An Unscientific Test PDF Print
Written by Vince   
Tuesday, 02 June 2009 13:25
Let me preface this post with the following:

My test is about as unscientific as they get but the physics behind the experiment is solid.

A cyclist approaches and hill and immediately starts to pedal as fast as he can. Somewhere in the middle of the hill, the cyclist loses momentum and begins to slow down. As the cyclist arrives at the top of the hill he struggles to get over and once at the top, he catches his breath, and then begins to pedal furiously down the backside of the hill to make up for lost time.

Yoichiro Nambu

 This scenario is all wrong from the beginning to the end but the part which involves my unscientific study is the portion after the cyclist crests the hill.

When descending, you basically have two options. You can either:

a. pedal
b. get as aero as possible and coast

Option b. is more efficient and is most likely faster.

The Physics

The energy needed to accelerate an object quadruples with a doubling of the velocity and also the air drag quadruples with double velocity. To explain it differently, an object moving 50mph may require 10 horsepower but that same object requires 80 horsepower to move 100mph.

Reality

I've been in situations where a rider in front of me is pedaling and I'm tucked in, knees on the top tube, and my pedals are at the 3 and 9 position. As I continue my descent I notice the rider in front of me getting closer even though I'm doing nothing but coasting and he is pedaling. This is what prompted my test. I wanted to see what the actual numbers would be given different situations and how pedaling would affect my speed.

The other day while riding, I was on a long descent which gave me several opportunities to play around. In the first situation, my speed was 40MPH while tucked and coasting. As soon as I started pedaling, the powermeter displayed an effort of over 300 watts and yet the speedometer showed a decrease in speed. In the second situation, it took 250 watts to increase my speed from 28MPH to 29MPH. And in the final scenario, it took 180 watts to increase my speed from 27MPH to 28MPH.

Let me recap:

@ 40mph – an effort of over 300 watts - decreased speed
@ 28mph – an effort of 250 watts - gained 1MPH
@ 27mph – an effort of 180 watts - gained 1MPH

I question one of the last two scenarios because of the 70 watt difference and only 1MPH between them but I think the most obvious point of this test is the first situation where I lost speed even though I was over 300 watts. At one point I soft pedaled and it appeared the act of pedaling itself created more drag and slowed me down.

If I can remember, I will try this experiment again at lower speeds to clear up the discrepancy in my 28MPH and 27MPH tests. Who knows, maybe after I put another 5 minutes into this project, I'll get a Nobel Prize for Physics. I mean really -- if Yoichiro Nambu (pictured above) can get one for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics, I can definitely get one for coasting downhill. :)
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written by Matt Getting, June 02, 2009
Ouch, my brain hurts :)
It's true
written by Keith, June 05, 2009
This is true. It is tactically a bad idea to put in the same effort throughout a race. I read an article with harder numbers than yours a year or two back. Basically, the energy you are personally putting into the wheels results in a better return on energy investment the slower you're going (because of wind). So, as you increase power up a steep hill, you see an immediate increase in speed. But, as you go faster and faster into the wind, the extra power you can put in with pedaling has a diminishing ability to increase your speed; mostly you should be aero and let gravity do it.

This is why a good cyclist will bias their output such that they are above average up hills (less wind, more meaningful ROI on power output) and do less down hill. Down fairly steep hills, you should entirely rest and just get as aero as possible. If somebody happens to gently pass you, by the time you get to the hill you've been resting for 20 seconds and can use that extra energy to burn them up.

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