Zinn technical FAQ: Gear choice for optimal efficiency, dead Di2, Mavic 10/11 switch

Published August 18, 2022 01:13PM

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I have often wondered if there is a difference in feel, torque, power, or anything else when using the big ring or small ring in a gear with identical gear inches – say a 43 x 19 or 52 x 23, which are both 59.79 with a 25mm tire on a 700C rim.

Indeed, which chainrings you pick to achieve the same gear ratios makes a difference. Using your example, frictional losses are greater in the 43 x 19 combination than in the 52 x 23 combination. This article explains why and includes a graph of friction depending on chainring selection to achieve the same gear ratio.

If you use a smaller chainring and cog to achieve the same gear ratio as with a larger chainring and cog, you increase the articulation angles of the chain due to the chain going around the smaller cog and chainring. In other words, the chain bends more around the smaller cog and chainring than it does around the bigger cog and chainring. This increases frictional losses in the chain, despite the fact that the chain speed is higher with the larger chainring (at the same cadence)—the amount of chain bend matters more than how often the chain bends per second.

Tension on the upper span of a chain is inversely proportional to chainring size. In other words, the smaller the chainring, the higher the chain tension. That’s simple physics: Power Output equals RPM x Torque transferred to the cogs by means of the chain tension. At the same power output and cadence, RPM and Torque are identical in both larger and smaller chainrings. Thus, the chain tension (force) with the smaller chainring is higher, as Torque is given by the equation Torque = Force X Radius, and the chainring’s radius is smaller. Conversely, a larger chainring has a larger radius, and therefore the chain tension is lower at the same RPM and power output. Higher chain tension increases friction within the chain by pulling harder on its pivot pins and by forcing its rollers harder against the teeth of the chainring and cog.

To obtain optimal efficiency with a 2 X 11 system, shift from the outer to the inner ring when shifting into the ninth-from-smallest (second-from-largest) cog. In other words, always cross-chain from the big ring down to the third-largest cog before dropping into the inner chainring as the road ramps up.

I have a Shimano R8070 groupset and came home from holiday yesterday. When I was going to use my bike that worked fine when I left home three weeks ago, the rear derailleur suddenly didn’t work, i.e., nothing happened when I pushed the shifter buttons. The front shifter worked as normal, but I anyhow charged the battery. Still no action from the rear derailleur. I wondered if it had gone into crash mode (even though there’s no reason to, as it hadn’t been used), so I held the junction box button down for 5 seconds to get a flashing red light simultaneously as I tried to shift — but nothing happened. I use manual mode, but I tried semi-synchro mode, and when I used the front shifter, the rear derailleur also changed gears. So, it is working somehow and could mean that the problem is in the shifter? I disconnected and connected the wires on the rear derailleur and on the shifter itself (and made sure it clicked as it should when I connected them again). Still nothing from the shifter. I did the same thing with the battery stored in the seat post, as I read somewhere that this could be seen as a “soft reset” of the system. But nothing works. I was wondering if you could have any clues to what is wrong? The real weird thing is that the only thing that has happened is that the bike wasn’t used for three weeks, so there’s nothing that actually “happened” and could explain (in my head) why the shifter seems completely dead…

“Based on Ole’s comments, my best guess is that there is an issue with either the wire that connects the rear shifter to the junction box or an issue with the shifter itself. Because the rear derailleur still works in semi-synchro mode, it’s not likely an issue with the derailleur or the wires communicating with the rear derailleur. More likely than not, it’s something with the wire from the rear shifter to the upper junction box, because sometimes they can get worn over time from friction or a crash, etc. and eventually lose connection. This would also explain the mysterious loss in function, because sometime the wire gets worn and then will randomly lose connection.

The best way to diagnose this issue is to take the bike to shop with a PC Link Device and they can run a system diagnostic check connected to the computer. This will help them narrow down the problem area and hopefully solve the issue. If that’s not an option, it’s probably best to pull the bar tape and inspect that section of wire and replace as needed.

Having your right shifter stop working while the bike goes untouched for three weeks is hard to imagine. So is the wire losing internal contact during your absence.

I would have guessed that the short wire from the shifter to the junction box was loose, since that would explain why the rear derailleur doesn’t respond to commands from the right shifter yet works in Semi Synchro mode (controlled by the left shifter rather than by the right shifter). If you disconnected it at both ends and reconnected it, then you eliminated it just being loose. I know you said it clicked in, which is reassuring. Ideally, you plug in and unplug Di2 wires with this tool; it ensures that the wire clicks in.

I have a Trek Madone 5.2 with a 10-speed Ultegra group. Also using Mavic Ksyrium Elite wheels. I find there is a point when there is a jump in the cassette that makes it either too much or too little gearing. I have researched a bit and have found it is possible to upgrade to 11 speeds, and with the Ksyrium wheels it would not be necessary to change hubs. Have you had any experience with this and any suggestions?

Yes, you should be able to do this upgrade with no problem on that bike. On recent Mavic Ksyrium wheels, there will be a spacer behind the cassette when running 10 cogs. You would remove this spacer when installing an 11-speed cassette.
― Lennard