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I am building a bike and I am using a saddle with carbon rails on an alloy seatpost. I read that it's not an issue as long as the shape of the clamps is compatible with the shape of the rails, and in my case it is.

However, the torque specified on the rails (6 Nm) and the torque specified on the post (12 Nm ~ 15 Nm) don't have any overlap. Which of the two should I go with?

I am afraid that if I go higher than 6 Nm, it will damage the carbon fiber of the rails, but if I go lower than 12 Nm, the saddle will slip while I ride, and as a consequence will again damage the rails.

Should I ditch the combination all toghether? Or something in the middle, say 9 Nm, is going to work just fine?

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  • Are you sure the 12Nm torque spec on the post is for the saddle rail clamp? I would think it’s the max torque that the seatpost itself can withstand from your frame’s seat clamp.
    – Weiwen Ng
    May 5 at 12:18
  • @WeiwenNg: Possible but normally it would rather be indicated on the seat-tube.
    – Carel
    May 5 at 12:28
  • @WeiwenNg Yes, I am sure, the torque is specified on the clamps. Funny enough, long time ago I was confused about this particular question as well, see bicycles.stackexchange.com/q/46888/1621 May 5 at 13:24
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If the max. torque on the rails is 6Nm then you shouldn't exceed that in no case. The 12-15Nm is the maximum when the clamp is used with metal rails.

In your case, start by taking the clamp apart and clean the bits that touch the rails of your saddle from any remains of grease with a solvent. Check the clamps for sharp edges and give them a sanding with fine paper if required. Then apply some carbon mounting paste, grease the bolts of the clamp and tighten to the recommended torque for the saddle.

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    Using carbon mounting paste sounds like an interesting solution instead of overtightening. Is carbon paste going to be effective between alloy and carbon though? I have always used between carbon and carbon. Also good tips on checking the clamps before mounting the saddle. May 5 at 13:25
  • @AlessandroCosentino: Yes, there should be no problem. With metal rails to metal clamp you should grease the interface. But carbon on metal, grease is a no-go, so yes carbon paste is the option.
    – Carel
    May 6 at 7:07

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