Sheared Aluminum Crank Bolt - GXP crankset

FWIW. Some things [components, tools (!?!), etc.] are apparently designed to be sacrificial, but I doubt that the crank bolt is one of them ...

Looking at the pic, I doubt that the bolt is aluminum ... aluminum chainring bolts are not uncommon, BTW ...

However, the bolt could be Titanium -- Titanium has been known to shear -- you decide.

The bolt head must likely developed a hair line fracture because it was over torqued when you installed the crank. The screw head finally sheared off while you were out riding. Check the provided spec sheet that was supplied with the crank when you purchased it for the correct torque value when you install your new bolt.

Link to Shimano Tech. Not TruVativ but very similar. bike.shimano.com11)%20Crankset%20Installation.pdfOpen ↗

Quoted post said:

Originally Posted by davereo [IMG]/img/forum/go_quote.gif[/IMG]

The bolt head must likely developed a hair line fracture because it was over torqued when you installed the crank. The screw head finally sheared off while you were out riding. Check the provided spec sheet that was supplied with the crank when you purchased it for the correct torque value when you install your new bolt.

Over tightening is common cause, perhaps the most common cause, of bolt failure on bike components. Also double check to see whether the instructions suggested lubricating the bolt.
With that said, I'd look around to see if there's a steel version of that bolt.

I am pretty sure the bolt is aluminum, the parts list says it is an "alloy" bolt. Other sites indicate that it is aluminum. The metal is much softer than the steel spindle it threads into, it definitely not steel or titanium. Whatever the material, it seems to be a poor choice for the application. The part number is:

11.6900.002.140

I have not identified a drop in replacement yet.

The failure may be due in part to my installation. On the first round of installation I followed the installation instructions to a tee, but it backed out soon after during a ride. Each time I corrected the issue the bolt would back out after a few weeks of riding. Some internet searching suggested that this may be a common issue. So I used some locktite and extra torque; perhaps it was overtightened or damaged due to the earlier mishaps.

The head actually popped of, not really sheared since the crank will not rotate due to the splines.

The pedal design does appear allow stress to be transmitted the bolt head when forces parallel to the axis of the spindle are applied. Such would be the case when riding out of the saddle rocking the bike back and forth. If the material is aluminum, then cyclic stresses could fatigue it to the point of failure.

Anybody else have experience with these? Maybe I should be carrying an extra in each of the bikes I have these installed on.

My wife has a Truv Elita triple on one of her bikes. Never any problems - no breaking bolts or backing out. Most likely your issue was too much torque. Alu bolt ... not going to find Ti or other exotic hardware on a $100 crankset. :wink:

$40 on Ebay with free shipping - the replacement bolt is going to cost me over 1/4 of that price.

How many miles does she have on that crankset? The bolt died a slow death, I estimate that I have 4k on mine.

Just a swag, but probably she has 2-3k miles over a couple of seasons on that crank. Still in like new condition. Have removed the crank a couple of times and retorqued to spec. First removal was to switch her to a light compact double, but that didn't last very long. ... reinstalled the Elita. Whatever makes her happy. 🙂

I received the new bolt in the mail today. The design is different from the old bolt - the transition from the threaded shaft to the head is much meatier than the old bolt. The head is thicker and more round as well - perhaps this is a newer design to address weaknesses.

I applied locktite liberally and snugged it up to 40 ft-lbs of torque and all is well.

Excellent. Bike's really curl their toes when you use a torque wrench on 'em.

Update,

I pulled my ol road bike out for its inaugural 2014 ride today. Two thirds though the ride, the replacement bolt sheared. I appears to have sheared in the same way as the previous victim. Luckily my family was kind enough to pick me up as it would have been a long cold walk home in 25 degree weather.

I have ordered replacement titanium bolts.

I have some video to share which I will post later.

The video is being processed now - the result of the broken crank bolt can be seen at the end of the video:

Quoted post said:

Originally Posted by maydog
Update,

I pulled my ol road bike out for its inaugural 2014 ride today. Two thirds though the ride, the replacement bolt sheared. I appears to have sheared in the same way as the previous victim. Luckily my family was kind enough to pick me up as it would have been a long cold walk home in 25 degree weather.

