New Fork - Real handling difference or imagination?
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  1. #1
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    New Fork - Real handling difference or imagination?

    Hi:

    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 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

  2. #2
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    You did a great job gathering the appropriate specs.

    I agree with you; the spec'd numbers of the original fork vs. your replacement shouldn't reveal that much of a change in handling.

    On a technical point, I think you may have a dimension backwards. I would expect a shorter A-C length to INCREASE head angle, not decrease it.

    My suspicion would be one of two things-

    1. The change in A-C is causing the handling change. I experienced something similar when I installed a headset with a different bottom stack measurement. Drove me nuts until I replaced it with something closer to the original headset. Here my suggestion would be to perhaps install the fork from another bike you own, one with a 370mm A-C measurement. If that solves the problem then you have your answer. It involves a little work but it need not be permanent.

    2. The replacement fork is out of alignment. Of course, carbon forks can't be aligned, so you're sort of guessing on this. Calfee Design has a white paper on their web site explaining the importance of fork alignment. View it in terms of your problem vs. the shimmy mentioned.

  3. #3
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    Twitchy/unstable could also come from an improperly adjusted headset or sticky headset bearing.

  4. #4
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    Thanks for the comments.

    First, DUH - I am embarrassed that you had to point out that the shorter fork legs actually steepen the headtube angle. That's what I was visualizing, but for some reason I went the other way in my calculations. Thanks for pointing that out. It did make the differences, on paper, a bit more pronounced, but still not very great.

    Second, I rode a pretty varied, hilly 30 miles today. Before riding, I checked the headset and it seemed perfectly fine. But I went ahead and loosened the stem and headset and re-adjusted it making doubly sure it was not too loose or too tight (Can you make a sealed cartridge bearing headset too tight? It doesn't seem to happen in all the headsets I've done). Anyway, I was confident the headset was as good as all my other bikes' headsets.

    Not too surprising - I got used to it fairly quickly and it doesn't bother me any more. Yes, it is a perceptible difference from my other road bike which is a relaxed sort of design (Felt Z). But it is just different, not "bad" and not a deal breaker. I would no longer call it twitchy or unstable, it's simply towards the more responsive end of what I'd call a normal/acceptable continuum as opposed to my other bike which is on the more stability oriented end.

    It could actually be what the original Litespeed designers intended (remember, I don't really know what the axle-crown was on the original fork so this new one could actually be the same).

    Anyway, I learned a lot looking at this aspect of frame geometry which I had never done before and I appreciate your comments. (especially being kind re: my backwards headtube angle adjustment!)
    Last edited by Camilo; 06-04-2019 at 08:20 PM.

  5. #5
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    Quote Originally Posted by Camilo View Post

    Not too surprising - I got used to it fairly quickly and it doesn't bother me any more. Yes, it is a perceptible difference from my other road bike which is a relaxed sort of design (Felt Z). But it is just different, not "bad" and not a deal breaker. I would no longer call it twitchy or unstable, it's simply towards the more responsive end of what I'd call a normal/acceptable continuum as opposed to my other bike which is on the more stability oriented end.
    Yeah, being different enough that you noticed just surprised you.

    When I first started riding by bike with low trail, I couldn't believe the difference, and was worried I'd made a mistake in going that route. But by the end of the 2nd ride I loved the difference and now prefer the ride characteristics. There is still an adjustment when riding another bike, but I've adjusted by the time I'm out of the driveway. And my trail numbers are quite different, and large, 30 for the low trail and mid 50's for the rest of my bikes.

    At one time I focused on chain stay length, thinking that was where the handling changes happened, but now I realize that it's front end geometry where most of the magic happens.
    Too old to ride plastic

  6. #6
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    Can u ride the bike without hands on the bars?
    If you can't control it without hands, using just balance, IMO you may have went too far!
    If you can control it, I would think that you are just aware of the 'change'. Ride it a while and see if you get used to it.
    BANNED

  7. #7
    'brifter' is a lame word.
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    You can adjust ANY headset too tightly. Doesn't matter what kind of bearings.
    I work for some bike racers
    I've got some bikes, some guns,
    and a bunch of skateboards

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