carbon steerer tube bung - how tight?

neeb

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.

neeb

On second thoughts, I guess the bung shouldn't be slipping anyway after the stem bolts are tightened? Is the problem with the stem / steerer interface? I've used carbon assembly paste and tightened the bolts to spec (8 Nm) - what else can I do??

softlad

I wouldn't be using assembly paste on the steerer bung - it's probably making things worse, not better. The bung is an expander, so the paste really isn't necessary...

neeb

I wouldn't be using assembly paste on the steerer bung - it's probably making things worse, not better. The bung is an expander, so the paste really isn't necessary...

Sorry, should have made that clearer. I'm not using assembly paste on the bung, just the outside of the steerer / stem interface.

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...

topdude

Most likely the bung is slipping when you tighten the top cap bolt so the headset / stem / spacers are not being compressed correctly. Then after some use a bit of play becomes noticeable. It is not likely the stem will slip on the steerer once tightened to spec.
You do have the stem sitting about 3mm above the steerer i presume ?
How about trying a different bung / expander ?

neeb

Yup, the stem is at least 3mm above the steerer (or rather the spacer I have on top of the stem is). The shape of the underside of the top cap meant that 2-3mm wasn't enough, to it's more like 5mm.

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...

desweller

There might be some grease or other contaminant inside the steerer tube (if indeed you are having trouble applying preload to the top cap). Give the steerer tube internal surface and the bung loading surface a good clean with a suitable solvent cleaner and a clean cloth (but make sure you don't get any on the headset bearings as it will wash the grease out of there better than anything)

Monty Dog

I had a similar experience with the FSA carbon one with the carbon top-cap - as a consequence of a flexy steerer the bung kept 'walking' - replaced it with a standard alloy one and it stayed put. I'd have no problem putting assembly paste down a carbon steerer as it improves grip.

neeb

if indeed you are having trouble applying preload to the top cap

I'm not convinced that's the case actually.

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).

desweller

As you say, the top cap does nothing once the stem is clamped to the steerer. The resisting load generated by the friction between stem and steerer is far above the load that can be generated by the bung. So if it's OK when you put it together, and then it drifts when you hit a bump, that's a slipping stem.

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.

Dales1

The instructions for my Easton EA90 stem say that the top cap should be tightened to 1.6 Nm (15 in-lb) if the headset manufacturer has not specified torque.

So I wouldn't be hauling on the bung's allen key myself.

Dales.

neeb

Dales1 wrote:

The instructions for my Easton EA90 stem say that the top cap should be tightened to 1.6 Nm (15 in-lb) if the headset manufacturer has not specified torque.

So I wouldn't be hauling on the bung's allen key myself.

That's a bit different, as the top cap torque will be just for the preload. The bung itself has its own (larger) allen key slot for tightening it inside the steerer. Still, interesting to have a numerical value for headset preload! Unfortunately my torque wrench doesn't go that low...

DesWeller wrote:

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.

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?

desweller

neeb wrote:

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.

neeb

Thanks!

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.

Yes, sanding the inside of the stem clamp is something I haven't done yet which is often recommended.

I'll do that, clean everything thoroughly, reapply the assembly paste, tighten an extra 1 or 2 Nm, and cross my fingers...