Cutting and installing a carbon steerer
1.2.02 by Dan Empfield
Sometime back, and to be honest I can't even remember the time or the place, I was asked about cutting carbon tubes. As I recall, the context was a Carbon X one-piece aero bar manufactured by Profile. At least that's what I had on my mind when I answered the question. I remember telling the asker that one could use a tubing cutter, and that this is what I'd recently used on a set of Carbon Xs that I'd recently adapted for use.
That answer I gave started to gnaw on me. Perhaps that was an incomplete answer. I thought it prudent to address the issue of all-carbon tubes, such as is the case with carbon steerer tubes on forks. I found the opportunity and the timing fortuitous, as I just this weekend mitered a pair of forks with two such steerers, in preparation for a road test. (I'm just now embarking on a Dura Ace vs. Record shootout, which will be fully covered on Slowtwitch, and I built up a pair of identical bikes, except each had one or the other of these fabulous road groups. But I digress).
Here's one thing in which you can rest assured. Everybody -- that means everybody -- who's spent much time in the service department of a bike shop or bike manufacturing facility has cut a steerer too short at one time or another. Anybody who says he hasn't either isn't allowed access to the cutting tools, or is hiding the awful truth. Heck, I've seen bona-fide"pro mechanics" thread the bottom bracket backwards. I've seen them rip the threads off a crankarm by using the crank puller without remembering to remove the washer. So don't fret. If you miter the steerer too short, just put an ad on Tribuy.com letting everyone know that you've got a fork custom made for a 42cm bike. (Just use an assumed name).
Bottom line, mitering the steerer too short won't make the fork any less wrecked, and you'll still have to go out and buy a replacement fork, but at least you'll have the satisfaction of knowing you're not the first and you won't be the last.
We're going to talk about threadless steerers here, for a few reasons. First, we're talking about carbon steerers, and these only come in the threadless (Aheadset style) variety. Second, heck, does anybody make bikes with threaded stems anymore?
In the case of the photo at top left, I've got the bike in the stand and I've turned it up on end (so the loose parts don't fall onto the ground). I put the fork in with all the head parts attached, plus the spacers, plus the stem. This is the part where most people blow it. They forget to put the stem in. They calculate the steerer length based on only the head tube length, the headset, and perhaps the spacers. You can measure all this without having to assemble everything, but the best way to make sure you're not forgetting anything is just to put the whole fork on, just like you're building the bike. Now make your mark, right at the top of the stem.
The top of the steerer needs to be about 3mm shorter than the top of the stem (technically, the top of the "stack," which is the stem and the spacers combined. You may choose to have a spacer or two above the stem, so that you can move the stem up a little if you want. Either way, your steerer has to be 3mm shorter than all that stuff combined). As you can see from the photo above right, I've market the steerer, and I'm scribing the outside -- really just scoring the outside -- with a tubing cutter. If you've got a mitering block, you don't need to do this. But I'm not going to cut this steerer in a mitering block, but just freehand. So I'm going to score the outside, all the way around the circumference of the cut, so that my hack saw blade will have a guide. Do this very, very carefully, with very little pressure applied to the tube. You don't want to break the integrity of the carbon along the inside diameter of the steerer tube.
If these tubes were carbon-reinforced aluminum tubes, as is the case with a Carbon X bar, I'd just (gingerly) cut the whole thing with a tubing cutter. But you can't do that with a pure carbon tube. You've got to saw it.
Now I cut the thing with a hacksaw. But you can't use your basic hacksaw blade, that won't cut through carbon. You've got to use a toothless blade, and there are many styles on the market. I use this same sort of blade when I'm cutting anything plastic -- like PVC pipe (necessary every time I back over a sprinkler head with the car).
When I'm done cutting the steerer, I get rid of any flash on the inside with a file. I've got a half-round file that's almost precisely the same arc as the inside diameter of a 1 1/8" steerer (above right). Then I very gently do the same on the top of the steerer with the flat side of the file. Now I'm ready to install the fork into the frame. If you do this right, it all fits fine and -- again -- that steer column is 3mm short of the entire stack (as is the case with the installed fork pictured at left). Make sure your carbon steerer has some sort of wedge on the inside that braces it against the tightening pressure of the stem. While steel steerers are quite capable of exhibiting a great amount of crush strength, carbon steerers are not, and so they need the wedge on the inside. If you've got a fork with a carbon steerer and you have no such wedge, consult with the manufacturer of the fork and ask about this.
FOR MORE ON CUTTING STEERERS, INCLUDING THE USE OF A MITERING BLOCK SPECIFIC FOR THE USE, CLICK HERE.