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Steering geometry mystery.

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Steering geometry mystery.

Old 10-02-23, 08:32 AM
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Steering geometry mystery.

I ride small bikes: 49 to 52. I have owned lots of them, with everything from an 89 Cannondale with 71.5 HTA and 54mm of rake to a Merlin with a 73 HTA. I have had bikes with too much trail, and know what that feels like (40mm rake fork on a 72.5 HTA), but my Serotta does something I don't understand.

The bike is a 1991 NHX, only produced one year. I saw a copy of the geo once, and it seemed very straightforward - 72.5 HTA with a 47mm rake. The original fork is long gone, replaced by a later Serotta carbon version, so chances are the rake is slightly different (45 or 43?)

Here's the weirdness:
At low speeds, especially when I turn out of driveway to start going slowly up the hill, the front end feels "floppy". As in, turning the bars back and forth slightly to counter the sway of pedaling slowly out of the saddle yields a kind of resistance that can be felt in the handlebars, that seems a little like what "wheel flop" sounds like. Otherwise, the bike handles very well at all speeds and it never feels unsafe or unpredictable. But I have no other similar bikes that do this.

Geometry otherwise is pretty normal for a 50cm - 51.5 TT, 41.0 CS, 70mm drop. I'm using a 120mm stem and 42cm bars over 25mm of spacers, and I'm generally on the hoods when I feel this.

The only comparable experience I've had is when test riding smallish women's bikes as a mechanic. Many of these bikes are recent, and likely have a lot of trail due to standardized rake forks and shallow HTAs. But those bikes generally also have relatively high handlebars with the hoods rotated up as well.

Anyone have any thoughts why my Serotta behaves this way out of saddle at low speeds? I'm not worried, just really curious.
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Old 10-02-23, 09:16 AM
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It IS flop that you are feeling. The more weight on your front wheel, the greater the sensation, for some given geometric level of flop.
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Old 10-02-23, 12:37 PM
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You would be better served with a fork with more rake, less trail, and less flop. The industry has been slackening head angles, so 55mm rake forks are much more common. Unfortunately, you might not be able to find one with the right axle-crown measurement.

I have a bike with a lot of flop, and if I haven't been riding it for a while it takes some effort to get used to the flop. I built a fork for it with more rake, but never got around to using it. Story of my life.
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Old 10-02-23, 01:22 PM
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Not knowing a lot about geometry that sounds like what the Tuesday Aug 1 bike that I owned for a short time. Being a cruiser I figured it would almost drive itself and be a cinch to ride no handed , but if leaning into a turn it seemed to want to fall on over into the turn.
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Old 10-02-23, 02:09 PM
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It's abundantly clear from all your posts that you know what you're doing, but just to eliminate a couple of possibilities that are easy to overlook (and easy to fix):

No chance that the headset is developing notchiness?

No recently added aero bars?

That out of the way, I'm looking forward to the ensuing discussion. Why hasn't this topic (i.e., high-quality frames that should ride beautifully but have unsettling quirks) come up before?

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Old 10-02-23, 02:59 PM
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There is plenty of discussion of forks and replacement forks elsewhere on this forum. Those threads usually result in a lot of conflicting anecdata.

Originally Posted by Hank McMauser
Not knowing a lot about geometry that sounds like what the Tuesday Aug 1 bike that I owned for a short time. Being a cruiser I figured it would almost drive itself and be a cinch to ride no handed , but if leaning into a turn it seemed to want to fall on over into the turn.
I looked up that bike and I have trouble believing it is low trail, but that sounds like low trail to me. I think that most people* agree that bikes with low trail don't ride well no-handed. I like the way my low trail bike rides, but it definitely seems to steer in on corners at low speeds and there is no way I could ride it no-handed.

*I'm sure the low-trail true believers disagree that anything could be wrong with low trail.
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Old 10-02-23, 06:45 PM
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Take Brochure Numbers With A Grain Of Salt

One thing to consider is that often times the "Geometry Charts" that manufacturers sometimes included in their brochures are not always accurate. The Italians were probably the worst in this respect, IMHO.
The numbers might not be accurate and honestly, the majority of overworked and under-financed bike builders / manufacturers place a really really low priority on such details in the brochures.
It might be a year or two before numbers get updated.

Which is a long way of saying that without some pretty careful measurements, you really can't always be sure of judgement based on a HT angle and fork rake numbers from a brochure.

Anyway - good luck and hopefully you can find a couple of different forks to try and feel out the differences.
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Old 10-02-23, 07:47 PM
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Like I said, I have experienced a lot of different rake/HTA combinations, and I feel like I would have run across this somewhere else. That Cannondale with the 71.5 HTA certainly didn't display flop.

I don't know what's going on, but it is strange. I'll try to look up Serotta carbon fork rake offerings.

Edit: Looks like the F1 was 43mm. So this may be a relatively high trail bike.


- Good headset, no aerobars.

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Old 10-02-23, 07:54 PM
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Originally Posted by unterhausen
There is plenty of discussion of forks and replacement forks elsewhere on this forum. Those threads usually result in a lot of conflicting anecdata.


