My Carbon Fibre Trek (mid 90s) came with an Aluminum fork which I slightly bet recently in an accident. More than one bike shop said I was going to die if I didn't replace it, but it is a 1" with a long steerer tube. I found one after buying 3 on ebay that fit. It is a 'composite' Kestral. It fits fine but I had to put a 3mm spacer on the upper part of the bearing. It turns perfectly when not riding, but when riding it feels completely unstable. With the bent AL original fork I could ride without my hands on the handlebar, but with this new fork, I feel like the bike with go out of control even if I look behind me and not let go of the handlebars. It is the worst stability on a bike I have felt, but the shop says everything is good. What could be going on ? The new fork is a little lower (3mm) but could that f*ck up the whole geometry?
Thanks