Technical FAQ: Frame aerodynamics, mixing parts, disc brakes, and more

Lennard Zinn addresses questions dealing with a variety of topics ranging from frame shapes to disc brakes

Aerodynamics of new vs. old frames

Dear Lennard,
Since most carbon fiber frame builders seem to be using kammtail (as modern as the ’71 Chevy Vega!) tubes for aerodynamic purposes, it got me wondering about the relationship between cross-sectional area and shape. Is a modern carbon frame more or less aerodynamic than my 25-year-old steel frame, using round cross sections but with a much smaller area exposed to the wind?
— Steve

Dear Steve,
The shape makes a big difference. I remember reading a statistic once about an antenna on an airplane; it was something to the effect that if it is round in cross section, the skinny little antenna has similar drag to one of the plane’s entire wings. I may be getting the relative magnitudes wrong, but you get the idea.

If you look at the figures in this drag coefficient chart, a round shape is about twice as fast as a square shape, but it looks like a round tube has to be less that one tenth the thickness of than a wing-shaped tube before it has the same drag. Of course, the round cross section applies to vertical tubes in the wind, approximating only the head tube, seat tube, and seatpost, although the air around the seat tube and seatpost is so “dirty” coming around the moving legs, that the magnitude of drag difference is less. The top tube, running parallel to the wind, adds no drag, and the down tube, which crosses the wind at an angle, is oval in cross section as far as the wind “sees,” and that’s faster than a round shape.
― Lennard

Using Dura-Ace 9000 with older wheels

Dear Lennard,
I ride a 2014 Specialized S-Works Roubaix with Roval carbon CLX40 wheels and a Dura-Ace CS 9000 11×28 cassette.

I’d like to set up an old (circa 2008) pair of Mavic Ksyrium SSC wheels as a backup. Will these wheels/hubs accommodate the CS 9000 11 x 28 cassette? If not, how do I determine which wheels will work with this cassette?

Also, if it does work with the Ksyrium, will shifting be clean and smooth? Will I need spacers?
— Eric

Dear Eric,
As I said here, your Mavic Ksyrium wheels will work with an 11-speed cassette, as long as you remove the spacer that is behind the 10-speed cassette.

I can’t guarantee that the first cog will be exactly the same distance from the dropout as it is on the Rovals, so you may need to tweak your cable adjustment (or electronic adjustment) a bit. Once that’s done, it will shift just as well.
— Lennard

Mixing 10- and 11-speed components

Dear Lennard,
I’ve read all the compatibility columns you’ve written, but none seems to hit my situation. I’ve just bought a Tri bike with an Ultegra (11-speed) setup. Previously, I’d built a PowerTap wheel for my road bike, which is 10-speed (mostly 105 components with a few Tiagra components, too). I’d like to use the PowerTap wheel for both bikes (switching back and forth). Bike shop put the 11-speed Ultegra cassette on the PowerTap wheel, no problem. However, they say the only way to make that wheel operative with the 10-speed is to upgrade the entire component set to Ultegra, which is a $1,000 project. Really don’t want to do that. Is there any way to make an 11-speed cassette work on a 10-speed bike?
— Matthew

Dear Matthew,
Sure there is a way to make it work! You can’t use the 11-speed cassette with your 10-speed drivetrain, but you can use that wheel; you’ll just have to switch cassettes when you switch the wheel between bikes. All you have to do is to put a 2mm circular shim behind the 10-speed cassette when you install it on the wheel. This is the same thing that’s long since been required to install a 10-speed cassette on a Mavic freehub body, so you can just get a Mavic shim if you don’t already have something to fit the bill.
— Lennard

Help with gluing tubulars

Dear Lennard,
I got myself into a little bit of a carbon conundrum while gluing my new wheels. I picked up a set of the new Easton EC90 Aero 55s and 2 Conti Competitions in 25mm. I’m using Conti Carbon specific cement. I haven’t glued tubulars in 20 years, so I’m trying to do it by the book. I read both tire and wheel manufacture’s literature. Here’s my issue: Continental recommends sanding the carbon rim before gluing. I even found a Conti video where they say it’s “extremely important.” Easton’s booklet says no wire brushing and no emery cloth. I did wipe down the rims with alcohol and a cloth. Obviously, a rough surface will give me a better bond, but I don’t want to damage my brand new wheels before mile No. 1.  I also don’t want to have a roll-off and break a collarbone. What’s the right way to go here?
— Drew

Dear Drew,
The main thing is to have the gluing surface of the rim be free of any mold-release substance, as glue won’t stick to this (and your tire will be able to roll off). There is no guarantee of removing the mold-release with solvent. So I lightly sand carbon rim beds and then clean them with alcohol before gluing. I make sure I don’t sand through any fibers; I just scuff the topcoat.

That said, I remember years ago in one of the many tests of tubular gluing published by C. S. “Chip” Howat Ph.D., P.E. of the University of Kansas Department of Chemical & Petroleum Engineering. Howat found that sanding didn’t make any difference to the glue bond on carbon rims. He did clean all of the rims very thoroughly with solvent (acetone, I believe).

I started sanding carbon rims because I had always sanded aluminum rims before gluing. Now I do it for peace of mind, because I know that mold-release coatings will be removed by light sanding, and I don’t want to be wondering whether I got any glue-repelling substances off with solvent alone. I sleep easily knowing my tires will stay on; rolling a tire is a much more present danger than is the possibility of rim failure due to light sanding of the topcoat. And judging by answers I’ve gotten from makers who said that I needn’t worry about it when I asked about loss of rim strength when chunks of carbon came off of the rim bed when peeling a tire off, sanding is no cause for concern. Wiping with alcohol ensures I don’t leave any sanding dust on the rim, and I don’t like breathing, touching, or depending solely upon more toxic solvents.
― Lennard

More on the disc-brake issue

Dear Lennard,
I’ve been following the controversy about road bike disc brakes, including concerns about riders getting lacerations, etc., because the discs are sharp. Is it possible to round the edges of a disc?
— Dave

Dear Dave,
Certainly one could round the edges of a disc, but I don’t think that would solve the problem. The fact that it’s a thin piece of steel (disc rotors are generally 2mm thick, and some are 1.8mm thick) is more of an issue than the condition of its edge. It’s still a knife if you hit it at high speed whether it has a rounded edge or a square edge. And then, of course, the times when people are getting cut by rotors would be during crashes, and, chances are, they will have been braking hard and the rotors would be extremely hot. So now you’re getting hit by a hot knife, and I think the fact that it might have a rounded edge would be of little comfort. A hot knife goes easily through cheese no matter how dull it is.
― Lennard

Feedback on component compatibility

Dear Lennard,
I read in your Q&A section that the 9000 won’t work with any Dura-Ace 10-speed components.

FYI, you can use a Dura-Ace 9000 front derailleur with Dura-Ace 7800 levers. It works better than the 7800 front derailleur; you should give it a try.
— Tim