How important is Compression Plug / Top Cap Tightness after stem is tightened?

[McGilli]

I have my bike setup - and used an Enve compression plug which is like 40g - tighten the stem - etc etc.

All good.

My new compression plug arrived today. It weighs only 6g. So, I removed the Enve - which is like 2" long, and replaced it with this which is like .5" long. Thing is - this new plug doesn't get quite a solid hold - Once I expand it - when tightening the cap bolt if I keep going the new plug will be pulled up slowly. So the bolt is tight - but never solidly tight like before.

But - since my stem was already tightened etc with the old plug - does the tightness of the top cap really matter any more?

[McGilli]

Here's the plug I was using:



Here's the new one - i know I had the inside upside down in this photo...

[ms6073]

Once the stem/headset assembly is properly pre-loaded and the stem bolts tightened, there are some in the WW community that go ahead and remove the top cap and bolt and replace it with a press-fit cosmetic cover. That said, I have to ask, did you tighten the Extralite Ultrastar to the specified toruqe? The newer Ultrastar 3 is tightened to 7-8 Nm of torque, but I have an older Ultrastar for which 15 Nm of torque is specified and I suspect that after using a digital torque wrench to ensure proper tighteness, I am confident I would over-load the headset bearing long before the Ultrastar slipped/moved while tightening the top cap!

[McGilli]

Thank you.

Is it also a concern if I currently have 1cm of spacer above the stem - and therefore the new compression plug would not sit inside the stem (after been preloaded with the longer plug) but would be in the steerer tube that is above the stem?

[ChiZ01]

putting some carbon grip compound inside steertube helps with the ultrastar.

[stuka666]

Zero, it's doing nothing there after the stem has been tightened.

It's only useful if you need to readjust or have a crash while on the road.

I don't have one installed in both bikes, from time to time or if I feel the bearings are a bit loose, I use a heavy one at home and then remove again.

[fujiac]

Hi, Im new to this community.

Anyway I want to contribute.
My local carbon repair shop (In Germany) recommends to use a compression plug or some socket to support the load of the clamping on the steering stem.
They told me that they saw broken steering stems when wrong compression plugs were used.

But of course the pre adjuster does not hold the clamp in its position.

[cyclespeed]

Hi, Im new to this community.

Anyway I want to contribute.
My local carbon repair shop (In Germany) recommends to use a compression plug or some socket to support the load of the clamping on the steering stem.
They told me that they saw broken steering stems when wrong compression plugs were used.

But of course the pre adjuster does not hold the clamp in its position.

Specialized also seem to like the idea of having the compression plug in, and in the right place, to act as a support for the steerer where it is compressed by the stem.

But I suspect they are just playing it extra safe; there is no reason why a good quality stem tightened to 5Nm should have any adverse effects on a carbon steerer.

In theory, and depending on your frame/bearings, you may not need a bung at all. Have a pal squeeze the fork to the stem with all his might while you tighten the stem up.......

[fujiac]

Lets assume the compression plug gives more structure integrity and supports the steering stem.
Now, say the plug is half the length of the clamp and mounted in way to sit at the uppermost part of the clamp in the half way between the first screw.
Then the momentum of the steering clamp e.g. if you go over a speed bump, acts mostly on the lower part of the steering tube.

So may be it is there to distribute the load.
By this argumentation you should either use a compression plug that fits the clamp OR none at all.

Anyway I am not a mechanical (engineer).

[cyclespeed]

Lets assume the compression plug gives more structure integrity and supports the steering stem.
Now, say the plug is half the length of the clamp and mounted in way to sit at the uppermost part of the clamp in the half way between the first screw.
Then the momentum of the steering clamp e.g. if you go over a speed bump, acts mostly on the lower part of the steering tube.

So may be it is there to distribute the load.
By this argumentation you should either use a compression plug that fits the clamp OR none at all.

Anyway I am not a mechanical (engineer).

As luck would have it, I am a mechanical engineer! (Kings College London)...but that was a long time ago....!

Even if the bung sits behind only the top half of the stem, the bottom half still benefits, as your rigid end point is now half way down the stem, not right at the top.

