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there aren't any headshok/lefty rigid options that will drop straight in I guess?
There are rigids. They are hard to find and expensive.
FWIW a working headshok can be had for about £100 and when locked out is rigid. And when unlocked is a very tidy light fork. I don't think they can be bought new any more.
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I found these 1.5 to 1.56 shims.
I have no idea how they would work with a steerer that's supposed to be a-head with a crown race and compression bung. cannondale 1.56" is an interference fit.
Might be worth a punt though if you are committed to that fork.
Probably not. This replacement headset might work however according to this chart. But I'm not sure. You'd be taking a punt. It seems to be that the Cannondale headtube has the same size bore as 'oversize' headtubes that existing headsets for 1.5" steerers are designed for. These might work your frame.
It's possible but unlikely - looking at the photos I'm pretty much certain the frame has the headshok headset. Folks used to describe the Cannondale standard as 1.5" because 1.5" didn't exist back then. Now that it does, some folks haven't realised that they are two different things and are still using 1.5" to describe 1.56".
The question to ask the seller is: "can I fit a headshok fork with the existing headset without adapters and whatnot?"
Reducer - drop in to existing headset. Takes 1-1/8th straight forks.
Headset 1-1/8th straight.
Headset tapered - takes tapered 1-1/8th to 1.5" forks
No problem.