Is it really so bad to slam a stem?

Discuss light weight issues concerning road bikes & parts.
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RocketRacing
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Joined: Thu May 10, 2018 2:43 am

by RocketRacing

So various opinions advise against a full slamming of the stem. The logic i have had explained to me is that in an accident, with a carbon steerer, sheer forces from the headtube, and stem would concentrate forces at one point of the steerer... increasing the chances of a failure. A spacer would at least spread those forces over a larger area.

Thoughts?

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Alexbn921
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by Alexbn921

Nope.
Slamming makes for a stronger junction with less leverage on the steer tube. If you crash hard enought to break either the stem or fork it would be toast regardless of the spacers.

by Weenie


velov
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by velov

I've never heard of that reasoning...it's about fit/flexibility for most people. I doubt a tiny spacer would have any benefits in a crash. Also, if the accident was big enough to reach failure at the steerer I would get the bike scanned anyway even if the damage wasn't obvious.

alcatraz
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by alcatraz

If looks are a priority then ideally you buy a frame with the right length head tube for optimal fit with a slammed stem.

Do you mean that with a larger space between upper headset bearing and the stem, it'd take more of a beating before failing?

RocketRacing
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by RocketRacing

alcatraz wrote:
Tue Jul 23, 2019 4:21 am
If looks are a priority then ideally you buy a frame with the right length head tube for optimal fit with a slammed stem.

Do you mean that with a larger space between upper headset bearing and the stem, it'd take more of a beating before failing?
That is what i was advised for the logic. My view was that if you have forces that might damage a steerer, you have bigger problems to worry about.

Mr.Gib
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by Mr.Gib

This is a bit weird. A few questions in this "what if I crash" genre. My recommendation is a logical one: set up your bike so that it is optimized for riding, not crashing. Capiche?
wheelsONfire wrote: When we ride disc brakes the whole deal of braking is just like a leaving a fart. It happens and then it's over. Nothing planned and nothing to get nervous for.

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Calnago
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by Calnago

Well, like most everything, it depends. I think Trek for instance still says to use at least a 5mm spacer under the stem as a result of some rather catastrophic failures back a few years. But the bottom line is that it has to do with the stress risers that can result when an abrupt edge is forced against the steertube. Either by a relativley sharp edge on the lower face of the stem, or a similarly abrupt edge at the bearing, with the compression ring. In Treks case, they were blaming the stem manufacturers and actually officially said not to use FSA stems, since those seemed to be on the forks that broke. And in the newest headsets supplied to Colnago from Acros for example, the compression ring is actually made of Delrin, or like material, to further cushion the steertube from such force. A compression plug that actually provides structural support to the steertube and extends through the stem and all the way past the upper headset bearing is really good insurance against this kind of thing happening, although it’s pretty rare. And even when Trek was adamant about the 5mm spacer under the stem, I would still see pics of pros setups without that spacer. Common sense should prevail here.
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icantaffordcycling
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by icantaffordcycling

I still use a 5mm spacer so that I am in accordance with rule #45 and to keep my warranty but otherwise I would like to slam it so that the steerer tube is flush. The expander in the steerer is what provides support for that area in the case of a crash so just get a really long expander like the Colnago one or slam your stem
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Djakninn
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by Djakninn

I thought it was a 5mm spacer above the stem that Trek advised. The instructions that came with my Factor also suggests a 5mm spacer be used above the stem regardless of spacers or not under it.

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Calnago
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by Calnago

Yes, they advise they as well. Top and below. The one up top is more to ensure the steertube can make contact with the full length of the stem and not be crushed or “coned” if the top of the steertube ends around it below the top stem bolt. I think it’s just food practice to have a 5mm spacer above but there are many designs now where the stem cap can sit flush. But those are in conjunction with a steertube compression plug that supports the upper part of the steertube usually. Depends on the construction. Canyon wasn’t using anything there for awhile. I’d go with the manufacturers recommendations.
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Hexsense
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by Hexsense

The spacer below can be removed if you use normal size headset cap.
Compare to very slim headset cap, normal headset cap is like cap+spacer 2 in 1 already.

JerryLook
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by JerryLook

I always heard to put a 5mm space above the stem too.
Right now mine is slammed with a “slam that stem” upper bearing cover, with the 5mm spacer above the stem. I have a normal expander, which extends down into the upper bearing area. I have heard this helps stiffen up the upper bearing area as well.
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RocketRacing
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by RocketRacing

Helpful comments.

Makes me want to fully dlam stem and get a headset expander plug that spans top of stem to the top bearing.

I use an extralite. Any longer, but light options?

An additional reason for dpacers above the strm was fit/resale.

by Weenie


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