They pedal going downhill on their light carbon bikes

[Hip Priest]

Reminds me of my first club run. I was worried I'd get dropped on the hills, but I was fine. It was on the descents where I got dropped.

 

[jefmcg]

This is the most likely of all explanations, makes a huge difference.

I used to think (I should mention I was already a middle aged woman, as this thought should belong to a 12 year old boy) that I had the fastest bicycle in the world, because i was always going faster downhill than the person in front of me. It took a couple of years before I reached the DERP moment, and realised this was because i was drafting them.

Similarly when another rider pulls away from me on the flat, then a car passes me and drives behind the faster rider. Suddenly I'm able to keep up with my friend without effort! Oh, yeah, I'm now drafting a car

 

[bpsmith]

Pedalling downhill is what makes the climb worthwhile!

 

[Pat "5mph"]

I've got a huge big chainring

 

[Dirk]

I've got a huge big chainring

Too much information!

 

[boydj]

Riding in a group, the people at the front should always pedal downhill, otherwise everybody behind them will be braking all the way down.

 

[jefmcg]

I've got a huge big chainring

How big? This big?

 

[oldroadman]

Right to go all scientific.

"Downhill

During descents, the negative slope of the hill in the power equation reflects the addition of gravitational potential energy to the power generated by the cyclist. In a freewheel (passive) descent, the cyclist's speed will be determined by the balance of the air resistance force and the gravitational force. As the cyclist accelerates, sv2 increases. Once kaAsv2 (plus the negligible power term associated with rolling resistance) increases to match giMs, the cyclist will reach terminal velocity. Any further increase in speed must be achieved by adding energy through pedaling. However, on steep hills, terminal velocities may reach 70 km·hr-1. At such high associated values of sv2, even the application of VO2max would result in only a minimal increase in speed.

Terminal velocity can be solved for in the cycling equation above by setting power at 0. If one assumes the rolling resistance term is also 0, and that there is no wind blowing (v = s), then the equation becomes:

kaAs3 = -giMs
or s = (-giM/kaA)1/2

Thus, the terminal velocity is roughly proportional to the square root of the ratio of M/A. Scaling reveals that larger cyclists have a greater ratio of mass to frontal area. They therefore descend hills faster as a consequence of purely physical, not physiological, laws. Since the larger cyclist has a greater mass, gravity acts on him or her with a greater force than it does on a smaller cyclist. (Note: A common misconception is to note the equal acceleration of two different sized objects in free fall in a vacuum, and assume that the force of gravity on both is equal. The force on the more massive object is greater, being exactly proportional to mass, which is why the more massive object is accelerated at the same rate as the less massive one.) While the larger cyclist also has a greater absolute frontal area than the smaller cyclist, the difference is not as great as that for their masses. Thus, the larger cyclist will attain a greater s3 before a balance of forces results in terminal velocity.

With lighter cyclists climbing hills faster due to their greater relative VO2max, and heavier cyclists descending faster due to their greater M/A ratio, one might assume that equal performances would occur in races involving equal up and down segments. However, ascents take longer than descents, so a speed advantage to small cyclists on the acsents produces a greater time advantage than large cyclists obtain on the descents. For this reason, smaller cyclists are generally superior competitors on hilly road races." http://www.sportsci.org/jour/9804/dps.html#downhill

If you cannot be arsed to read it - it basically says heavy bloke/bike is faster than light.

Whoever suggested terminal velocity on a fast descent is 70 kph is talking out of the wrong orifice. Personal fastest recorded in this country (in a race situation) before spinning the 53x13 (which was the smallest sprocket in the distant past) a bit - 85 kph, add a few good revs and the clock was at 98 kph. Only the bends slowed it. Focuses the mind a bit, over 90 kph - speeds as recorded on a moto (with calibrated speedo) close by. And I was a skinny 58kg in those long gone days, going faster than bigger blokes on descents. A nice compact position is all that's needed.

 

[Pat "5mph"]

How big? This big?

Not quite

 

[totallyfixed]

How big? This big?

Nah, this big, I lent the Tour of Cambridge organisers my lo-pro vintage bike to show them the gear I prefer to use for climbing.

 

[ColinJ]

My bikes are like that, only in reverse!

(OK, not quite but the smallest chainrings are all smaller then the biggest sprockets - Cannondale road 28/29, Basso 26/28, Cannondale CX 34/36, MTB 22/32.)

 

[classic33]

I don't think even the hills up north are that steep.

@Accy cyclist Plug in the numbers here. http://www.gribble.org/cycling/power_v_speed.html

There's this one. Still falling as you pass the house on the right-hand side of the picture.

Or this one, 40 degree slope. Only half a mile though.