broken GXP locking nut - BB90 compatibility problem

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jules21
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broken GXP locking nut - BB90 compatibility problem

Postby jules21 » Thu Aug 09, 2018 3:13 pm

I broke the locking nut on my GXP cranks. see shared album for photos.

it broke when I tried to tighten it - again, after it came loose again. the nut has sheared halfway along its axis, leaving half the nut still threaded into the crank spindle.

Q1: how should I remove it? I can slide an 8mm drill bit into it, but it doesn't grip. the surface is smooth there (cylindrical), even though the other end (broken off now) has the 8mm hex fitting.

the problem seems to be that there was side play in the cranks. I'm running GXP cranks in a BB90 BB shell (Trek Madone 7s). I honestly can't remember where I sourced the BB bearings, but I think they were already installed when I bought the bike 2nd hand. It came with GXP cranks (different ones, but the same model).

I'd stacked it with several spacers, but they weren't enough to take up the lateral slack.

the BB bearings are shown in the album, and the NDS one is just a spigot bearing installed directly into the BB shell (no plastic BB bearing housing). I didn't have time to check the DS bearing - I assume it's the same bearing (different internal diamater as per GXP).

I was going to : replace it with this BB from fleabay

Q2: is the extra width of the plastic shell going to take up the lateral slack that I have now, with the directly-installed bearings?

any advice appreciated!

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Duck!
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Re: broken GXP locking nut - BB90 compatibility problem

Postby Duck! » Thu Aug 09, 2018 7:09 pm

Use a broad flat-blade screwdriver to get the broken-off bit of bolt out. Give it a firm whack to get the sides of the screwdriver to bite into the bolt; with no tension from the bolt head (because it's not there!) to pull up against the threads, the stump should screw out fairly easily.

Part 2: That linked bottom bracket won't work. BB90 is a Trek proprietary system which has the bearings fitted directly into the frame. Standard press-fit units won't fit because the housings are too big for the frame bore.

The root of the problem could be that it's a Trek. I don't know what vintage your frame is, or if Trek have improved their manufacturing recently, but the tolerances of at least earlier BB90 (and MTB BB95) shells were frankly dreadful. It could be that the entire bottom bracket/crank assembly was floating in the frame! The GXP system is designed so the inner shell of the left bearing is clamped between the step on the spindle and the left crank arm, while the right side is effectively floating, applying no lateral compression on the bearings. Sometimes a wavy spring washer is fitted between the right crank & bearing, but is not usually needed.
I had a thought, but it got run over as it crossed my mind.

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jules21
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Re: broken GXP locking nut - BB90 compatibility problem

Postby jules21 » Thu Aug 09, 2018 8:40 pm

Thanks Duck much appreciated. I'll give that a go.

OK I got the tensioner bolt (not nut) out of the spindle easily enough. Now to figure out the lateral slack.

jasonc
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Re: broken GXP locking nut - BB90 compatibility problem

Postby jasonc » Fri Aug 10, 2018 2:58 pm

Just throw a Shimano crank and BB on.
Image

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Duck!
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Re: broken GXP locking nut - BB90 compatibility problem

Postby Duck! » Fri Aug 10, 2018 4:12 pm

Shimano BB won't fit.
I had a thought, but it got run over as it crossed my mind.

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jules21
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Re: broken GXP locking nut - BB90 compatibility problem

Postby jules21 » Fri Aug 10, 2018 5:14 pm

you can make it fit though can't you? without the reducer on the NDS bearing? making them both 24mm

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Duck!
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Re: broken GXP locking nut - BB90 compatibility problem

Postby Duck! » Fri Aug 10, 2018 7:54 pm

Wrong end of the stick. I said the bottom bracket won't fit. Because BB90 is a proprietary system, you need a BB90 bottom bracket, which additionaly needs to match the chosen crank.
I had a thought, but it got run over as it crossed my mind.

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jules21
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Re: broken GXP locking nut - BB90 compatibility problem

Postby jules21 » Mon Aug 13, 2018 9:38 am

FYI I have 'fixed' it. the NDS bearing was toast and replacing it has made it all much better.

there is still a small amount of play in there, almost indistinguishable but detectable when I grab the crank arm and wobble it.

as the DS bearing is pressed in but the (now replaced) NDS one just sits in its housing (frame shell), I'm guessing shell wear or manufacturing tolerance is the cause.

thanks again for advice. it's perfectly rideable at the moment at least.

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