Loose crank - fix ideas?

[OkiePhlyer]

I noticed after a long ride last weekend that the left crank was a little loose. I tightened it back up and rode again a few days later. Same thing after that ride. Loose. I took it off and tried blue loctite this time. I let it sit for a few days before riding the bike again.

Today while riding I noticed it was loose again, and when I looked down the crank was nearly off! When I stopped to inspect the crank it actually did come off then. I noticed some fine metal shavings on the crank bolt so I'm guessing the crank bolt threads have worn some or are damaged.

Am I down to replacing the spline and bolt? Would Teflon tape wrapped around the bolt help it stay in place? A friend suggested tapping the bolt threads with a hammer to flatten them and make it harder to turn, but I don't want to ruin parts that might still be good.

SRAM told me it was still under warranty as long as I had the receipt to show the bike shop. I don't think I have it so that option it probably out.

Crankset is a Truvativ Elita.

Any other ideas? Thanks!

 

[dabac]

Pretty much all cranks are quite sensitive to being installed correctly, with the correct torque from the beginning.

Once cranks start to wiggle they very quickly damage the important fit between crankarm and BB spindle beyond repair.

With square taper people have had some luck with either various fillers, or by carefully filing the surfaces back to flat before reassembly.

Splined fit, I wouldn't try it unless the situation was fairly desperate.

Replacing the bolt is unlikely to do any good.
It's the crank arm rocking that works the bolt loose, not the bolt letting the crank arm move.

Can't figure out what you mean by "replacing the spline".
The splines are integral to BB spindle and crank arm, and can't be replaced separately.

The BB spindle, being steel, is very rarely damaged by the much softer aluminium crank arm.

If it was me, I'd try to get a new left crank.
Doesn't have to be the exact same model as long as fit & length is right.

Install using torque wrench and hope for better luck this time.

 

[CAMPYBOB]

Would Teflon tape wrapped around the bolt help it stay in place?

Please. Do not even attempt that. It's dangerous.

I posted some info about using a Helicoil if the female threads are stripped out. The bolt is usually harder than the spindle, but if it is the component with damaged threads it is cheap to replace. The Helicoil repair may not leave a structurally sound spindle depending on how much wall thickness there is to work with and the repair would likely cost more than a replacement bottom bracket would.

If your splines are worn it's likely the crankset is or at least the arm (and possible the spindle) will need replaced.

Do not attempt any amateur repairs in this area. If you lose a crankarm while pedaling it can cause loss of control of the bike and that, sir, can result in serious injury or death. I had something similar happen when I snapped a crank arm into two pieces. NOT fun flying across a lane of oncoming traffic!

 

[dhk2]

I'm not familiar with the Truvative crank, but on tapered and ISIS cranks a proper interference fit is a must. On my older ISIS crankset, the "push-on" dimension is spec'd. That's the distance "up" the tapered spindle that the arm should fit with hand pressure. Then while the bolt is being torqued, the crankarm is pressed onto the spindle as the torque comes up smoothly to the full spec value. If the push-on dimension is exceeded, you stop and get new parts.

If the crankarm is too loose (failed), it will push on past the spec limit, then bottom out long before the bolt hits spec torque. Worst case, it will slide onto the spindle fully, and you can screw the bolt on all the way by hand. In that event, you have no interference fit, and your only option is to replace the crankarm (as a minimum). Trying to glue the bolt or crank in place won't fix the problem; new parts are needed.

 

[OkiePhlyer]

Here is a fairly good illustration of what my setup looks like. I confirmed with SRAM that the spline is steel so I wouldn't think the aluminum crank bolt would damage that. I think the crank itself is ok. I didn't see any wear on it, plus when the bolt is tight there is no slipping or popping. After cleaning the grease and gunk from the crank bolt I found the threads on it were worn like I suspected. I also found some fine thread pieces that were left inside the spline itself so obviously the crank bolt was loosing material. I got a part number from SRAM so I can order a new crank bolt. They also gave me the torque specs so it will go on properly when it arrives. The tech also thought blue loctite would be a good option when it is reassembled. With a good bolt shouldn't a proper torque keep it from backing out or is loctite just extra insurance?

Hopefully this will get me up and running again. I appreciate all the cautions and comments. [HASHTAG]#NeverStopLearning[/HASHTAG]

 

[dhk2]

Appears to be an ISIS spindle...10 splines. That means it's likely more than 10 years old? The bolt may be aluminum rather than steel; if so, a replacement steel bolt is the way to go. But my guess is the crankarm is done.

You'll still need to have a proper interference fit. Do the hand-press check I mentioned above, with everything clean and a thin coat of grease on the splines. How far onto the splines does the crankarm go by hand? The joint works by stretching the crankarm hoop as it's pressed onto the tapered splines. If the old arm stretched (yielded), that means you'll never get a tight interference fit.

