Originally Posted by
hokiefyd
Length: to my understanding, I cannot (or should not) clamp a threadless stem around the threaded section of the steerer. My plan would be to cut the steerer tube at the very bottom of the threads (where the cone nut is in the picture) and use spacers under that for height, as with a normal threadless steerer. Is this correct?
I assume that the cone needs to match the race, and that it would be best to buy a 1" threadless headset, and at least use the top race and cone out of that. Is that the best option?
With respect to *not* clamping the new (threadless) stem on the threaded portion of the old steerer tube, you are absolutely correct. Unless you do not value life & limb.
In my role as mechanic at a busy bike Co-op, I regularly see the frightening products of backyard mechanics. But this ghetto conversion is among the scariest: where a threadless stem is simply clamped on the top of a threaded fork steer. Often, the stem is barely secured - hanging on by 10mm or less to the very end of the steerer.
So the main problem here is not as much the clamping forces of the new stem, but it is that the threads introduce as major stress riser to the steerer tube. Think about the constant back and forth forces on the steerer in the gap right under the stem. Reinforcing from the inside, doesn't fix this. It may only buy you a few miles of the bars hanging on to the rest of the bike when the steerer fails.
Bottom line: a threadless setup simply requires a smooth, strong steerer.
And yes, get a 1" threadless headset, or at least the top parts.