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I have a bike (~2016 year of manufacturing) with a fork which has what is called a straight 1⅛" steering tube, used with threadless headsets.

I have always thought that, compared to tapered 1⅛ – 1½ steerers, the straight meant the same diameter all the way down from the top. A strictly cylindrical piece of tube that is.

However, at the latest overhaul I measured the diameters of the steering tube at different positions. Closer to the top of the tube it was 28.6 mm or 1⅛ inch as expected. However, at the crown race it was 30 mm.

Studying the documentation to the bicycle confirmed my measurements, as the lower headset cup was listed as EC 44/30, which means 30 mm internal diameter of the bearing to match the steerer. The fork is still called as "straight steerer" by the vendor.

The fact is, my "straight 1⅛ fork" is slightly but still tapered at the crown. This poses a few questions in terms of (in)compatibility.

Are all straight 1⅛" steerer forks actually tapered to be 30 mm at the bottom? Are/were there forks with truly cylindrical tube with 28.6 mm all the way down?

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Yes this is totally normal. The crown race is fitted to the base of the fork steerer with an interference fit. If the steerer didn't have a slight lip at the base, the interference fit would be all the way down the tube!

The reason it is called a straight steerer is because it has the same size bearings top and bottom. Tapered meanwhile means having larger bearings (like 1.5") at the crown race than at the top race. Much more noticeable.

I don't think the diameter of the crown race seat is totally universal, I've had to file one down before at least. So that is a good example of where the EC 44/30 SHIS code you notice is so useful in getting this kind of thing right.

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