lowering stem on carbon steerer

jackc6549

i'd like to lower the bars on my poggio a little bit. Not to look pro I'd just prefer a slightly lower position. Theres about 50mm of tube above the headtube (1x10mm bearing cover & 4x10mm spacers).

Would be ok to lower the stem by 1-2 spacers (10-20mm) and put them on top of the stem? Or would I need to cut the fork tube? My main concern is, I guess, the expansion plug provides some internal strength to the tube, so lowering the stem without lowering the plug would result in the stem clamping on a hollow tube. Or am I just talking bollox? first bike with a carbon tube, so thought I'd risk ridicule on here before the possible crunch of carbon

thanks

edit: this bloke has lowered the stem a lot without cutting http://blog.wiggle.com/2014/02/10/revie ... oggio-1-5/ but no update to say whether he has any problems, etc

lapavoni10

I would run it with the extra spacers above the stem for a while to make sure before cutting. There should not be a problem with the steerer as long as you dont go nuts, I dont have a bung and never had any issues.

rrsodl

jackc6549 wrote:

i'd like to lower the bars on my poggio a little bit. Not to look pro I'd just prefer a slightly lower position. Theres about 50mm of tube above the headtube (1x10mm bearing cover & 4x10mm spacers).

Would be ok to lower the stem by 1-2 spacers (10-20mm) and put them on top of the stem? Or would I need to cut the fork tube? My main concern is, I guess, the expansion plug provides some internal strength to the tube, so lowering the stem without lowering the plug would result in the stem clamping on a hollow tube. Or am I just talking bollox? first bike with a carbon tube, so thought I'd risk ridicule on here before the possible crunch of carbon

thanks

edit: this bloke has lowered the stem a lot without cutting http://blog.wiggle.com/2014/02/10/revie ... oggio-1-5/ but no update to say whether he has any problems, etc

For what is worth.... I don't think it's a stupid question at all. It's a grey area so you might not get a definitive answer. It took me a long time to get assurances from the fork manufacturer to say it was OK to have 40mm of spacers, they will only advice 25mm max. I thought this was not right so I kept asking until the fork designers said it was OK. A lot depends on the stem length and bars reach that most people don't know about. Other fork manufacturers accept 40mm max as normal.
Back to your question, I think your concerns makes sense. I have asked myself the same question, I moved 20mm of spacers to the top and I feel OK on the bike but for longer rides 50+ miles might be too much so I'm reluctant to cut the steerer off. I'm considering one of these in the mean time I'm spending a great deal of time on the drops for anything up to 25 miles and then mostly on the hoods.

rrsodl

lapavoni10 wrote:

I would run it with the extra spacers above the stem for a while to make sure before cutting. There should not be a problem with the steerer as long as you dont go nuts, I dont have a bung and never had any issues.

You say you don't have a bung, may I ask what do you use instead with a carbon steerer?

Cheers

wishitwasallflat

jackc6549 wrote:

..the expansion plug provides some internal strength to the tube, so lowering the stem without lowering the plug would result in the stem clamping on a hollow tube.

This is correct, whatever else you may do re spacers and/or cutting make sure your stem clamps only to the part of the steerer which contains the bung - it's there to support the part of the steerer which experiences compression forces from the stem being clamped on.

antfly

Two spacers would be fine, according to the bike manufacturer I asked.

Matthewfalle

Cut away - it'll be fine.

You can run no spacers above or below with perfect safety - look at everyone who "slams" their stem.

If you don't have a bung then some people even use a star nut without fatal consequences.

No need to call fork manufacturers etc.

I also wouldn't worry about all those people who say "oh no, don't cut it, you might want to sell it at some point" - if you thought like this you'd never take it on the road.

antfly

Somewhat cavalier advice, there. A star nut is not a good idea on a carbon fork steerer.

Matthewfalle

Agree that a star nut is not perfect but people do use them and don't die.

Don't really see what is "cavalier" about the post though - steerer is too long, remove spacers, cut down excess, set fire to waste.

Simples, n'est ce pas.

slowbike

Matthewfalle wrote:

Agree that a star nut is not perfect but people do use them and don't die.

Don't really see what is "cavalier" about the post though - steerer is too long, remove spacers, cut down excess, set fire to waste.

Simples, n'est ce pas.

If you're going to offer advice that is not best practice then you should state that it isn't. That's the second bit of advice you've given today that is (IMHO) incorrect.

Tightening up steerer clamps around the stem will put a sideways crush stress onto the stem. It is partially held in place by the front and back of the clamp. A star nut will not add any (significant) opposing force to the crushing effect of the clamp. An expanding bung does add support to the steerer which is why it is used for carbon steerers.

Matthewfalle

It's not a sideways crush - it's a central fit. Stems do up at 5nm 7- that's not enough to crush if you fit a stem properly.

If you're doing things up so tight that they crush I humbly suggest that you give your tools away (IMHO).

A star nut is an option, not a recommendation.

slowbike

Matthewfalle wrote:

It's not a sideways crush - it's a central fit. Stems do up at 5nm 7- that's not enough to crush if you fit a stem properly.

