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.
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.
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.
It is not likely that the bolt failed in shear, due to the way the loading is applied.
A longer shoulder won’t help since it will keep you from tightening the seat post clamp properly.
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.
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.
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.
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.
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.
imjustme, I had exactly the same thing happen to me a few days ago (hence finding this thread and writing this reply).
Did you ever come to a solution to prevent it happening again?
Also, where were you able to get a suitable replacement bolt from? I contacted some bike shops but they were only able to sell the whole saddle fittings pack.
Being BMC, it was typically overpriced and there’s no guarantee the bolt wouldn’t just snap again.
I think I found a solution because the bolt has not broken again.
The shop were I bought the bike talked to a BMC rep. The rep said the bolt could be tightened to twice the specified torque. This means tightening to 20 Nm instead of 10 Nm.
The shop tightened the bolt to 20Nm and I’ve kept it at 20Nm since. Of course, this meant buying a torque wrench capable of reading 20Nm.
This thread is old but maybe this will help someone else in the future with a similar issue.
You are exceeding the elastic range of the bolt and introducing it to the plastic range. What this means is that every bolt has a yield point and once you surpass that point the bolt will stretch without the ability to compress back to original shape.
The point now in plastic elongation will eventually fracture and failure after so may load cycles (vertical or horizontal). You are not able to replicate the original stress factor or shape no matter what outside of replacing the bolt. Just because a bolt is a “certain” grade does not change the proper torque value for which the engineered bolt was designed to hold - thats a complete other conversation.
What does that mean for you? To find the proper torque value and with a accurate torque wrench torque the bolt to that value. If the bolt allows slippage at that value retorque with 2 additional NM (for cycling) until the bolt no longer allows slippage and never surpass double the original value of torque.
If you are needing to surpass double the value you need to find an alternative rather it is a shim/washer or paste.
The bolt came with the brackets(?) that hold the saddle rails. While I was waiting for the bolt to arrive, I tried finding a similar bolt on my own. The problem was finding a bolt with the size shoulder found on the OEM bolt. I was able to find a full threaded bolt.
Thanks again. I really appreciate your getting back to me on this.
Whereabouts were you able to find the full thread bolt?
I doubt the full thread would cause any issues but maybe I’m missing something. Did it work okay?
I seem to be able to find cap head but not hex head bolts which would otherwise fit the criteria. I can also find plenty of hex head bolts with the correct dimensions but not at that grade. Aaargh! So near but yet so far.
Sigma Sports said they could get me the bolt but only as part of that pack with the other fastenings. That would be £37 plus shipping. I can’t justify that when all I want is a bolt costing next to nothing.
Did you try to contact BMC directly about it? I have but, going from previous experience, I fully expect them to completely ignore me.
The bolts are self-tapping with axial slits cut into them, often used to clean out rust and gunk in female threads. Common in automotive applications, and often under light loads, e.g. air filter boxes, skid plates, etc.
You can see the bolts sheared right where the slots end, where there’s a stress riser. Also, the entire batch probably wasn’t heat treated properly.
I’d find an equivalent screw without the slits. Also make sure the female threads are clean and lubricated. Use blue Loctite, anti-sieze, or grease.
It’s also where the saddle assembly meets the seatpost so that might be part of the issue too.
Add it to the list of design issues with that BMC bike. In retrospect I absolutely wish I’d bought something else. Unfortunately we don’t have the luxury of road testing bikes before making a purchase.
The interface between the two components (seatpost and clamp) is precisely where one would expect the bolt to fail. Did that bolt also have the groove in the threads?