I looked up that bike and I have trouble believing it is low trail, but that sounds like low trail to me. I think that most people* agree that bikes with low trail don't ride well no-handed. I like the way my low trail bike rides, but it definitely seems to steer in on corners at low speeds and there is no way I could ride it no-handed.

*I'm sure the low-trail true believers disagree that anything could be wrong with low trail.
My experience is that high trail is hard to ride no-hands at low speeds, and only low speeds. Low trail bikes are supposed to be vague at speed, but stable at low speeds.

I've never ridden a low trail bike, but I trust people like Tom Kellogg if he says the above is the case. It matches my experience.
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Old 10-03-23, 09:15 AM
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Originally Posted by Kontact
My experience is that high trail is hard to ride no-hands at low speeds, and only low speeds. Low trail bikes are supposed to be vague at speed, but stable at low speeds.

I've never ridden a low trail bike, but I trust people like Tom Kellogg if he says the above is the case. It matches my experience.
That may have been it, I only rode it a few times, and traded it off I just remember it being sketchy as all get out, so down the road it went.
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Old 10-03-23, 09:38 AM
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Originally Posted by Kontact
My experience is that high trail is hard to ride no-hands at low speeds, and only low speeds. Low trail bikes are supposed to be vague at speed, but stable at low speeds.
The problem with descriptions like that, and mine, are that it's subjective. Tom Kellogg is probably better at riding no hands than me. I feel that the way bikes feel is fairly sensitive to small changes in geometry. And I can get used to anything, it's just that I'll notice differences in edge cases, like slow speed turns. I'm not going to build a series of frames that demonstrate a wide range of handling qualities. Maybe someday I'll build a variable rake fork, but probably not.

I have wondered if trail really fully describes the way bikes ride. For example, you can get to the same trail by having a bike with a lot of rake and 73 degree head tube, or not so much rake with a steeper head tube. Do those bikes really handle the same? Another difficulty is that there are usually other things that change between bikes, like tire size and handlebar widths. I just recently saw someone make a bold, wide ranging judgement based on a bike he had that was much too large for him. Fit is fairly important to subjective handling impressions. A while back, someone put pro riders on an old racing bike that was far too small for most of them. They thought it was scary. It probably had exactly the same geometry as a carbon bike of the same size. Racing geometry hasn't changed much since the '70s. Okay, it could have been a crit bike from the '70s, they were intentionally scary, that's what we wanted back them. But a bike that's far too small is probably going to feel scary.
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Old 10-03-23, 11:55 AM
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Originally Posted by unterhausen
The problem with descriptions like that, and mine, are that it's subjective. Tom Kellogg is probably better at riding no hands than me. I feel that the way bikes feel is fairly sensitive to small changes in geometry. And I can get used to anything, it's just that I'll notice differences in edge cases, like slow speed turns. I'm not going to build a series of frames that demonstrate a wide range of handling qualities. Maybe someday I'll build a variable rake fork, but probably not.

I have wondered if trail really fully describes the way bikes ride. For example, you can get to the same trail by having a bike with a lot of rake and 73 degree head tube, or not so much rake with a steeper head tube. Do those bikes really handle the same? Another difficulty is that there are usually other things that change between bikes, like tire size and handlebar widths. I just recently saw someone make a bold, wide ranging judgement based on a bike he had that was much too large for him. Fit is fairly important to subjective handling impressions. A while back, someone put pro riders on an old racing bike that was far too small for most of them. They thought it was scary. It probably had exactly the same geometry as a carbon bike of the same size. Racing geometry hasn't changed much since the '70s. Okay, it could have been a crit bike from the '70s, they were intentionally scary, that's what we wanted back them. But a bike that's far too small is probably going to feel scary.
The bike with too much trail got stable by going from 40mm to 45mm of rake.

The Cannondale I had with the shallow HTA handled like all of my other bikes, due to having neutral trail.

Even more radically, a custom Seven we had built with a 70.5 HTA and Seven's variable rake fork at over 50mm of rake rode normally for me due to the neutral trail. The best way to shorten reach and keep 700c wheels

So count me as a believer in the power of trail to set handling more than any other factor on road bikes
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Old 10-03-23, 12:31 PM
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There is no doubt that trail is important for handling. There are a batch of bikes now that come with forks where you can change a puck that changes the rake. I think they usually come in the rearward/ higher trail position. One of the mechanics at the lbs expressed some dissatisfaction with the handling of his bike, and I suggested switching to the position that gives more rake. It cleared up his issues.

It's just that discussing this in general terms doesn't always work out. It's a lot easier to look at a particular bike and successfully make predictions about how a change will affect handling.
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Old 10-03-23, 01:26 PM
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Originally Posted by unterhausen
There is no doubt that trail is important for handling. There are a batch of bikes now that come with forks where you can change a puck that changes the rake. I think they usually come in the rearward/ higher trail position. One of the mechanics at the lbs expressed some dissatisfaction with the handling of his bike, and I suggested switching to the position that gives more rake. It cleared up his issues.

It's just that discussing this in general terms doesn't always work out. It's a lot easier to look at a particular bike and successfully make predictions about how a change will affect handling.
Yup. Those bikes have two rakes to deal with tire size. I restrict my comments on handling to road race style bikes.
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