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Left crank arm slipping

Old 04-23-23, 10:46 PM
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Left crank arm slipping

I just had my LBS install a new bottom bracket. When I took the bike out for a test ride, as soon as I put torque on the left pedal, the crank arm moved on the bottom bracket axle(?). Has anyone got any thoughts on what would cause this?
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Old 04-23-23, 11:08 PM
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Improper reassembly. Take it back to the shop. Don't ride it until they fix it properly. Damage to the crank arm will result.
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Old 04-23-23, 11:19 PM
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Originally Posted by JochenRindt
I just had my LBS install a new bottom bracket. When I took the bike out for a test ride, as soon as I put torque on the left pedal, the crank arm moved on the bottom bracket axle(?). Has anyone got any thoughts on what would cause this?
Did they do any other work, like service the rear axle or install a Freewheel?
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Old 04-23-23, 11:30 PM
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Originally Posted by Bill Kapaun
Did they do any other work, like service the rear axle or install a Freewheel?
They did not work on the rear axle or cassette (it is in fact a cassette and not a freewheel). They did adjust the front and rear derailleurs, replace shifter cables, and replace the cable sheath near the rear derailleur.
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Old 04-23-23, 11:32 PM
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Originally Posted by base2
Improper reassembly. Take it back to the shop. Don't ride it until they fix it properly. Damage to the crank arm will result.
I made it about 3 feet before i realized what happened and that the position of the cranks was no longer opposed. Do you think that was enough to damage the splines in the crank arm?
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Old 04-24-23, 06:27 AM
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For this to happen, the bolts would have to be really, really loose. Take it back and have them fix it.

I doubt any damage.
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Old 04-24-23, 11:05 AM
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Thank you to everyone for the information and advice. I took my bike back into the shop this morning. It was a different guy this time, the owner. At first he thought perhaps it was the wrong bottom bracket his mechanic installed, but when he got the old one out of the trash, he saw it was identical to the new one. Basically what happened was that the cranks hadn't been properly seated all the way on the axle. He said there was no damage to the crank arm because it wasn't engaged far enough to do any damage. He apologized for the work his mechanic did and said he would talk to him. I would like to give that shop my future business, but I don't know if I can be certain that the owner will be the one doing the actual service.
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Old 04-24-23, 11:30 AM
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If the shop keeps good records this return service visit has been retained for a future reference.

Most crank arms are Al and most BB axles are steel. The softer Al can get damaged if the arm is augured back and forth when they are not properly tightened against each other. It doesn't take much to ruin a crank arm if it comes loose of the axle. If your arm is damaged than the arm will come loose again, and generally fairly soon after the "corrective tightening". If not the arm will likely stay put and only need the usual annual check.

I suggest returning to that shop in a few rides and ask then to double check the arm's retaining bolts for proper tightness. If the boss isn't there and if they recorded the earlier return service then they should see the need to do this recheck for free and on the spot. If the bolt has stayed properly tight all is good. If the bolt was a bit loose this needs to be known as it could be that indication that some damage did happen. Assuming all goes well during this follow up check buy a tube or something from them while you are there. The size/cost isn't the issue with this, it's their treat for treating you right. Andy
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Old 04-24-23, 12:00 PM
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Originally Posted by Andrew R Stewart
If the shop keeps good records this return service visit has been retained for a future reference.

Most crank arms are Al and most BB axles are steel. The softer Al can get damaged if the arm is augured back and forth when they are not properly tightened against each other. It doesn't take much to ruin a crank arm if it comes loose of the axle. If your arm is damaged than the arm will come loose again, and generally fairly soon after the "corrective tightening". If not the arm will likely stay put and only need the usual annual check.

I suggest returning to that shop in a few rides and ask then to double check the arm's retaining bolts for proper tightness. If the boss isn't there and if they recorded the earlier return service then they should see the need to do this recheck for free and on the spot. If the bolt has stayed properly tight all is good. If the bolt was a bit loose this needs to be known as it could be that indication that some damage did happen. Assuming all goes well during this follow up check buy a tube or something from them while you are there. The size/cost isn't the issue with this, it's their treat for treating you right. Andy
He did make a copy of my original receipt, and he seemed quite confident that there was no damage to the arm and that it would no longer slip. Nevertheless, I will be vigilant for any movement over the season.
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Old 04-24-23, 01:09 PM
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[QUOTE=JochenRindt;22869916]He did make a copy of my original receipt, and he seemed quite confident that there was no damage to the arm and that it would no longer slip. Nevertheless, I will be vigilant for any movement over the season.[/QUOTE

The problem with this is that by the time most will notice movement the damage has been done. That's why I suggest a proactive (not reactive) stance and have a follow up tightness check soon. The goal is to not do something after the arm comes loose but to not have the arm come loose to begin with. Andy
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Old 04-24-23, 03:30 PM
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On the topic of tapered diamond cranks, the BB shaft is a very hard metal.

Cranks on the other hand, are casted with a much softer metal, which is why we often see them round out on a loose or missing bolt or nut.

I always blue loctite these. Prevents any wobbling out, and also doubles as an anti-seize.
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Old 04-24-23, 09:32 PM
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I would want all the work done on the bike re-checked.
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Old 04-24-23, 10:08 PM
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this does not sound good. if it was a square taper BB with alloy crank arms, there was some damage to the square broach.

sorry to say

/markp
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Old 04-25-23, 08:25 AM
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But he only went like 3 pedal strokes. How much damage could've been done? At worst the mechanic installed the bolt finger tight & got pulled away by a customer & never revisited the job where he left off. It's not like the OP let it waggle back and forth for 3 months of 40 mile daily commutes, in the snow, uphill both ways.

