A shim between bottom headset cup and bearing

Question

I ended up in a situation in which my new fork with an integrated crown race imposes lower headset bearing (1.5" 45deg) stack restriction, i.e. I need an additional 1.5mm of bearing height so the fork doesn't rub lower headset cup (bonded to CF tube, by the way).

I see two ways to alleviate the issue:

1. Grind a bit of headset lower cup so the fork clears. It's aluminium and hanging part that rubs the fork serves no structural purpose, at least that's what I think.
2. Use a shim between lower cup and a bearing. See the CAD drawing for details. I reckon the support of cup from top (45deg surface) and outside (54mm inner diameter) should be enough, after all it's the same mechanism that keeps the bearing in place. But to my surprise, I failed to find such a product!

So, the question(s): is the shim approach a viable solution? If no, why exactly? If yes, can I buy one?

Update: in perfect scenario, I want to use the shim with another, less tall but lighter bearing I already own. I do not want to purchase a taller bearing.

Here's a fork crown photo: A the tight situation:

Update 2: as promised, the bottom headset cup: The tall bearing height is 8mm, the less tall lightweight bearing is 7mm.

Update 3: the 2.5mm shim looks too thick even for a 7mm bearing, the gap is ~2.4mm:

Answer 1

The custom shim works, I settled with a ~1.4mm option, this was the minimal thickness machinist agreed to make, he said "it would bend in your pocket otherwise". After properly preloading the headset bearings I did not notice any play during pedaling efforts or when applying front disc brake during ~200km the bike made since assembly. That's not much distance, so if anything happens and I survive I'll update the answer.

Apparently, some people experience the same problem.

To get your own shims, find a local machinist and use the blueprint from the question, adjust thickness as needed. My shims were made from a 2024 aluminium alloy plate. The outer diameter tolerance should be in the ballpark of +0mm -0.03mm, the rest doesn't matter.

Answer 2

I bought a bike from Van Nicholas in the Netherlands. It's the Skeiron road bike. The frame came with the fork so I would guess it should have been machined together for bearings and all without a problem. They sent 2 shims same as you described but I can't see why 2. The fork fits nicely, very close, but nicely with the Cane Creek headset bearing in the crown. The top of the headset was a different matter however, the headset cover did rub the headtube and one of those shims placed between the bearing and the headtube race made for a prefect fit. Almost a year of riding and no issues at all.