Sram cranks 30mm spindles

Discuss light weight issues concerning road bikes & parts.
ti473
Posts: 11
Joined: Thu Jan 25, 2018 9:29 pm

by ti473

Thanks JAQ1 for the good info. Like I said, in the end I ordered a 3rd crank since I wanted to get this damn build over with. I will keep that in my back pocket though in case I run into the same issue again.

NickJHP
Posts: 82
Joined: Wed Feb 14, 2018 2:22 am

by NickJHP

I don't know if the SRAM cranks allow the same sort of modification, but this is what I did with a set of Deore XT cranks to make them narrower for use on a touring bike.

I bought a pair of Deore XT M785 2x10 cranks with 38-26 chainrings on closeout for $A106. The cranks are intended for use on a 73mm BB with a 48mm chainline, and come supplied with two 2.5mm spacers for installation on a 68mm BB. As supplied, even if the spacers are omitted, to get a better chainline and narrower tread, the mating splines on the BB axle and RH crank won't allow the cranks to snug up to the bearings on both sides. I had a look at the splines on the inside of the RH crank where it mates with the axle, and decided that even if I ground off 5mm of splines, there would still be 10mm length of spline to engage - more than enough. I clamped the crankarm in a rubber-jawed vice, and a few minutes with my Dremel and a small grinding wheel did the job. As assembled now, the end of the axle protrudes about 1mm past the hole in the crank. I thought about putting the axle in my threadless stem saw guide and cutting a couple of millimetres off it, but a quick scrape with a file showed that the steel in the axle was much too hard for a hacksaw blade, so I didn't bother.

A 1" headset spacer is exactly the right size to fit over the end of the axle and in the crank recess around it, so I set the preload by threading in the plastic compression cap with a 5mm 1" headset spacer underneath it, and removed it once I had the crank bolts tightened. The compression cap isn't threaded right to the end, so even without the spacer there, it sticks out too far to be left in place in normal use.

The end of the axle doesn't actually protrude past the non-recessed part of the crank, so I don't think it'll cause me any problem.

Removing the 5mm of spline actually only loses me a couple of mm of spline engagement compared to fitting the crank on a 73mm BB shell, as in that situation the end of the axle sits about 3mm inside the recess on the crank. So I'm pretty sure it'll hold - if I weighed 120kg it might not, but I only weigh 65kg.

Images of the modified splines and assembled crank.

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by Weenie


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