What is the truth behind bike weight? Does it really help THAT much?

Crankyfeet

Yeah... but the larger friction is compensated by the larger component lateral force that a heavier rider has... doesn't it? You know... centrifugal force (or is it centripetal??? I always get confused on those two terms)

__________________

TheDarkLord

Yeah, that's true. To clarify, the force felt by the rider during a curve is centrifugal force, while the actual acceleration itself (due to change in velocity) is directed in the opposite direction and is called centripetal acceleration. So, I guess the coefficient of friction is the primary factor when it comes to cornering without the tire washing out. So, why would a heavier rider be able to corner better?

dhk2

I used 155 lb rider with 20 lbs of bike and equipment, then added 2 lbs....not exactly a kg, but then all these numbers are just approximate due to the input variables not being exactly known. Anyway, looks like a 1.3% increase in weight yields about a 0.6% increase in speed. This is expected, since the equation balances force (weight) against the square of the speed. (If you square 1.0064 you'll get 1.013).

Actually, that's the reason us heavier guys don't have an advantage in the hills. For the same power, that extra kg we drag up that steep hill costs a straight % of our total weight, while going down, we only get the square root of that % back.

TheDarkLord

Yes, it agrees with theory. I was gonna post this before, but was waiting for your input regarding the parameters you used... I posted yesterday that for a drag force that is dependent on velocity, the terminal velocity is proportional to mass. Wikipedia has an approximate analytic solution for the case where the drag is proportional to velocity square (involving hyperbolic functions; you can see it here), and as you say, the terminal velocity is proportional to the square root of mass. So, a 1% reduction in weight will give rise to 0.5% reduction in terminal velocity.

But is it really true that the power output of a heavier rider is proportional to the weight (Cranky's post)? Don't see why climbers like Chicken would be so lean then...

Crankyfeet

Maybe because of the effects that Eden mentioned. The lighter rider is getting a more wavering frictional effect due to bumps and undulations, which bounce the lighter rider more.

The proverbial chain (in this case - grip on the road throughout a corner) is only as strong as the weakest link. Once slipping starts, the force required is much less to sustain slipping (less than the force required to initially cause it). It is the same theory behind anti-lock brake systems in cars (which is - prevent skidding => more braking force).

__________________

Eden

Now when I was discussing that I felt more stable with all that weight in my bag with my husband, we also thought it might have something to do with the distribution of the weight as well as the fact that there was more of it. We thought where the bag was sitting was possibly changing my center of gravity enough to make the bike feel stickier in the corners.

Crankyfeet

The heavier guys just get square rooted...

__________________

Crankyfeet

I think the ramifications of this debate so far are clear. If you can somehow get two water bottles full of sand and a full messenger bag from your team at the top of the hill... it could make things a lot easier on the descent. That doesn't break any rules... does it?...

__________________

dhk2

The drag power is a function of velocity cubed, but since the "power input" on a descent is also a function of velocity, when equating the two sides to solve for steady-state velocity, I get the following function of v squared:

V **2 (m/sec) = (2*kg*9.8*Gradient)/ (rho*cdA)

Believe in general a bigger, heavier rider will have more power output, but excess body fat doesn't contribute to that power. So, for top pro's already at their peak power development, getting down to minimum healthly bodyfat is necessary to optimize their power-to-weight ratio.

dhk2

Good plan. On the way up, you can also throw your two water bottles to your support crew, saving up to 1.5 kg, and have them handed back at the summit.

It is fun to pass the lighter guys on the descents, but the cruel reality of the physics is hard to overcome without superior fitness or wheel-sucking skills. Last weekend, I was smoking by a 150 lb guy on his new 46mm Reynolds tubie super wheels....at 40 mph, my extra 35 lbs beat his expensive aero wheels hands down

Crankyfeet

I wasn't saying that power was proportional to weight. I was showing an example where this was the case to illustrate how the effects of extra power can compensate for the disadvantages of extra weight on the upslope... but you get an advantage with both power and weight on the downslope.

My point was not that power is proportional to weight.

__________________

Crankyfeet

If only some races were just downhills intermixed with some flats (no uphills). You could get an entirely different physical specimen who specializes in these type of races...

__________________

dhk2

Actually, it is surprising how strong some overweight cyclists are on the flats. I've learned never to judge a rider by first appearances.....that big guy with 30 lbs of extra bodyfat is great to draft behind, and may just end up pulling you home at 22 mph

Crankyfeet

The extra weight can give them more momentum that makes it easier to sustain speed into a headwind. Same as the flywhel effect in a car (though that is mainly to sustain momentum during gear changes). And under that fatty cushioning, there might be some serious muscle that remains hidden from view...

__________________