Good uphills but not great on flats ?

[ColinJ]

You need to understand the relationship between power-to-weight and power-to-drag. As your body weight increases, your drag increases, but by a much much smaller amount, and given that power required to overcome air resistance is a function of the drag you and the bike produce and the speed you are riding at, a large increase in muscle mass results in a relatively small increase in drag.

The power required to overcome gravity on the hill is a linear function of the gradient and your weight, so as your weight increases so does the amount of power needed to maintain a given speed.

For example, say you weigh 65kg and can produce a steady 200W of power, and I weight 90kg but can produce 250W power, on a hill of 10%, I can travel at 8.2kph, whereas you would be travelling at 8.8kph. However on the flat I can do 34kph to your 32kph, even if I could only produce 200W I'd only travel a fraction slower at 31.5kph.

Light = fast on the climbs, heavy = fast on the flats (assuming greater weight = greater power).

Yep.

I had a very skinny*** teenager come on one of my hilly local forum rides. We started one steep climb together but he shot away up the hill at double the speed that I could manage. The gradient eased over the summit as the road went onto open moorland, where we were encountering a stiff headwind. I could see him ahead of me coming almost to a standstill in the wind. I had no problem with it, caught him up, and went past at double his speed!
Light on climbs, good; heavy, bad. Lightweight body but light on power too into headwind-from-hell, very bad; heavy but powerful, okay.


*** When I say 'skinny', I mean 'skinny' ... I asked him where he'd found leg-warmers small enough for his super-slim legs. He replied that he couldn't find any, so he wore arm-warmers on his legs instead!