So Why do heavy guys do better on Flats? - Page 2

Originally Posted by Thylacine

They don't do better on the flats. You're assuming they do because the light guys do better on the climbs.

Mark Cavendish has won more flat stages this year than anyone. Is he a 'heavy' guy?

Originally Posted by JAPANic

You think they go fast on the flats?
You should see them going downhill in a straight line...

Our club trains every Sunday on a flat stretch and depending on the wind the heavier guys have an advantage. Especially if there is a tail wind. They are disadvantaged in a head wind though as the momentum is fighting the wind. Assuming a heavier rider is bigger they also have more surface area/drag.

Originally Posted by 11ring

This thread is full of crap science. There is only one reason why bigger riders tend to be faster and that is becuase they produce more power in relation to the aerodynamic drag forces that act on them.

Momentum has almost nothing to do with it unless you are trying to be a human battering ram- or if you go down a big hill onto a flat, the bigger rider will hold their speed because they contain a greater amount of kinetic energy. Because momentum = 2 * KE/V then momentum is interchangeable with KE if we control for velocity (as in you are both going down hill at the same speed.) So in a very convoluted way and in a very special circumstance, it may be vaguely important.

But this cannot be applied to racing situations, unless you have a sprint very close to the base of a descent, and even here I can't see the effect being that great.

There are two other foces that a rider must overcome- gravity and rolling resistance. Both scale according to weight so in these areas the rider with the highest power to weight ratio will benefit. Thus on very dead roads, or climbs, those with high power to weight get an advantage, whereas on silky smooth roads, or flat or downhill sections, those with high power to drag get an advantage.

Originally Posted by JAPANic

You think they go fast on the flats?
You should see them going downhill in a straight line...

Our club trains every Sunday on a flat stretch and depending on the wind the heavier guys have an advantage. Especially if there is a tail wind. They are disadvantaged in a head wind though as the momentum is fighting the wind. Assuming a heavier rider is bigger they also have more surface area/drag.

Originally Posted by longfemur

Weight does not matter as much when on the flats as it does climbing, once you're already accelerated and rolling.

Since heavier guys have bigger muscle mass (since as cyclists, the weight probably is not fat), they potentially can produce more power to keep themselves rolling than lighter guys on flat terrain. But muscles themselves weigh a lot (the quads being the biggest muscles in the body as far as I know), and when the road starts sloping up, the weight starts being more of a handicap than the power advantage, and so lighter guys have the potential to climb more easily, within reason (assuming they have pretty strong, well-trained muscles too). This is why "rouleurs" are rouleurs, and climbers are climbers, and why climbers need the rouleurs to keep them in the race in between the climbs.

Aerodynamics don't really come into the picture that much in this scenario, because there isn't really that much difference in frontal area between heavier guys and lighter guys.

But, as we all know, there are other factors that confound all this, such as overall talent, strategy, team ability, the effectiveness of the performance enhancement some can get away with, etc.

Originally Posted by dhk2

You think a pair of Zipp 808s save 40 watts over standard rims? My guess is that the savings is more like 4 watts at normal TT speeds of 25-30 mph. Wheels just don't have that much frontal area compared to the rider. Besides, if the tires stay the same, not much changes.

If we could save 40 watts @ 25 mph just by switching to aero wheels, my guess is everyone would have them by now. Just imagine the time I could gain on the fall club century with 40 more watts....I'd pay $1500 to save 40 watts tomorrow.

Originally Posted by alienator

First some facts for the misguided: according to Zipp, their 808's save between 2 and 7 watts at 30 mph.

Originally Posted by doctorSpoc

AHHHH no... Zipp claims the new shape yields about 2-7 watts savings... i.e. new shape vs old shape...

"The new shape yields to a savings on the road of 2-7 watts of power for a pair of wheels at 30mph"

http://www.zipp.com/wheels/detail.php?ID=21#

Originally Posted by alienator

That's your interpretation. They don't seem to imply that at all.

At any rate, choosing Tour Magazine results as a point of reference is questionable, at best. They're not known for conducting scientifically sound tests.

Originally Posted by Josh @ Zipp

This is actually a repeat of a similar test they did last year. Funny enough I was just talking with Marcus from Triathlon DE just Monday of this week about the test. What they do is have the riders ride at a constant speed for so many laps and then take the average wattage from that run. They do all the wheels at the same speed and use the wattage difference at the same speed to determine the savings, so those wattage numbers they are quoting are watts saved at the same speed.
What we have been discussing with them is that this test really only holds water for indoor track usage as the airflow is completely stagnant in at the track they use, so from an engineering perspective we can consider all of the tests to be conducted at exactly zero degrees of yaw. Unfortunately, zero degrees of yaw will statistically occur less than 1% of the time in the real world, so to really make use of this data we have to either add similar testing done outdoors or wind tunnel data.
At zero degrees of yaw, not only are most wheels very similar, but many aero wheels with large surface area are at a slight disadvantage due to the predominance of skin friction drag over pressure drag, but with just a few degrees of yaw, the pressure drag becomes an order of magnitude more important than skin friction drag.
If you look at our white paper 'A Note on Rim Width' : you will see that all of the data is pretty compressed at zero degrees and that a disc actually gives up a little to both the 404 and 808, this is due to the predominance of skin friction at this angle (note that the zipp disc has less drag here than any other discs with the same shape...thank you dimples!). With just a little wind angle, though, the disc, or 808 suddenly becomes vastly superior to less deep rims. This wind angle phenomenon is part of the cleverness of the Xentis wheel, which has the spokes angled relative to the wind centerline so that at zero degress the spokes are at 10 degrees or so to the wind.
This works excellent at zero degrees, but you can also have too much wind angle and at the higher angles the drag begins to increase again as the Tour magazine test determined: you can see in their tour article Download Translated PDF wind tunnel graph that the Xentis is slightly faster than either the 808 or the disc at zero, but by 4 degrees the disc and 808 have a significant advantage. Using the numbers at zero from the Tour magazine test we extrapolate a 1.2 watt savings at zero over the 808 and 2 watts over the 999 (the data in the Zipp white paper mimics this identically), which is almost exactly what Triathlon DE found in this test.
As with any testing, the problem is determining the limitations of the test itself. In my opinion the Triathlon test is dangerous as they don't really adequately explain the very limited nature of the usefullness of the data. But when we combine testing methodologies you can start to really have something that is worthwhile. I think that this test is very interesting in that it more or less validates one of the data points seen in wind tunnel data, but it is dangerous in that wheel decisions made from this type of test will be very limiting in the real world. This also shows the give and take necessary in the design of these products. We spend about 50 hours per year in the wind tunnel trying to tweak and refine these shapes, but the reality is that you almost always have to give up something to get something.
Lately we have been giving up very low and very high yaw angle performance to improve mid range performance, and I am convinced that this methodology is really the future of design since real world wind angles are generally nothing close to zero degrees. I'm currently writing another white paper on this describing the vector resolution and wind angle distribution in percentages that we are now using for design.
For more information on Rim Shape and the aerodynamic “sweet spot” see our engineering white paper on rim shape:
Rim Shape: (PDF)

Originally Posted by doctorSpoc

http://www.zipp.com/support/askjosh/aerowheels.php#
after you get to the page click on "Wind Tunnel" then you can see Josh's comments by clicking on Josh's view.. hmmm.. who's opinion to i go with here... this is pretty hard... i think i'm going to Josh's... lol.