Technical FAQ: Drafting an aero bike and derailleur setup

Lennard talks drafting science and derailleur setup in this week's Tech FAQ

Dear Lennard,
With all the hoopla about aero bikes, do they provide a better draft for the guy riding behind? Perhaps no different when directly behind but maybe at certain angles?
— Freddy

Dear Freddy,
Certainly when riding directly behind an aero bike, there is less draft. In a crosswind, I suppose it’s possible that there might be more draft since it would tend to have a higher side surface area, but if the aero bike is also efficient in a sidewind, then it also should not be providing as much draft as a standard road bike. That’s because draft is a function of turbulence—the more turbulence, the more draft. But aerodynamic efficiency means that turbulence behind the object is reduced—that the wind reattaches better coming off of the object. So, I wouldn’t count on finding more draft behind an aero bike whether straight behind or off to the side in a crosswind.
― Lennard

More on gear range combinations

Dear Lennard,
How thick should the washer be between the mech and hanger to use a 11-32 tooth cassette on a short cage mech?”
— Mike

Dear Mike,
Tim has successfully used a 32T rear cog and a short-cage rear derailleur with his spacer method. He provided this answer to your question and even included a photo of his washer!

I’ve been using a 1mm thick washer to gain an extra few teeth. Specifically, it’s a 1mm x 10mm ID x 20mm OD washer. I filed down one side to clear the b-screw and tab, and it fills out the space behind the knuckle nicely. I’ve fit a 32t 10-speed cassette with an Ultegra 6700 derailleur (I did have to also run a longer b-screw). Of course, each frame’s hanger will vary the effect, but this can always help. I’ve attached a photo [above] for clarity.

I figured this out when I had a thin-hangered frame with a cheap derailleur that wouldn’t shift to the smallest cog (A Yuba Mundo cargo bike, so a long cable run, to boot). At first I thought it would just help get the derailleur back in the spring tension zone, but when I had to let off on the b-tension, I knew I had a really useful trick on my hands.

I wouldn’t recommend more than 2 spacers, although I haven’t really tried a third. I suppose it mostly depends on how solidly you can bolt your derailleur, and taking up 3mm of your hanger seems pretty shady.
— Tim

Thanks for your question.
― Lennard

Dear Lennard,
One combination of shifter, derailleur, and cassette that seems to work essentially perfectly is to use 11-speed Campagnolo shifters with 9-speed Shimano rear derailleur and cassette. There is actually a 0.1mm difference in the cable pull per gear between the two systems, but if the derailleur is adjusted so that it is centered on the central cog of the cassette, the float in the top jockey wheel is more than enough to take up the 0.4mm difference at either end of the cassette.

I have this combination installed on two bikes, and the shifting works just as well as it did with the original Shimano shifters. I also found that new 11-speed Chorus shifters were pretty much the same price as sellers were asking on eBay for NOS 9-speed Ultegra shifters.
— Nick