Carbon steerer - expander plug (Hed Doctor) not gripping

GyatsoLa

I've had a constant rattle from the front of my Enigma steel framed bike since I got it a year ago. It has an enigma etape carbon fork with a carbon steerer, and a Hope threadless headset. I partially dismantled it a few times to try to identify where the rattle was from without success.

To cut a long story short, when I took it to my LBS they puzzled over it for a while and decided that the Hed Doctor (the expander plug that Hope use instead of a star nut) wasn't gripping the steerer. They broke successively two tightening screws trying to get it to grip, before concluding that it was hopeless. The carbon steerer was visibly swelling at the upper end under pressure (to the extent that it was difficult to fit spacers over the top). In the end, they replaced the Hed doctor with an unbranded expander plug which has a steel tightening screw and heavier grooves on the side for grip, and added some carbon anti-slip grease for good measure. It seemed to work, but just 40 km later.... its rattling again, I haven't opened it up, but I am pretty sure the expander plug has come loose again.

So is there a solution? It it a flaw with the fork steerer?

balthazar

The expander plug (and bolt) is only used to fine-tune head bearing preload, during initial setup. This misunderstanding is persistently reported here. After the stem bolts are tightened, the plug/bolt could be removed entirely, with no consequence but to make future headset adjustment difficult. The stem –alone– holds the assembly together!

The expander plug only needs to grip enough to permit a gentle preload. I've never measured it, but even with tricky headsets I'd expect that to be less than 5Nm. Yet by your account, the plug was deforming the steerer tube, whilst still failing to grip. That seems contradictory to me. I can only guess that something undiagnosed is wrong, perhaps within the headset.

Also, I'm alarmed by your account of the deformed steerer. That is a thick-walled tube of considerable structural importance: if it is damaged to the extent you report, I can only visualise that it is broken.

topdude

The carbon steerer was visibly swelling at the upper end under pressure (to the extent that it was difficult to fit spacers over the top).

You have got to be joking, that steerer sounds broken to me, replace the forks before it fails and you die horribly :shock:

GyatsoLa

Thanks for the replies - now I'm worried!

Balthzar - I know the expander plug only requires gentle preload - the problem is, that it just doesn't seem to have gripped, whether through gentle load or severe. I know its safe enough to ride without one (on a different bike a couple of years ago, i rather nervously negotiated 2,000 metre climbs and descents in Taiwan when the star nut disintegrated on me), although obviously thats not exactly ideal.

Balthazar and topdude - I'm surprised you say that the fork is broken - when I saw 'deformed', its just visible on close inspection - we only noticed it because it was so hard to fit the spacers over the top 5 cm or so... once forced through,they dropped down neatly. I suspect it may have been my (and my LBS's) attempts to get a firm grip that did the damage.

Maybe i should mail Enigma and ask for their comment.

balthazar

GyatsoLa wrote:

Maybe i should mail Enigma and ask for their comment.

I would. Without pictures, it's hard to judge, but I cannot imagine how the bung has at once swelled the steerer and failed to grip it.

By the way, the preload I'm referring to is the bolt into the expander plug, (which just serves as a fixed point). I'm sorry to labour that point, but too often here people have assured that they know how this works... but subsequently revealed that they in fact didn't, and were bearing down on the preload bolt as if it held their stem on. Without knowing you, it's hard to be sure you're not in their number.

Through regular queries in this forum, I've noted the headset/stem/preload bolt is one of the least understood interfaces on the bicycle.

GyatsoLa

Balthazar - yes, I understand that - or at least to the extent that when putting on a headset I've always only put just enough pressure on the preload to stop any movement and no more - thats what I was taught to do and what my Park Blue Book advises. If the damage to the steerer was done by overtightening, that was done by my LBS, when I asked them to investigate the rattle.

I emailed Enigma this morning, and remarkably I got a reply within a couple of hours (kudos to them). They said I should have used the expander they supplied, which surprises me as I've no memory of there being one with the fork. They also said that if the distortion was visible then the steerer was compromised - i.e. I need a new fork. Oh well...

Ben6899

Could be that if the headset has never been properly set then the bearings are knackered. And that means it'll never feel right. Good luck... and find a new LBS!

andyrr

I've an expander that has 2 bolts - 1st one os tightened for it's outer surface to grip the inner of the steerer then the 2nd bolt is tightened to compress the stem/headset. I guess the Hope one works in a simialr way and the overtightening is happening when the tightening of the expanded - steerer is being done ?
Anyway, iit does sound like the fork steerer has been overstressed and a wise move would be, unfortunately, to replace it.

maddog 2

:shock:

My understanding was that the Hed Doctor was for steel/alu steerers. I've not heard of one being used on a carbon steerer. Anyway, the second bung sounds like a better option.

As said, the bung is just to preload the bearings. After that it's the stem that holds everything in place. If it's coming loose then either it wasn't preloaded enough in the first place or the stem has moved.

GyatsoLa

Thanks everyone - I suspect it was my LBS that did the damage by being overenthusiastic with trying to tighten it - but I can't be certain of this, so I won't name and blame! It might well have been my fault when I originally did the work.

maddog - you might be right about the Hed Doctor for steel/alu (although I've heard at least one bike mechanic say he thought it was for carbon only) as its more an mtb part and most sus fork steerers tend to be metal, if I'm not mistaken. The website isn't very specific on this point.

Anyway, its an expensive lesson learnt, I've just ordered another fork.

on-yer-bike

Tightening the head cap bolt can pull up the expander if it is not tight enough even though there is not much torque on the head cap bolt. This has happened to me with a Trigon fork and their unusual expander device. This caused the expander to rise above the the top of the steerer and prevent the headset bearing from being preloaded sufficiently. However, even with this diagnosed I to still have a clicking noise when out of the saddle sprinting or climbing. The mind boggles at the thought of a professional mechanic giving it enough welly to deform the steerer.