2old

Don't know if this was mentioned, but Atomic Cycle in LA has a headset that supposedly (only read about it) allows the larger fork to be used with the smaller head tube. Call them.

Redshift96

Thanks so much for the tip. AtomicCycles.com is the same shop as GenuineBicycleParts.com. I called,Atomic, and it sounds like this is the "RetroRyder" adapter mentioned in other threads, designed to use a 1" steerer in a BMX/older-Schwinn-type headtube using a different 1" standard headtube seat ID of 32.85mm. Won't work for a headtube with the ISO 1" standard of 30.10mm. Unless the headtube insert/seat has enough meat to be reamed out to 32.85mm. Doubtful that this would be wise in my case, as the OD of the carbon headtube is 39mm. I'm going to see if i can determine the wall thickness of the headtube insert once I remove the original headset, but again, highly doubtful that this is a viable solution.

Nonetheless, huge thanks for the pointer!

Redshift96

Thank you. Turns out this is dead on. Having worked with CrMo steerers exclusively, I was more than a little shocked to discover that the wall thickness of the aluminum steerer on the fork I'm intending to use is a mere 1.95mm. Holy frigging safety margin, Batman. I guess I should be talking to the weight weenies about this.

I may look into this approach (sleeving the ID), but I would definitely use aluminum rather than steel. Especially if intending to bond them, as differences in the coefficient of thermal expansion between materials would be an invitation to bond failure and differential stresses over time.

It appears that the viable choices are:

1.) Ream the headtube seat from 30.1mm to 32.85mm and use the Atomic adapters with the unmodified 1-1/8 aluminum steerer.I suspect that this will not work as it would likely thin the headtube insert too much, if not completely.

2.) Sleeve the aluminum steerer with machined aluminum tubing of the same grade. This may indeed require boring the ID of the steerer in prep for a tight bond.Not planning an interference fit, but this might be an option if there is some way to manage the temp stresses on the carbon/aluminum bond. Then machining the OD of the steerer to the ISO 1" spec, 30.1"/~26.0mm (25.4mm nom.).

3.) Fab up some external adapter cups which mount the 1-1/8" headset bearings outside the headtube, I'll need to calculate the effect on head tube angle, but I'd expect it to be pretty negligible. I suspect that the adapters would probably need to be steel, though I'd prefer aluminum. Again, stress calculations in store. I'll post up a drawing if I end upgoing this route. The big question here is: Would a machined lip which locates the adapter inside the headtube seat be thick enough to allow the 28.6mm steerer to pass through while still being structurally sound? (That lip would be roughly 0.75mm, hence fabbing the adapters from steel may be necessary vs aluminum here.) Or would the the adapter need to locate via the OD of the headtube? I'm not convinced that a slip fit over the headtube OD is appropriate on a carbon headtube.

Again, thoughts/recommendations welcome. I'd like to do as professional a job as humanly possible here... not afraid of the machine shop. Just don't want to cut corners or compromise on safety/reliability.

base2 

Old vintage Schwinn Varsities use the old BMX standard head tube diameter. Schwinn 1 inch steerers are not current industry standard 1 inch. I forget exactly what makes them different... Steerer thread pitch, wall thickness, & internal diameter for the quill stem to insert in to. Maybe all 3. I don't remember any more. Whatever. At 30.x mm head tube diameter for a 32.x head set cup to slide into leaves precious little material in the head tube to hold the works together.

The solution though is pretty easy. A lathe, an interference fit, a press, a bit of welding, a reamer & a facing tool.
Witness the fitness:


headtube upper cup by Richard Mozzarella, on Flickr


headtube lower cup by Richard Mozzarella, on Flickr


headtube overall by Richard Mozzarella, on Flickr

Non-drive side front 1974 Varsity by Richard Mozzarella, on Flickr

Or, switching tracks, & if you could find one, you could also use a VP-H755 headset to use any generic current industry standard 1 inch fork into a vintage Schwinn/BMX 32.x (🧐🤔 ) frame. You might need to ream the 21.15mm top locknut to use a 22.2mm quill though. I think the VP-H755 has been discontinued. In searching for this thread, nothing useful presented itself.

Redshift96

Thanks much for the reply and the pics. If I were working with a steel or aluminum frame, I'd be all over that solution.

At the moment I'm leaning toward boring/honing the inside of the aluminum steerer, then sleeving it with a precisely machined aluminum tube, epoxying that in place, and turning down the exterior to ISO 1" spec. Not thrilled with the idea of less than 0.030" of the original steerer wall thickness remaining though. Ugh.

However, the stock wall thickness of the aluminum steerer is only 0.077", so maybe losing half of that isn't so drastic, presuming the sleeving is done well. I'm intending to use at least 0.100 wall aluminum tubing for the sleeve.