Replacement fork has forked up my bike A little while ago, a year or so, I changed the cheapish steel fork on my commuter for a cheapish carbon bladed fork I had hanging around. The headset on the bike is a little non-standard but the shop that did the work for me (moved the crown race over) assured me that it would be fine.

Since then the handling of the bike has been atrocious. The new fork has a greater rake and a longer steerer but the handling of the bike is now really unsteady and twitchy. The other issue is that it's suddenly really difficult to get the loading right on the headset, a sixteenth of a turn in either direction and it's far too loose or far too tight.
The new fork has a greater rake ... but the handling of the bike is now really unsteady and twitchy.


The greater rake reduced the trail.  The trail the distance between where the pivot axis of the fork meets the ground and where the tire meets the ground.  The greater rake moved the front wheel forward, towards the pivot axis of the fork.

It's like the front wheels on a shopping cart.  The wheel points in the right direction because of the force from having the contact point behind the pivot axis.  A smaller trail reduces the forces that keep the bike going straight.

See the Wikipedia article on bicycle and motorcycle geometry:  https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Trail


  The other issue is that it's suddenly really difficult to get the loading right on the headset, a sixteenth of a turn in either direction and it's far too loose or far too tight.


That sounds like the bearings might not have been properly installed - an incorrect crown race (wrong angle?) or something like that.  How do you know they moved the old crown race over?  They're not hard to get off with the right tools, but a little impatience during removal can result in a bent crown race. Maybe they cleaned off all the old grease and didn't re-lube it when installing the new fork? 

Without details, it's almost impossible to do anything other than guess at the cause of this problem.