Just a curiosity really, but would this crazy idea work...

Threadless fork, install a really deep star nut, use a long bolt to pretension and clamp a stem on as usual.
!Then, remove the pretension bolt and top cap.
Install a quill stem into the fork above the existing threadless stem.

(yes, Im aware that this would require dissasembly for future headset adjustments)

So, how well would that work? Quill capable of supporting handlbars and associated weight? Threadless fork capable of supporting quill wedge without wall deformation? Etc....?

Possible applications include:
using the normal threadless stem as headlight mounts
poor-man's stem riser
frankenbike points

So as I see it, you'd have 2 stems on the bike, since you can't remove the threadless one which is keeping the headset adjustment. I won't comment, since you already categorized it as crazy.

sounds awful.

+1. What possible benefit does it have?

there are products you can mount to your bars, which are cheap and hold heavy lights securely.
The most ideal position for a light is just to the left or right of the top of the front wheel is anyways.

check this out

If I wanted a twin stem bike, I'd just get an uncut threadless fork and drop two stems on it. Maybe even put a lock collar below the bottom stem so that I could rearrange stems without affecting the headset adjustment.

You get a second stem for your second set of handlebars.

Indeed this is ideal, done that before.
But this question is mainly a curiosity for what to do with an existing short cut fork, whether a quill stem would be workable or if the wedge would just deform the steer tube.
Are threadless forks made thinner walled than threaded ones?

I imagine non-carbon threadless steerer tubes could handle a quill wedge. If you can put the wedge where the threadless stem is that would give you some peace of mind. If your upper bar is for accessories, you wouldn't really need to reef on the quill bolt much.

Like this: https://www.sheldonbrown.com/harris/surly-rohloff/

What is ID of the fork's steerer tube?

dunno. I've heard it can vary. I've only 1 1/8 threadless forks around, but no 1 1/8 quill stems, so I can't test fit here.

I have a great fascination with "Frankenbike's".

Sheldon has been probably the most useful single source of information, but not all of his contributions can be found on his site.

I recall he had mounted an ahead stem on a long quill stem; someone, with his guidance, had mounted a threadless stem on a threaded stem riser, which would seem to allow the use of two stems; and he had used a threadless fork without a star nut (I believe that is on his site).

A long steerer tube would still seem like the best option, but there seem to be numerous ways to mount two (or more) stems, and various other ways of mounting accessories.

to answer the OP's question: no, it wont work at all.

You'd need a really long head tude in order to use a really deep star nut. If you can deal with installing the fork and stem without the star nut (much more difficult to get proper preload) a quill stem CAN be used in a (steel) threadless fork.

With the right tools many (not all) threadless forks can be converted to threaded. If you do a forum search you should find the relevant info. The install depth for the quill should still place the wedge in the head tube, so you'd need a fairly tall stem, but that's not that unusual an item.

Forget the jamming the star nut in, instead, find a Bike shop with a Bike Mine account,
BM in the US distributes BBB parts a NL based company,
Their BHP21 is a quill type internal or stem raiser.
It replaces the star nut, , so the star nut is not needed , their design uses a stem bolt,
of their design, which is threaded inside it's hex socket.
so the headset pre load adjustment is retained via bolt thru top cap.

I got one to restore height to a steerer tube cut too short for my needs,
and I got to add a 2nd stem to mount my handlebar bag upon.

BBB likely contracts thru Taiwan too, thats where the bike manufacturing capital is..
so retail price was about $23, same as other stem raisers on the market..

https://www.bbbparts.com/headparts_bhp21.php

as I used it, : https://www.cyclofiend.com/working/20...clark1008.html

Thanks fietsbob.
I hadn't seen the threadless steerer extension before.
I'd seen a threadless stem riser, which clamped over the steerer and had an area for the stem to clamp above, and I'd seen threaded to threadless adaptors (which were basically the same design as what you posted), but I hadn't seen anything advertised as a steerer extension.

I would imagine that, as with a threaded stem or threaded to threadless adapter, this would be slightly less rigid/solid than a standard threadless setup (but not to an unsafe degree). Is there a perceptible difference in feel?

I dont have the Luxury of 2 nearly identical bikes but for that one difference,
in order to have a proper comparison, I used the BHP21 to regain the height lost to the hacksaw
in a distant prior build up, of this bike ..

The Quill is a strong extruded tube with a keyway slot,
and the stack of shims are made with the matching key tab.

Another solution, common in most Wholesale parts inventory a LBS can order
is a steel tube made to have a smaller diameter on the bottom .. ID of fork
and a larger OD at the top.. you could insert another quill stem, or you could
add a few shims and clamp a threadless stem around that .

from Harris site probably from QBP, wholesale
Stem Raisers for Standard Expander/Wedge Stems $19.95



See:
SM247I inch BMX.833" / 21.15 mm for older American bikes, BMX, early MTBs
SM2481 inch Standard.875" / 22.2 mmfits most bikes with threaded headsets.
SM2491 1/8 inch1.0" / 25.4 mmfits oversized forks with threaded headsets.