[Critter Cameraman]

If my steerer tube top is about a 0.5mm (1/2mm i.e. a couple of frog's hairs) below the top of the stem (almost flush) so my steerer cap cannot compress correctly and I add 5mm spacer above the stem top and compress the steerer cap... is 5mm too much? Do I need to use a 3mm spacer? or is 2mm and 2 frog's hairs extra not crucial from the usual recommended 3mm space between steerer tube top and steerer cap?

And why is 3mm the magic space between the steerer tube top and the steerer cap?

Thanks and Merry Christmas!

 

[TraxFactory]

If my steerer tube top is about a 0.5mm (1/2mm i.e. a couple of frog's hairs) below the top of the stem (almost flush) so my steerer cap cannot compress correctly and I add 5mm spacer above the stem top and compress the steerer cap... is 5mm too much? Do I need to use a 3mm spacer? or is 2mm and 2 frog's hairs extra not crucial from the usual recommended 3mm space between steerer tube top and steerer cap?

And why is 3mm the magic space between the steerer tube top and the steerer cap?

Thanks and Merry Christmas!

Ideally the steer tube should pass completely through the stem with some stick-out so you get full bite on the stem when tightened. I usually put a 10mm spacer on top of stem, just in case I want to fine tune my setup.

In your case I dont think 3 or 5 mm will matter since there is zero stick-out. Perhaps you can remove/reduce any spacer on bottom of stem if possible so you get the stick-out on top.

 

[jan_kaspar]

What ever spacer you put on top, the setup is safe. Let’s start with that. It may not be ideal but you won’t break anything by having 0.5mm less clapping area on the steerer. The only thing you’re trying to achieve proper preload on the headset bearings. I would use the smallest spacer that still allows the top cap to not touch the steerer when tightened. This will allow you to achieve optimal headset preload. Then tighten the stem and do a final light cinch on the top cap again and you’re in business. Alternatively you can get a flatter top cap.

 

[mack_turtle]

Stem Removal & Installation: Threadless

This article covers the full process of how to replace threadless stems, including component selection, spacer orientation, and centering and headset adjustment.

www.parktool.com

The stem or spacers should be slightly above the top of the steerer tube (approximately 3mm). If they are flush with or below the steerer tube, headset adjustment will be impossible. Acquire additional or differently-sized spacers to reach the appropriate height. It is also possible that the new stem is taller which may require you to remove a spacer.

When dealing with a carbon steerer tube, it is critical to have the steerer tube go all the way through the stem*, in order to reduce stress on the end of the steerer tube. It is then necessary to have a spacer on top of the stem.

*This is not true of carbon fiber Cannondale SI forks found mostly on their road bikes. Their proprietary headset system has compression assembly that reinforces only part of the steerer tube, so the stem has to be about flush with the top of the fork so that the stem clamps the reinforced area.