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

The seat is attached to the post by a single stainless steel bolt. KSFC 12.9 appears on the head of the bolt.

The first time I had adjusted the bolt and may have overtightened it. The second time, the shop that sold me the bike installed the seat and tightened it to manufacturer's spec. The seat is positioned with the clamp in the middle of the rails. The seat is a Selle SMP Glider.

The second bolt broke after 23 rides averaging 75 minutes each. The first bolt lasted far longer.

I am not pushing high watts ever. I am not overweight (6 ft and 175 lbs).

Any experience with this? Any idea why the bolt fails? These are the seat clamp parts. Seatpost with first sheared bolt. Seatpost with second sheared bolt and bottom part of seat clamp. The seat is positioned farther back than it was when clamped but the seat won't balance in place without a bolt in any other position. enter image description here

UPDATE AS OF December 10, 2020

LBS talked to BMC. BMC said bolt can be torqued to 2x recommendation. LBS set it to 20Nm (2x the recommended 10Nm). I bought a torque wrench capable of 20Nm and check the bolt regularly. So far, no issues.

  • 1
    Can you add a photo of the area around the bolt? My guess is that BMC went for an exotic solution without checking that it actually survives in use, but it's difficult to tell from promo photos what exactly they did.
    – ojs
    Aug 18, 2020 at 13:15
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    I think I might have an idea, but could you add a photo of the full post, clamp and saddle assembly to verify it?
    – ojs
    Aug 18, 2020 at 14:21
  • 2
    BMC provided the bolt. Aug 18, 2020 at 17:55
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    I think the trainer is a key factor here. My guess is that when you ride on the road you are rocking the bike side to side possibly without even knowing it. This could be because your hips are not flexible enough for your riding position or maybe your seat is too high. In the trainer, the bike is fixed, so your hips are rocking side to side and putting stress on the bolt in a way it is not designed for. Maybe a lower seat position would help or increased flexibility depending on what the source of the rocking is
    – Andrew
    Aug 18, 2020 at 19:24
  • 4
    If that's a stainless steel bolt, it looks like there's something wrong. Per boltdepot.com/fastener-information/materials-and-grades/… a 12.9 bolt is alloy steel, quenched and tempered. Also note the strengths listed at that link - stainless steel is a lot weaker than a 12.9 bolt. Every 12.9 bolt in this Google image search is dark: google.com/search?q=12.9+bolt&tbm=isch None of those are the bright silver of the bolt that failed. Aug 19, 2020 at 0:32

3 Answers 3


Any idea why the bolt fails?

Bottom line: I believe the bolt fails because the seat post clamping mechanism design causes fatigue in the bolt.

ASTM E1823-10a, Terminology Relating to Fatigue and Fracture Testing, defines fatigue as: The process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations. "Fastener Fatigue" Fastenal.com

The article linked above contains many pictures and detailed descriptions of how to diagnose many aspects of fatigue failure.

What it boils down to is that the bolt was flexed back and forth - subjected to "fluctuating stresses and strains" in the same place until it broke.

The fastener assembly process is one of the most important, but often overlooked, contributors to fatigue performance. The cyclic-stress amplitude imposed on a given fastener (and therefore fatigue performance) is highly dependent on preload. Specifically, increased preload results in decreased cyclic stress-amplitude, particularly at loads below the clamping force imposed by the fastener. Fatigue of Threaded Fasteners

So, in general tighter bolts are less susceptible to fatigue failure.
The "Fatigue of Threaded Fasteners" article goes on to say:

The concept of higher preloads resulting in increased fatigue performance may be counterintuitive, which might otherwise suggest that increasing the tightening force of a bolt may increase propensity for failure. Insufficient preload has been attributed to the cause of the majority of fastener fatigue failures.

Later it says that excessive preloads are bad. It should be tight but not too tight.

Possible issues

  • Riding your bike generates the forces that caused the fatigue failure..
    Riding the bike is normal usage - the clamping mechanism should be designed to handle the force it's being subjected to.
  • Since the bolt was tightened to spec by the shop on the second bolt we can assume that tightening was not the issue.
  • A bad bolt? It is possible but I think unlikely to have two bad bolts in a row.
  • Is the bolt too weak? a rating of 12.9 is very very good. The bolt should be up to the task.
  • Does the design of the clamp place cyclic stresses on the bolt?
    I think an argument could be made that since the break is just above the bottom seat angle adjustment mechanism of the clamp that the design focuses cyclic stress on the bolt at that point, especially if both bolts broke in the same place.
    If you had a bolt that was shank just past the point where this bolt broke it might last longer - a longer grip length.

