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Tech FAQ: Tapping derailleur hangers, shimming carbon and rear aero wheels

  • By Lennard Zinn
  • Published Apr. 3, 2012
  • Updated Apr. 3, 2012 at 12:56 PM EDT

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

Dear Lennard,
I’m traveling. When I arrived in here in Spain, I had an almost impossible time putting on my rear derailleur. Upon close inspection, the first thread on the hanger (NOT replaceable) is damaged.  The derailleur is on now and it shifts fine, but I don’t dare take it off.  When I get back home, it will require a repair.  Can you recommend what I need and who can do this?
— Ted

Dear Ted,
You just need to get the derailleur hanger tapped out. Any bike shop should have that tap.
And if the threads are shot, wheelsmfg.com used to sell a “Dropout Saver,” which was essentially a T-nut. You’d drill out the threads and insert this T-nut and then screw your derailleur into it. I don’t see it on the Wheels Mfg. website anymore, however…
― Lennard

Dear Lennard,
Your beer can shim trick saved my sanity on a previous bike (aluminum frame and alloy seat post).  I now have a Scott CR1 carbon frame with a Ritchey carbon post, and I am getting the annoying creaking sound even when I have it torqued to the max and a little beyond.  Is it safe to insert an aluminum shim from a soda/beer can with both pieces being carbon?

And thanks for your books! I have both the road and mountain bike maintenance books.  They have severely reduced trips to the bike shop, and help me be more intimate with my preferred mode of transportation and sport.
— Kurtis

Dear Kurtis,
Yes, it’s safe to insert an aluminum shim from a soda/beer can with both pieces being carbon, as long as you don’t make it too wide; the seatpost must still slide in without forcing it. Obviously, make it tall enough that you can hang onto the top of it and not let it get pushed all of the way down into the seat tube. Before doing that, you might try expanding carbon-assembly paste; it could very well fix the problem with less time, muss and fuss.

BTW, creaking from a seatpost can also come from the head (grease the bolts and rails), from the edges of the saddle rubbing the seatpost clamp (grease those as well), or from the bottom end of the seatpost moving around in the seat tube. The latter is often an issue in a bike that has a cylindrical seat tube shim sleeve; the bottom of the seatpost moving around below the shim can make creaking noises. In that case, if you cut off the seatpost so that when it’s inserted to the proper saddle height for you, the end of the seatpost is flush with the bottom end of the shim, the noise will stop.
― Lennard

Dear Lennard,
You recently wrote:
“I agree. I think that would be an ideal way to approach the problem. Yes, the rear 25mm would have slightly more wind drag than a 23mm, but on the rear, the air passing by is already so ‘dirty’ aerodynamically that I would be surprised if it were even a measurable effect. And the rolling resistance savings would be higher on the rear where you have more weight concentrated.”

This is really the vexing question I’d previously sent and I’ve never found any info on (looking elsewhere). Shouldn’t we all be doing the exact opposite of what we see with rear wheels being more aerodynamic than fronts (e.g., Zipp’s 343: 303/404)?  The front should be aero but the rear doesn’t matter and should probably be more based on rotational weight.

This is something that’s always puzzled me because you invariably see pros riding with a deeper rim on the back than on the front.  I know this is supposedly a sacrifice for steering stability in wind but there’s another wrinkle and that’s weight (rotational and real).

Using as deep a rim on the front could pay aerodynamically and overcome any weight advantage of a more box-like climbing rim.  But surely the rear can offer so little savings on wind that it’s worth going light as possible there.

So instead of seeing guys riding the Zipp 606 (404 front and 808 rear) we should be seeing 808 fronts and 101 rears, right?

A related question I’d also like to know is what difference this makes if you’re not a domestique or soloing to victory?  I’ve never seen wind tunnel info for a guy who rides mid-peloton (as most star riders do).  Is there much advantage to aero wheels there, when you’re on someone’s wheel?

I strongly suspect that the research so far has been based on less than real-world racing reality (and the fact that pros have to build up their bikes to 6.8 kilos . . . so why not have aero wheels?).
— Brian

Dear Brian,
In general, I think your assertions are correct. The rear wheel makes so little difference aerodynamically for most riders in mass-start road races that it makes more sense to have a lower-inertia rear wheel that requires less energy to accelerate or to drag it up a hill. An added benefit would be enhanced rider comfort due to the more compliant wheel under the rider’s butt.

In pro road racing, there are protected riders and there are domestiques. Whether a domestique is pounding away on the front of the peloton, bringing the entire group back up to a breakaway, riding in a small, day-long breakaway, or schlepping water bottles back up to the pack from the team vehicles, they need aero equipment. When he is busting his ass doing his job, he is bucking the wind, and it makes sense to have his equipment as aerodynamic as possible. However, for a guy in a local race just hanging on in the pack, doing as little as he can and hoping for a good placing at the finish, aero schmaero. It just isn’t going to make a significant difference for him. He is better off with a lightweight bike and lightweight wheels of low rotational inertia to reduce the energy required for him to stay with the group when it surges in speed.

However, your statement about most stars not needing aero equipment because they’re always mid-pack needs a little tweaking, I think. I agree in the case of climbers and GC riders in major stage races. When they need to do their solo thing, they are moving slowly relative to the wind and upward against gravity. They need the lightest bike they can get away with. Of course, as you say, it is so easy these days to get a bike down to the UCI-mandated minimum weight of 6.8kg, so they might as well throw aero wheels on it. However, as Lance Armstrong commented during his comeback, attacks from the GC riders on climbs are extremely explosive. That means a lot of acceleration, and low rotational inertia wheels accelerate with lower energy cost. If the rider is rolling at a steady pace and pedals absolutely smoothly, it doesn’t matter where the weight is on the bike — if it’s a 6.8kg bike with heavy wheels or it’s a 6.8kg bike with light wheels will be irrelevant. But races don’t go like that.

Where your statement needs tweaking is related to team leaders whose prowess is in winning races in rolling or flat terrain on solo breakaways or in sprints. In either case, aero wheels, bike, clothing and bike position can make the difference between the top step of the podium and disappointment.
― Lennard

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Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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