Think of standing on a diving board. Stand 1/2 way from the rigid support and the board deflects little. Stand right at the end, it deflects a lot.

Anyway, I've tried to destroy offcuts of carbon steerers in the vice, with hammers, etc. and they are very strong. I think you would need to have a huge M8 bolt tightened to 20Nm+ to risk any damage.

[racingcondor]

They told me that they saw broken steering stems when wrong compression plugs were used.

This I can believe. There will certainly be some people who have used old star bungs in carbon steerers, load one of them up and then tighten the stem and the star will cut into the stem very quickly.

I can see why a company like Trek, Cannondale or Specialized would recommend a large bung (idiot proof, removes a lot of risk from home mechanics without torque wrenches).

I don't think there is any real risk from using an Extralight Ultrastar (correctly), I can't prove it though. It's just my feeling since I've watched this forum for a long time and I'd expect to have seen a failed stem with one hanging out by now if it was more common than 'freak accident'.

[Sacke]

Has anyone here actually broken a steerer tube by over tightening the stem bolts?

I didn't put the recommended aluminum sleeve in my Cervelo steerer tube, but just an expander bolt.

Now if some brands say it has a structural support function, I might just glue it in there, just to be safe.

[glepore]

Once the stem/headset assembly is properly pre-loaded and the stem bolts tightened, there are some in the WW community that go ahead and remove the top cap and bolt and replace it with a press-fit cosmetic cover. That said, I have to ask, did you tighten the Extralite Ultrastar to the specified toruqe? The newer Ultrastar 3 is tightened to 7-8 Nm of torque, but I have an older Ultrastar for which 15 Nm of torque is specified and I suspect that after using a digital torque wrench to ensure proper tighteness, I am confident I would over-load the headset bearing long before the Ultrastar slipped/moved while tightening the top cap!

This. However, if its not a real Ultrastar but rather the J&L, I've had situations where I was unable to tighten it enough to prevent slipping, and eventually destroyed one trying to. However, even if 'loose", if you can preload the headset appropriately, who cares.

[11.4]

This isn't a simple answer. First, there are many stems out there with different clamping characteristics. And second, there are many kinds of damage that can occur with or without the plug. The first point is self-evident and there's no way to address it in this forum. But as for the latter ...

Have you ever noticed how the plug sometimes won't engage properly until you've clamped down partially with the stem? That tells you right there that the stem is creating a concentric clamping pressure that is altering the dimensions of the steer. Steers are thin enough these days that they also can go out of round, based on how effectively the stem clamps on them. And a number of newer headset designs such as Cane Creeks and the new version of the Chris King don't insist on a specific outer diameter for the steer and just adjust a wedge ring until it fits. That means you can't really on the headset to provide stabilizing support for the steer because that wedge ring doesn't tighten enough to protect the steer fully.

We see some of the most common problems with improper plugs when working on the track. Riders may swap between aero and classic bars a couple times or more during a single evening of training or racing, so there are numerous setups involved, and they are always running into loose plugs or having to over tighten something to cope with a plug that isn't really sized properly for the steer. If one is reducing the outer diameter of the steer because the plug isn't properly supportive, then strong leverage on the stem, particularly if the plug isn't aligned precisely to support the entire stem clamp, can cause both slipping of the stem and crumpling of the steer wall.

In short, I would never consider it safe to do without the plug or to have the plug loose in the steer. I agree that the plug's only basic role in assembly is to allow accurate compression of the stack so the headset is adjusted properly. However, in actual riding, with the newer and thinner steers that have become commonplace since threadless headsets arrived, the plug has gained a second and important role. Having seen the damage that can happen to the clamping surfaces of the steer when the plug isn't working like it should, I'd never take that risk. You could consider aluminum sleeves or other alternatives that accomplish the same purpose, but modern steers require some kind of internal buttressing that goes the full height of the clamping pressure of the stem.

[McGilli]

Thanks everyone who replied and offered input. The compressor is in fact a J&L. As everything is already clamped in place and has been fine for a while now I'll put this one on and if I notice any slop in the headset bearings down the road I'll either reset with the Enve first again, or will just switch back full time.