Problems in deepest darkest France

pralston · Jul 10, 2007

Hi All,

Been riding (or trying to) an R3 SL from Cervélo. So far I've broken two seatpost clamps. These clamps are scary light at 9grams each and are a lot different from the standard R3.

The French distributor tells me I'm the ONLY person ever to break one let alone two and Cervélo Canada very nicely sent me new ones but I don't feel I'm at the end of this saga.

So, anybody else out there having a similar issue? Yes, I did use a Torque Wrench and Yes, it has been calibrated and double checked against my LBS's same tool. The frame is a size 58 (does that matter?) and I weigh a miserly 70kg... my thighs ressemble Chicken Legs Rasmussen and not Freddy Cancellera, so in theory, this shouldn't be happening and it is spoiling the experience of one FINE frame.

For the record, Cervélo Customer Service Canada (forget the French clowns) have been excellent but I can't believe I'm the only one...

What say you please?
PaulR

bobbyOCR · Jul 10, 2007

pralston said:
Freddy Cancellera,

huh?

Anyway, I think your seat clamp may be mis-aligned or some other alignment problem. Something is putting stress on your seat clamp that shouldn't be. It is a usually, relatively unstressed area (doesn't hold weight, in other words)

pralston · Jul 10, 2007

Thanks BobbyOCR,

I don't see what can be misaligned - it is a VERY simple assembly.

A seatpost clamp supports primarily the mass of the rider in the Z axis (stops the post sliding down...) but also must support a certain amount of X and Y forces caused by the mass of the rider leveraging on a pole (the seatpost) as he/she moves around or the bike goes over a rough surface. If the frame is thin (it is seriously thin in the case of the R3 SL) my belief is that too little of this X and Y force is absorbed by the frame right at the top of the seattube and it is the seatpost collar that is paying the price as the forces try to ovalise it.

I welcome comments on whether my amateur Engineering mind is correct or not, but in reality, if I am right for whatever reason, I cannot be the only one getting this problem (unless my frame has a fault right at the point where the seatpost goes into the seat tube?)

PaulR

bobbyOCR · Jul 10, 2007

On a properly designed frame, all the seat clamp should do is narrow its inside diameter so there is enough friction created between the seatpost and frame for the seatpost not to move. The seat clamp bears no weight. Also, the seatpost whould be absorbing most of the torsional forces, not the seat clamp.

How did it break exactly.

pralston · Jul 10, 2007

How did it break exactly.[/QUOTE]
I'll try and describe it's shape and the breakage (then a photo if I can find it...)

The shape of this clamp is two thin hoops siamesed together at the rear where the bolt applies the clamping force. It is one of the hoops each time that snaps. Never during initial torquing, just a handful of days later while out on the road, you hear a loud ping/crack (like a stone throw up by a car when it hits your frame) and there goes another one...

Curses, curses, curses...

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Problems in deepest darkest France

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