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crank and BB installation torque: does it really matter?

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crank and BB installation torque: does it really matter?

Old 03-11-12, 09:04 PM
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powitte
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crank and BB installation torque: does it really matter?

I'm used to square taper stuff, where I was taught you tighten the crank bolts as much as my puny arms can torque it. It seemed to do the job always.

Now I'm about to install a GXP BB and Rival crank, and all SRAM's torque specs have me worried. How much does this matter? Do I seriously need to employ the services of a torque wrench to do the job right? Is there even an adapter that will fit the right BB tool? I suppose the crank bolts are more important to get just right.

Thoughts?
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Old 03-11-12, 09:08 PM
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have you ever stripped out an aluminum part by over tightening a bolt?
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Old 03-11-12, 09:18 PM
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Yes. Doing the job right requires a torque wrench. Specifications exist for a reason.

Now, you can, as you previously experienced, install something by feel in such a way that it works and it's good enough. But it's not technically correct. Much like whoever it was that taught you to just wail on square taper crank bolts you may get replies to this thread saying it's not a big deal and just tighten by feel, but they're not correct either.

The proper adapter is the Truativ GXP adapter. http://www.amazon.com/Truvativ-Howit.../dp/B000QSZRWM

Park Tool also makes the BBT-19, supposedly suitable for a wide variety of external BB cups. http://www.parktool.com/product/bott...et-tool-bbt-19
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Old 03-11-12, 11:12 PM
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Outboard BB cups aren't too critical. The goal there is for them to be fastened against the frame firmly so they don't loosen up in use. You'd have to do a serious job of overtightening them to shear the flange off, otherwise it's a binary thing: either it stayed put, or it didn't. The bearings don't care, they're out of the loop.

The crankarm on GXP is slightly more important, but has a lot of tolerance for error. Basically, the inner part of the non-driveside GXP bearing cartridge gets sandwiched between the shoulder on the GXP spindle and the NDS crankarm, which also swedge-fits to the splines in the process. But ultimately, it bottoms against the bearing and this can be percieved by the mechanic as a noticable ramp-up in resistance, similar to how Octalink characteristically ramps up when the crank bottoms on the spindle as designed. If you overtighten it, the bearing will simply be sandwiched more firmly... the crank's not going any further onto the splines. To put this into perspective, square-taper is more sensitive and error-prone than GXP, since it has the option to keep swedging further and further onto the spindle.

Then again, I've encountered people who can't feel that effect. So if you're not sure, then bust out a torque wrench. The beam style will suffice.
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Old 03-12-12, 06:38 AM
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for gxp, octalink, isis i do it by feel because it is quicker. sq taper and other tapers i use a torque wrench
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Old 03-12-12, 07:30 AM
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for GXP, you have one bolt doing the job of two on a square taper type crank.

it's slightly more prone to loosening if you don't tighten it enough.
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Old 03-12-12, 09:02 AM
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Originally Posted by AEO View Post
for GXP, you have one bolt doing the job of two on a square taper type crank.
Not really.
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Old 03-13-12, 06:23 PM
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Thanks!
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