Wilmingtech

One of the things overlooked in this thread is that Carbon manufacture and design has constantly been changing at a great pace over the years. In addition the makeup of a fork and the components that hold it all together on each manufacturer's bike will be different as well.

There is a great difference between the carbon fork on my 2013 road bike and 2017 endurance/gravel bike. The expanders are different, the carbon thickness on the forks are different and I am certain the layup of the carbon is different as well.

Best practice is to stick to the manufacturer's recommendation until you take it all apart and understand the engineering behind it. What works fine on one bike doesn't mean it's going to be the same for all bikes.

-Sean

2old

I had a carbon steerer tube cut by a good shop, then when I disassembled the bike several years (but not too many rides) later, there were filaments separating from the steerer tube. I would put spacers above the tube to prevent needing to cut it, but probably not more than two inches in height.

MinnMan

Update.

I went to the LBS from which the bike was bought, which is of course, a Felt dealer. They say that a small spacer (5 mm) on top of the stem is actually necessary, so that the force of the bolt securing the stem does not crush the top of the steerer tube.

When the fork arrives, I'll be bringing it back to this LBS for installation. They do much more work with high end bikes than the first LBS.

reburns

After reading this thread, I posed the question to Advanced Composites, maker of the Wound Up Duo 2 all carbon fork used on one of my tandems. They have no problem with putting spacers above the stem.

DrIsotope

In the assembly manual for my R3, it explicitly states not to have any spacers above the stem, as well as the max amount of spacers below the stem. The top of the steerer is supposed to be ~3mm below the top of the stem. I also went full commitment and glued in the aluminum sleeve, because I've seen what an expander is capable of, if it happens to sit below the clamping area of the stem.

I imagine every mfr. has their own recommendations. I know some forks can be run with no expander/insert at all.

jadocs

Ideally you want the compression fitting centered where the stem attaches to the steerer. So you need to know the length of the fitting. Some are shorter than others. The type of top cap and compression fitting lip determines how much you need to add to the cut on steerer. In this case it was 5mm plus an extra 2mm to the steerer.

In this example you could run it without a top spacer, but using a spacer on top actually centers the compression fitting better. I will also echo the notion that your top spacer did not cause your steerer to fail.

Sy Reene

Combining these 2 guidances yields what I've always thought was the 'rule'... namely steerer tube cut about 2.5mm above top of stem clamp, 5mm spacer over that 2.5mm. Steerer tube top protected by the spacer that's extending above by 2.5mm. The stem clamp is fully interfaced with the steerer tube.

FWIW, this is also what happens to be in Zinn's book as a "Pro Tip": "If the fork has a carbon steering tube, always place one spacer above the stem. That way, the entire stem clamp is clamped onto the steerer, and there is no chance for the upper part of the clamp to pinch the end of the steerer. This is a good idea for a steel or aluminum steering tube as well."

jadocs

Pinching is not an issue with a compression fitting that interfaces with the top of the steerer like the one I posted above. You will note there is a small rim that sits on top of the steerer. It’s impossible to pinch because it makes the top of the steerer solid. This rim is 2mm and adds to the length of the steerer. The type of cap I am using requires 5mm above the steerer to compress properly, which is where the 2mm + 5mm I referenced above comes from.... but it is still optimal to run a spacer above so the fitting is centered in between the clamping mechanism of the stem.

78rr

The manufacturer does not have a uniform standard and I will follow the manual of the current accessory for installation.

TimothyH

The issue isn't the spacer itself above the stem but as others have said, lack of support for the stem inside the steerer when using short expansion plugs.

If the expansion plug does not extend far enough down into the steerer then the bottom of the stem can crush the steerer. Steerers are constructed with fibers running along the length of the tube. They are very resistant to bending loads but not at all resistant to crushing loads. It only takes 1 or 2 percent elongation to start cracking the fibers.

An example is below. The Specialized plug is on the left. The Enve plug is on the right. The Enve plug may not be long enough to support the inside of the steerer at the bottom of the stem.




-Tim-

jadocs

That Enve compression fitting looks a lot like mine on my Litespeed. If it is the same that lip is 2mm not 1mm based on me physically measuring it.

TimothyH

This video explains the issue with steerers and expansion plugs well.



-Tim-

jadocs

Why the heck would someone do that? I've heard people say the top cap is not necessary after adjusting the headset, but removing the compression fitting? That's kind of dumb, why would anyone remove something that contributes to structural integrity?

TimothyH

To save weight.


-Tim-