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Well, you kind of can. Isaac used one mould for a track/TT frame and just stuck a spacer between the fork crown and the lower headset bearing to fix the difference in BB drop they thought necessary.
In practice, your main problem is that the steerer has a short oversized section at the bottom to fit the crown race, or with some carbon forks the bearing seat is moulded in place. Those are not insurmountable issues, but need custom parts made to interference fit tolerance levels, not just the generous clearance fit you have with normal spacers.
If you go more than a few mm, you have to start thinking about whether the steerer is really fit to take such cantilevered loads.
In the end, why would you even try? Forks can be readily obtained in a wide variety of axle to crown lengths, and if you want something that's not available off the shelf, they can be made by a frame builder.
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gbj_tester
ltc
I'm sure there are reasons so can someone with a better (better than non existent) understanding of engineering explain why you can't run spacers under a headtube in the same way that you can above it?