Technical FAQ: Di2 options, hot brakes, and more

Lennard Zinn addresses a variety of questions ranging from using two Di2 modules to follow-ups about exploding tires at the Tour of Oman

Using two Di2 control modules

Dear Lennard,
I had another thought in terms of mixing an XTR Di2 rear derailleur with a Dura-Ace or Ultegra Di2 front. Could this be done with two control modules — one for each derailleur — or would that require two batteries? Being unfamiliar with the mechanical aspects of Di2, it occurred to me Shimano may use the power wires for the control signals, in which case it wouldn’t work with just one battery. But if that were the case, how big a penalty would it be to have two batteries, or is that just not feasible?
— Steve

Dear Steve,
If you have two separate batteries, the trim function won’t work, which is a key part of Di2. In other words, the front derailleur moves over at two different points during the rear derailleur’s movement across the cassette to provide rub-free performance in any cross gear. This obviously cannot happen if the two derailleurs are running as separate systems.
— Lennard

Dear Lennard,
I’d like to set up my 1×11 drivetrain (SRAM X1) MTB up to ride on a Wahoo Kickr trainer until the weather improves a bit. The Kickr is a wheel-off setup and I’m concerned about compatibility of the cassette that comes with it. You can get the Kickr with an 11-speed cassette, but I suspect it’s a road cassette, and I don’t know if it will work with the rest of the drivetrain. I thought maybe I could just pick up a compatible cassette and replace the one on the Kickr, but the least expensive SRAM 11-speed mountain cassette is about $300 – more than I’m excited to spend on a cassette that will live out its life on a trainer. Do you think the 11-speed cassette that comes with the Kickr will work? Or can I buy one of the less expensive SRAM or Shimano 11-speed road/cross cassettes and use that?
— Chester

Dear Chester,
Either choice will work; the spacing is the same on an 11-speed road or MTB cassette as your X1 cassette, so the derailleur will shift just fine with the Kickr stock 11-speed cassette or any SRAM or Shimano 11-speed road cassette. The freehub body is different, however, so you cannot use a SRAM 11-speed MTB cassette on that trainer.
— Lennard

More on exploding tires at the Tour of Oman

Dear Lennard,
Bontrager (and others) has introduced tubeless-ready versions of their D3 Aeolus carbon wheels. Could a good tubeless rim/tire/sealant combination have mitigated the danger of exploding tires?
— Eric

Dear Eric,
Some of the dangers would disappear, but I still foresee plenty of problems in that kind of heat with prolonged braking.

No, you could not tear off valve stems as with a tubular with liquefied rim cement. And you could not melt inner tubes as with a clincher.

But with full-carbon tubeless clincher rims, you still run the risk of the rim walls folding outward like a limp taco shell if the Tg (Glass Transition Temperature) of the resin is exceeded.

The potential for the tire itself to fail would be the same as with a standard clincher. The enormous heat and pressure could cause the tire to fail and would certainly do so if its melting point were to be exceeded.

With rims converted to tubeless with plastic rim strips, whether aluminum or carbon rims, another concern could be melting the rim strips to the point that the air pressure blows out through the spoke holes.
― Lennard

Dear Lennard,
If the peloton would switch to disc brakes, the heat would not be a problem anymore!
— Todd

Dear Todd,
That is not entirely true. Yes, the overheated rims and tires we saw in Oman would cease to be a problem with disc brakes. In those conditions, I believe the heat at the caliper and rotor would be a problem, however, and, as I said in my column about Oman, could lead to total brake failure.

Even if the UCI were to allow disc brakes in road racing, for them to gain wide acceptance, road racers would insist on superlight versions that are also low in profile. Racers are not going to be interested in brakes that render their $10,000 aero road bike less aerodynamic than road bikes of the 1970s. Calipers and rotors sticking out in the air create drag, as do the crossed spoking patterns and more spokes required with disc brakes.

But answering some of these demands by reducing the size and weight of the calipers and rotors means there will be very little thermal mass in the system, making them subject to overheating. Too much heat in the caliper can boil the fluid and can burn through the resin pads that good performance with tiny rotors requires.

