Cutting a carbon steerer tube below stem

Discuss light weight issues concerning road bikes & parts.
Alpine318is
Posts: 152
Joined: Tue Aug 18, 2015 6:30 am
Location: Seattle, WA

by Alpine318is

Hello,

Currently on my S-Works Venge, I have an ENVE stem with a 5mm spacer above the stem similar to below. I can't stand the look of it and want the top cap to sit flush with the stem. I have heard in the past that you need to leave 5mm of carbon steerer so that the stem clamps the whole steerer and you don't risk damaging the top of the steerer. With the ENVE compression plug I am using it sits on top of the steerer tub instead of just inside of it and I would think would provide enough structural support to keep the stem clamping forces from damaging the steering tube. Am I correct in assuming that?

Thanks!

Image

Image
2012 Specialized Venge S-Works
eTap
ENVE 4.5
2013 BMC Granfondo GF01
eTap
ENVE Classics 45
2016 Cervelo P2
Ultegra 6800
ENVE 6.7

by Weenie


NickJHP
Posts: 274
Joined: Wed Feb 14, 2018 2:22 am
Location: Canberra, Australia

by NickJHP

No, because as your photo shows, the expanding part of the bung will be about half a dozen millimetres below the top of the steerer, and the stem will have to sit slightly above the top of the steerer in order for the top cap to fit without a spacer.

Nefarious86
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by Nefarious86

I just make sure I cut my steerer to the mm so that when the headset is compressed the cap is basically touching the expander. I actually had to shave the underside of this cap to get it to tighten up properly lolImage

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spartacus
Posts: 108
Joined: Mon Apr 04, 2011 6:53 pm

by spartacus

I used to do it but now I use a 5mm spacer when I cut forks just for peace of mind.

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nycebo
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Location: New York, NY

by nycebo

In fairness OP, the 5mm space call doesn't look bad. But to answer your question, I think you're better off with a slave but may be alright with a 3mm spacer. I've seen some online.

The shave the underside of the top cap is a cool looking solution.

TobinHatesYou
Posts: 4197
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

It depends on the stem as well. If an alloy stem is hollow at the joint, more clamping force will be applied throughout the rest of the steerer clamp. If the bolts are at the extremes of the clamp, that also isn't optimal. Some stems like Bontrager's high-end XXX carbon stems are larger diameter for the last 4mm or so and designed to be used without the need for spacers on top.

In general I think it's pretty safe to leave about 3mm with most stems where the bolts would sit below that mark.
Last edited by TobinHatesYou on Thu Mar 28, 2019 9:15 am, edited 1 time in total.

gurk700
Posts: 315
Joined: Fri Jan 20, 2017 7:40 pm

by gurk700

5mm above looks great to me. Also comes with a piece of mind.

TobinHatesYou
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Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Here's an example of a stem (bar/stem combo in this case) designed to be clamped with 4-5mm to spare.

Image

If only more stems were designed this way...
Last edited by TobinHatesYou on Thu Mar 28, 2019 8:13 am, edited 1 time in total.

NiFTY
Posts: 1323
Joined: Sat May 26, 2012 11:26 pm

by NiFTY

I don't for my bikes. I also think it looks rubbish and when descending on the top tube i rest my chest on the stem and the spacer is uncomfortable. I have extralite expanders in both bikes with nil issues.
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corky
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Location: The Surrey Hills

by corky

Anyone ever damaged the top of the steerer by not having 5mm extension?

mattr
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Location: The Grim North.

by mattr

corky wrote:
Thu Mar 28, 2019 8:36 am
Anyone ever damaged the top of the steerer by not having 5mm extension?
Yes, when used with a
TobinHatesYou wrote:
Thu Mar 28, 2019 7:08 am
If an alloy stem hollow at the joint,
the steerer had cracked where it was only supported by a ~2mm lip where the top of the stem "hole" was.
(Not mine, a friends)

mattr
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by mattr

Thinking about it, this is one of those places that an old standard (using a spigot on the bottom of the top cap to centre and locate the cap) is no longer valid. And is causing issues (only on the look of it, mechanically, it's fine)
What you need is a recessed top cap, with a 2-3mm location socket fit in the bottom. You still need *some* steerer above the stem. But at least you can use a neat, one piece cap. Rather than a spacer, and a cap with a spigot fit.
I might see if i can knock one up on the printer at the weekend.

happyon2wheels
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Location: Denmark

by happyon2wheels

I used to run the top cap flush with the stem (Ritchey WCS 4Axis and later the C260) but after starting to read posts on this forum I have added a few mm above the stem on new bikes with carbon steerer. Instructions from various manufacturers vary and opinions certainly do and I can't say it has made a difference compared to when it was cut flush...so I guess it's a matter of piece of mind more than anything else and I am now used to the look of a 5mm spacer above the stem :noidea:

TobinHatesYou
Posts: 4197
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

mattr wrote:
Thu Mar 28, 2019 9:05 am
Thinking about it, this is one of those places that an old standard (using a spigot on the bottom of the top cap to centre and locate the cap) is no longer valid. And is causing issues (only on the look of it, mechanically, it's fine)
What you need is a recessed top cap, with a 2-3mm location socket fit in the bottom. You still need *some* steerer above the stem. But at least you can use a neat, one piece cap. Rather than a spacer, and a cap with a spigot fit.
I might see if i can knock one up on the printer at the weekend.

Cervelo and a few others have used that “bottlecap” design with top caps. It does look marginally better, but thoughtful stem design looks even better.
Last edited by TobinHatesYou on Thu Mar 28, 2019 9:24 am, edited 1 time in total.

mattr
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Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Cool, someone obviously had the same idea as me. Dammit ;)

by Weenie


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