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Old 09-13-05, 09:29 PM   #1
Supertick
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How/Why did this happen?

I went riding with four friends today. I am 6'1" 200# and was riding my touring bike with 700X28 tires at 100 PSI and Deore hubs. Two of the other riders were my size and the other two were quite a bit smaller. One was on a cross bike and the other three were on road bikes. We climbed a very steep hill and I was able to stay with them without any trouble at all. When we got to the top of the hill we stopped and all started at the same time. We coasted downhill about 2 miles and none of us pedalled during the downhill run. My bike is no doubt heavier than the others. All of them passed me on the downhill and walked away from me as we were all just coasting. I would have thought that because of my weight and the weight of the bike that I would have been the one going the fastest. Could someone please explain this to me. Were their wheels that much better? What am I missing here.
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Old 09-13-05, 09:36 PM   #2
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Probably a few variables in there. More than likely the one with the smoothest hubs wins.
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Old 09-13-05, 09:39 PM   #3
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oops wrong post
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Old 09-13-05, 09:39 PM   #4
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Over a relatively long distance like two miles, many factors could explain your slower speed. I believe one of the biggest factors is the aerodynamics. I doubt all the riders equally tucked and offered the same drag. Head, arm, leg positions all factor into the overall drag. Berarings and mechanical drag are probably a lesser contributor.
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Old 09-13-05, 10:55 PM   #5
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A lot of drag from the 700x28 tires compare to standard road tires which is 23"s. Road bikes tend to put you in a more aero postion.
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Old 09-14-05, 01:00 AM   #6
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Yeah, but total drag from rolling resistance is minimal, probably less than 5% of the energy pushing those bikes on the downhill. It's most likely all aerodynamics. Was your face 1" away from the stem with a flat? Were your elbows tucked tightly against your side? Just changes in position makes a 15-40% difference in speed on most of the downhills I do around here.
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Old 09-14-05, 05:21 AM   #7
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OK, I run 6'3" and 220 pounds. My bikes run loose bearing hubs with high grade bearings and high quality grease. tires are 700 x 25 at 95 psi on my main road bike. And I run a rather aggresive riding position, with a 4" drop from the saddle to the tops of the bars.

I always walk away from lighter riders on downhills.
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Old 09-14-05, 05:57 AM   #8
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My first guess would be tire pressure. I don't know about the physics, but seems that could make a bigger difference than aerodynamics.

Otherwise, weight is huge factor - my wife and I have similar bikes, tires, etc. and when we go downhill, I go a lot faster all other things being equal.
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Old 09-14-05, 08:00 AM   #9
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Funny you ask, I had a similar question a few days ago.


Difference in rolling resistance.
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Old 09-14-05, 08:11 AM   #10
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I agree that tire pressure, hub bearings, etc. are very minor components in this case.

Your aerodynamics were bad enough that they outweighed (pun intended) your weight advantage.

I have a friend who outweighs me by 25 pounds and we ride nearly identical bikes with the same tires, tire pressures and hubs that I overhaul and adjust for both bikes. I always beat him down long hills because he doesn't tuck-in as much as I do.
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Old 09-14-05, 08:34 AM   #11
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Weight will make you go down hill faster? That's a whole new law of physics. Galileo may have to rise from his grave on this news!

Remember from high school science: Velocity of a falling object = acceleration of gravity X time. There is no mass (weight) in this calculation.

Weight gives you momentum; not acceleration!!! At the bottom of a 2 mile hill, a 200# rider will have a bigger impact on a brick wall than a 75# rider; but, if they are on comparable bikes with equal consideration for wind resistance, they will hit the wall at the same time!!!!!
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Old 09-14-05, 11:00 AM   #12
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Quote:
Originally Posted by JKfromPBurgh
Weight will make you go down hill faster? That's a whole new law of physics. Galileo may have to rise from his grave on this news!

Remember from high school science: Velocity of a falling object = acceleration of gravity X time. There is no mass (weight) in this calculation.

Weight gives you momentum; not acceleration!!! At the bottom of a 2 mile hill, a 200# rider will have a bigger impact on a brick wall than a 75# rider; but, if they are on comparable bikes with equal consideration for wind resistance, they will hit the wall at the same time!!!!!
Your equation for a falling body is correct for the moon where there is no air resistance. However, on earth with an atmosphere you have to add the force caused by the air resistance. Assuming equal aerodynamics of two riders moving at the same speed, the air resistance imposes equal force on each rider. The resulting decelleration component from this force is less for the heavier rider (acceleration = force/mass) with the result that the heavier rider coasts down the hill at a higher speed than the lighter rider. Thus, the heavier rider not only hits the wall at the bottom of the hill harder, he hits it sooner than the lighter rider.

However, all things are generally not equal and in the case above, the lighter riders were probably more aerodynamic for some reason than the OP, resulting in a lower air resistance force and a higher resulting speed.
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Old 09-14-05, 02:49 PM   #13
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If you are heavier, you should go down hill faster despite the fact you are a bigger 'wind sheild', but many factors can come in to play which will tip the scale the other way, i will just list a few so you get the picture.

If they are in a very aero position e.g. in the drops and you are leaning back. If you are wearing baggy clothes and htye are wearing lycra. Low presure tires. Breaks rubbing. Extra friction fomr seized up bearings in the hubs. Any of these could be part of the problem and there are lots of other possibilities, you could have even been zig-zaging slightly which would slow you down (by this i dont mean cutting from side to side i just mean a bit of minute movment of the wheel, could even be due to a loose headset)
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Old 09-15-05, 03:51 AM   #14
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yeah, the frontal surface-area increases with the square function of size (^2), but weight goes up by the cube function (^3). So if you take a small guy and double his physical dimensions (x2) in all directions, he's going to block four times (2x2=4) as much wind as before, but will weight 8x heavier (2x2x2=8). So the bigger guy will block 4x as much wind, but have a lot more weight to push that wind aside, so he'll go faster than the smaller guy on downhills.

Position on the bike really makes a huge difference as well. Tucking in the knees together reduces turbulence and help stabilize the bike. I always try to get as low as possible with my back perfectly flat and horizontal. That way, my entire upper-body only blocks as much wind as my shoulders. Then having one arm behind my back also reduces drag a bit as well, typically gives me an additional 1-2mph at 50mph. Rubberized skinsuits are about 1-2mph faster than normal skinsuits @ 50mph. Normal skinsuits are about 1-3mph faster than separate jersey/t-shirt & shorts depending upon how tight-fitting they are and the rear-pocket configuration.
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