Bicycle Mechanics - Follow torque number on clamp or seatpost?
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08-01-12, 02:23 AM
I just got a new bike with a carbon seatpost. The seatpost sticker says the recommended torque is 6.2nm, but the seatpost clamp says 5nm on it. Which one do I follow?
Also, does this mean that it will be impossible to destroy my seatpost due to overtightening because the clamp will fail before the post (I won't be testing this)?
Try 5Nm, if you can twist it, go up to 6. I'd say the seatpost's spec is the critical value here, right?
Have to be a pretty flimsy seatpost clamp if it can only take 5Nm.
08-01-12, 06:00 AM
I agree w/Kimmo. Go w/the lower number first. All that matters is that the post not slip.
Carbon post? Do you plan to use assembly paste? You might be able to use even less than 5nm.
This is a perfect example of why I hate a paint by numbers approach to mechanics using torque specs. There's no way a seat post maker can specify a torque spec. A post maker might specify maximum clamping compression, but even that depends on how perfect the fit is, and how round the tube.
Then there's the issue that clamping force depends on the construction of the clamp and the thread of the clamping bolt, so unless the seat post maker is psychic and knows what clamp you're using he can't have any idea of a torque spec.
Use carbon assembly paste and tighten the clamp enough to hold so the saddle doesn't twist or slip when you ride, staying below the 5nm of the clamps spec. Also remove the clamp and grease between the clamp and frame, then install with the slot not aligned with the slot in the seat tube. That allows the clamp and seat tube to support each other across the slots to keep the tube round which greatly reduces local stress on the post.
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