If you're doing things up so tight that they crush I humbly suggest that you give your tools away (IMHO).

A star nut is an option, not a recommendation.

It is a sideways crush - you're obviously not mechanically minded. Most stems do recommend 5-7nm torque.
Star nuts have sharp edges that are designed to grip the aluminium steerer - they will cut into carbon steerers and damage them.

I've not crushed anything - but then I use the right tools and the right kit for the job.


In your opinion a star nut may be an option - but it's not recommended practice.

lapavoni10

I dont use a bung, and have never had any issues.
The same applies to my Campag carbon seat post, which has thinner walls than my fork steerer and also does not have a bung (and which I think has greater force applied)

Matthewfalle

lapavoni10 wrote:

I dont use a bung, and have never had any issues.
The same applies to my Campag carbon seat post, which has thinner walls than my fork steerer and also does not have a bung (and which I think has greater force applied)

Exactomondo. Grazie.

Next please.

jackc6549

Thanks for all the help and replies

homers_double

Matthewfalle wrote:

Don't really see what is "cavalier" about the post though - steerer is too long, remove spacers, cut down excess, set fire to waste.

Good lad, bloody namby pamby roadies.

Cut it, it's fine. I cut carbon fibre tubes all the time, specifically 10-11m long ones designed to steer an 8lb angry carp away from reeds. If it's above the stem it isn't under any sort of force at all except a small amount of compression from the top cap.

antfly

In which case he doesn't need to cut it.

tuneskyline

I slam my stem No spacer just a cane creak AER headset [ lowest stack height]. On my other bike when it was built up a Guru Photon I had the stem virtually inside the top of the frame Here's a photo


Here is my other bike a Trek , its an old photo but I still have the same stem -17 x130cm set



No spacer's on sight ...go low and fast

gozzy

So, discussions about star nuts aside. Is the conclusion that, yes it's mostly considered ok to fit the stem with spacers above and below, so that the stem is fitted below the bung, before cutting the tube?

homers_double

I'd reposition the bung, then cut it.

jackc6549

homers double wrote:

I'd reposition the bung, then cut it.

I forgot to mention, the bung has a collar (about 3mm high) that rests on top of the steerer tube so repositioning it like you can with a star nut isn't an option. Unless I buy a different bung. Do they make compression bungs that don't rest on top of the tube?

To be honest, the main reason for lowering it (apart from preferring a slightly lower position) is:
1. I don't like the idea of 50mm of tube above the head tube. I'm probably being paranoid but it just seems needlessly high and increasing leverage on the tube.
2. Even more so, I don't like the fact the stem is about 5mm higher than the top of the tube so its clamping the very top. Its not so bad because of the aforementioned bung resting on top, but I'd prefer it clamping a bit lower and having a 5mm spacer on top.

I've cut all tube on my previous bikes and have no problems doing it, but all of them had alloys tubes. Just wanted to get the full sp before cutting this. I'll probably get round to it at weekend, after getting some different sized spacers and a decent saw blade.

jackc6549

tuneskyline wrote:

I slam my stem No spacer just a cane creak AER headset [ lowest stack height]. On my other bike when it was built up a Guru Photon I had the stem virtually inside the top of the frame Here's a photo


Here is my other bike a Trek , its an old photo but I still have the same stem -17 x130cm set



No spacer's on sight ...go low and fast

As good as that looks, its making my neck and back hurt just looking at it.

Matthewfalle

Looks cool and fast to me and no neck/back pain from this neck of the woods.

tuneskyline

Matthewfalle wrote:

Looks cool and fast to me and no neck/back pain from this neck of the woods.

Spot on.

homers_double

Jackc6549, I'm assuming the bung reinforces the carbon tube as traditionally they don't like being clamped.

If you can, remove the bung, cut and refit. Honestly the bit sticking up is doing nothing at all.

gozzy

Gozzy wrote:

So, discussions about star nuts aside. Is the conclusion that, yes it's mostly considered ok to fit the stem with spacers above and below, so that the stem is fitted below the bung, before cutting the tube?

Ha, I didn't make my point too clearly last night.

My query was, I'd like to try out different set ups first, then eventually I will cut the steerer.
So, can you safely re-arrange the stem and spacers to try out different heights, even if this means clamping onto unsupported tube as the bung only fills the top inch or so of the tube. So if I move it down 30 mm, I will be clamping onto un-bunged steerer.

keef66

When I built up my CR1 I found that Scott recommended no more than 5mm spacers above the stem. Their supplied bung sits at the top of the steerer; I assumed their advice was to ensure the stem was clamping onto that part of the steerer supported by the bung.

Having said that, the resulting offcuts of CF tube were unbelievably strong when I tried to crush them.

I think there's probably a reason the use of star-nuts in carbon steerers has fallen out of favour.

gozzy

As it happens, it's a CR1 that I've got; it's good to hear that the steerer offcuts were quite strong.

mitchgixer6

I did a quick test with the off cut when I cut the steerer on my new Felt. A quick test jumping up and down on it with my full weight (75kg) resulted in zero damage.