If You Give a Mouse a Cookie

Shimano’s latest electronic Di2 shift system does not allow you to cross chain from the small chain ring to the two outermost rear cogs.* Can’t do it. No over-ride, no opt-out. What!?!?

Read about it in Lennard Zinn’s recent VeloNews Technical FAQ. **

Every racer learns that when you get a flat you shift to the small chainring – small cog combination to facilitate getting your wheel out and a replacement in more easily.*** It’s also a good idea anytime you have to change a rear wheel. If you’ve got the latest Di2 and you’re running compact or mid-compact chainrings, you can forget that.

This all started with carbon braking surfaces. Rubber pads and aluminum rims make a great braking combination. There is a good consistent friction factor. They work reasonably well, wet or dry.

Then along came carbon fiber rims. The first generation retained the aluminum braking surface. So far, so good. The carbon is bonded to an aluminum hoop. The carbon is in some wheels structural (e.g. the original Zipp wheels), and in some a non-structural fairing (e.g. early Mavic Cosmic Carbone wheels; in current Mavic Cosmic wheels the carbon is still a non-structural fairing but it extends over the brake track. A complete aluminum rim is hidden inside the carbon shell. How weird is that?)

Inevitably suppliers did away with the aluminum and began making the entire rim out of carbon fiber. Well, it didn’t take long to learn that:

  1. You need different (apparently very expensive given the price of a set of carbon brake pads) materials to make brake pads that will stop but not grind down your carbon fiber rims.
  2. Even the best carbon rims with proprietary -enter marketing language here– surface treatment don’t stop as well as aluminum rims when wet.
  3. Heat build-up in the rim during extended braking can cause temperatures in excess of the melting point of the carbon fiber resin. (It’s not really the melting point of the resin we are worried about. It’s the “glass transition temperature”, whatever.)
  4. Aluminum conducts heat away from the braking surface better than carbon fiber. (I could be wrong about this but I don’t think I am.) Score a point for carbon fiber brake tracks. Your tires won’t get so hot. But the surface temperature of the carbon fiber brake track goes up more, and the pads get hotter too. Braking generates heat. It has to go somewhere.

So the designers borrowed disc brake technology from mountain bikes. Take the braking function off of the rim entirely. It’s elegant, but I feel like I’ve been driven by a series of small steps to a place I would not have gone all at once. If you are wondering what this has to do with being locked out from the last two cogs, hang on a bit longer.

Brief wheel design digression: Because a disc brake acts on the hub, braking force results in a torque between the hub and rim that is not present on rim brake wheels. This torque must be transmitted via the spokes to the rim. This is why you will not see radially spoked disc brake wheels. I recommend Jobst Brandt’s classic book “The Bicycle Wheel”.

Disc brakes generate eccentric loads, so frames become more complex and ultimately require thru-bolt axles to withstand the eccentric loads. (And to keep the front wheel secure in the fork dropouts.)

So we’ve come to the point where our road bikes have thru-bolt axles, because we wanted carbon fiber rims. What’s this got to do with Di2 and cross-chaining?

Did you know that thru-bolt hubs are wider than quick-release hubs – like 10mm wider.  I suspect this has something to do with the mountain bike origins of thru-bolt technology. As described in Zinn’s FAQ column, the result is that the cassette on a thru-bolt wheel is about 5mm farther to the right.

But the chainrings are where they’ve always been. Cross-chaining is apparently so bad that the big chainring tries to pick up the chain if you are on one of the outer two cogs, especially on a racing geometry bike with short chainstays. So Shimano in their wisdom won’t even let you go there with Di2. I suppose if they could figure out a way to do this lockout with mechanical shifters they would?

Riding next to my friend Vic last week, I observed the lockout in action. Even though his bike was equipped with rim brakes and quick-releases, his shifters would not let him shift to the last two cogs while on the small chainring. (52-36 crankset) ****

All because you wanted carbon fiber rims.

And what’s this got to do with giving a mouse a cookie. If you have kids, you know.



*if you input chainring size difference of more than 14 teeth. So a 53-39 combination is OK; a 52-36 or 50-34 combination will invoke the lockout. I suppose you could hack the lockout by lying to the system about your chainring sizes.

** If you’re using SRAM ETAP I guess you’re on your own.

*** And if you have a rear flat you raise your right hand, your left for a front flat, to tell neutral support what kind of wheel you need. Does anyone know how to signal quick-release vs thru-axle?

****Dan, alias “6-0” just got a new Canyon Ultimate with Di2, disc brakes, and a compact crankset. With the chain on the small chainring and the third cog it really wasn’t too difficult to remove the back wheel because there is no quick release nut sticking out to the right.