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Does a tapered carbon steerer (40mm at the crown) typically require a slotted crown race if the steerer lacks an integrated bevel on which the lower bearing can rest?

I have a Cane Creek Forty headset with an unslotted crown race whose inner diameter is 39.8+ mm and a tapered carbon steerer whose outer diameter is 40mm "on the nose".

Having read about the dangers of tightening a stem onto the upper section of a carbon steerer without a compression plug inserted into the tube and properly torqued to provide resistance against the inward forces, I am concerned that this Cane Creek crown race might not be the right one to use with this particular carbon steerer.

What are typical tolerances here? Is this steerer typical at 40mm or a bit chubby? Is 39.8+ mm typical for unslotted crown races, or a bit skimpy?

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The Cane Creek 40 system uses cartridge bearings. The crown race of this bearing system is generally to support cartridge bearing, ie: it doesn't actually have bearing balls "racing" on it. Those are contained between the 2 halves of the cartridge bearing. What this means is that one can use a split crown race without interfering with the bearing function or setting of the bearing preload. As you infer in your question, a split crown race requires far less force to seat, making installation onto a carbon steer tube far less risky. Even if it's proven that proper installation of a closed crown race upon a carbon steer tube is 100% harmless there's the peace of mind factor to consider using a split crown race.

One can split the provided crown race using a cutting wheel on a rotary tool or a hacksaw supplied with an appropriate fine-tooth blade. It may be best to split the provided crown race as it's typical protocol to use a crown race that comes together with the headset. One can obtain a separate, split crown race that will work if it shares the same dimensions as the provided race. Apparently you will have to obtain a split crown race from another manufacturer as no listing for a split crown race is found in the "replacement small parts" section of Cane Creek's 40 web information. Indeed, specific directions or cautions for installing Cane Creek headsets on a carbon steerer tube are absent, at least within the 40 section of their website. I favor splitting the provided crown race and gently installing that as it would be most economical and preserves the unity of the headset.

Regarding your tolerance questions, I quote from Park Tool's Threadless Headset section discussing pressed cups and bearings: "There must be a slight diameter difference between the two pressed surfaces. Typically, the pressed headset race outside diameter should be between 0.1mm and 0.25mm larger than the head tube inside diameter." Your posted diameters differ to within this range, and so your parts are sized as they should be.

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  • I did opt for peace-of-mind and cut a slot in the Cane Creek crown race with a fine-toothed hacksaw as you suggested. It simply "clicked" into place. The Cane Creek web page "everything you need to know about headsets" is silent with respect to carbon forks. Cane Creek support advised me that it's OK to force their unslotted race onto a tapered carbon steerer; their rationale is that the carbon of the taper is much thicker than elsewhere on the tube. The tolerances you cited are for the press-fit not the race-taper area. I wouldn't infer they're similar for race and tapered carbon steerer.
    – Tim
    Jan 17 at 13:16
  • @Tim Yes, that line I quote from Park Tool's website is just before explaining pressing of the cups, however the section on pressing the crown race refers back to this introductory section discussing the acceptability of a press fit, ie the tolerances, inclusive of the crown race and it's seat.
    – Jeff
    Jan 18 at 8:29

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