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So my riding buddy can never keep up with me downhill.

  • I'm slightly heavier than him (5-10lbs).

  • I have an older non aero bike. He has a newer aero bike. His is probably a few lbs lighter than mine.

  • I have a better aero position than he does.

But I don't just go a little faster than him, I smoke him!

My suspicion is wheels.

Can lower end wheels be the cause? Anything else?

ebrohman
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John_911
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    Stiffer wheels and harder tires make for better downhill speed, but the main factors here are firstly your better aero position and the weight difference. – andy256 May 23 '16 at 01:37
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    Just to clarify: you mean, you're both coasting, and from the same starting speed? – R. Chung May 23 '16 at 01:59
  • There are a number of answers to similar questions already, try for example How can one estimate drag for a bicycle? Wheels contribute very little to drag, and are usually so light that the difference in inertia or rolling resistance between wheels is negligible. But R. Chung can give you an exact answer if you provide a little more info, I'm sure :) – Móż May 23 '16 at 03:18
  • Without being facetious, peddlng helps. I've routinely kept up and passed people on the downhill purely because I pedal up to speed when cresting a brow, without waiting for gravity to get me going. Mass helps as does a good aero position. Finally, confidence in your own riding abilities helps you keep the hands off the brakes. Nervous braking slows you down early and heats your brakes, making them worse and making you brake earlier, a viscious cycle. – Criggie May 23 '16 at 04:00
  • Is this a road or an off-road downhill? – Criggie May 24 '16 at 04:09
  • I read this and thought of downhill mtb. Took me a while to realise why no one was mentioning skill. – Holloway May 24 '16 at 08:43
  • @holloway For both downhill road and offroad, its about the rider's balance and reaction time, and the bike's grip. Also a lot to do with anticipation and looking ahead. Where they differ IMO is downhill off-road, its more of a controlled fall, whereas downhill road is about pushing to the limits of grip on corners and maintaining momentum through the corner. – Criggie May 24 '16 at 10:44

3 Answers3

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It's really simple, gravity/weight moves you forward, and drag/friction keeps you from going fast.

He can't do much about the weight difference, short of drinking more beer or getting a heavier bike.

But drag and friction is a different story. There are many things slowing you and your buddy down, the largest factor being wind resistance. Wind resistance is related to the square of your speed, so twice as fast means 4 times the resistance. Secondary factors would probably be your tires (knobby vs smooth, soft vs hard), and even your hubs. I once had an old hub serviced and started keeping up with my downhill-coasting buddies.

Lee-Man
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Another point to consider - confidence. I had a washout on a road, which lead to a slide on a downhill ~4 months ago, which ended up off the road and down a hill.

I am now much more leery of turns at speed, to the point I brake down to a slow speed and coast through any leaning part of the curve, only applying pedals again when exiting the corner.

So, is your riding partner braking excessively for corners? Slick tyres also contribute to the step-out-happiness of a wheel, so check and compare the tread too.

Finally consider some elimination. Swap bikes for a trip, and see if the speed goes with the bike or stays with the rider.

Criggie
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  • "coast through any leaning part of the curve" isn't that a given? If I pedal through corners, the pedals hit the ground. – Holloway May 24 '16 at 08:45
  • @holloway its hard to describe. 1) a gentle veer can always be pedalled. 2) some corners you have to coast through. But there's not a clear cut boundry. Also, you know the feeling of riding on ice? Very careful and quite vertical - I feel like I'm doing that on a fast downhill corner where in the past I'd lean more and trust the tyres to hold on. In a trip today I consciously used the camber of the road to help with the turn, and leaning was much more confident. I guess its a case of "once bitten (slipped out), twice shy." – Criggie May 24 '16 at 10:39
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Apart from setting up his suspension and his tyre pressure (around 27-29psi if talking about downhill bikes, tubes and tyres), he can't do much... It's all about his skills. He can only practice. Bikes don't make riders. Lack of brains does (always with the good meaning ). I have found myself riding faster than guys with serious downhill bikes, on a 120mm full suspension trail bike. Push your friend to go harder.

Ride hard, ride free

Chris Tsiakoulas
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    29 psi on a road bike will likely have you bouncing the rims on the road surface. OP said "aero bike" which sounds like a road bike, and the question doesn't say but is probably about road. – Criggie May 24 '16 at 04:08
  • I actually thought we were talking about downhill mtb here! – Chris Tsiakoulas May 25 '16 at 20:39
  • We don't actually know which the OP was referencing. So we're inferring from clues in the question, but its always possible. Your points are certainly related. – Criggie May 25 '16 at 22:40