Broken chainring bolts.

Discuss light weight issues concerning road bikes & parts.
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NiFTY
Posts: 1088
Joined: Sat May 26, 2012 11:26 pm

by NiFTY

Question.

I have a Cannondale SISL with the evo 110BCD spider, and 52/36 Q rings (a brand new set of non-aero, from starbike). I have grinded down theoutside of the 52 tooth chainring to fit the cranks without hitting the inside of the crankarm and it is now well clear of the inside of the crankarm.

My issue is that it is impossible to torque any chainring bolts beyond 6Nm as they all fail. The failure is not the threads stripping but a single side of the lip of the nut (the bit fed in from the inside) snapping. It is always just one side that snaps. I have tried several sets of alloy chainring bolts (KCNC and FSA) and all do the same thing. It is as though something isn't straight and they are being torqued on an angle causing the failure. The q rings are both orientated the right way and the spider is in its correct orientation. Any ideas why this is happening. I use alloy bolts on all my cranks and have never had this issue before.

At the current level of torque there is a lot of creaking from the crank so I don't want to leave the torque really low. Also i am a weight weenie so adding steel bolts is not a palatable thought either.

Any ideas? Thanks in advance.
Evo 5.02kg SL3 6.77 Slice RS 8.89 viewtopic.php?f=10&t=110579" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

finnrambo
Posts: 33
Joined: Thu Dec 18, 2014 6:27 am
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by finnrambo

Can you please post pictures? I'd honestly just suggest switching brands, could be worth a try?
Canadian Junior racer and Product Reviewer: Finn Blackburn
Blog: https://finnblackburncycling.wordpress.com/

by Weenie


Valbrona
Posts: 1484
Joined: Fri Oct 28, 2011 3:25 am
Location: United Kingdom of Great Britain and Northern Ireland

by Valbrona

Maybe it is not the chainring bolt/nuts. Have the recesses in the chainrings been machined correctly so that they support the nut/bolt heads?

elfuinha
Posts: 727
Joined: Sun Aug 08, 2010 10:18 pm

by elfuinha

Pictures would be great.

Otherwise try titanium bolts, to keep the weight low.
I think POP PARTS OF PASSION bolts, are ok to a max of 10Nm.
Check this ones.

Qman
Posts: 115
Joined: Wed Oct 23, 2013 9:55 pm

by Qman

You should not be breaking chainring bolts. Based on your description of the failure, it sounds like something is wrong with the surface that is in contact with the side of the lip that is snapping. As you suggested, the bolt could be going in crooked. Or the surface of the chainring is uneven where it contacts the bolt.

Is it happening at one bolt, or all of them?
Is this a new install, or did you successfully install bolts previously?

As others have suggested, photos of a failed bolt and of the inside and outside surfaces of the chainrings (with one bolt removed) would be helpful.

In the meantime, I wouldn't suggest buying more bolts. There is no way that a FSA or KCNC bolt will fail unless you have an issue with the rings and/or the installation.

Qman
Posts: 115
Joined: Wed Oct 23, 2013 9:55 pm

by Qman

I forgot to mention the grinding of the chainring. Why was this necessary, and is it possible that the grinding is a cause? If you've ground off in the area of the bolt counterbore, then that would cause an uneven surface and break your bolt.
Again, photos would help.

by Weenie


NiFTY
Posts: 1088
Joined: Sat May 26, 2012 11:26 pm

by NiFTY

Update.

I had to grind off the chainring because compact BB30 has limited clearance under the crank for solid disk rings. The area grinded is well away from any bolts. (36 degree) I too the crans off and had a look at the chainrings. Two of the chainring holes were slightly smaller in that a steel chainring bolt had to be forced in. This is due to the overlapping nature of OCP holes on the Q rings. I sanded down just enough of the extra material that the bolts could slide in easily and the crank was quiet even at low bolt torques.

Thanks everyone for your input.
Evo 5.02kg SL3 6.77 Slice RS 8.89 viewtopic.php?f=10&t=110579" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

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