anotherhillyay

TL/DR: non-drive side crank arm fell off midway up an incline 89 miles after LBS performed annual tune-up.

Hello. I'm new to the forums and looking for any assistance with my current circumstance. My LBS performed an annual tune-up on my 2021 BMC Team Machine ALR01 on June 3rd. This past Sunday, July 10th, I was on mile 20 riding up a mild incline when my left foot "shifted" out and away from my bike. It was a similar feel to clicking out of the pedal, but far more awkward. Once I came to a stop, I looked back and found the crank arm (pedal still attached) and a large stubby screw laying a couple feet back.

After calling for a ride, I called my LBS. I informed them of what had happened and told them I would bring it in later that day so they could take a look at it. When I got home I covered the left crank opening with saran wrap and a rubber band, wrapped the screw in parchment paper, tossed it in a zip-lock bag with the crank arm, loaded the bike on my rack, and drove to the LBS. They said they would look into it, they were ordering a part (which I did not get the name of, unfortunately) and told me they would give me a call Tuesday.

Today is Tuesday, I got a call about an hour ago. The service mechanic asked me one question, "Did I experience anything leading up to the crank arm falling off, creaking, flexing, anything like that?" I replied that I hadn't noticed anything, which I had not. The service mechanic informed me that their determination, based on the information I gave them, was that it was NOT a service issue, and that I had a couple of options. 1: Try to warranty the crank set with Shimano; 2: Buy a new crank set. Essentially they are absolving themselves of any responsibility, aside from the assistance in filing the warranty claim. I asked the mechanic if he had any idea of what the warranty issue might be, and he replied that he was stumped. He also mentioned that he had never seen a non-drive side crank arm fall off, but had seen it happen on the drive side.

Since the tune up performed on June 3rd, aside from one wash and re-wax after a ride in light rain, I haven't performed any maintenance on the bike what so ever. She's kept inside my bedroom, on a rack, and has never been dropped or mishandled in any way. It was only the 5th ride since the tune-up. I'm calling ******** on the LBS, but I am open to being proven wrong.

Thank you for taking the time to read this and I would greatly appreciate any sage advice.

- anotherhillyay


For the curious, the pics are in my album. I couldn't link due to this being my first post.

cxwrench

If they took the crank off the bike to clean it as part of the tune up then it's pretty damn obvious that they didn't install it properly. The most likely thing is that the 2 pinch bolts weren't tightened to the proper torque value. I've seen it numerous times and been guilty of doing it myself. The 2 pinch bolts and the 'crank arm fixing bolt' back each other up. The black plastic (or alloy in some cases) fixing bolt draws the NDS arm onto the crank axle and sets bearing preload. The pinch bolts tighten that arm onto the axle and also tighten the axle onto the fixing bolt. The pin on the 'plate' engages a hole in the axle and provides back up to the other fasteners. If the pinch bolts aren't tight and the NDS arm starts rocking on the axle the fixing bolt will loosen and work it's way out of the end of the axle. At that point the plate isn't strong enough to hold the arm on the axle and there ya go. Blame for this almost surely lies with the mechanic that did the work, which obviously wasn't checked by anyone.

Litespud

good luck getting the wrench to admit responsibility. This is why it’s worth learning to do your own bike maintenance if you can - bikes (at least for now) are pretty simple machines and there are innumerable resources out there to help

Outrider1

OP, what level of service or 'tune up' did you have performed. At my LBS the service packages range from a basic look over, featuring tightening fasteners, etc., to a thorough going over with new cables, all bearings greased, etc. Check to see if lubing the bb/crank arm was part of the service that you elected. It's most likely on their website. If it was part of the package, cxwrench gave you all the ammunition above. Talk the manager.

anotherhillyay

Thank you all for taking the time to reply. The crank was disassembled as part of the tune up package, my mistake for not mentioning that in the post. Ironically, I paid extra for it. It's good to hear that it isn't exactly uncommon for this to happen. One other thing I forgot to mention - there is a portion on the splines inside the crank have been worn smooth. As you mentioned, without those bolts being properly torqued, wouldn't it have allowed the mechanism to more or less spin incorrectly, causing those smoothed patterns? That's something that would have been noticed during the tune up I am guessing as well. They signed off on all parts being in good shape so it would have had to happen after the tune up.

