New Fork - real handling difference or imagination?
 

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

Your assumption that the shorter fork will slacken the HTA is in error. The shorter fork steepens the angle, so that puts #2 at 54mm trail and 15 flop. It also lowers your saddle to bar drop, steepens your seat tube angle and lowers the bottom bracket. I would consider 3mm of ATC change to be insignificant, but the combined effect may make a noticeable change.

I should play with a trail calculator more, 3mm of atc and 2mm of rake makes that much difference? Huh

But going to less trail will increase the "twitchiness." You will get used to it.

One of my friends bought a used bike with a replacement fork. It must have had a longer ATC and possibly less rake than stock, because it really didn't want to turn at low speeds. Looked perfectly normal.

The new fork results in a steeper head angle and with more rake there is less trail. All in all pretty minor differences though. The Columbus Minimal is a quality piece and I agree with unterhausen that you will get used to it. Keep riding.

Agreed that the new fork steepens the HA although the numbers on paper are small changes in themselves. I wonder if some of the handling feel is also due to the small weight distribution change. But I would exchange the forks and pick the one I liked the handling of better.

Years ago I built a frame/fork and decided that I wanted to try a fork with less rake/more trail. So I built a second fork matching the A-C dimension and found the handling to be only slightly different and didn't solve the problem I was trying to "fix" (camping load shimmy). I left the second fork on the bike because overall I like a longer trail. Andy

I had to replace the original fork on my Merckx Team SC - previous owner had put a star nut in the carbon steerer. D'oh! Replaced it with another Merckx fork with 2mm more rake (45 vs 43) and it's made a subtle but noticeable difference in the handling (not for the better). So yeah, a small change makes a difference.

Thanks for all the comments. It really helped.

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, thanks for the reinforcement of my opinion that this change isn't likely to be a big deal, and for the "just ride it" advice. I rode a pretty varied, hilly 30 miles today. I'll have to say, that you are absolutely right - I got used to it and it doesn't bother me at any speed. 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'd have to say, it's simply a more responsive as opposed to a more stable sort of ride. Definitely within what I'd consider "normal" handling. 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).

Anyway, I learned a lot looking at this aspect of frame geometry which I had never done before and I appreciate your very kind words (especially on my backwards headtube angle adjustment!)

You may come to love the quicker steering. I raced a bike with VERY quick steering many years ago. So quick it took a month to be comfortable, two or more to be comfortable no-hands. (For scary 3-4 races it was a Godsend. It would steer though any crash.)

My Mooney is maybe barely on the quick side of "classic". My custom ti "good bike" is quicker. My custom ti fix gear is race quick. Not as quick as that old racing bike but I have to pay attention. I absolutely love it. But I do find really quick bikes get me to ride faster. Now the real fun is when you can ride that really quick bike and leave a perfectly straight tire track.

Ben

A couple of things I've learned from painting and repairing frames is that the original specs and what they actually measure are often not the same. Another unknown in this equation is whether either of the forks are still accurately aligned. Any discrepancies can also impact the feel of the ride.

Thanks Doug, I was going to mention that handling differences might not all have to do with geometry, i.e. only with rake and A-C

Doug, I think you're saying that true geometry, that accurate measurements of the actual bicycle might not match the geometry that is published, say in a geometry table.

I take Doug's comments to be this too. I have spent some time with Doug and have talked about spec drift. Back before the interweb was Gore's dream we use to believe what was in print:) Now I know better. Andy

When I went to England in ’75 to learn how to build frames, I used my Italian Masi Gran Criterium I had gotten in Milan in ’72 as a geometry pattern to build my own frame. I very carefully measured the seat/top tube angle with a vernier protractor. It read 73º30” so I made my new frame the same angle. Except later I discovered that the seat tube’s real angle was just 73º to the ground because the top tube was tilted. My discovery got several Masi owners on the Classic Rendezvous list to measure their frames and everyone’s top tube was not level by 6 to 9mm.

Re twitchy-ness By now , as a local low speed user of folding small wheel bikes..

Brompton about a 35 mm trail, front mass , of mt gear in the front bag or panniers on my bike Friday

and the handling is quite steady , and maneuverable .

I know the OP has a taste for race bikes..




continue group speculation.. :innocent:





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