Technical FAQ: Aero, synchro shifting, wheels, and frames

Lennard Zinn addresses questions relating to coasting vs. pedaling downhill and other topics.

Coasting vs. pedaling downhill

Dear Lennard,
I often wonder what is at play as I pass my friends on a downhill while drifting compared to them pedaling. Is there an aero loss to the moving legs? Does it boil down to some combination of weight difference, wheels, and aero positioning?
— Jeff

Dear Jeff,
Drafting is playing a primary role as you approach and pass them. When you are far behind, this contribution to your speed can be ignored. So if you close in on them from a far distance back, then you really are rolling faster.

Assuming you are rolling faster, then the next contributions are your weight and frontal area. If you have more weight and/or less frontal area, you will also run them down. Yes, the relative aerodynamic drag of your wheels vs. theirs will contribute to your speed difference as well. Finally, differences in tire rolling resistance as well as friction in the clicking freehub will contribute. [related title=”More Technical FAQ” align=”right” tag=”Technical-FAQ”]

As for you passing pedaling riders while you are coasting, we all know that at a certain point it is faster to coast than to pedal on a fast enough descent. The question is, at what point is that true? Often, the tradeoff in aerodynamics to hold a riding position in which you can pedal costs you more speed than just keeping quiet on the bike and coasting, even though the speed is not so high that you are spun out. This is more obvious if your friend is pedaling in a higher body position than you. In any case, there will be more turbulence around pedaling legs than around ones that are not moving.
― Lennard

Shimano Synchro shift

Dear Lennard,
I have a question regarding the new direction of Shimano Synchro shift. The new TT bar setup only has one button on each brake lever and one button on the end of each aero extension. I like riding in the big ring most of the time, up hills included. I want to be able to ride in the 53/25 while an 11-28 cassette is installed. How do I do this with the new Di2 TT setup from Shimano? It seems crazy to me that they have eliminated this option from their setup. There literally isn’t a way to shift the front derailleur at all, which sounds crazy. Clarification is needed here.
— Matthew

Dear Matthew,
I think you’re all set going in, with the default setup with, for instance, ST-R9160 Shimano Dura-Ace Di2 TT levers. And if not, you can reprogram it with your phone or PC.

The S2 (fully Synchro) mode in the default configuration on Shimano Di2 2 X 11 keeps the chain on the large chainring when shifting down from the smallest cog through the cassette all of the way until the second-to-largest cog. And that would be your 25 on your 11-28 cassette. Only when you ask for one more lower gear will it drop to the inner chainring (while simultaneously shifting up to the fourth largest cog), rather than shifting to the largest cog.

If you want to change on which cog it will trigger a front shift, you can reprogram it with the e-Tube app on either a smartphone via Bluetooth or with a wired connection to a PC. You can also reprogram the rear shift that occurs simultaneously with the front shift to make it move as many cogs as you wish, although I recommend sticking with the default. (I have tried having it do a triple rear shift with a front shift, since looking at a gear chart it seemed like that might be a smoother step down, resulting in less cadence change. But in addition to a lot more clunking during all of that shifting, it actually made the shift less smooth on the legs. So I went back to the default double rear shift with each front shift in Synchro mode.)

You can read more about what I’ve written regarding the use of reprogramming here and here.
― Lennard

Balancing a wheel

Dear Lennard,

I recently purchased a deep-dish aero road bike wheel and noticed that its weight is slightly out of balance. Can you tell me how to weight balance a bicycle wheel?
— David

Dear David,
With the bike in a stand, let the wheel spin until it stops. If it always stops with the same point at the bottom, then that is the heavy spot (if it is inconsistent, your hub bearings need work). Obviously, drop the chain off of the cogs if it is a rear wheel.

You will need to weight the spot on the rim opposite the heavy spot. You can tape small weights to the rim opposite the heavy spot. Often, the heavy spot is opposite the valve stem (i.e., the wheel always stops with the valve and the top), and you can screw more valve collars onto the valve until it no longer favors stopping with the valve at the top.
― Lennard

Bike fit help

Dear Lennard,

Not all 6-foot-1 guys get to that height the same way, and I get there with shorter femurs and a longer torso. My bike fitter recommends a 78-degree seat tube angle to accommodate the short femurs. I found that STA on the road with an aero frame, grudgingly. Unfortunately, almost all size 58 frames for CX bikes come with 73-degree seat tube angles. Even with a zero-setback seatpost and the seat slammed forward, I can’t quite get the full 78 degrees if the bike’s STA is 73. I compensate with shorter 170mm cranks, but that is not ideal for CX. Maybe I’m not the only one with this problem: I’ve noticed that many pro riders above six feet tall have their seat slammed as far forward as possible (such as Matthieu Van Der Poel), and/or they downsize to a size 56 frame with a longer stem. I’ve tried both and don’t like either option very much for CX, because I’d like flex in the seatpost and a shorter stem. Any recommendations?
— Matt

Dear Matt,
Being a custom framebuilder, I’ll give you the same answer any custom framebuilder would: get a custom frame!

Other than that, your options are a turned-around seatpost like triathletes used to use before tri bikes came with steep seat angles. Perhaps if you were to use a Thomson setback seatpost, which achieves the setback with a bent shaft, you could still get some flex out of the post when you turn it around so it bends forward rather than backward. Obviously, this forward position on the bike requires that your stem be much longer to get your bars out to a comfortable position, especially since you say you have a long torso.

A custom frame will be just the ticket for you!
― Lennard