This is a little crazy and you shouldn't even have to think about this but it might be possible to make it last longer by riding softer. Sometimes it's possible to mitigate poor design by changing behavior.

enter image description here

The bolt in your post is 12.9 which is a metric rating.
There is no bolt greater strength than 12.9

Metric Class 12.9 The highest metric class for strength, it exceeds Grade 8.

Some of the comments reflect the fact that there are counterfeit bolts - one that has a rating stamped on it that it does not deserve.
If you broke a real 12.9 bolt there is no better bolt to get. To make sure you are using a real 12.9 bolt you need to get one from a reputable source.

  • 1
    I wonder if stainless steel is the wrong material for this bolt. A plain steel bold would have a higher elasticity.
    – Criggie
    Aug 18, 2020 at 22:43
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    That doesn't really look like a fatigue failure - there's no noticeable fatigue zone (where the original crack propagated slowly) that transitions to a rapid failure zone. (google.com/search?q=fatigue+failure&tbm=isch) The failed surface looks the same all the way across the bolt. Weird that a 12.9 bolt would fail like that - twice. Bottom line, is, though, that bolt shouldn't be failing either catastrophically or from fatigue. I gotta agree with @Criggie - get a good steel bolt from a reliable source, and make sure it's greased. Aug 19, 2020 at 0:21
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    There are a lot of counterfeit bolts out there. You can’t count on the rating unless you know the manufacturer. Chinese sources are particularly suspect.
    – Eric S
    Aug 19, 2020 at 22:32
  • Would these bolts be better than the stainless steel 12.9? mcmaster.com/screws/hex-head-screws/… Aug 20, 2020 at 17:57
  • @user1757436 I have updated my answer to include bolt rating information. Bottom line, if you have a real 12.9 bolt there is nothing better.
    – David D
    Aug 20, 2020 at 18:28

It looks like the way you positioned the seat and the seatpost clamp bolt are both working hard against the bolt's metal fatigue rating. You selected the farthest back hole in the seatpost and also have the seat setup far back on the rails. You picked the worst combination in terms of leverage on that bolt dooming it to fail. Set up the seat clamp mounting more forward on the rails and pick a bolt hole further forward on that post.

  • That is an excellent observation - welcome to the site!
    – Criggie
    Sep 12, 2021 at 22:53

I have had several seat clamp bolts break on my BMC Roadmachine. The clamp does not bed well on the seatpost when the bolts are tightened to spec. They use 5mm bolts which are not strong enough. TO complicate things further, their proprietary D-shaped post makes it impossible to use other manufacturers posts.

So far BMC has only offered new bolts, which is disappointing. Now, after 4 failures, I'm getting fed up. They need to do a recall and come up with a real solution that is safe!

  • 2
    Try sourcing a 5 mm 12.9 bolt from a reputable bolt dealer. I bet you get a dark one instead of a shiny bright silver one. And maybe print out this page and send a copy via registered mail with return receipt to BMC's corporate counsel - you'd think BMC would have learned to competently source a proper 12.9 bolt by now - because that's what this problem seems like to me. In every single image of a bolt I find online that shows the 12.9 on the head, the bolt is dark, not shiny silver like the ones shown here. Seems like someone sourced "12.9 stainless steel bolts" from Alibaba. Nov 25, 2021 at 16:38
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    And you thought I was joking. "4.8/ 8.8/ 10.9/ 12.9 Stainless Steel Hex Head Bolt" Umm, 12.9 bolts aren't stainless steel... Nov 25, 2021 at 16:48
  • Hi, welcome to bicycles. While this is interesting information, it doesn't really answer the question. When you earn some reputation you'll be able to leave comments, but in the meantime please only post answers that answer the question.
    – DavidW
    Nov 25, 2021 at 16:49
  • LOL. That link now states "DIN933 DIN931 Stainless Steel 304 316 Thread Bolt Heavy Hexagon Hex Head Nut and Bolt" Busted? Jun 24, 2023 at 13:33

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