Geometry discussion?

[unterhausen]

Originally Posted by Kontact

I've had a hard time finding these super tight geometry crit bikes of the '80s

a couple of people have mentioned my comment in passing. I think the extreme geometry bikes were never that common, and probably got retired relatively quickly. I am not aware of any production bikes like that. When I was racing in the mid '70s to early '80s, most of us were on European brands. And there were lots of Paramounts around too. Most American builders of that era were building perfectly sensible bikes that had geometry that was very similar to European bikes of that time.


I feel like some more than one person is mixing up trail and rake. Or maybe it's my mild dyslexia.

[Kontact]

Originally Posted by cmg

more trail, more stable, less fork rake. less trail, quicker steering, twitchy, more fork rake. i ride small frames with 72-72.5 HTA i like to use forks with 50mm rake. that's the happy medium.

This is generally not considered to be true. High trail is stable at high speeds, but twitchy at low. Low trail stable at low speeds, vague at high speeds.

Nothing is so simple as more of X is only stable. Bikes are made low trail for very good reasons.

[dddd]

I think that there is semantics at work here. People usually refer to a twitchy bike as one that darts around at high speed, not low. I would call that low-speed issue steering flop, but that's semantics as well I suppose.

And likewise I would call the low-trail bike twitchy at speed, not "vague".

But perhaps we're all trying to say basically the same things?

[marciero]

Originally Posted by dddd

I think that there is semantics at work here. People usually refer to a twitchy bike as one that darts around at high speed, not low. I would call that low-speed issue steering flop, but that's semantics as well I suppose.

And likewise I would call the low-trail bike twitchy at speed, not "vague".

But perhaps we're all trying to say basically the same things?

The terms "twitchy" and "vague" are not necessarily in conflict or contradictory. We typically use the former to describe how the bike responds to steering input; that is, while changing the turning radius. The latter, at least in my mind, seems to suggest how the bike feels when trying to go straight or maintain a constant turning radius. The "on rails" feel that high trail bikes have cornering at higher speeds is really a resistance to changes in turning radius- equivalently, a lesser sensitivity to steering input. Low trail bikes are less resistant/more sensitive to changes in turning radius. For some riders this causes the bike to feel "vague" when maintaining a given turning radius and twitchy when actively changing the turning radius.

[ripvanrando]

Low trail bikes have a tendency to shimmy at speed compared to high trail bikes although a leather clad handlebar bag and wide supple tires tend to mitigate as does the fat of a chubby rider, especially if wearing a blue woolen jersey. It is a system. Not just the bike. We don't have the equations or the eigenvalues; so, we all just worship at our respective alters.

[dddd]

Originally Posted by marciero

The terms "twitchy" and "vague" are not necessarily in conflict or contradictory. We typically use the former to describe how the bike responds to steering input; that is, while changing the turning radius. The latter, at least in my mind, seems to suggest how the bike feels when trying to go straight or maintain a constant turning radius. The "on rails" feel that high trail bikes have cornering at higher speeds is really a resistance to changes in turning radius- equivalently, a lesser sensitivity to steering input. Low trail bikes are less resistant/more sensitive to changes in turning radius. For some riders this causes the bike to feel "vague" when maintaining a given turning radius and twitchy when actively changing the turning radius.

You described all of that pretty well.

I had one steep-angled bike that was a bit on the big side and I used a 9cm stem on it. I would often steer into higher-speed corners only to have my trajectory come in too far too early, so I often took such corners in a less-than-smooth line at more cautious speeds. I thought the the trajectory "vagueness" was in my head until I downsized to a smaller frame of the same model (1991 Specialized Epic) and fitted a 10cm neck. The second bike handled just about perfect and I still have it! I still call the first bike twitchy though, if only because I think it's what the term means to the most people.

[Mark McM]

Originally Posted by marciero

The terms "twitchy" and "vague" are not necessarily in conflict or contradictory. We typically use the former to describe how the bike responds to steering input; that is, while changing the turning radius. The latter, at least in my mind, seems to suggest how the bike feels when trying to go straight or maintain a constant turning radius. The "on rails" feel that high trail bikes have cornering at higher speeds is really a resistance to changes in turning radius- equivalently, a lesser sensitivity to steering input. Low trail bikes are less resistant/more sensitive to changes in turning radius. For some riders this causes the bike to feel "vague" when maintaining a given turning radius and twitchy when actively changing the turning radius.

Furthermore: The steering inputs required to keep a bike in balance (either in a straight line or in a turn) vary with speed. A steering input which causes a small lateral deflection at low speed will cause a much larger lateral deflection at high speed. And steering trail acts to oppose steering inputs

At low speed, large steering inputs are required to stay in balance, and since trail tends to oppose steering inputs, a bike with high trail may seem "unstable" at low speeds, because the rider has to work harder to apply the steering inputs to keep it in balance. A bike with low trail will react more readily to steering inputs, making it easier for the rider to provide the steering inputs to stay balanced.

At high speeds, a bike which reacts quickly to steering inputs may feel "unstable", because the bike may have large reactions to even the smallest steering inputs. A bike with low trail may therefore feel "twitchy" at high speeds. With high trail, the bike reacts less to applied steering inputs, so will better tend to stay on track, both in a straight line and when turning.