HillRider

I agree the mechanic didn't install the crank properly. Either the fixing bolt (the "large stubby screw" you found) wasn't tightened properly and/or the pinch bolts weren't torqued properly. This is the shop's error and they should make good on it. Don't take no for an answer.

BTW, on any recent (HT II) Shimano crank, the nds arm is the only one than can fall off as the spindle is fastened permanently to the ds crank arm.

Outrider1

Makes total sense. I would definitely speak with the manager or owner. Someone else may be attempting to cover up the potential mistake.

anotherhillyay

Thank you. This is really good to know.

zandoval 

Bad news... Is there a way for you to easily notice a poorly installed crank arm on a newer type bike. Is it normal to check and double check work on your bicycle even after paying someone else to do the job.

I don't have a local bike shop so I do my own maintenance and repairs. I have made mistakes and suffered the consequences. Of late my "Pre-Flight Check List" is getting unreasonably long...

rm -rf

The mechanic: "I've never seen a non drive side crank fall off". Ha, that's hilarious! Of course he has. I've seen it happen on group rides a few times.

They didn't re-install the crank correctly.

It's the type with the two side bolts near the axle? These are reliable if they are installed correctly. I like the design.

The crank is installed with the two bolts loose. The center "stubby screw" is tightened just enough to take the play out of the bearings. It's a "bearing preload" adjustment, not a structural "keep the crank arm attached" piece.
Then two bolts are tightened to the correct torque, and the arm is secure and can't fall off.

I'm guessing they forgot to tighten the bolts, and the preload piece kept it together temporarily. It's so stupid to lie about this. Just own the mistake.

~~~
The loose crank could mangle the splines as you pedal. It's likely still rideable if the spline damage isn't too extensive. Perhaps you can keep the crankset, but get some other discount in compensation. The preload threads could be stripped too.

I pull the crank off the bike every few years, to inspect the bearings by rotating them with a finger, feeling for roughness. I wipe off the splines with paper towels, then regrease them. So any spline damage would be obvious--but they only get damaged if the bolts are loose. Then the reinstall is easy, once I learned the steps to do it.
~~~
Park Tool repair guides
These are step-by-step, carefully explained. I've learned how to do lots of maintenance procedures here!

Remove and install a crank.
go to the "two piece compression slotted" section -- that's Shimano and other similar cranks.
Hmmm, surprisingly, this repair guide kind of skipped over the bearing preload details -- I tighten just enough so there's no play when I wiggle the crankarm. Too tight, and it won't spin very far with the chain off -- bearings are too tight.

79pmooney

You might contact Shimano as the shop suggested, tell them what happened, what the shop said and the work they did plus the date. Yes, the shop is who needs to make up to you. But having Shimano know what they did might just make their seat a little warmer.

anotherhillyay

I'm pretty new to the scene so I am learning as I go. I suppose it's not a bad idea to double check the work done. Having said that, from this point forward I'll be doing as much maintenance as I can on my own and double checking that.

anotherhillyay

Yes, exactly, it has the two side bolts. I still can't post images yet or I would, they show the splines. Thank you for your suggestions and for the link. I'll get these bookmarked for the next go around.

cyclezen

Sadly, you shouldn't need to 'follow-up' on every repair/checkup you have an LBS do, but always better to find an issue than experience it.
If the LBS is worth any of the 'salt on their bread, they should make good on their obvious error. I would remain insistent and not take any answer other than for them to make good.
I've personally seen a couple arms come off - in both cases the splines had some light damage, but both were still good enough to still properly set the Non-Drive arm at required specs, and have worked fine for what is prolly 15 mos. of hard, regular use.
what condition are the splines on the arm and axle? - post your 10 replies somehow and then put up some good pics.
Ride On
Yuri

Polaris OBark

This could also happen if they tightened the pinch bolts first, before tightening the pre-load plastic bolt. I'm not sure there is a reasonable way to double-check it, except to take it apart and put it back together again, tightening in the right order (pre-load first, then alternate between the two pinch bolts, after making sure that the retainer plate is in there correctly).

