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I see a lot of new aero stems are clamped at the very top of carbon steerer tubes. Wondering how this is safe, does the design of them get around the issue of previously needing 3 mm of more above the stem?
You could always get away with clamping close to the top of a Steerer, but having it extend slightly above and fitting a 5mm spacer was just prudent.
I suppose the key thing is making sure if you cut it down you really mean it because unlike before you can't just go re-stacking spacers.
the issue of previously needing 3 mm of more above the stem?
Do you really need 3mm though, probably not....
cut steerer to top of stem, add 5mm (or any size) to tension headset. clamp stem then remove spacer and replace top cap now flush with stem, up to you if you take a 5mm spacer with you, but when has a headset ever come loose during a ride if properly adjusted in the first place?
Provided the top bolt was clamping steerer tube (and not crushing the end - which is unlikely since there will be a plug not a star nut, I've never thought it a problem. My 3T stems only have one bolt anyway! Some of the other designs have more than 3mm above the bolt and hence the top of the stem is effectively the spacer. And as Rocket says, you can always just pretension and remove. All about the looks in my opinion.
And yes you can extend a once-cut-too-short steerer tube. I have one of the extenders in a fork and it has been faultless. I've since swapped the fork for a lighter Ritchey, but the offender will be in another bike presently.
I’ve since swapped the fork for a lighter Ritchey, but the offender will be in another bike presently
Wouldn't it be better to just cut it the right length this time 😉
Ask the last owner :p - I wanted to raise the stem slightly and swap from their slammed 140 mm!!! Epoxy bonding is an option, but it's really just a nicely engineered longer carbon bung.
I don't think you really need 3mm of the steerer above the stem.
IMHO this came about from the 5mm spacer being common and cutting the steerer 2mm short of that to ensure there is clearance between the top cap and steerer to ensure preload.
Personally, I use a 2mm spacer with about 1mm of the steerer above the stem and a top cap with a profile that doesn't foul on the steerer tube.
The stems with an integrated top cap are designed to sit without a spacer on top so that is obviously fine with the big IF that the designers have done their job right.
The issue with traditional stems tends not to be the clamp at the back but their small area mating onto the front of the steerer tube due to the cutout where the stem section is. If the steerer is sitting below the top of the stem then much of the mating area towards the top of this interface is gone which greatly increases the risk of the stem camming or causing impact damage in use or during a minor crash etc.