The most coveted wheel in the peleton: Stign Vandenbergh
Several riders in the local peleton are universally beloved for their large asses. This is not necessarily homoerotic or 40 ounce bounce-type love; this love is directed toward the supposed larger pocket of rarefied post-ass air they offer. I'd say a good 30% of conversations after races involve rating and debating the drafts offered by various asses. Of course, this conversation is much more common in the lower ranks, since large asses are rare in the higher categories with a few notable exceptions such as Johan Vansummeren, Stijn Vandenbergh and, a few years ago, Magnus Backstedt.
Boonen receives an unfair advantage drafting from Johan Vansummeren
I've always felt as if larger riders offered a significantly different draft, but I've never seen a study on exactly how many watts savings riding behind a six and half foot 250 pound rider gets you compared to, say, a five and a half foot 135 pounder. When Maggie Backstedt, at around 6'8", launched a break at Clarendon Cup several years ago, I couldn't help but feel he gave more draft than he got.
I brake for massive booty: Maggie Backstedt
Unfortunately, no study yet exists comparing the draft of different size asses. Ideally, I'd find test subjects and do the data collection myself with my own power meter. Unfortunately, this is not an option, since I don't have the nerve to put up an "ISO Wide Loads for test study" on the MABRA forum. Also, the government is presently unwilling to fund such a study.
Never fear. I believe through sheer brain power and confusing numbers games, I have come close to answering the question of how much bounce you can get per ounce. I will use data from two related studies to extrapolate (a big word meaning "to lie about") an exact answer to our question of whether what's in the trunk is just junk, or if it's actually useful.
Study 1: Watts Required to Sustain 20 mph behind (a) Car and (b) another Cyclist
The first study I charted the power output of a cyclist at 20 mph in four states--upright, on the drops, behind another cyclist, and behind a car.
Upright: 260 watts
Drafting a cyclist: 180
Drafting a car: 120
The bigger the draft, the less power need to go 20 mph. This means that drafting behind a bigger thing is better than drafting behind a smaller thing. Uh, we knew this already.
What we want to know is when that size increase starts to really matter.
This requires that we turn away from booty, and focus on the frontal area, since that's what determines the size of the draft. We'll need to figure out the frontal surface area of a car and various-sized riders. From there, given our data, we can answer the question of whether big booty is as wonderful to ride as we make it out to be.
To find an estimate for frontal surface area (FSA) of different-sized riders, we turn to a second study.
Study 2: Frontal Surface Area of different sized ridersThis study gives us an idea of surface area of cyclists. Cyclists at 5'8" have a FSA of .379 meters. Cyclists around 5'10" have a FSA of .402.
This gives us a rough idea of the difference in surface area between riders: for every 2 inches of height, there is a .03 difference in FSA.
So, with a bicycle's FSA added in, the FSA of a 5'6" rider is .449 meters; a 6'6" rider comes in at .622 square meters. Compare that to The FSA of a car (a Mazda sedan, in this example) is about 7.763 meters.
We calculate the coefficient:
Difference in watts / difference in FSA = coefficient
(140-80) / (7.763-.449) = 8.203
This means that, assuming a constant coefficient, for every 1 meter increase in FSA in the object to be drafted, the rider gets an 8-watt savings.
We end up with a chart like this:
Drafting behind a normal-sized rider at 20 mph saves you 80 watts; drafting behind a 6'6" rider saves you 81.48 watts.
This is crude and assumes a lot of things remain constant.
Still, this suggests that drafting behind a larger person does save a small amount of energy. Over an hour or two, this adds up.
Also, at higher speeds, a big booty will offer even greater savings in power output. My guess (commoner for "extrapolation") is that at 30 mph, drafting behind a large person compared to a smaller one is probably at least a 20-30 watt advantage.
In summary, our data suggests that the condition commonly referred to as Too Much Booty in the Pants offers significant advantage to those who pursue it. For drafting purposes, naturally.