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To expand on what I just said a moment ago...
Adding some maths in...
If we say the 2 riders are on the same bike, wearing the same clothes, using the same tyres, and that neither are pedalling (its too steep to have any effect any more), then the only variable is their weight and frontal area.
We'll say the 2 riders are 65kg and 100kg (roughly 10st and 15st)
We'll say the hill is a constant 15 degree slope.
We'll also say that the heavier guy only has 20% more frontal area (hence only 20% more drag) when they're both in an aerodynamic tuck.
If we say that the max velocity reached for the thinner guy is 80km/h (50mph), we should be able to work out the predicted max velocity of the heavier guy... (I'm going to assume drag goes up in a linear fashion, I know it doesn't, but my maths doesn't extend that far).
Using F=Ma... Force acting upon the smaller guy vertically is... 9.81m/s2 x 65kg = 637.65 Newtons
But we're not interested in the force vertically, we're interested in the force in line with the slope...
Which is... Sin15degrees x 588.6 = 165.04 Newtons propelling him forward in line with the slope.
So what we're saying is that when he reaches his terminal velocity of 80km/h, he has 165.04 Newtons still acting to propel him forward, but he is also now experiencing 165.04 Newtons of deccelerative force due to aerodynamic drag.
For the heavier guy, we're saying he is experiencing the following accelerative force...
sin15 x 9.81 x 100kg = 253.90 Newtons
Which is directly proportional to the amount he weighs.
If he has a 20% larger front area though, at 80km/h he is only going to be experiencing the following amount of deccelerative force...
165.04 x 1.2 = 198.05 Newtons
Clearly, he is still accelerating!
So assuming drag is a constant (as I've stated before I know it isn't, but for the purposes of making it easy to explain I'm saying it is), we can then apply the following...
253.9 Newtons / 198.05 Newtons = 1.282
1.282 x 80km/h = 102.56km/h (or 64.1mph)
So in conclusion, once again, Fatties go faster!!! 😉
If we're to actually do it properly, as aerodynamic drag goes up as a square of the speed increase, the fat guy is likely to only hit 90.58km/h on the same stretch of road (by my crude calculations), which is still some 10 and a bit km/h (or 6.6mph) quicker than the skinnier guy... All just cos he weighs more!
That help?
Not one argument with my logic?
What's wrong with this forum these days? 😕
😉
in my younger thinner days i hit 56mph on a rigid 1996 rockhopper with slicks blown up 110psi on a london to brighton charity bikeride