Koyote

This happened on one of my bikes shortly after it had been rebuilt at the shop. The mechanic had been using a torque wrench which he later found to be faulty, so the bolts weren't torqued properly. The shop took responsibility and replaced it very quickly and at no charge.

Rolla

Curious what exactly the tune-up procedure was that you paid extra for.

cxwrench

I find it absolutely ridiculous that the mechanic is denying any wrongdoing on his part. I'm the service manager at the shop I work at and would not stand for this. We double check everything and those 2 bolts are definitely something that gets checked on every bike that has Shimano HTll. The only way this happens is the bolts not being tightened.

Rogerogeroge

Sorry, getting Shimano in the loop isn't going to do a damn thing for the OP, nor will it make the shop's seat any warmer. But I agree this is 100% the shop's fault.

pdlamb

Out of curiosity, does the NDS arm need to be replaced after this maintenance flop, like an older square taper arm would need replacement?

Moisture

Even if those pinch bolts/ preload bolts are tightened to the correct spec, some designs will still loosen unless you use a bit of thread locker.

ClydeClydeson

Either he's completely BSing you, or this is a dangerously inexperienced tech. NDS cranks arms fall off ALL THE TIME. It used to be a very common failure in the days of square taper bottom brackets, and is less common now, but low cost bicycles are still made with square taper BBs and are still assembled by low-paid workers in a factory trying to get as many bikes finished as possible, and higher-cost bicycles generally have L crankarms that require several steps to install, and therefore have more possibility for human error, so it still happens (as you found). Perhaps this was the guy's first week as a bike tech?

Anecdote:

As a tech in a shop in the mid 90s (where probably 10% of all repairs I did involved a NDS crankarm that had fallen off) I was given a bike to do a 'minor tune up' - brake and derailleur adjustments, some cleaning, oil, air, minor adjustments, and a 'once over' of the rest of the bike. The bike was the highest-cost CCM BSO from the box store Canadian Tire. It had proudly marked on the frame 'ALUMINUM CONSTRUCTION' - which was the only 'feature' this bike had in common with any higher cost bike. It made no mention of what grade of aluminium it was, but I can only assume one with a high Cheeze Whiz content, as about two days after the bike was picked up post minor-tune up, the threads pulled out of the bottom bracket shell of the frame. The customer came in assuming we had screwed something up, but we stuck to our guns that the BB was not removed or otherwise touched as that was not part of the minor tune-up package.
If I had removed and reinstalled the BB, even if it had gone in smoothly and we believed my work was 100% correct, and the customer came back two days later with that problem, we would have had to admit it could have been our fault.
In OP's case, since the crank had definitely been removed and reinstalled, it is 100% the responsibility of the shop to fix it. Weird that they would even argue, because a replacement L side crankarm is not that expensive.

ClydeClydeson

100% yes.

The fit between the crank and spindle need to be very close, and riding with the arm not properly tightened results in movement that will wear the inside of the soft aluminum splines against the hard steel splines of the spindle. Even if the wear is not visible to the naked eye, you have to assume damage and that replacement is necessary. The arm moved enough to ratchet the fixing bolt off, and this is definitely enough movement to cause damage to the arm's splines.

Polaris OBark

Reading your last two posts, are you saying the whole crankset should be replaced, or saying only the left-side crank-arm needs to be replaced, because the steel spindle splines are likely still intact?

cxwrench

Yes.
No, grease should be fine.
Normally just the NDS arm.