When everything is correct, the crankarm will be hard to pull off after the bolt is removed. You'll need an ISIS threaded crank removal tool to get it off, as it takes some force to remove. My guess is you don't have the tool, and the arm is just pulling off the spindle with light hand force?

If so, time for a new crankarm. It's only aluminum, and once it's yielded, due to either improper install, too many on/off cycles, or just a decade of time, corrosion, or just heavy loads, it's shot.

 

[dhk2]

PS: The bolt torque is important too of course. With a new crankarm, the bolt holds everything in place and keeps the arm from backing down the tapered splines. The torque spec is 25-30 ft-lbs IIRC. (You should look this up before you tighten the bolt). It will require a 3/8 or 1/2 inch torque wrench along with the proper hex socket (M12?) If you don't have or want to buy the right tools, just take the bike to a good LBS mechanic. This stuff isn't hard, but has to be done right.

 

[OkiePhlyer]

Appears to be an ISIS spindle...10 splines. That means it's likely more than 10 years old? The bolt may be aluminum rather than steel; if so, a replacement steel bolt is the way to go. But my guess is the crankarm is done.

It is a new crankset. At least to the bike. I'm not sure how long it sat on the shelf. There are no more than 400 miles on it. I'm guessing the crank bolt wasn't torqued properly on initial installation so it failed. I read through SRAM's GXP crankset service manual and found the torque specs (38ft-lbs). I'll get it back together when the bolt arrives on Monday and see how it works. Hopefully that will have me up and riding again.

 

[dhk2]

Okie, I hope it works too. Let us know. Sorry to say that I'm not too confident, as it seems you're ignoring the details and questions I gave you above. Without the proper interference fit, it's not going to be safe.....just torqueing the bolt to 38 ft-lbs doesn't do the job. If the crankarm presses all the way to the shoulder stop (or very close) with just hand pressure, it's junk and shouldn't be used.

I'm just repeating the warning because I'd hate to see you bust your ass on a sprint or climbing a steep hill. As CB and dabac's posts above tell you, this is one of those places on the bike that needs to be done right for safety sake.

 

[OkiePhlyer]

dhk2...Sorry if you thought I was ignoring your warnings. I really do appreciate all your advice. In reading your posts and others it has caused me to do more research and learn a little more about the crankset I have.
The expanded photo I posted probably wasn't the best visual aid. I just couldn't find and expanded photo of mine, and wasn't close by to take my own photos. My set being the Elita GXP, if I understand correctly, looks similar but is not the same as an ISIS spindle. The left side crank has a threaded retaining ring so a crank puller isn't necessary to remove the crank. From what I have seen and read so far with this particular setup the crank is much easier to install on the spindle than on an ISIS spindle. I've also watched a few GXP install videos and the new cranks seem to slide easily on to the spindle like mine does. Basically turn the bolt by hand with your 8mm hex socket until the crank is seated and then torque it to spec.

 

[OkiePhlyer]

UPDATE:
I installed the new crank bolt and torqued it to specs from SRAM. I've gone on several rides since and have had no issues. One ride in particular was hill training so I as putting a lot of stress on the cranks. I'm fairly certain the original bolt wasn't installed correctly to begin with and this started the process of it backing out. I am planning a long ride (75+ miles) this weekend so we'll see how it does during that ride.

Thanks again for all the comments and suggestions.

 

[OkiePhlyer]

LAST UPDATE:
Including the 77 mile ride a couple weeks ago I've ridden 200+ miles since installing the new crank bolt, and the crank is still nice and tight. In the past I've kind of ignored torque recommendations, but this issue has me thinking differently now. Never stop learning...

 

[dhk2]

Good to hear all is well so far. You might want to periodically check the torque on the crank bolt just to make sure it's holding. Don't loosen it first, but just put the torque wrench on it. Apply torque to near the spec limit, say maybe up to 35 ft-lbs, and if the bolt doesn't turn, all is good. Wouldn't hurt to check this now, then at 500 miles, then every 1000.

Only takes a minute to check, and if it's holding torque you won't have to worry about it coming loose again.

 

[OkiePhlyer]

Good to hear all is well so far. You might want to periodically check the torque on the crank bolt just to make sure it's holding. Don't loosen it first, but just put the torque wrench on it. Apply torque to near the spec limit, say maybe up to 35 ft-lbs, and if the bolt doesn't turn, all is good. Wouldn't hurt to check this now, then at 500 miles, then every 1000.

Only takes a minute to check, and if it's holding torque you won't have to worry about it coming loose again.

Good idea. Not only in this particular case, but as a periodic check on any bike. Having the crank come undone isn't fun.