Analog things tend to be non-binary. Severity of a mechanical defect situation tend to scale starting from zero increasing in severity with force, use & time. 3 pedal strokes hardly qualifies as abuse & the OP did the correct thing.

I foresee many happy miles.
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Old 04-26-23, 05:54 AM
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Agreed.

Just torque that nut back on and ride.

Wheter the OP can DIY this themselves is the surprise.

Then again, that's what a LBS is for.
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Old 04-26-23, 07:16 AM
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Originally Posted by base2
But he only went like 3 pedal strokes. How much damage could've been done? At worst the mechanic installed the bolt finger tight & got pulled away by a customer & never revisited the job where he left off. It's not like the OP let it waggle back and forth for 3 months of 40 mile daily commutes, in the snow, uphill both ways.

Analog things tend to be non-binary. Severity of a mechanical defect situation tend to scale starting from zero increasing in severity with force, use & time. 3 pedal strokes hardly qualifies as abuse & the OP did the correct thing.

I foresee many happy miles.
Given the way cranks and spindles connect, I don't see how anyone could assume that no damage was done. The crank would have to move laterally several centimeters to be in a position to rotate on the spindle without being harmed. That can't happen with a bolt that the is even finger tight.
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Old 04-26-23, 08:34 AM
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Originally Posted by mpetry912
if it was a square taper BB with alloy crank arms, there was some damage to the square broach.
It will probably be a bit chewed, but that's nothing to worry about if it stays put when correctly tightened.
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Old 04-26-23, 08:50 AM
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I disagree ! if it's a "bit chewed" from at least what I imagine happened based on the OP's description then the arm is compromised.

the square broach is rounded out

On re-installation, there will be the same loads distributed on a smaller contact area and will likely fail again. Or not run true. Or both !

the shop owes the OP a new crank arm in my view

/markp
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Old 04-26-23, 09:00 AM
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Originally Posted by Kontact
I would want all the work done on the bike re-checked.
I used to test ride new builds and repairs, I'd be slightly concerned about any mechanic who wasn't prepared to do that, although obviously there are situations where it's not feasible. I was lucky that there was a steep hill less than 200 yards from my workshop so I'd test the gears going up and the brakes coming back down. Lesser machines I'd ride 500 yards around the block, even BMX and kids' bikes would be ridden around the yard. This is a better final check than anything you can do with the bike in a work stand.
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Old 04-26-23, 09:09 AM
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Originally Posted by grumpus
This is a better final check than anything you can do with the bike in a work stand.
Same as aviation maintenance. A test flight is worth 100 ground tests. Even better have the mechanic go on the test flight with you.

I once had a partial engine failure on the first flight after "engine repair" (top end). fortunately I had not strayed far from the airport and had just enough power to make it back to an acceptable landing.

I expanded my vocabulary of profanity after that. Bikes are not so different.

Something like a quill bolt on the bike's stem or a front quick release can spill you and result in a possible injury.

So a real test ride is mandatory in my opinion

In this case the shop owes the OP a new crank arm. full stop.

/markp
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Old 04-26-23, 09:30 AM
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Originally Posted by mpetry912
I disagree ! if it's a "bit chewed" from at least what I imagine happened based on the OP's description then the arm is compromised.t
The middle of the flat face is where slight damage usually occurs in this scenario, and that part carries little to no load when the crank is correctly tightened (and in this situation with an extra "that ain't goin' nowhere"). When a crank is run slightly loose the edges of the axle taper erode a divot just shy of the broached corner, that's the damage you need to worry about because it can cause the crank to loosen with use no matter how hard you tighten it.
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Old 04-26-23, 09:42 AM
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Originally Posted by JochenRindt
I made it about 3 feet before i realized what happened and that the position of the cranks was no longer opposed. Do you think that was enough to damage the splines in the crank arm?
The OP made no mention of what type of crank they have but this quote sort of indicates that it isn't a square taper
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Old 04-26-23, 09:55 AM
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Originally Posted by mpetry912
this does not sound good. if it was a square taper BB with alloy crank arms, there was some damage to the square broach.

sorry to say

/markp
My guess is that this was not square taper. However, with Shimano's Octalink interface, if the splines are lined up with each other (stacked), rather than properly seated with the splines in opposing grooves, you could actually tighten the crankarm bolt. On the road BBs I'm familiar with from that era, the overlap isn't very deep, and the crankarm not being seated isn't as noticeable as it is with other interface types.
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Old 04-26-23, 10:07 AM
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Originally Posted by Eric F
My guess is that this was not square taper. However, with Shimano's Octalink interface, if the splines are lined up with each other (stacked), rather than properly seated with the splines in opposing grooves, you could actually tighten the crankarm bolt. On the road BBs I'm familiar with from that era, the overlap isn't very deep, and the crankarm not being seated isn't as noticeable as it is with other interface types.
good point. As usual we are working with no pictures, minimal detail and lots of speculation.

if it was a cottered crank, it would not matter at all

/markp
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Old 04-26-23, 10:23 AM
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Originally Posted by mpetry912
good point. As usual we are working with no pictures, minimal detail and lots of speculation.

if it was a cottered crank, it would not matter at all

/markp
Accurate.

This is one of those "Yeah...I did that" issues - LOL. Thankfully, it was on my own bike, and I caught it before I actually tried to ride. With the Octalink BB, you need to take a moment to feel that splines are properly interlocked before tightening the crankarm bolt. The way the BB spindle interfaces in the recessed crankarm, you can't actually see the splines come together.
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