I have a Merida Reacto 4000 with carbon seatpost and alloy saddle rails. There's no torque rating specified for the bolts which clamp saddle onto the seatpost.
What torque should I use for them?
I've tried 6 Nm but it doesn't stay fixed on bumps. I can try increasing it by 0.5 Nm if nothing else works but would love to know the general idea for any such unspecified bolt.
Images of the saddle-seatpost clamp-
This Merida manual on page 130 says 5–7Nm or 6–8Nm, depending on which bolt size the clamping mechanism has. I’m not 100% sure the manual applies to your seat post but I assume going up to 7Nm should be safe.
You could try carbon installation compound or a retaining compound (e.g. Loctite 603). I’ve also used liquid climbing chalk (magnesium carbonate) when I had nothing else at hand. Use a waterproof marker to mark the position of the saddle rails so you know if it got loose.
The best approach is to have a manufacturer specification. It’s always annoying when there is no specification or it’s unclear for which product the specification actually is. I really don’t understand why they can’t just print it on the part and/or provide a proper manual for each part.
The good news in case of the saddle clamp is that it’s not safety critical.
https://www.manualslib.com/manual/807720/Merida-Bike.html?page=130#manual
8Nm is a pretty good generic saddle rail torque.
Edit:
Oh, that’s not a regular 2-bolt seatpost head. If there’s no torque spec anywhere on it, the necessary torque is likely to be even higher, in the 10-12Nm range if not more. This design is basically a fancy single-bolt style post where one bolt tightens wedges that create the clamping force. This is a far less direct process than bolts that directly clamp down, so the required torque is higher. Send Merida an email asking for the correct torque spec before you go cranking anything down though.
A few photos of the seatpost head clamp would be nice.
