Technical FAQ: Compatibility of GRX shifters with SRAM MTB derailleurs, carbon belts, e-bikes

Zinn comments on adaptations — like converting a hardtail to a gravel bike or using a "city" e-bike off-road.

Photo: Spot Bicycles

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

Dear Lennard,

I’d like to add drop bars and narrower tires to my Niner hardtail to bodge a gravel bike rather than spend for an entire new bicycle. The Air9 RDO currently runs a 2 x SRAM XO ten-speed drivetrain.

My thought was to purchase the 400 series GRX ten-speed shifters, link them to the XT brakes on the bike, and use a JTek shiftmate to run the current front and rear derailleurs.

The charts don’t show GRX, only Shimano 10-speed road, and the Shiftmate 6 appears to show compatibility. What do you think? Also, Shiftmate now has front adapters. Will I need one of these as well?

And, will the GRX shifters work with XT single piston brakes?


Dear KC,

I would guess that those front and rear Shiftmates would work, and that is only a guess. It sure looks like, based on JTek’s chart, that the Shiftmate 6 and 7 is what you need. And yes, running an MTB double front derailleur with a road double shifter does require a Shiftmate.

The levers will certainly activate the XT brakes, and the fluid is the same; whether the leverage is correct so that they work as designed is an entirely different question that I don’t know the answer to. Shimano has an enormous online page of compatibility of boatloads of combinations of its products here.

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.


Dear Lennard,

I had thought carbon belts were going to revolutionize drive trains but have yet to even see one. Any thoughts about why they seemingly have not taken off? Is this because people are invested in normal chains, or did they not deliver on performance and efficiency?


Dear Jeff,

A belt drive requires an internal-gear hub and a frame with sliding dropouts and a right seatstay that can be opened up to install the belt. That’s the reason you don’t see so many of them; it requires accepting a heavy rear hub and buying a frame with those two specialty features. That said, a high percentage of high-end bikes with internal-gear hubs do have a Gates belt; you probably just are not seeing many high-end, internal-gear bikes.

A minor detail that may also scare people off is that, while reliable, there is no way to fix a broken belt, and not many shops will have a full range of belt lengths if you walk or hitch a ride to a bike shop. Some folks use internal-gear hubs (like Rohloff) for extreme long-distance third-world touring (like around the world or Alaska to Patagonia) because of their reliability; they don’t use belts because they need to be able to fix the bike in remote places, and bike chains are findable in the third world, belts not so much.


Dear Lennard,

I read your recent answer to Mike’s question. I had major back surgery April 2021 and had to come up with a recovery plan.  My surgeon and PT both said I would need to ride a recumbent only from then on (both non-cyclists).  I thought no way in heck that was going to happen, but I did realize I needed to get another bike with a more compliant ride after 50+ years on road race frames, the latest being a 1994 Columbus TRX steel DeRosa and a 2010 carbon Idol.  Great bikes but can’t handle tires bigger than 25C.

So, I bought a carbon Creo nearly identical to the one pictured in the article.  I have two sets of Roval carbon wheels with 42mm S- Works Pathfinder Pros for gravel and 32mm Continental 5000TRs for the road. Also running a PNW dropper suspension seat post and changed to an 11×46 cassette so 1:1 low end to help conserve battery life for rides with a lot of California vertical.

No back issues since I got it in August 2021, but I’ve only done rides up to 60 miles and have done some rides with 6K climbing. You were spot on in comments to Mike about the Class 3 E-Bikes being a great equalizer in giving the physically challenged the opportunity to ride with his hammerhead friends.

With the exception of my e-bike wife, I mostly ride solo. Had an interesting experience in Bend a couple of weeks ago when I saw another solo rider approaching me from behind; I was doing +/- 20 mph on a flat road just before a short steep climb. I upped the wattage, and the obviously fit local racer could not catch me to the top. He caught up to me shortly after and said, “Thank god you’re on an e-bike; I thought I had lost some fitness.” I told him it enables me ride at level I was at when I was his age (probably late 20s …).

Byorn (Byron)

Dear Lennard,

A brief note about using a “city” e-bike on gravel.

We live in NH near the VT border, so we have access to a network of beautiful dirt roads. Last year my wife bought a “city” style e-bike for errands and recreation. Her first attempt at riding off-pavement was not a success. A quick check revealed:

1) 700×45 tires at 60psi. A reasonable size, but at that pressure — fine for smooth pavement — the ride quality on dirt was abysmal.

2) When I lowered the pressure, I realized that the tires were a multi-belted puncture-resistant construction offering minimal compliance. I could barely deflect the tread at 0psi. Great for urban hazards, but not for gravel.

So off came the stock tires, and on went a set of 700×50 Gravel King SKs. Set to a reasonable pressure, my wife said that ride comfort and control were vastly improved. We’ve had some great dirt-road rides together.

The stock setup wasn’t bad — it was just wrong for the desired use.


Dear Lennard,

Reading the new FAQ about racey e-bikes that are limited to class 1, I believe something very simple was overlooked. Both examples Pinarello and Look cited in the answer are European brands. Class 1 20mph speed is roughly 32kph. By law the EU restricts e-bikes to 25kph. So, a European brand can make a software change to up their bike from 25kph to 32kph with only a trade-off in lower range. If they changed a bike designed for 25kph to class 3, roughly 45kph, the trade-off in range would probably be too great to still be marketable. To increase the range, you need a bigger battery, which will need a bigger downtube to fit. Now you have two different bikes, and that probably makes less business sense than trying to sell a 20mph limited race bike.



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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