Technical FAQ: Back surgery and road cycling, shiftmates, hydraulic caliper compatibility

Zinn comments on WiFli RD capacity, shiftmates front and rear, Shimano hydraulic MTB/Road caliper compatibility, wire flats, and more.

Photo: Nick Payne

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Have a question for Lennard? Please email to be included in Technical FAQ.

Dear Lennard,

I read Byorn’s letter to you regarding riding a bike after major back surgery. I had my fourth at age over 50, and it was done by a spine surgeon with decades of experience, both as a spine surgeon and a road cyclist.  His recommendation may be reassuring to some cyclists in the same situation: most patients can return to cycling (even on a road bike) after surgery, and it can be beneficial to do so.

Contrary to what many may assume, an endurance position on a road bike need not stress the intervertebral disks, because in an endurance position (spine at 45-60 degrees), the lower back can stay relatively straight with the weight shifted to the pelvis, powerful leg muscles, feet, and hands. Sitting upright in an office chair, or perhaps even a recumbent bike, may actually be more stressful, because sitting upright, it is harder to engage the core and leg muscles to take the load off the disks.

The most important rehab you can do is to work on flexibility of legs and hips and strength of lower core (transverse abdominis), in addition to the legs if you experienced weakness while waiting for the surgery.  Low-power, high-cadence cycling is actually often recommended after back surgery, because the rapid movement of the legs prevents the formation of adhesions, namely post-op scar tissue that entraps nerves. Moreover, with the back relatively straight or even in slight flexion (as would be on a road bike), both the central canal and neural foramina are opened up, reducing stenosis in these areas. In fact, in a TT position, the neural foramina of the lower spine can actually be at its largest, provided one has good flexibility and no disk bulges.

– Marc

Dear Marc,

I have never had back surgery (knock on wood). However, I have lost two inches in height due to complete collapse of all of my intervertebral discs from L1 through the entire lumbar area to S1. Indeed, my experience over the past nearly 40 years that I have lived with this degenerative condition has been that road riding is very good for my back, and decidedly better for it than sitting in a desk chair. So, I’m not surprised to hear that it is often recommended for post-op back surgery patients.

– Lennard

Dear Lennard,

In your 6 September Tech FAQ , you published a letter regarding the max cog size that an 11-speed eTap WiFli RD can cope with. I can say that, although only being rated for a 32-T cog, the WiFli RD definitely works OK with a 36-T cog, as I have one installed on a gravel bike where I’m running an 11-36 cassette. The B-screw is at about the mid-point of its travel to allow the top jockey pulley to clear the largest cog.

And in your Velonews column of September 27, you say, “running an MTB double front derailleur with a road double shifter does require a Shiftmate”.

In my experience, if you are only using two chainrings, an MTB FD will work fine with a road double shifter without any device to change the cable pull. I have:

  • One bike using 10-speed Ultegra STI shifters and a Deore XT M786 2×10 FD
  • One bike using SRAM Force double-tap 11-speed shifters and a Deore XT M786 2×10 FD
  • One bike using SRAM Force double-tap 11-speed shifters and a SRAM X9 2×10 FD

On all of these bikes, the front shifting works perfectly. I use MTB FDs, as all these bikes are using 38 or 42-tooth big chainrings.

And going in the other direction, I converted one of my wife’s bikes to flat handlebars, and it is now using SLX 11-speed shifters with the original Ultegra 8000 FD, and the front shifting is fine with that setup as well.

I’ve attached photos of a couple of the bikes.

– Nick

Nick’s MTB with FD road shifter (Photo: Nick Payne)
Nick’s MTB with FD road shifter (Photo: Nick Payne)
Nick’s WiFli setup (Photo: Nick Payne)

Dear Lennard,

Regarding your answer to KC, just FYI GRX 10s is like the latest Tiagra 10 4700. The rear mechs are basically 11-speed, and the shifters pull different cable pull amounts than other Shimano road 10s shifters.

So, the Shiftmate for Shimano 10s road won’t offer the right adjustment.

– Andrew

Dear Lennard,

In reply to KC you wrote:

“The chart shows no compatibility of the GRX RX400 10-speed shifter with a BR-8100 single-piston XT brake caliper. You’re on your own on this; you could always try and see what happens. Brake carefully when trying it.”

I can confirm that it works. In fact, every hydraulic Shimano brake caliper since at least 2011 until today works without any restrictions with every brake lever / brifter of that same period. It’s truly choose and mix as you like. The piston diameter in the BR-M8100 caliper is exactly the same 22 millimeters as in all their Flat-Mount calipers up until the latest 11-speed generation (BR-R9170, BR-R8070, etc.).

For their 12-speed flat-mount calipers (BR-R9170 and BR-R8170) Shimano reduced the piston diameter to 21 mm, and they’re no longer made from ceramic. (I suppose it’s because of that 9 percent reduction in piston area that they claim the pad retraction has increased by 10 percent compared to their previous calipers, although piston travel and hydraulic ratio are not strictly correlated.)

I guess you can still combine those 12-speed calipers with older brifters and vice versa, as a 9 percent change isn’t dramatic, but the lever stroke and firmness of the lever will be different.

By the way, even their truly excellent 4-piston calipers like BR-M9210, BR-M8120, BR-M8020, … down to the BR-MT520 – they’re technically all the same – work perfectly with road bike brifters, as they have the same hydraulic ratio. (Which is no wonder as both the 2-piston and the 4-piston calipers are officially compatible with the same mountain bike brake levers.)

In 2018 I have started testing BR-M9120 on one of my gravel bikes and now run ST-RX815 and ST-R785 combined with BR-M9120 on all road and gravel bikes. After testing them thoroughly, we also installed that combination on some customers’ bikes in the past few years. For guys who love descending or need a lot of perfectly controllable braking power because of their bikes’ system weight, it’s addictive how well that combination works. As a nice side effect their much longer pads live much longer and they don’t get as hot as the tiny ones in the Flat-Mount caliper pads. Maybe something you would like to recommend to some of your extra tall customers.

– Horst

Dear Lennard,

Absolutely agree with Jobst Brandt’s ideas on flats.

I’m flatting much less since retiring – much less predawn, wet weather, and in-town rides.

One of the worst flats I’ve had was inflicted by an “industrial” (single edged) razor blade, which the front tire flipped up just perfect for the back tire slice; I heard the front tire slap it. I was late to work that day …

My guess is there’s many more steel-belted tire shards on the roads than other small metal hazards, however, I believe the nails, small screws and staples tip up easier and find their way in better as well. Staples, all kinds, are king here on the Central Coast of California (per my experience), as the dreaded puncture vine is still a rarity; glass bits are a close second.

– Tom

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom huge bikes a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

He holds a bachelor’s in physics from Colorado College. 

Follow @lennardzinn

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