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Old 06-04-10, 10:31 AM   #1
Steely Dan
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Cutting down a carbon steerer - DIY or let a pro wrench handle it?

so i got a new motobecane from bikesdirect.com several weeks ago and now that i'm satisfied with the vertical position of the stem, i've got about an extra 1.5" of steerer tube sticking up with spacers in between the stem and top cap. everything works perfectly fine, but i'm obviously not crazy about the aesthetics of the extra 1.5" inches sticking up from the stem.

i've cut down a steel steerer before with a pipe cutter, but the steerer on my new motobecane is carbon and that's a material i'm just not familiar with. my question: is cutting down a carbon steerer literally as easy as measuring things up and taking a pipe cutter to it, or is futzing around with carbon a job better left to a professional at a bike shop?
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Old 06-04-10, 10:41 AM   #2
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I've never taken a saw to a bike in my life -- so don't put too much faith in anything I say on this -- but I think you're supposed to use a hack saw on carbon rather than a pipe cutter, I believe the pipe cutter puts too much sidewall stress on the carbon. But you better check with others....
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Old 06-04-10, 10:44 AM   #3
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Don't use a pipe cutter for anything. Hacksaw with something to guide the cut.
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Old 06-04-10, 10:47 AM   #4
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High speed is your friend.
Cuts the fibers and seals the resin.
Use a diamond blade (tile saw) or cutoff wheel (dremel).
NEVER A HACKSAW
Use a jig to guide the cut.
Sandpaper the edges and you are done.

The cutting of carbon fiber composites is more of a grinding process then a cutting process; essentially, you want to grind away the fiber and resin slowly to make the cut.

Enjoy

Last edited by powers2b; 06-04-10 at 10:50 AM.
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Old 06-04-10, 11:37 AM   #5
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Wrap the steer tube with electrical tape at the cut point and use a hacksaw. You can clamp it (lightly!) in a vice to help guide your cut. Easy.
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Old 06-04-10, 11:41 AM   #6
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Cut it outside and don't forget to wear a respirator.
carbon fibre dust is very similar to asbestos.
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Old 06-04-10, 12:56 PM   #7
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Quote:
Originally Posted by powers2b View Post
High speed is your friend.
Cuts the fibers and seals the resin.
Use a diamond blade (tile saw) or cutoff wheel (dremel).
NEVER A HACKSAW
Use a jig to guide the cut.
Sandpaper the edges and you are done.

The cutting of carbon fiber composites is more of a grinding process then a cutting process; essentially, you want to grind away the fiber and resin slowly to make the cut.

Enjoy
This actually isn't as tricky as the above implies. Cutting with a fine tooth hacksaw works perfectly fine, at least for me and pros who have told me to do it this way - including the Park Tool website:

http://www.parktool.com/repair/readhowto.asp?id=111

The cut with a fine tooth hacksaw is very smooth without any fiber breakage or raggedness.

This job is actually quite simple, but the old saying of "measure twice, cut once" should be followed, even if you're a very confident person!! Also, since you have so much to work with (1 1/2"), I might suggest that you do a practice cut, maybe take 3/4" off it, just to see how your technique works.

I've cut a couple using my miter box and hack saw. The miter box isn't an optimal way to guide the saw for this though and I might try some other sort of guide next time. However, it worked fine in the end.

With my miter box, I wasn't able to get it perfectly square cut, and had to finish the job with a large flat file to get it squared up. It was a little fussy to carefully square up the cut. I found the high spot using a carpenter's square, sighting towards a window to see the light peeking through the lower area. I marked the high area with a pencil, and then carefully flat-filed to bring down that area. Re-sight, repeat, etc. Took about 5-10 minutes (easy does it!). You could do the same thing with a bench belt or wheel sander - ASSUMING you know how to use it, select the correct grit and are very, very careful!!!

I'll probably do it the same way next time since it's not that tough to square it off with a file, and I feel comfortable doing it, but really, it would be much easier to either buy a dedicated cutting guide that are available for cutting steer tubes, or I've read that two hose clamps can be used as a guide.

I also lightly dressed the edges - inside and out - with a file (flat file for outside, round/rat tail file for inside). This was just a tiny, tiny bit to take the sharp edge off. Not really noticable visually, but you can feel it. You could do the same thing with sand paper.

Finally, if you're at all uncomfortable, take the bike - as is - to whatever bike shop you have the best luck with and tell them to cut the steer tube and install the stem in exactly the same position you currently have it. It should be a minimum shop charge for that (whatever their minimum is). I'm thinking $25 or so would be reasonable.

Last edited by Camilo; 06-04-10 at 01:04 PM.
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Old 06-04-10, 01:13 PM   #8
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I wrap the steerer tube with tape (blue painters tape is good) to minimize fraying and get a clear marking line.
Use a NEW fine tooth (metal blade) hacksaw and cut away, paying attention to get a straight cut.
Wet the area with a spray bottle or your bidon as you cut. This keeps the dust trapped in the water and cleans the blade teeth.
Do this outside, you don't want to breathe carbon dust.

sand it lightly to round out the edges a bit and that's it!
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