A 1 1/8 straight fork tube going into a Trek Headset VP Zero Stack (Semi-Integrated) 1- 1/8+ Tapered to 1.5.
I think i need some kind of adapter for the lower 1.5 inch end... do they make these and if so what are they called / how to get one?
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The good way of doing this is to use a reducer crown race. That way you're not doing anything weird to the stack height. The big headset makers all have their own version of such a part and they mostly interchange, assuming a common ZS56 type lower bearing with a 45 degree crown race contact angle. SHIS nomenclature for a complete lower (cup, bearing, and crown race) that does what you need here is ZS56/30.
For example, if you wanted to get a complete lower, it would be something like:
Or, if you can ascertain that the headset you have uses the same kind of bearing as another crown race you can buy alone, or can take such a bearing, you could buy just the race, which would be something like:
One of the imperfect parts of SHIS is that, to my awareness, it doesn't have a great way of naming reducer crown races. The number 52 appears in the description for the above because it would also be used along with an IS52 lower. But, it would also play nice dimensionally with the ZS56 headset above.