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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-

enter image description here enter image description here enter image description here

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  • Who do you want to sue for a seatpost? 0:-) Aug 18, 2021 at 13:18
  • I think it was meant to be "use", no "sue" :-) Fixed now Aug 18, 2021 at 13:26
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    @GrigoryRechistov Ah, of course. Somehow may brain didn't feel like making the connection today XD Aug 18, 2021 at 13:41
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    Study fastener/bolt relationship between clamping force and torque. If searching Google, skip anything that mentions Bicycle. The Bicycle Industry has sold us the idea that hell freezes over if you incorrectly tighten a fastener (true to a point), specifies a torque, and pretends in the real world of reused fasteners, lubrication (or not) etc have no effect. Studies I have seen have indicated real world variations of up to 50% in clamping force for for the same torque. Unless you have a calibrated torque wrench and a and new, lubricated faster, the torque is merely a guide.
    – mattnz
    Aug 18, 2021 at 23:19

2 Answers 2

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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

enter image description here

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  • I think "carbon paste" is the term I usually hear in the US, although I bet everyone will understand carbon installation compound. I have not heard of people using Loctite for this purpose. Actually, though, the question is asking about the saddle clamp, not the seat clamp (the clamp mounted around the seat tube; the Merida in question appears to have a seat binder rather than a traditional clamp). I have not heard of people using carbon paste for saddle rails, which are aluminum in this case, and the clamp is presumably aluminum
    – Weiwen Ng
    Aug 18, 2021 at 13:56
  • @WeiwenNg: „Paste“ sounded too German (my native language) to be right ;) It’s made with “high friction micro particles” which should work for metal-to-metal interfaces as well. I think it only has carbon in the name because carbon components often use low tightening torque which often leads to slipping. LOCTITE® 603 is specifically made for metal-to-metal interfaces and should probably not be used for carbon components.
    – Michael
    Aug 18, 2021 at 14:25
  • @Michael Thanks for the answer. That manual, however, is from 2008 (specified on page 138). I don't think it's wise referring to a manual more than a decade old. Thoughts? Aug 19, 2021 at 4:26
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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.

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  • You might want to know this got auto-flagged for being short; maybe add a reference? Also, is 8Nm safe for the carbon seatpost?
    – DavidW
    Aug 18, 2021 at 16:30
  • @DavidW 8Nm isn't out of the question for carbon in general; my Parlee's seatpost clamp is marked up to 8Nm. Also, while I can't clearly see how the clamping mechanism is constructed, it's presumably aluminum clamps, and the bolts involved might be tightening two aluminum pieces together independent of the seatpost's structure. Think of how an Enve seatpost is constructed and how the clamp hardware is independent of the carbon structure - also, many custom titanium seatpost manufacturers buy Enve's hardware and just enclose it in ti.
    – Weiwen Ng
    Aug 18, 2021 at 17:57
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    @DavidW Sorry, no official reference. 8Nm is just the torque I nearly always see for saddle clamps. With the edit, the real torque spec is likely much higher. Note that the carbon only sees the saddle rail clamping force; even if the bolt tension is extremely high, a lot of it is not being converted to vertical force because of the wedge design.
    – MaplePanda
    Aug 19, 2021 at 0:11
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    @MaplePanda I admit I didn't dig enough to figure out how the clamp worked either; I was just concerned that the answer be good enough not to be auto-flagged. The edit makes this much better.
    – DavidW
    Aug 19, 2021 at 0:22
  • @MaplePanda It is indeed a two-bolt clamp. I've added the pictures now. Aug 19, 2021 at 4:22

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