I have ordered replacement titanium bolts.

I have some video to share which I will post later.

First, I'd say 40 ft-lbs of torque is a bunch for alloy bolts. My FSA crankset (ISIS spindle) came with alloy M8 bolts, and I believe the spec was 35 ft-lbs max. . My guess is the bolts failed in storage due to the torque stress, likely aided by corrosion. If the bike was stored in a damp, unheated garage, that would certainly speed the corrosion process. I replaced my OEM alloy bolts after about two years with steel ones. After removing the crankarms a few times and retorquing, didn't want to rely on the aluminum ones.

Judging by the past problems you've had, I'm thinking something isn't right with the fit of your splines. On the ISIS spindle, with proper fit the crankarm should press on by hand a specified amount, then basically bottom out as the torque spec value is being reached. I've never had the bolts loosen, and torque them with ti assembly lube or plain grease on the threads, not Locktite. If your crankarm is bottoming easily, say when pressed on by hand, or with the first 10 ft-lbs of torque, that may well be the cause of the problem.

The arm was installed per spec. The locktite was added to prevent loosening. 40 ft lbs is the max spec. The bolt survived more than two years, last post September 2011, and the break is clean. It doesn't appear to be corroded. Its a typical cyclic fatigue failure.

I have 3 other GXP cranksets and loosening without locktite has been an issue with each. A quick googling reveals that this may be a common problem.

I suspect that the design is marginal and aluminum is not up to the task. I will snug up the titanium part to a little less torque and use a good helping of locktite.

Our snow is 98% gone now. You still have LOTS left on the ground!

Your roads are as bad as ours. This winter killed them. Every bump you hit made my teeth hurt.

At this point I think I would be thinking about buying a different crankset design.

The roads are not that bad. The mount I was using is too large for the diameter of the bars and was loose, that emphasized each little bump.

I wouldn't buy another GXP crankset but I would like to get the proper mileage out of the ones I have. The new bolts are pretty cheap in comparison and will give them a try.

Quoted post said:

Originally Posted by maydog
The video is being processed now - the result of the broken crank bolt can be seen at the end of the video:

OMG!!! Maydog, that loooks so much fuuun!!! So it's March there too huuuuh??? [IMG]files.cyclingforums.combig smile.pngOpen ↗[/IMG]

I also went for a ride... Wanna seee??? No snow at aaall, daaang it... [IMG]files.cyclingforums.combig smile.pngOpen ↗[/IMG][IMG]files.cyclingforums.combig smile.pngOpen ↗[/IMG][IMG]files.cyclingforums.combig smile.pngOpen ↗[/IMG]

[IMG]cyclingforums.com1000Open ↗[/IMG]

I let YouTube stabilize the video, so not it should not be as painful to watch.

V'nx are you not getting enough attention? You are not adding any value to the conversation.

My intention with this thread is to keep it on topic and informational in case someone else is having the same issues I have.

maydog, you probably know more about failure mechanisms than I do, but I don't see how cyclic loading would be factor in bolt failure unless your spline/crankarm joint was loose. Why is movement occurring when you pedal to cause any cyclical loading on the bolt? If there is any movement, wouldn't that cause the internal alloy splines on the crankarm to wear?

I went back and looked at the ISIS specs, which may or may not be relevant. They recommend a fit of 3-6MM when an alloy crankarm is pressed on by hand, plus bottoming on the spindle shoulder before the peak torque is reached (using grease on the splines during assembly). Also, they state the bolt must not bottom out; recommending a check by trying the bolt without the washer under the head to insure there is clearance. I'm guessing you have already checked to insure the bolt isn't bottoming when it's torqued up to spec,

With a proper interference fit and lightly greased (or ti-lubed) splines, there will be a certain feel and progression to the torque as the arm is being stretched until it bottoms home. If your crankarm bottoms easily on the splines when you torque the bolt, because it has yielded or worn, doubt you'll ever be able to fix the problem with stronger bolts or more locktite.

Sounds like you've checked out tthese points already, but you didn't mention it in your reply. In that case, accept my apology for wasting your time with the obvious. I've told you all I know about this issue.........so I won't beat it any more.