Pedal fell off AGAIN!

[si_c]

The GRX spindle is AIUI aimed at a 73mm BB so if you have a 68mm BB then as @fossyant says you need spacers to fit correctly. This will be either a 2.5mm spacer on each side or a 5mm spacer on the drive side. This will impact the chainline so you'll also need to adjust the front derailleur to compensate.

There are, as noted above, two separate means by which the arm is retained on the spindle, the pinch bolts and the safety tab. In addition you can leave the preload cap in place which should also prevent the arm falling off. For all three to fail indicates a serious quality control issue by the manufacturer.

Can you measure the outside-outside width of the BB cups to check what width it actually is?

 

[Willam]

Is there a spacer on the driveside crank (not behind the driveside BB, but on the crank itself)?

No spacers on crank.

 

[Willam]

I think you need to know the exact model number of the crankset, and find the tech docs for it, and the bb. The model number is usually stamped on the back of the cranks, it should be FC-something. It definitely looks like you have 5mm too many spacers fitted somewhere though.

Checked it, recommended the bb I have, the RS 500

 

[Willam]

The GRX spindle is AIUI aimed at a 73mm BB so if you have a 68mm BB then as @fossyant says you need spacers to fit correctly. This will be either a 2.5mm spacer on each side or a 5mm spacer on the drive side. This will impact the chainline so you'll also need to adjust the front derailleur to compensate.

There are, as noted above, two separate means by which the arm is retained on the spindle, the pinch bolts and the safety tab. In addition you can leave the preload cap in place which should also prevent the arm falling off. For all three to fail indicates a serious quality control issue by the manufacturer.

Can you measure the outside-outside width of the BB cups to check what width it actually is?

Ok literally just tried that, it didn’t work, was worse than previously, the left crank would not stay on without me holding it as the spacers reduced the spindles available, so much even less of the axle was hanging out of the non drive side to be able to attach the arm.

 

[Willam]

Unless some else has any ideas I’m going with the pic in my op look correct and I just didn’t tighten the crank arm bolts enough…all the videos I’ve watch say to lightly “pinch” the bolds, so I guess I took them at their word when looking it up it’s around 13-15nm.

 

[Ming the Merciless]

I have a XTR ht2 crankset designed for mtn bike 73mm bb fitted to my 68mm bb on my recumbent. You have two spacers on the drive side and one spacer on the non drive side. Make sure the crank is pushed fully through the BB, you sometime need to give it a good thump with the palm of your hand. That should ensure enough threads are free to engage. Also ensure the pre tension is fully wound in when first mounting LH crank.

 

[figbat]

The thump is a good call - mine takes a couple of decent thumps to seat fully into the BB. You also need to use the preload cap with some force - I believe there is a nominal torque value but I have no way of applying a torque wrench to the tool; my understanding is that the diameter of the tightening tool is designed so that when tightened as much as possible by hand, it achieves about the right torque.

Then close the locking tab (you did make sure it was open before fitting!) and tighten the two pinch bolts, doing each in turn over several goes until tight; so nip one up, then the other, then back to the first, then the second and repeat by tightening each one more each time until you achieve the recommended torque.

 

[Willam]

I have a XTR ht2 crankset designed for mtn bike 73mm bb fitted to my 68mm bb on my recumbent. You have two spacers on the drive side and one spacer on the non drive side. Make sure the crank is pushed fully through the BB, you sometime need to give it a good thump with the palm of your hand. That should ensure enough threads are free to engage. Also ensure the pre tension is fully wound in when first mounting LH crank.

If you look at my pic in the op you will see adding spacers will make matters worse, even though I thought this the case, I did try it when I posted above, I could not literally get the arm on with 2x2.5mm spacers fitted.

Sorry if I’m coming across as ungrateful here, I appreciate all the replies.
This bike is my commuter so a little stress may have come across in some of my replies.

I’ve not mentioned I’m also waiting on a new gear shifter so contending with cycling in top gear for the last few days, hopefully it’ll get here in the next couple of day…get really for help with that too lol

 

[Willam]

The thump is a good call - mine takes a couple of decent thumps to seat fully into the BB. You also need to use the preload cap with some force - I believe there is a nominal torque value but I have no way of applying a torque wrench to the tool; my understanding is that the diameter of the tightening tool is designed so that when tightened as much as possible by hand, it achieves about the right torque.

Then close the locking tab (you did make sure it was open before fitting!) and tighten the two pinch bolts, doing each in turn over several goes until tight; so nip one up, then the other, then back to the first, then the second and repeat by tightening each one more each time until you achieve the recommended torque.

Yeah thanks, did all that but have to say the preload screw may help a little but it’s not there for that reason, it is there to preload the bearing in the bb, the safety pin is only there to help check the crank is in the right place, a 1g piece of plastic is holding nothing on.

 

[Gunk]

That’s too far offset so the plastic safety tab will not seat properly, sometimes the spindle is not pushed right though the BB and you have to give it a light tap with a rubber mallet.

I’ve just refitted one (Shimano XT) and used two washers both sides, it seats perfectly but I did need to tap the drive side home by about 2mm

 

[Ming the Merciless]

Does this crank arm fitting look correct? Just fitted a new crank and BB but after one day the arm fell off, now I’m thinking maybe the BB is too big?
The BB is to spec for the crankset HT RS500, the BB size on bike is 68mm, I fitted the road BB with no spacers, have I got the wrong BB for the bike or some other issue? Would I need to use the mtb BB without spacers?

View attachment 625049

Something that has been bugging me is how deep the threads are and how shallow the splines are. I’m sure you should not have that many threads as they are just for the top cap.

See this image and note the inside of LH crank seems to have much deeper spines. The silver threaded bit in your image just looks wrong.

 

[Ming the Merciless]

Or zoom in on this grx one. Looks very different to your photo. What the hell is the silver threaded bit in your photo? I don’t think it should be there.

 

[faster]

Looks fine to me. I suspect you've just not done up the pinch bolts tight enough.

Someone above had said to preload as much as you can by hand with the tool. Ignore this advice and preload as little as you can get away with without there being any play. As long as it's been 'thumped' through from the other side (as others have mentioned), once you can sense any tension on the adjuster at all, just turn a tiny bit more, as little as you can manage, then do up the pinch bolts evenly (don't do one up tight then the other - tighten them both a bit at a time). If there is no play, there is enough preload.

Bear in mind that running it loose may already have caused damage.

 

[faster]

Or zoom in on this grx one. Looks very different to your photo. What the hell is the silver threaded bit in your photo? I don’t think it should be there.

View attachment 625161

The spindle.

 

[Willam]

That’s too far offset so the plastic safety tab will not seat properly, sometimes the spindle is not pushed right though the BB and you have to give it a light tap with a rubber mallet.

I’ve just refitted one (Shimano XT) and used two washers both sides, it seats perfectly but I did need to tap the drive side home by about 2mm

Could you still see some of the spindles when the arm was in place?

I’ve double checked I can not hit it home any more than it is, the picture may make it look worse than it is?

Got me to work without falling off so heres hoping.