland Drive at the same speed as a very average 220lb rider sitting up. (Experimental results because the club regroup point is at the top of the climb).
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Old 09-12-05, 06:41 PM   #7
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Quote:
Originally Posted by jessica3333
Is there a general equation for the relationship between the weight of the rider and the speed of a bike as it coasts down a hill, all else being equal?
http://www.kreuzotter.de/english/espeed.htm
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Old 09-12-05, 07:09 PM   #8
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http://www.kreuzotter.de/english/espeed.htm

Thankyou. For once I didn't have to post this site first.

This site has been very helpful in the past for doing certain preparations for different events on different terrains. For instance, I was torn between taking my tailfairing with me on a 7 day tour in Tennessee. I was worried that the extra 6 lbs worth of weight would hurt me on the long climbs. After plugging in some wattages with the stock bike setup and the tailfaired bike setup on different grades, it appears that the extra weight of the tailfairing will on the average only slow me down by .2 mph at the most.

The benefit will be on the flats where I can gain 5-6 mph over the stock setup. I'd rather cruise at 28 to 30mph on the flats and pay a .2mph penalty on the uphills.

oh, if you want to get faster on a downhill, try one of these.

http://groups.msn.com/BicyclingForum...o&PhotoID=7408




on a -5 percent grade, a 200 lb rider in the drops will reach a terminal speed of 35.7mph

on my tailfaired lowracer on the same downhill 5% grade I will hit 56.1 mph

( been there done that) many times

Last edited by lowracer1; 09-12-05 at 07:15 PM.
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Old 09-12-05, 07:31 PM   #9
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Downward force on a hill is m*g*sin(phi)

m is the mass of the object (rider+bike)
g is the gravitational acceleration on Earth (9.81 m/s^2, or 32.2 ft/s^2)
phi is the angle that the hill is inclined

MASS does matter!!!

If a bike slid down a hill (rather than rolling), then mass would cancel out of the equations, but rolling resistance is difference than kinetic friction.
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Old 09-12-05, 08:52 PM   #10
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Originally Posted by lowracer1
For instance, I was torn between taking my tailfairing with me on a 7 day tour in Tennessee.
So after your ride, let us know if you thought it was worthwhile.
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Old 09-12-05, 09:02 PM   #11
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Quote:
Originally Posted by lowracer1
The benefit will be on the flats where I can gain 5-6 mph over the stock setup. I'd rather cruise at 28 to 30mph on the flats and pay a .2mph penalty on the uphills.
I am training intensely to increase my cruising speed on the flats as you say. I want to go on a 3000 mile tour with some hot shots. Unfortunately my average on the flats is 20 to 22 MPH. Tell me your secret to get up to 28 to 30 MPH. I think I would pay for advise with measurable success.
I am an old guy at 63, however.
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