Technical FAQ: Di2’s fail-safe for short chain stays; more on hoop stress

Lennard answers a reader's question about his Shimano Di2 shifting and explains why hoop stress can damage rims.

Heading out the door? Read this article on the new Outside+ app available now on iOS devices for members! Download the app.

Dear Lennard,
I really need help with my Di2. It’s stuck in full synchro, S2 mode, and I cannot get it out. The bike originally came with 2013 Dura-Ace Di2. I added a climbing shifter years ago, and it works fine. Recently, I changed to the new battery and an Ultegra RD-R8050 GS derailleur. It worked all right for a while, allowing me to change from manual to S1 to S2 using the mode button on the Junction A box. Even in manual mode, however, I could only get down to the third cog when in the smaller chainring. This makes it a little tough to remove the rear wheel.

Then I added the Bluetooth connector, EW WU111. Now, the mode button no longer works to change the shifting mode.

I just want to use it in manual mode, but I cannot get it there. I tried to fix it in the E-Tube iPhone app by changing the rear shift to 0 on front shift.

My bike shop called Shimano, and I understand Shimano said it cannot be changed back to manual. That makes no sense. Any ideas on how to get it back to manual?
— Don

Dear Don,
Manual shift mode still would not allow you to do what you’re trying to do. You’re seeing the effects of a fail-safe Shimano built into its newest Di2 derailleurs (your Ultegra RD-R8050 and the Dura-Ace RD-R9150) in order to address some geometry issues on racing bikes with through axles, disc brakes, and compact cranks. On disc-brake road bikes, the cassette is further outboard, but the chainrings are in the same position as for a rim-brake bike, and this can create some chain issues.

[related title=”More Tech FAQ” align=”right” tag=”Technical-FAQ”]

A disc-brake racing bike will often have a short chainstay (410mm or less) along with its 142mm overlock dimension (dropout spacing) for the (12x142mm) through axle, and a 16-tooth jump on a compact crank from a 34T to a 50T or from a 36T to a 52T chainring. If the bike is cross-chained from the inner ring to the smallest cog, it will often hit the inside of the big chainring. I’ll bet some of you reading this have even experienced this on short-chainstay bikes without disc brakes. Then the chain pick-up teeth on the inboard side of the big ring can grab the chain and drag it up.

To address this, Shimano built in a feature into its latest road Di2 rear derailleurs called Gear Position Control (GPC), which blocks the smallest two cogs when the chain is on the inner chainring on a system where the user has told the computer interface that there is a 16-tooth difference between the chainring sizes. This is in effect even in manual shift mode.

This does create the issue you mentioned for removing the rear wheel. To do so, put the chain on the big chainring; otherwise, you won’t be able to get it into the 11-tooth cog.

Switching out of S2 should still work, though. I have a similar GPC on my XTR Di2 mountain bike, where it disallows full cross-chaining, but toggling between M, S1, and S2 still works.

I’m sure you used to switch between modes with a double click of your mode button on your Junction A. I recommend trying this again. If you have done all of your firmware updates (using the app or at a shop with an E-Tube interface), it should work. You cannot switch between modes in the app; that is only for changing in which gear positions you want the synchro shifts to occur.

If you have done the fast double-click of your mode button properly, both the red and green LEDs on Junction A should blink in unison. On your Garmin or other computer synced with your D-Fly, it will display M (manual), S1 (Semi-Synchro), or S2 (Full Synchro), and your Junction A LEDs will also tell you what mode you’re in. One blink of both the red and green LEDs means M, two blinks means S1, and three blinks means S2. Try using your fingernail on the mode button if you’re not seeing this happen. Here’s more on setting up Synchro shift and syncing with your cycling computer.

If you can’t get both LEDs to blink simultaneously with a good, quick double-push of the mode button, there may be a problem with the firmware update on one or more of the components in the system. The smartphone app may not resolve it, so you may need to go to a dealer equipped with the E-Tube PC interface.
― Lennard

Hoop stress on wide tandem tires

Dear Lennard,
My wife and I have been tandeming for over 40 years, starting on 700x28c tires rated for 96 psi, but pumped up to 120 psi due to our combined weight on a 42 lb tandem. Once Conti came out with the GatorSkin 700x32c tires, we switched and dropped our max pressure down to 115 psi. We often ride and even tour with panniers on rough and dirt roads. We have worn out many tires over time, but we have had only two carcass failures in that time. One was an obvious defect noted within 20 miles. So I don’t think the maximum pressures noted on tires are always representative of what they can actually stand up to.

On the other hand, I have had two pinned rims separate at the rim joint after switching from 28 to 32c tires. So I switched to welded rims — Mavic, HED, and H Plus Son, and have had no rim problems since.
— Christian

Dear Christian,
I’m not surprised on the tires, as manufacturers generally build in an extra safety margin beyond what is imprinted on the tire.

Even though you have gotten away with it, I recommend you read this about hoop stress. As you can see in the linked article, that 115psi on a 32mm-wide tire is the equivalent stress on the tire casing and rim to a 23mm tire at 160psi. That is a lot of stress, particularly when you plunk two people on top of it.

The pinned-rim failures with your tire switch may have had to do with the fact that the hoop stress is higher with a 32C tire at 115psi than with a 28C tire at 120psi. Where your 32C tire at 115psi exerts the equivalent stress on the tire casing and rim as a 23C tire at 160psi, the 28C tire at 120psi exerts the equivalent stress on the tire casing and rim as a 23C tire at 146psi. So that rim was having to deal with more than it was accustomed to.

I think you are overinflating your tires unnecessarily, potentially creating a more dangerous situation than the one you’re trying to avoid by pumping them up so hard. You want to avoid a blown tire or failed rim on a tandem, so I recommend using as little pressure as you can get away with to not have pinch flats. Latex tubes can help with the pinch flat issue, since they are tougher against impact, but they do require pumping every time you ride since they bleed air slowly.
― Lennard

Freehub swapping

Dear Lennard,
I was just wondering if there is a way of substituting a Swiss-made Edco Multisys Freehub body that accepts Shimano, SRAM and almost all Campy cassettes with a different freehub body shell. Primarily I’m interested in replacing it with a Campy 10s freehub.
— Greg

Dear Greg,
No, because Campagnolo’s freehub ratchet does not properly engage the teeth inside the Edco hub. As former Edco CEO Paul Lew says, “Although it may engage in your hand, the tolerances are not matched, and if you try to ride it, you’ll find yourself stranded when the engagement fails.”

Why you don’t just keep using your Edco Multisys freehub body? A 10-speed Campagnolo cassette should fit on it.

As an alternative, Edco makes a 10-speed Campagnolo Monoblock (one-piece-machined) cassette; it will fit your Edco wheel.
― Lennard

An American in France

What’s it like to be an American cyclist living in France? Watch to get professional road cyclist Joe Dombrowski’s view.