First time working with threadless headsets on a new build
 

Sorry for the newbie question, but I've searched and searched and can't really find the answer I need.

I'm building a new bike from scratch for the first time in about 10 years. One of the "new" things, to me, this time around is the threadless headset.

First, I notice threadless headsets come in different stack heights. Does it matter what height I go with? Does stack height come into play with setting what used to be "stem height" in the days of threaded headsets?

Or, is "stem height" now a combination of stack height plus using headset spacers?

Or perhaps to put it another way, how do you know where to cut the fork in a threadless setup?

Sorry, but this is just a little confusing yet.
Thanks in advance!

Yes, thats it.

Total steerer tube length = Head tube length + Headset stack height + Stem steerer clamp height + Total spacers - 2 or 3mm. You need the extra couple millimeters because if the steerer were flush with the stem, you wouldn't be able to achieve adequate compression.

Start a little longer than you think you might need, you can always add spacers above the stem and you can cut the steerer shorter once you dial in the right handlebar height (though it's a little tricky shortening a steerer with the star nut already installed--you just have to cut a little at a time and reset the star nut further down the tube). [Edit: fixed a couple problems in the equation above.]

In order to get the handlebar height where you want it you can assemble the whole thing before cutting the steer tube or setting the compression. Just don't ride it until you cut the tube and set the compression. And even then you can cut it long and use a spacer above the stem if you want to ride it awhile before making a final cut. Spacers are available in several thicknesses and are used between the headset and stem to set the height of the stem. You could also leave a 10 mm spacer above the stem as long as you want to. As previously mentioned, the top of the steer tube needs to be a few mm below the top of the stem or top of a spacer above the stem so that you can control how much compression you can apply with the top cap and bolt. Tightening the stem's pinch bolt(s) is the last step.


Al

Just so I understand about "spacers"....

I can use them "temporarily" above the stem, to avoid cutting the fork tube, and also possibly below the stem depending on needed stem height. Once I've found the proper stem height, I may need spacers above the headset/below the stem, permanently to obtain that height, correct?

Any recommendation on spacers? I've seen both carbon and aluminum. Weight is not important.... at least not in this part of the bike.

Unless you've got a really big frame, you'll want to cut the steerer at least some as you experiment with stem height--otherwise, you'll have tons of spacers. You will likely need some spacers below the stem when you're dialed in. I like at least a small spacer above the stem so that the stem completely clamps around the steerer, but that's not necessary.

Until you install the star nut, it's not good to ride the bike because the headset will likely be sloppy. Do you have another road bike (even with a quill stem)? You can use it to estimate where you want the new stem (same saddle to bar drop, seat post to bar length, etc.).

Can't go wrong with aluminum spacers.

http://sheldonbrown.com/handsup.html

According to this, you can use a seatpost binder on the stem and not need the star nut, or spacers. This would let you play around with height without pressing the star nut in. Then once you figure out where you want things, cut and install the nut.

That's an interesting tip.

Park tool's website has service / repair guides: Overview. Threadless headsets. Cutting steerers.

I've used the threadless guide, and it helped me a lot.

If the fork has a carbon steerer tube you must not use a starnut. (Yes, Cannondale has a carbon fork that uses a specific starnut but I believe that isn't the case here)

Carbon steerer forks use an expansion plug which can be removed and replaced until you get the steerer length finalized. BTW, if the fork does have a carbon steerer, I recommend leaving it long enough to require at least a thin spacer (say 3-5 mm) ABOVE the stem. This assures the stem clamp is completely supported by the steerer and protects the steerer's top edge from the crushing forces.

Do you know how exact your "FIT" is going to be on your new Build?

On my build this year, I left the steerer tube as one of the last things I did. I got the bike finished, and then took it to the LBS that I work at. We did a full "FIT" on this build for me and dialed in the height of the stem, choose the angle and length. We added spacers where we needed too. Then we marked, measured TWICE and then CUT the steerer tube.

This worked very well on my build.

Chris