Technical FAQ: Seatpost slippage, crosswinds, and more

Lennard Zinn addresses a variety of questions — including one regarding the use of a soda can to fix a slipping carbon seatpost.

Using soda can shims on a seatpost

Dear Lennard,
I was driving a SAG vehicle when a retired pro rider was stranded with a slipping carbon seatpost on a Cervélo R3 (it wasn’t his bike). He was being helped by a moot-sag driver who asked me if I had a Coke. I thought using a piece of aluminum cut from a soft drink can should be not used on a carbon bike and seat post. I worried the seat post or seat tube could fail and injure the cyclist. I did have paper towels like one uses in a bathroom and a paper plate. Fine dust on the road side was also available. Would these have been a better choice? (The moto-sag was eventually able to improve the slipping seat post by adjusting the torque on the seat post fastener.)
— Scott

Dear Scott,
If the seatpost slips because it is undersized relative to the inside diameter of the seat tube, I doubt you would hurt anything by slipping an aluminum pop-can shim in there, as long as you don’t make it too big and force the seat tube slot open any wider. After all, early carbon bikes always had aluminum sleeves inside the seat tube, head tube, and bottom bracket shell. And the edge of the seat lug will always dig more deeply into a carbon post than will the very thin edge of a pop can.

Paper towels or paper plates would disintegrate too fast to offer a lasting solution. Fine dust probably would not be sufficient to stop the slippage if it had become chronic and the seat binder already had been fully torqued when it was slipping before. Putting sand on the post might work in preventing slippage, but if you’re concerned about damage to the carbon, I can’t imagine this solution would do less damage than a properly-sized and properly-placed pop-can shim, and it could do more.
― Lennard

Carbon frames and crosswinds

Dear Lennard,
Is it my imagination or are carbon fiber frames more sensitive to crosswinds than steel or aluminum frames? Or could it be the deeper dish of the wheels and bladed spokes?
— Bill

Dear Bill,
Sidewind performance is dependent on total side surface area and shape, not on the materials used to create that shape. And yes, if your carbon frame has tubes with taller sections, and its wheels have wider blades on the spokes and deeper sections on the rims than your other bike, then that will explain it.
― Lennard

Taped tubulars slower than glued?

Dear Lennard,
I am about to start using tubular tires for my time trial bike. I have read that tape installation is quite a bit easier than gluing, specifically in getting the tires true on the wheels. However, scuttlebutt says that tubular tires that are installed with tape are actually slower than glue installations. Do you know of any laboratory testing that verifies this?
— Will

Dear Will,
I have heard this for years, back when Tufo tape was the only gluing tape that anybody on this side of the pond was using for road tires. I have never seen any data to back this up.

With a skinny road tubular that fits into the rim bed tightly, it makes sense, since you can end up with a very thin layer of glue holding the tire on, and the tape will have a certain thickness that will almost certainly be greater. A thicker layer would seem likely to have higher hysteresis losses.

When it comes to cyclocross tubulars, however, I don’t think this argument holds much water. That’s because the poor fit of a 33mm tire to a rim intended for a 23mm tire means that the space in the center of the rim bed will end up being filled up with something, whether it’s just air (allowing the tire to move up and down), a super thick layer of rim mastic alone, mastic plus “Belgian tape,” a mastic-slathered layer of cotton tape, or straight tubular gluing tape with a thicker, conformable layer on the tire side like Effetto Mariposa Carogna. And in that case, it would require some real testing to be able to say which of those offers the lowest hysteresis loss.
― Lennard

Feedback from prior columns

Dear Lennard,
Your recent post on thumb injuries really hit home with me. I have a repetitive stress injury to my right thumb from stupidly continuing to use a bad keyboard (bad spacebar) many years ago. It got bad enough recently that I was going to look in to a brace, like the ones you showed. But quite by accident, I seem to have stumbled on the solution to my issue — electronic shifting. What seemed to exacerbate my injury most was the constant shifting on the rear cassette — having to grip with my right thumb while rotating the lever for downshifting. So it was a pleasant and unexpected surprise to find that in four rides I’ve done with my new electronic drivetrain, I haven’t noticed the pain in my thumb at all — not once. Score one for high tech. I also suspect I’m getting some benefit from the disc brakes, as they require significantly less effort than my old caliper brakes, which translates to a much lighter grip while braking. It’s a one-two punch I had not anticipated.
— Steve

Dear Steve,
Yes, that has been a great solution for many people with thumb issues. Andy Pruitt himself told me it fixed his thumb problem, and I have sold at least one Di2 group with one of our custom frames due to exactly this concern.
― Lennard

Dear Lennard,
You may have a physics error in your article about Rotor. For numerous shifts, like 100, something has to provide energy to move the derailleur, whether there is one spring or many. The rider is providing, one way or the other, that energy. Most likely, moving to a larger cog, the rider provides that energy with the large stroke, force X displacement, to provide the energy to move the derailleur and put energy in the standard derailleur spring.

To move down, the rider releases the indexing that let the spring(s) move the derailleur toward the small cogs.

The other possibility is one of the springs is a clock spring that needs to be rewound. It is most likely not the case, since you did not mention rewinding the spring.
— Pierre

Dear Pierre,
You are absolutely right. I guess I was too subtle in my comment, namely, “I did find that it takes a hard push for front upshifts, so either Rotor set the feel for sending this signal quite hard, or you are indeed helping the front derailleur’s downshift spring with your hand.”

Yes, it is impossible to have a perpetual-motion machine, which this would be if all of the energy really were to come only from a pair of springs, neither of which were getting rewound with each shift.
― Lennard