PaulRivers

If gravity affected you going downhill equally regardless of mass, then it would also affect you going uphill regardless of mass. You'd be able to pull a 500lbs behind you with the same speed as you could pull 1lb up a hill.

PepeM

BF physics are the best physics.

Carbonfiberboy 

IME mass doesn't make as much difference as position and body configuration. That's everything. I've weighed from 160-145 and so far have never ridden with anyone who coasts faster than I do. A couple who've coasted as fast as, though. On maybe 1000 group rides? Position and short legs, plus a nice helping of courage or maybe just experience downhill racing and MC riding. Downhilling is similar - same emphasis on position, fitness, and not being worried by the feeling of speed. I've given lessons, but didn't seem to help very much. Some of it's maybe simply feel. Anyway, it's fun to be known for doing something well, even if it's not climbing. I also sprint pretty well, though I doubt the two things are connected, but hmm.. Descending muscle? Just gotta have it where it counts.

asgelle

Carbonfiberboy 

OMG. Equations! They're beautiful and even helpful, but in the end, they don't win bike races, not even coastiing races. That said, heavy riders are pretty fast, but short heavy ones don't have good position. Tall heavy ones fail in the CdA part. It's complicated because the interactions of the variables are not shown in the equation, plus there's that mental thing. And that said, one of the fastest coasters I've been with was a tall guy with reasonably low BMI, a good climber. His position was perfect though. His bike was interesting, 650 deep carbon wheels, HED I think. Looked weird but he was fast. Local crit champ. We were coasting wheel-to-wheel at about 50 on a 7% grade IIRC. That would have been in ~'03 for those into our local riding culture.

asgelle

No, but they give the insight to understand how various factors such as weight and drag area affect speed to answer questions such as which is more important, weight or drag (which is what was being discussed, not winning races).

burnthesheep

FWIW, most of the larger guys who benefit from the weight downhill have three strikes against them:

-their bike setup is likely already more "slack" than their skinny counterpart, meaning only so low you can get

-they weigh more so tucking like a skinny guy hurts the arms more, especially if we're talking descending in the mountains for 10 to 15 minutes at a time

-they punch a larger hole in the air, a smaller rider not only can tuck lower they are already smaller

I don't know many club riders that can even ride in the "pursuit" position like a machine gunner with a flat back for very long. I usually don't, as it's rude on a group ride to the folks resting behind you.

But, if on a hammer ride...........I most certainly will do this on my turns.

hsuehhwa

Most of the time riders with a lower power to weight ratio climb slower but descend faster.

wphamilton

In the sense that you've got that square-cube thing going on (frontal area vs mass per one-dimensional length) you are right. Asgelle is also right, because he rolls up the square-cube part into "body configuration (CdA)", specifically the "A". Kind of tricky of him since I suspect you were thinking solely of positioning by "body configuration" and he includes size and shape.


Reading that seems kind of muddled even to me, so let me put it this way. I am about the same size as you, and if I am next to a guy 50% heavier, say 225 pounds, and he's the same shape scaled up, he's only going to have about 30% more frontal area (the A in CdA). Big advantage to him, and in reality I can't fully make up that difference in my higher density by being leaner. My amount of muscle mass cannot get dense enough to make up for that amount of difference. It can make up some, but nothing like that amount. So, in order for you or me to be better or equal in coasting there would HAVE to be some additional difference in shape, positioning or both. From that perspective (rather than absolute numbers) I have to agree with you that the position and shape would necessarily be larger factors than weight in the speeds you achieve coasting downhill.

Carbonfiberboy 

Yes, nicely done.

asgelle

Actually I wasn't thinking of size, and isn't body position the same as shape? But this is why referring to the equations is useful. We know the drag force is equal to 1/2 rho CdA v^2, with many factors contributing to the drag area CdA. So the question becomes how does CdA scale with weight, and the answer is that there is no one formula that relates the two for all riders. Sure there are approximate allometric scalings, the simplest being A=m^(2/3), but these are very approximate and small changes in body shape or position can easily have large effects on CdA. That's why I think it's misguided to try to link drag with the gravitational force. For all intents and purposes, they're independent.

Carbonfiberboy 

On group rides with my single, I've always been a slower climber than most of the faster folks. Thus I've focused on getting back on during descents and flats and so I've long experimented with position. I've also worked on being able to maintain high intensity for long periods - most riders back it way off as they come over the top. Anyway, I've been messing with this for a long time. Most club riders just don't make the effort and thus don't have the trial-and-error experience, or don't even want to get it. Who wants to keep their chin 2" off the stem cap if they don't have to? It's hard on the back as well as the arms. Of course we're talking about club riders. Top racers have this down.

wphamilton

Not from my understanding of it. Area is a projection of the body shape, given a particular body position, onto a 2-dimensional plane. For example, if you have very wide shoulders and big chest, and I have a long and skinny torso, we have differing shapes. If we assumed the same body position, low and parallel to the ground, you would present a larger area. Same position, different shapes, different area, so I do not see them as the same.

At a given orientation, "A" scales with size and is modified by the particular mass distribution within the body (shape). Cd is also dependent on body shape, but to a much lesser degree than is area IMHO. Since the situation is dynamic, I believe that the actual Cd also has dynamic properties depending on the person's motion and adjustments and I suspect that's in fact part of Carbonfiberboy's edge. I think that he adjusts moment by moment to optimize his drag coefficient.

Brian Ratliff

10 years later and we are all still arguing something you can just calculate:
Why do heavier cyclists descend faster?