I don't understand how these things fit together... could one put a threaded headset on any frame, i.e. it's just a matter of the proper forks & head bearings?
Hi Jim!
The assembly diagrams comparing threaded to threadless headset should help visualize the difference. See:
http://sheldonbrown.com/images/382px-Bicycle_headset_exploded_view-en.jpg ...and...
http://sheldonbrown.com/images/Bicycle_headset_(threadless)_exploded_view-en1.jpg...full story here...
http://sheldonbrown.com/headsets.html Threaded headsets use a threaded top race and locknut that engages threads on the steerer. Threadless headset components simply stack together, held in place by a clamp-on stem (or other clamp on the steerer, like a Thorn Accessory T-bar). Threadless headsets use a star-fangled nut to remove excess play in the system until the stem clamp can be tightened on the steerer, locking the adjustment in place.
On a threaded assembly, the stem has a quill that fits *inside* the steerer. It has nothing (much*) to do with bearing adjustment, and simply holds the handlebars in place. The quill is held in the steerer by a split wedge or expanding wedge that allows it to be raised or lowered (largely*) independent of headset bearing adjustment.
Did Thorn make such bikes?
So far as I know...no. SJSC did briefly sell some Tange-tubed road frames under their label that were made in the Far East and I have seen a few of those on eBay with threaded steerers/quill stems, but those aren't Thorns.
could one put a threaded headset on any frame, i.e. it's just a matter of the proper forks & head bearings?
Ehh, no.
Almost, but not quite.
The problem is, threaded headsets were mostly produced for 1" (nominal outside diameter) steerers. A very few larger-diameter steerers were made for quill-stemmed MTBs.
At the same time, there are not a lot of threadless headsets available these days for 1" steerers.
It isn't just steerers; the headtubes are sized for the steerers.
Assuming you could get a match in steerer diameter and head tube diameter, then as far as the frame is concerned -- yes! -- you could freely switch back and forth (changing stems appropriately).
Best,
Dan.
*Tightening the stem quill wedge can pre-load the bearings to a small degree, so it is wise to check bearing adjustment after the stem quill bolt is tightened as well as before. The wedge must never, ever be in the part of the steerer that is threaded else fractures can occur with predictably tragic results. The wedge must also never be down in the lower, butted portion of the steerer else it won't expand and clamp properly.
