http://davesbikeblog.blogspot.com/2007/05/trail-fork-rake-and-little-bit-of.html
^that explains it
Although your new fork may also be a little longer which will , in effect, slacken the head angle relative to the fork, reducing the effect of less trail (this is sometimes the case with putting road forks on track bikes)
@rpm started
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http://davesbikeblog.blogspot.com/2007/05/trail-fork-rake-and-little-bit-of.html
^that explains it
Although your new fork may also be a little longer which will , in effect, slacken the head angle relative to the fork, reducing the effect of less trail (this is sometimes the case with putting road forks on track bikes)