Hi:
I hope this is an appropriate forum. I'm thinking frame builders can help.
I acquired a 1996 Litespeed Classic frame, size 53cm. I bought a new fork which I thought was appropriate, but it seems to have changed the steering/handling/stability quite a bit. The bike felt great before I switched forks.
The used frame came with an aftermarket quite beefy "aero" type CF threadless fork (original was threaded). This fork has a stated rake of 43mm and (as far as I can tell) a axle-crown length of about 370mm.
According to old Litespeed catalogues, this frame originally came with a 45mm rake fork and an axle crown length unknown (best I can tell would have been within that ~370 mm ball park)
[Interestingly, the next year, with the same geometry for that size frame, it came with a 43mm rake fork. It looks to me like in '96 they used 45 for smaller frames and 40 for larger. In '97 they switched to 43 across all sizes.]
Anyway, for various reasons, I bought a new Columbus Minimal CF fork with a stated 45mm rake and 367 A-C length.
It seems to me that the steering has become noticeably more twitchy and the bike just seems more unstable at than it was with the aftermarket fork that it came with. I haven't really had a chance to look at it at slow vs high speed etc. Just that my initial impression on a 10 mile ride was kind of twitchy and unstable.
I am absolutely no expert on frame/fork geometry, but did some online calculations of trail and "flop" for comparison, thinking there might be an obvious geometry change affecting handling. For the life of me, based on the tiny or no differences the calculator gave me, I can't figure out why this change is actually noticeable.... maybe it's imaginary?
I have read that decreasing the A-C length by 3mm (i.e. 370 to 367) slackens the head tube angle by about .3 degree. So I used the actual HTA of 73 for forks with ~170 mm A-C, and 72.7 for forks with A-C ~167.
The calculations I did were:
#1 Bike as purchased: HTA 73 (actual) Rake 43 A-C 370 Trail = 58 Flop = 16
#2 New fork (shorter A-C) HTA 72.7 (adjusted for A-C) Rake 45 A-C 367 Trail = 58 Flop = 17
#3 Bike as originally sold HTA 73 (actual) Rake 45 A-C 370 Trail = 56 Flop = 16
#4 My other road bike HTA 72.5 (Actual) Rake 50 A-C 370 Trail = 54 Flop = 16
So, it seems to me that, all in all, I would expect the new fork wouldn't show much if any noticeable handling difference compared the aftermarket fork that came with the bike. And, the difference with how the frame was originally designed, is very small as to not be noticeable. Finally, before changing the fork, it really did feel quite similar to my other road bike which I've had for years. I had spent quite a bit of effort getting the saddle position (over the pedals, angle, etc) and the saddle-bar drop and saddle-bars and saddle-hoods very close to identical to my other bike. I felt that they were identical in fit/comfort and similar enough in handling as to entirely satisfactory.
What other factors could be affecting this? The "old" fork was a pretty beefy "aero" type fork with a steel steer tube. The new fork is much slimmer with a CF steer tube; much lighter. Possibly fork flexibility difference? I checked all the fit angles and measurements after installing the new fork and they are very close (within a few mm, as close as can be).
Any words of wisdom? Something I don't understand and/or am missing?
Thanks