If hydraulic fluid boils in a brake system, the brakes do not work. Hydraulic brakes work because fluids are essentially non-compressible, so pushing on one end of a column of fluid will result in just as much push at the other end of the column of fluid. Gases, however, are compressible. That’s why we ride on pneumatic tires. So if the fluid boils, gas bubbles will appear in the fluid, and the push from the master cylinder piston in the lever will not push the pistons in the calipers out hard enough to stop the bike.

We’ve discussed this here in the past. Different brake fluids have different boiling points, but all of them do have a boiling point.

As for the pads, resin pads get a stronger initial bite on the rotor than do metallic, a.k.a. sintered, pads. And if the rotor size is going to be tiny, like 140mm (or smaller yet) in order to satisfy the desires of road riders for low weight and low wind drag, you will need a lot of brake bite. But resin pads will rapidly be destroyed with the kind of heat that slowing down a road bike on a steep, fast descent in high ambient temperatures requires.

As an example of the issues involved, I can point to experience with a super strong, super-tall, 330-pound customer of mine who owns five custom Zinn titanium bikes we’ve built for him here in Boulder. The last two have been with hydraulic disc brakes, because he had so many brake and tire problems with rim brakes similar to the ones I discussed in my article about Oman.

This man rides a lot, all over the world, and he does things like back-to-back six-week riding camps in the Alps in summertime. (He gets stronger and stronger, but, contrary to what you might expect, his weight does not change significantly.)

With Shimano hydraulic road disc brakes, he was getting just one day (!) out of organic (a.k.a. resin) pads, even on cool days, and that’s using XTR rotors, 180mm front, 160mm rear, not the 140mm rotors that come with the Shimano Di2 road disc brakes. Yes, his disc brakes work way better for him than rim brakes ever did, and they eliminated the tire problems he was having, but daily pad replacement was overly burdensome for him. So he switched to sintered pads, and on his trips to the Alps, he now goes six days between pad changes. He’s not overjoyed about the hassle, but he can deal with weekly, rather than daily, pad replacement. He also has to forgo the nice initial bite of a resin pad.

Despite Shimano’s insistence that he should need neither the large rotors nor the sintered pads, that is what he ended up with, because he could not stop with 140mm rotors. Imagine the braking power it takes to slow his bike down while descending the Col du Tourmalet. He and his bike weigh 350 pounds, the same as a 175-pound man and his 135-pound wife on a 40-pound tandem. If you’ve ever ridden a tandem down a steep descent (like the Tourmalet!), you know how fast it picks up speed. I know from personal experience how terrifying exactly this circumstance can be. Now imagine the minimal sense of security you would feel if all that stood between you and your partner and flying off a cliff in the Alps were a pair of road disc brakes with 140mm rotors.

It is my opinion that prolonged braking in 120-degree Fahrenheit (49C) temperatures would bring about similar brake problems for a rider of a more average weight.

Cable-actuated disc brakes could solve some of these problems, but they are not likely to be used by road racers. When coupled with sintered pads, cable-actuated disc brakes would eliminate most heat problems at the rim and at the caliper, other than perhaps warped rotors (or even melted sandwich-style rotors). But they are bulkier, heavier, and less aerodynamic than hydraulic discs can be, and they have less power and modulation.

My point is that I don’t think a magic bullet yet exists for being able to do prolonged braking on a steep descent at 120-degree temperatures. The best thing you can do in the meantime is not hold onto the brakes but instead brake harder and let off for a few seconds in between, rather than letting them continue to heat up. Or alternate front and rear braking to let one cool at a time if you have to maintain a slow speed, like in a neutralized descent.
― Lennard

Patching tubes

Dear Lennard,
Can latex tubes be patched the same way that butyl tubes can? Will the vulcanizing Rema Standard Patch Kits, #21 work on both types of tubes?
— Brian

Dear Brian,
Yes and yes.
― Lennard