Chainring bolt stress
 

A slightly technical question asked in a non technical way :)

When the cranks are turning and the crank arm/pedal is at a given position between 12 0' clock and 6 o'clock, where is the most stress placed and at what position (12-1-2-3-4-5 or 6 0' clock) etc?

I ask this as I had a few chainring bolts come loose and actually lost one completely on a recent ride and wondered at what position the most stress/power occurs. Would the most stress be on the chairing bolt above the crankarm (trailing/pulling) or below the crankarm (leading/pushing) at a certain position as this is where I either lost the bolt or where the most loose bolt was.

TIA

Well in a perfect world the crank spider would fit in the ring (note how I reverse the way most would look at things) and the only stress the ring bolts will see are rotational ones. The spider arm, it's shelves, it's bolt holes would contact everything evenly with no slop. The bolts would fit within the holes of the rings and spider arms perfectly. The ring would be perfectly round and even with a fixed gear set up the chain tension would not vary as the drivetrain was spun.


But in the real world non of this happens. So if the chain tension is well set then the ring will slightly rotate WRT the bolts being centered within the arms' and rings' holes. The bolts will hit the holes' inner bore nearly equally for all the arms/bolts/ring holes. (But if any are not perfectly located around the bolt hole diameter then some bolts will still be "loose" WRT tension from the chain).


Now if a non der set up has the chain tension wrong (as in any tension at any point along the spin of the system) then the rings will try to shift around differently as the cranks rotate, back and forth. And the bolts will under go their own wiggle within the holes. Often this will lead to the bolts loosening up as they try to equalize their sharing the loads and while the rings try to rotate differently then the spider arms do.


I grease the surfaces of the bolts and tighten them pretty well. Beyond what AL bolts like if possible. A well set up system using steel bolts shouldn't see bolt loosening. A sure indication of bad assemble, set up and poor follow up after assembly. Andy.

Thanks for the reply, Andrew. I had switched steel bolts for alu and that's when I lost them/they loosened. Think I may go back to steel again. Never had a problem with steel, first time I swap out to alu and this happens. The proof is in the pudding, as the saying goes :)

The loosening could be poor machining and tolerances of the alu bolts. or not enough tightness torque when installing.

Try some Loctite.

Loctite will certainly help and if not double sided allen heads use this tool to make sure they don't spin as you tighten them.

blue loctite is the stuff and gets recommended by Jan Heine for the aluminum bolts on TA

@trailflow1 @blakcloud @bulldog1935

I bought some blue Loctite just after the incident. Never used it before, but I will now :)

Strangely enough, I would have been certain that the bolts were tightened way enough. They are made by Aerozine and actually have a hex/allen bolt fitting to both nut and bolt part of the chainring bolts (5mm for nut, 6mm for bolt), that is the main reason I bought them from that company specifically. I thought having a hex/allen at either end would do away with the chainring bolt tool/large bladed screwdriver and most definitely be able to be tightened up enough. Bolts say 8-9nm, maybe I should have gone tighter? I also now have a torque wrench, so I will see at what value there is effort, if any to tighten.

Thanks folks!

well your switch to alu + inadequate torque , because you worried about stripping the alu bolts,

& Not re checking them before every ride, may be an issue ..

they use Torx or hex fittings?

LockTite + Alu fasteners + hex wrench may not come loose but round out the hex, trying..

Heating the bolt may be needed, it breaks down the thread lock chemistry ..

I hope I never am the wrench that has to take apart your Loctited parts :) Especially small, fine threaded, AL bits. I try to avoid using Loctite for threaded fittings if at all possible. In fact I use grease to lube all mating surfaces (including the outsides of the female half of the ring bolt) because I find in the rust belt I live in corrosion is the greater problem and a lubed fitting can be tightened more with less damage to the tool sockets or slots. Andy.

I'm a big fan of Loctite, but three places I never use it:
- bottom bracket threads
- chainring bolts
- pedal threads

I use antisieze compound on these.

Really?!! The guy who will almost kill a cow to make leather washers for his fenders recommends Loctite for his (non-standard) chainrings??

oh, an expert on me - well that's worth a block.

a psychologist could make a career on this forum.

blue loctite comes apart with normal tools. There is a purple loctite for fine threads, but Jan Heine is a pretty good resource.
It's all too easy to shear the heads from the TA aluminum chainring bolts if you're over-torquing them, and they're not easy to replace.

Steel fasteners stay tight by stretching slightly when properly torqued.

Al doesn't behave that way so the only way to keep them from loosening up is with thread locker...

Blue Loctite isn't that hard to 'break' actually.

Copper grease prevents rusting of bolts and threads together better than most other greases, while it doesn't help loosening from normal use like some greases can do.

So my preferred choice would be steel bolts + copper grease on the threads (and all the mating surfaces where bolt touches the thing it holds tight).

If using aluminium bolts, use the weakest (blue) loctite you can find.

The alu bolts were torqued to spec - 8-9nm. They use hex/allen fasteners at both ends, male and female.

Never thought of that. Good point!

ps - also from a phone call with Jan, I use red loctite on Italian-thread cartridge bottom brackets when they won't stick by torque alone - that joint breaks easily with normal BB tools.

A few comments:

"Copper grease" is also known as "anti-seize".

All fasteners stretch slightly as they are torqued and that applies to steel, aluminum, Ti, and every other material. Steel is stronger and will tolerate more torque without breaking but aluminum fasteners behave the same way.

You can't choose Locktite just by color. Blue (Grade 242), a low strength grade and Purple (Grade 222), the lowest strength grade, are unique. However, there are numerous Red and Green grades and some are weak enough to allow disassembly with hand tools and some require significant heat to break the bond. Be sure you use a suitable grade.

OP, replace the Alu with Chromed steel bolts.

Bolts, properly kept tight, the crank arm and chainrings work as if a single piece.