Technical FAQ: Chain lines on road disc brake bikes

Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
In your recent VeloNews Technical FAQ article titled Cross-chaining relief, you wrote this about the effect of disc brake axle lengths on chain rub when cross-chained:

“This spacing increase from 130mm to 135mm for disc-brake bike frames has moved the cassette out 2.5mm further to the drive side than it was with rim brakes, yet the crank manufacturers did not change the chain line of the cranks with the change from rim brakes to disc brakes.”

This is not strictly true. SRAM has introduced their “wide” series of cranks (available in Force level and below) in part to address this issue. These cranks have a chain line that is increased by 2.5mm to match the increased cassette offset of disc brake wheels. Campagnolo has also increased its chain line for disc brakes. Starting with their 11spd HO cranks (HO stands for “Hydraulic Optimization”), they have increased the chain line by 1.2mm. These cranks are intended to work with either rim or disc brake wheels, so their chain line splits the difference between rim brake and disc brake cassette offset.
— Mark

Dear Mark,
Thank you so much for pointing that out. While I’m familiar with recent increases in crank widths to accommodate the wider tires and consequent wider chainstays of gravel road bikes, I had somehow missed a change in chain lines to correct only for the added width of disc-brake rear hub axles. I learn something new every day writing this column, and I enjoy and appreciate that.

Chain line (the perpendicular distance from the center of the seat tube to the central plane between the two chainrings) had been around 43.5mm for most road double cranks (Campagnolo and Shimano) for as long as I can remember. Why split hairs down to the half-millimeter? Well, with nominal 5mm chainring spacing, the inner ring was 41mm from the center of the seat tube and the outer ring was at 46cm; halfway between them is 43.5mm.

While Shimano did not increase the 43.5mm chain line with 2×11 disc-brake groups, indeed, SRAM “wide” for 2×12 is wider; at 47.5mm chain line, the SRAM Force 43/30 Wide crankset for eTap AXS is 4mm wider than the old 43.5mm standard. Same goes for Rival AXS 43/30 Wide. And the crank designated simply as “SRAM Force” in the eTap AXS 2×12 family has a chain line option that is even wider, at 51mm https://www.sram.com/en/sram/models/fc-frc-d1 (the other option is 45.0mm). As you said, Wide cranks are only Force and below; SRAM Red eTap AXS 2×12 is at 45.0mm, 1.5mm above the old 43.5mm spec.

And indeed, Campagnolo HO chain lines on its H11 drivetrains are wider, but only by 1mm, not the 2.5mm required to full adjust for the wider disc-brake rear spacing. As you can see the chain line of cranks on Campy H11 hydraulic-disc-brake groups went to 44.5mm in 2018, from the 2015 chain lines of 43.5mm for rim-brake 2×11 cranks.

While Shimano went to a 47cm chain line for 2×11 gravel riding with its GRX crank (both FC-RX810-2 and FC-RX600-11), it is sticking with the 43.5mm chain line for 2×11 road bikes. Current 2×11 Dura-Ace cranks (FC-R9100) still have a 43.5mm chain line. This also applies to Ultegra FC-R8000 2×11 cranks and 105 FC-R7000 2×11 cranks. Shimano’s road 2×12 Dura-Ace FC-R9200 cranks do have a tiny 1mm increase in chain line, to 44.5mm from prior Shimano road cranks. Same goes for 2×12 Ultegra FC-R8100.

Looking around at other chain lines, the variation is interesting. SRAM Force 22 (2×11) has two options wider than the old 43.5mm spec, at 45.0mm or 45.5mm; neither offers the full 2.5mm increase that would create the same chain angles with a disc-brake hub as a 43.5mm-chain line crank has to a rim-brake freehub. Rival 22 offers either 45.0mm or 47.5mm, while SRAM Red 2×11 has a 45.5mm chain line. FSA is at 44mm chain line with its K-Force Light cranks.

If I were to answer that question from Yi again, I would first suggest that he measure the chain line of his Force 22 crank (average the distance measured from the center of the seat tube to the centers of the teeth of both the inner ring and the outer ring). A chain line under 46mm would mean that the chainrings are further inboard relative to the cassette than with a 43.5mm-chain line double crank on a rim-brake bike. And, as I explained last week, 43.5mm-chain line double cranks on rim-brake bikes already were prone to the chain touching the large ring when cross-chained from small ring to smallest cog with compact 34/50 chainrings. The caution I expressed about spacing his crank over to the drive side still applies, so I would also suggest to Yi the option of getting one of the wider cranks we’ve just discussed.

I also considered including frame alignment when I wrote my column last week and chose not to because Yi has a carbon frame. It can be an issue with metal bikes, which many readers may have, so I’ll go ahead and address it. If the bottom bracket is not “normal” (i.e., perpendicular) to the plane of the bike frame and is specifically tipped so that the right crankarm is closer to the right chainstay than the left crankarm is to the left chainstay, then this chain snag issue is exacerbated. In that case, the chainrings are not parallel to the plane of the bike, and the trailing edge of the larger chainring is angled inward toward the chainstay and hence is more in the way of the chain’s path to the small cog when cross chained. This misalignment can happen with a metal frame, because during the expansion and contraction with the heat of welding or brazing, alignment of the bottom bracket (or any other part of the frame) can change. This can be allowed for with a careful welding sequence, and it can be corrected on an alignment table after welding. For it to occur on a molded carbon frame would require a complete screwup of the machining of the mold and is hence highly unlikely. If the end of the drive-side crankarm is closer to the right chainstay than the end of the left crankarm is to the left chainstay, then the frame’s bottom-bracket shell could be misaligned.
― Lennard

Dear Lennard,
Regarding Alex Moulton’s testing, you left off the last bit!
*** 32-369. ***
Without the AM tire size, my last sentence leaves the reader hanging.
— Doug

Dear Doug,
Oops. I don’t know how I let that slip by. Yes, it certainly leaves one hanging wondering what Alex meant! Correction made.
― Lennard

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), 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 on Twitter.