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When I'm riding with my friend, I notice that he rolls down hills significantly faster than me. We can be riding next to each other, both in a similar aerodynamic position, but if we both coast he pulls away from me surprisingly quickly.

I'm a fair bit taller than him and probably weigh a little more (but only a few kilograms/pounds). Our bikes are similar. We both tuck into a similar position.

Apart from our size/shape, the main difference I can see are wheels. I have Campagnolo Khamsin and he has a set of American Classics that he purchased after he got the bike and spent about $800 on. They have a much deeper V than my wheels. My main theory is that the wheels are what's making the difference, possibly with aerodynamics and also better rolling resistance in the hubs, but I'm not sure how to confirm or deny this.

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I've noticed that I tend to coast much faster than other riders, despite not being particularly fast, and on any of my bikes. Would love to see the answer to this. –  Neil Fein Jul 24 '12 at 2:45
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It's most likely that he gets into a slightly better tuck than you. You being taller may be a major part of that. On a downhill minor rolling resistance issues would be swamped by air resistance. –  Daniel R Hicks Jul 24 '12 at 2:51
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Your max speed downhill is affected by your aerodynamics, and your rolling resistance. Aerodynamics is only marginally affected by tire width and spoke count, but your ability to tuck will be a much larger factor. Rolling resistance is affected in part by your hubs, but largely by your tire and tire pressure. The narrower your tire and the higher the pressure, the lower your rolling resistance, but as you make tires narrower you make it harder to corner and to brake. Obviously your ability to confidently corner at speed can be a major help on curvy downhill sections. –  Mike Samuel Jul 24 '12 at 4:35
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Have you tried swapping bikes to see if it's the rider or the bike? –  Tom77 Jul 24 '12 at 13:50
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Friction is working against you some where. As Tom77 said you could swap bikes to see if it's your tuck but make sure you try from a full stop and let gravity pull you both down the hill. You isolate out the aerodynamic part if you start from 0mph. If his bike moves faster from a stop then it's the wheels and you can look at the hub or tire inflation issues. If you both move at the same rate then swap out wheels and isolate out the aero rims you pointed out. After that look at yourself. Maybe your knees aren't pulled in as much as his... –  Chef Flambe Jul 26 '12 at 17:46

3 Answers 3

up vote 12 down vote accepted

The following forces are acting on your bike as you roll downhill (in more or less decreasing order):

Gravity

The force of gravity is proportional to mass. If two riders have the same aerodynamic profile, the heavier rider will descend with a higher maximum speed. This is easily seen in the formula for terminal velocity. The intuitive explanation is that the two will experience the same force due to drag, but the heavier rider will have a higher force due to gravity.

As you are heavier than your friend, gravity can't explain why he is faster.

Drag

Drag has a huge effect on descending speed. It's possible that your friend is getting into a tighter tuck, which as @ChefFlambe pointed out can even include your leg position. The aerodynamic properties of the bicycle (the frame, rims, spokes, etc.) can also make a difference.

My guess is that rider position is the most likely explanation for your difference in speed. Since you are taller than your friend, it may be harder for you to get into a tight tuck. You mentioned that you think it might be the rims. If this is the case, the front wheel will make a bigger difference than the rear wheel, so you could try swapping front wheels to rule this out.

Pedaling

A cyclist who continues to pedal downhill will go faster than one who is coasting. A bicycle with a higher top gear can be pedaled at a higher speed, and a cyclist who can ride at a higher cadence may have an advantage.

To rule out pedaling as the explanation, try going downhill with both of you coasting.

Rolling Resistance

Tires with high rolling resistance are slower than tires with low rolling resistance. Factors include not only tread type and width, but also the type of rubber and casing.

If you both have similar 700x23c wheels at roughly the same pressure (or slightly higher for the heavier rider), then you can rule this out.

Friction

Friction in the hubs will slow down a bike, but this force is probably insignificant unless there is a problem with bearings.

Another source of friction is in the freehub or freewheel, which will be ratcheting (clicking) while coasting. The friction may be difficult to compare against your friend's, however, unless you remove a lot from each of your rear hubs.

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FWIW, at really high descent speeds, attempting to pedal will actually increase aero drag beyond the benefit gained from any conceivable power input though cranks. That's why pros don't even bother to pedal on mountain descents and tuck into weird positions that make pedaling impossible (eg lying on top tube). –  Angelo Jul 27 '12 at 15:10
    
@Angelo, that's really interesting. Does this happen before or after the speeds where the top gear maxes out (around 30-35 mph). In other words, if I hypothetical bike had a really high gear, would this change the speed where it would make sense to stop pedaling? –  amcnabb Jul 27 '12 at 15:53
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after. We're talking 40+ mph. Elite sprinters on 53x11 can achieve high 40's for a few seconds on a flat. There was an account of this topic by the famous Jobst Brandt (here's what I found, there may be another more detailed blurb): yarchive.net/bike/high_speed.html –  Angelo Jul 27 '12 at 17:05
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Gravity is important, but it's the same for all! You need to take it into account only if you are riding on different planets, when gravitational acceleration changes:) "If two riders have the same aerodynamic profile, the heavier rider will descend with a higher maximum speed" - this seems to be wrong. Gravitational acceleration has completely nothing to do with the mass of the rider. See Gravitational_acceleration. You mention the terminal velocity on Wikipedia. However, it depends on mass only without considering buoyancy effects. S –  Andrey Sapegin Mar 13 '13 at 14:57
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Sorry, but a heavier object, given the same frontal area, will reach a higher terminal velocity. It's only in a vacuum that all objects fall at the same rate. And, compared to wind resistance, rolling resistance (for a good road bike) is vanishingly small. (The buoyancy force acting on a bike and rider is even more vanishingly small.) –  Daniel R Hicks Mar 13 '13 at 15:19

You could use the following equation; (Gravity equivalent power in watts) x (Coasting time in seconds for 1,000' elevation change) = 1360 x (Total weight in pounds)

170lb. bike & rider vs 340lb. bike & rider

(The heavy rider coasting at equivalent 462w no Pedal input) = (light rider gravity 231w + 231w pedal input to keep up)

231w pedal input (like superman/woman)

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I haven't seen it mentioned, but have you made sure your wheels are true? Am having the same issue and worry there is something like brakes rubbing. After I get the wheels aligned we''ll try try again, starting from 0. If he takes off then well try swapping bikes and seeing if its the bike or the rider!

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Hi John, this might be better as a comment. When you have enough Reputation you'll be able to add things like this as comments to the original post, or other answers. Welcome to the site though :) –  Mere Development Apr 2 '13 at 14:47

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