Technical FAQ: Cross-chaining relief

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Dear Lennard,
I have a SRAM Red 22 eTap drivetrain on a Giant TCR disc bike and have issues with cross chaining. Yes, I know you’re not meant to cross-chain, but it happens. I find that when I’m in 34-11, the chain crosses so much that it tends to touch the upshifting pins of the chainring. This causes the chain to try to upshift, but never fully make it (since the FD is still in the lower position).

Is this just me using it wrong? Or is there an issue with my wheel dish that should bring the cassette inwards? Or should there be another spacer on the crankset to move the chainrings outboard? Please help!
— Yi

Dear Yi,
No, it’s not just you. And, no, wheel dishing would not change what’s happening, because, incredible as it may seem, what is happening on your bike is actually by design.

The chain catching on the shift pins of the big chainring when fully cross-chained from the smallest cog already was a problem with 2 X 11 (and even 2 X 10) drivetrains on many rim-brake bikes and originated with the advent of compact cranks. The interference problem rarely existed with the larger-diameter 39-53 chainrings and the 14-tooth size difference between them (36% jump); it reared its head with the 34-50 compact crank’s 16-tooth (47 percent) jump exaggerated by the smaller chainring diameter (14/39 = 36 percent and 16/39 = 41 percent, but 16/34 = 47 percent).

(You may protest that your bike instead has a 12 X 142mm through axle—not a 10 X 135mm quick-release axle, but that actually doesn’t change the fundamental issue here. With a through axle, the dropout faces have recesses in them to accept the bearing end caps, and those recesses are 3.5mm deep. Thus, the spacing from inner dropout face to inner dropout face is still 135mm, even though the distance from end of bearing cap to end of bearing cap on the hub is 142mm).

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. So, while your chain might not have gotten snagged by the shift pick-up pins with your drivetrain on a rim-brake bike, that 2.5mm further outward that the small cog sits from the centerline of the bike increases that potential chain interference. And, obviously, the shorter the chainstays, the sharper the angle of the chain out from the inner ring to the smallest cog; a bike with a longer rear end would have less of a problem with small-small-gear-combo chain snagging.

Because of this issue, Shimano programmed many Di2 electronic road groups to prevent shifting to that combination. But with a cable-actuated system and with your electronic eTap system, it is up to the user to not get into this full cross-chain situation.

In answer to your third question, yes, a spacer to move the chainrings further outboard would reduce or eliminate this problem. However, if your bike has SRAM GXP cranks, you definitely should not add a spacer on the drive side. Adding a spacer to a SRAM GXP crankset will result in either binding and/or rapidly frying the bottom bracket bearings or in having your left crankarm fall off while riding.

The GXP system has a step-down spindle diameter with a shoulder on the non-drive end of the spindle that the non-drive bearing seats against. The drive-side bearing has a 24mm ID, and the non-drive-side bearing has a 22mm ID to fit over the stepped-down splined end. The lateral position of the cranks is fixed by tightening the left crank fully against the inner race of the non-drive bearing, pinching it between the crankarm and the shoulder on the spindle. As long as the bearing is fixed in the frame, the crank cannot move laterally. There is a little extra room on the right between the drive bearing and the right crankarm, and if you were to put a spacer between the right crankarm and the drive bearing any thicker than this little space tolerance, you would be pulling the shoulder on the opposite end of the spindle away from the non-drive bearing. Then, if you tighten the crank bolt, you push the non-drive bearing’s inner race inboard relative to its outer race, binding the bearing and accelerating its rate of wear. Same on the drive side. Furthermore, the spacer would prevent the left crankarm from moving inward enough to pinch the inner race of the non-drive bearing between it and the spindle shoulder when tightening the crank bolt, the left crankarm would not be fully engaged on the spindle splines and could work its way loose and conceivably fall off while pedaling.

If you instead have a SRAM DUB crankset, you still may not be able to put a spacer on the drive side, or at least not a thick enough one to eliminate the chain snagging you are experiencing. The SRAM DUB system eliminates lateral play and establishes lateral crank positioning with a threaded preload adjuster on the non-drive end of the spindle, adjacent to the left crankarm. You can only fit as many spacers on the drive side of the spindle as there is room between the preload adjuster and the non-drive bearing when the adjuster is fully backed out toward the left crankarm. If you put in any more drive-side spacers than that, you will create the exact same problem I described above in reference to a GXP crankset.
― Lennard

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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.