carbon steerer tube bung - how tight?
I recently finished a new build on a Dedacciai frame and fork (same as the Ribble Scuro RS) and am having problems with the headset coming slightly loose because of the steerer expander bung slipping a little when the forks take a big hit. The bung is this one:
http://www.dedacciai.net/eng-frame-cycl ... pander.php
It's tightened with an 8mm allen wrench. The thing is, I really did it up pretty tight last time and obviously I don't want to crack the steerer tube...
It doesn't help that it didn't come with any instructions and none seem to be available online. I do have a torque wrench I could use if I had any idea what the torque should be.
http://www.dedacciai.net/eng-frame-cycl ... pander.php
It's tightened with an 8mm allen wrench. The thing is, I really did it up pretty tight last time and obviously I don't want to crack the steerer tube...
It doesn't help that it didn't come with any instructions and none seem to be available online. I do have a torque wrench I could use if I had any idea what the torque should be.
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But in any case, it shouldn't be the bung / topcap that's holding the headset together once the stem bolts are tightened. So the bung slipping is more a symptom than a cause (although I've always found that tightening the top cap a little after tightening the stem bolts is good for helping to keep things in place).
I can't see what other options I have other than to exceed the recommended torque for the stem bolts, although this doesn't seem like a good idea. The stem is a deda, so it should certainly be compatible with their own steerers...
You do have the stem sitting about 3mm above the steerer i presume ?
How about trying a different bung / expander ?
It's just annoying that there is no information anywhere on how tight this bung should be. Someone told me it can be really tight as the steerer is very strong and relatively resistant to internal pressure, but obviously I cringe after a certaim amount of torque has been applied with a whacking great 8mm allen wrench...
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The first time it came loose, there was no obvious sign of the bung having slipped (it has a lip that sits flush against the top of the steerer, so if it has moved it's obvious once you take the stem off). That was before I used assembly paste on the steerer though. The second time I used assembly paste on the steerer and also tightened the top cap after tightening the stem bolts, so if the stem had slipped it would have pushed the bung up too. Indeed, on this occasion the bung had slipped up by a couple of milimeters.
If the bung slipped when I was applying preload, it's than bung that's at fault, but if it slipped later, it's the stem / steerer interface that's the problem (although perhaps having a tighter bung might make it less likely to happen).
Perhaps your torque wrench is reading low? They are not very accurate devices, believe me! Give the clamp screws a couple more Nm - you won't hurt it, there will be a fairly substantial safety factor in that target number and 2 or 3 extra Nm isn't much.
It's not impossible that the stem has just not been manufactured correctly and is not within tolerance for roundness or parallelism or something. You might just have a duffer.
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So I wouldn't be hauling on the bung's allen key myself.
Dales.
The wrench is a few years old and I have dropped it once or twice..
As for having a duff stem, you'd like to think with the prices Deda charge for their zero 100 servizio corse, they could get the quality control right... :evil:
This was sort of my reasoning too though. If it's slipping, it's not tight enough, specified toruqe or otherwise. Is it possible for it to slip and also be tight enough to damage the steerer at the same time?
Only if it's defective, I would say, or if the mating surfaces of either or both the stem and steerer are unusually conducive to smooth running. Friction is not always the most predictable of phenomena.
The two parts might just be at either ends of the tolerance for size (although I would have hoped that the tolerance would be selected in such a way that would still allow it to work) which would cause the contact area to be substantially reduced.
If there are high spots in the stem bore, then I would say it would be very easy to damage the steerer without exerting excessive clamping load. It would also be prone to 'rocking' across the axis of the steerer, which could lead to movement along it.
You can do a few manual checks though. If you have a narrow 6-inch steel rule, you can hold it edge-wise along the the inside surface of the stem. If it can rock, then there is a high spot somewhere inside. Obviously you'll have to do this at a number of points around the inside of the stem, and it will have to be nice and clean beforehand. You can also check for burrs or ridges at the edge of the stem bore and the edges of the pinch slot; they will also help prevent the stem from clamping correctly.
Like you say, given the provenance of the stem it's a little unlikely, but you never know. High price is not always an indicator of very rigorous quality control, and even the best QC processes sometimes let a bad one out the factory - you might just be unlucky.
You might get some mileage from lightly sanding across the inside of the stem and the steerer surface (around the bore and tube, not along them - across the direction of suspected movement) before cleaning them and reapplying the carbon paste stuff. Take care to protect the bearings from dust if you're going to try this.
Note that this is not based on faults that I've seen on bikes, it's just what I would do as an engineer.
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I'll do that, clean everything thoroughly, reapply the assembly paste, tighten an extra 1 or 2 Nm, and cross my fingers...