Seat bolt sheared off - twice

Hi,
I use a BMC TM02 on CycleOps Magnus. The seat bolt has sheared off twice while riding on the trainer.

Does anybody have any experience with this? Suggestions on how to avoid it?

BTW, this is not a ‘I am too powerful!!!’ boast. I’m 175lbs with below average w/kg achieved through sporadic adherence to the low volume plans. I am not jumping up and down out of the seat. To the contrary, I stay anchored 100% of the time even if Chad’s tips are telling me to get out of the seat. Sorry Chad.

Any insight is appreciated. My engineer son suggests getting tungsten or titanium bolts.

Pic of setup and failed screw would be really helpful to see.

Ti would not be my suggestion unless the existing screw is aluminum (which I doubt, but is possible). Steel in one grade or another is likely best.

One of the pictures shows the broken bolt still in the seat post. The other pictures show the same broken bolt and another unbroken bolt of the same type. KSFC 12.9 appears on the head of the bolts.

12.9 is usually the highest grade of steel bolt you can find, I wouldn’t get tungsten or titanium bolts, it’s just money down the drain IMO. Looks like those threads are dry, I would get new steel bolts from a reputable source like a local fastener store. Make sure you grease the threads, or use anti-seize. I would clean the threads on the post and double-check they aren’t damaged.

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Thanks. I found this explanation of shear strength of bolts with shoulders. https://www.fastenal.com/en/79/structural-bolts It suggests I should find a bolt with a longer shoulder because the current bolt’s should is not long enough to prevent the threaded area from being in the shear plane.

A few things.

  1. It is not likely that the bolt failed in shear, due to the way the loading is applied.

  2. A longer shoulder won’t help since it will keep you from tightening the seat post clamp properly.

  3. I note that you have the clamp in an extreme rearward position. Does the clamp sit in the middle of the saddle rails or is it more fore/aft? Having the clamp far from the center may cause a greater bending moment (lever arm) on the bolt.

  4. Are you using a torque wrench to tighten the bolt? Hex head bolts are generally easier to over torque than most hex bolts on bikes because open/box wrenches ae usually longer than hex keys for a similar bolt size. Having higher tension than needed will accelerate fatigue failure. which is my guess as to what is going on here.

  5. Can you send a picture of the bolt head, that should have the grade info on it which may help decide in picking another bolt.

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I’m with @clasher on this one! Get a class 10.9 or 12.9 bolt & give that a try. Just make absolutely sure you understand the true nature of the problem! Sticking class 12.9 bolts in there could just mean the next weakest thing breaks instead.

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Higher strength/grade bolt might help, but might hurt. Generally higher strength bolts have poorer resistance to fatigue since they are less ductile.

MILKMAN DAN!!! Bane of Karens everywhere.

I hate you, Milkman Dan.

OP said they were 12.9 bolts, so there’s likely something else going on… that or they are fake, which wouldn’t surprise me either.

Thanks for the ideas. To answer some of the questions-

  • The first time the bolt broke, the bolt as in the second position from the front. In that location, the clamp was far forward on the seat rails. The rear-most position puts the clamp in the middle of the seat rail.
  • I did not use a torque wrench.
  • The bolt head says it is a 12.9