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Cutting a steerer - why subtract 3mm?(6 posts)

Cutting a steerer - why subtract 3mm?Fez
Aug 19, 2003 7:24 AM
Every instruction I have read on cutting a fork steerer says subtract 3mm from the top of the stem for "adjustment clearance."

Not questioning the instruction, but can anyone explain why this is necessary?
re: Cutting a steerer - why subtract 3mm?zero85ZEN
Aug 19, 2003 7:30 AM
When you tighten and set the headset it can "settle" tighter than where it was when you measured for the cut. (The instructions usually are not referring to cases where you have put spacers on top of the stem and tightened everything down for test riding purposes...)
Also, you will need some clearance for the top cap, which usually seats into the top of the stem, to be able to tighten up and not bottom out on the top of the steer tube.
re: Cutting a steerer - why subtract 3mm?Calvin
Aug 19, 2003 8:30 AM
This figure is commonly stated for threadless headsets. The steering column shound engage as much of the stem as possible, but it should not be flush with the top of the stem. See also http://www.parktool.com/repair_help/howfix_forksize.shtml
Preload adjustment.Spoke Wrench
Aug 19, 2003 9:00 AM
The purpose of the top cap is to set the headset preload adjustment. For it to be able to do that, it has to be able to press the stem onto the headset bearings. If you have no gap at the top of the steerer tube, the top cap can bottom onto the steerer tube and consequently can't press downward on the stem. You need about 3mm because most top caps are thicker in the center than they are at the circumference so they dip down toward the steerer tube.
Unless you are running a carbon forkMShaw
Aug 19, 2003 11:55 AM
'cause then you want about 3mm stiking out above the stem. That way, the stem does not clamp around the cut, making it less likely to crush the edges of the steerer tube.

Put a spacer on top of the stem, and adjust as normal.

Mike
Unless you are running a carbon forkdivve
Aug 19, 2003 10:22 PM
...normally a fork with a carbon steerer should include a long body top cap that inserts 25 to 30mm into the steerer to prevent crushing.