I would think we'd have to go by inches instead of pounds in this case, not pounds.

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Find out what it is in life that you don't do well, then don't
do that thing.

It depends on the shape of your gut and whether you have cellulite or not. Cellulite is dimpled and as we all know, dimples are more aero, so losing weight could counterintuitively make you less aero. Similarly, a well-rounded gut can actually smooth the airflow over your back wheel and make you faster.

Too much fat on your thighs makes it tough to keep your legs in tight to the bike. Less weight would also allow you to get lower if your knees are banging into your belly.

I too completely agree with cartsman, cellulite is a skin condition which gives dimpled appearance on the skin and is caused by too much fat on your thighs. There are various ways through which you can reduce the thigh fat like by following a body challenge guide and by having a healthy diet daily. By reducing the fat on your thighs you will be able to keep your legs in tight to the bike easily.

I think you missed the joke!

0,7 km/h for 10 kg.

According to kreuzotter.de (has an english version)

I entered my data (84 kg) and 230w which gave a speed of 36,6 km/h: changing my weight to 74 gave a speed of 37,3 km/h.

The difference is due to the smaller frontal surface of the body, which is one of the parameters entering in the aerodynamic drag equation.

I think you missed the joke!
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Probably on purpose since their post is a sales pitch.

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Take a short break from ST and read my blog:
http://tri-banter.blogspot.com/

Don't expect that to be accurate.
These calculators are using typical values based on a range of inputs. They can be pretty accurate so long as certain assumptions hold true but manipulating weight isn't likely to be useful in determining the real effect as the algorithm likely reflects norms in body proportions as much, or more than body fat. For example, I have pretty wide shoulders, regardless of weight but my build also makes me more prone to be a little heavier (I have a naturally muscular upper body). Dropping weight will come mostly from some excess body fat at my stomach and maybe a little from my legs. It won't effect my shoulders which are far more influential on drag.

Frontal area is not really predictable based on weight. Frontal area is also not the sole determinant of drag. As a previous poster said, the impact of weight on drag could be positive, negative or neutral depending on the starting body shape and position and how the additional weight affects that.
I would think additional weight on legs, upper arms or shoulders will likely increase drag. Additional weight on the torso could go either way. It's very possible it could reduce drag if you were starting quite thin but will be specific to each individual.

Losing weight might give you a smaller frontal area but that will be offset by your formally tight racing skinsuit now flapping in the wind.

If I use this while riding on my Zipp disc, will it get rid of the dimples?

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My latest book: "Out of the Melting Pot, Into the Fire" is on sale on Amazon and at other online and local booksellers

No, but it's exactly half the equation. There's no conceivable way to gain frontal area by losing weight. Losing weight can't really hurt your frontal area and will almost certainly reduce your frontal area, so you've improved half the equation, at least.

And while I agree that the change in shape could theoretically result in an increase in Cd, I find it hard to imagine anyone with 10 kilos to lose actually having their Cd increase so much that it offsets the decrease in frontal area. Maybe, if you were a mutant with a camel-like hump on your lower back that made your body profile a foil, but for the average joe with a little blubber on the belly/hips/thighs, I just don't see it.

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"They're made of latex, not nitroglycerin"

I have no idea what you mean by "exactly half the equation".
Losing weight may or may not reduce frontal area but for moderate weight changes with uncertain causes, there's no reason to assume it will make a significant difference. Changes to the shape of the body seem as likely or perhaps more likely to impact drag. I stand by my main assertion which is that it's not likely you can make any usefully accurate correlation between weight loss and drag reduction.

I'm not sure what's confusing you. For a given speed and air density, Drag Force = Cd x Frontal Area.

I'd really like to see someone who lost 30, 20, or even 10 lbs and didn't reduce their Frontal Area at least a little.

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"They're made of latex, not nitroglycerin"

I know the equations well. My point is that half the equation doesn't mean half the impact unless the changes in both variables are proportionate. And that's an assumption that makes no sense to me. Thus stating it's "exactly half the equation" doesn't mean anything useful to me, it seemed you were implying something i couldn't see any relevance in.
I know cycling is obsessed with frontal area because understanding of aerodynamics in cycling is, by necessity, dependent on generalities and direct measurement. It's not like cyclists are all the same, either one to the other or moment to moment. But IMO this specific question is one where it's important not to make sweeping assumptions because the error margins in the rules of thumb are likely in the same order of magnitude as the changes we're discussing.

Your confusion is evident here.
There is no "A" generating drag force.
There is a lump factor "CdA" between air speed, air density and drag force.
The "A" in "CdA" is a 'normalizing' factor with the purpose of quantifying the 'aerodynamic shape coefficient' "Cd", separate from the physical size of the object. This "A" can denote frontal area, surface area, side area or whatever area is relevant to the comparison.

In the weight loss case, you can't change frontal area without changing the shape. "CdA" will change in symbiosis. "Cd" will not be constant.
There aren't 'halves' in the equation.

I never said Cd was constant. I said we know that one factor of the two factor equation (Cd x A) would improve with weight loss. So if you lose weight you've improved "one half" of the equation. You can't accurately predict what will happen with the other half of the equation. Cd may stay the same, may go down, may go up. But you'll only fail to see a gain in the unlikely result that the change in shape yielded an increase in the Cd of a greater magnitude than the magnitude of the decrease in A. It's a possible outcome, but one that's probably far out there on the bell curve.

Are you guaranteed to improve your aero by losing weight? No, not guaranteed. But the odds are strongly on your side.

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"They're made of latex, not nitroglycerin"

I'm headed to A2 tunnel in a couple of weeks. Currently 20lbs over race weight and will baseline it then. I'll be happy to go back in May after the weight is lost. I'll do the retest Publius style with all of you chipping in to pay for the trip. Who's in?

endurance1234 wrote:

What time savings in term of aerodynamics can be saved when losing 10,20,30 lbs etc on the bike?

The last time i looked at this was in 2006 at beta=0 with n~30 individuals. Across individuals, and using a generalized linear model that included a weight predictor:

CxA ~ 0.00078976 * weight (lbs)



so, this model would predict that for 10 lbs less you'd decrease your CxA ~0.008


YMMV,


-k

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