I run modern Pace carbon rigid forks in the retro Ti hardtail and my guess it is 20mm longer than "design" was bitd. I quite like the handling this produces and so do other riders that had a go with it. Note the extra 20mm in a rifgid fork have a very direct effect on head angle, no sag there. So in my book anything in the 20mm change you should be fine, but you may or may not like the resulting effect
There are other parameters in the steering equation, notably the fork rake. Head angle works with fork rake to determine trail? and a lot of the steering feel. But weight distribution over the front and rear also affects it. Remember gary Fisher genesis geometry? Combines short seat stays with long top tubes and short stems to create a different steering feel to his bikes.
However, I also ride modern All Mountain rigs with 68-69 degree head angles, my perception of steering probably has shifted.
Enjoy!!
There are other parameters in the steering equation, notably the fork rake. Head angle works with fork rake to determine trail? and a lot of the steering feel. But weight distribution over the front and rear also affects it. Remember gary Fisher genesis geometry? Combines short seat stays with long top tubes and short stems to create a different steering feel to his bikes.
However, I also ride modern All Mountain rigs with 68-69 degree head angles, my perception of steering probably has shifted.
Enjoy!!