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Old 09-02-10, 09:54 PM   #1
rothenfield1
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Fork Offset & Steering Characteristics

I recently bought an 83 Trek 560 and was disappointed with the twitchy nature to the steering at speed. I posted a thread in the C&V Forum about it here: http://www.bikeforums.net/showthread...ed-83-Trek-560

I have read comments about some bikes not feeling stable at higher speeds, but I have never read why this would be. I would consider myself an experienced rider of mostly lugged steel road bikes, but I have never owned a bike with steering this sensitive. I once owned a high end Italian bike that road as if it was on a rail, this is the opposite of that. I would not consider riding this bike hands-free unless on a very smooth surface at slow speed.

I first thought that it may have to do with the headset as it was badly brinelled but not loose. I replaced it with little to no effect on the handling. I started reading about fork offset and fork trail. Then I remembered that the bikes I had owned in the past had a fairly straight fork. This one has somewhat more of a curve to it. Iíve considered that the fork may be damaged in some way, it there is no obvious or unusual misalignment that I can tell.

Has anyone had this experience or know why one bike could track rock solid, while others ride erratically?
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Old 09-02-10, 11:08 PM   #2
unterhausen
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lower trail bikes (more rake) do feel a little twitchy to me. They settle down if you put weight on the front wheel. Some people like the feel, it gives the bike a "lively" feel. In my opinion, it's not particularly good for riding in a pack. I don't have problems at high speed, it's possible that your bike has some small alignment problems
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Old 09-03-10, 11:16 PM   #3
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I took the fork off and looked at it. It appears to be correctly aligned, as well the frame. I’ve also ridden it a lot lately and am understanding that it is the rider, not the bike.

What makes one bike handle differently from the other; if the rider is out of the equation?

Solid


Not

Last edited by rothenfield1; 09-05-10 at 10:22 PM.
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Old 09-04-10, 12:19 AM   #4
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So many little things go into what makes a bike handle the way they do... my Cooper road bike has pretty aggressive geometry and should be twitchy as sin but it is one of the most stable vintage bikes I have ever ridden.

I have also found that Bianchi tends to make a very well laid out bike that handles very well even when the bike has aggressive geometry.

Smaller frames tend to be more stable and with some companies they design a specific model and as the frame gets larger the frame angles are changed to adjust for that and to retain the desired ride and handling qualities... sometimes the maker misses the mark and the larger frame models are not as stable as they should be.

I work with some very tall riders and they often have complaints that their off the peg bikes, and we are talking about frames in the very largest sizes are not stable at high speed and this is usually because the trail isn't correct for the frame.

The Trek might benefit from having the forks adjusted a little to give it a little more trail and if you were to measure the Bianchi against the Trek you would probably discover that these measurements are different.

Once upon a time most road bikes has forks that had curved blades and then this Colnago fellow convinced everyone that a straight bladed fork with an offset at the crown was a better design.

Besides it's shape, a fork's materials and how it is made can affect it's feel and ride qualities.

To demonstrate how frame angles can affect steering...

I have a folding bike with semi vertical dropouts so moving the wheel back also moves it up and drops the rear enough to change the head angle by a few degrees... when I ran the bike as a fixed gear the low gear (bigger cog) dropped the axle to the bottom of the dropout which gave me a steeper head angle than when the wheel was set with it's small cog which dropped the rear a little.

Now as a geared bike I run the wheel set as far back as it goes and the trail seems to be perfect.

Some old Campagnolo long semi vertical dropouts are like this and moving the wheel back also brings the axle up and lowers the rear enough to affect the head angle and how it handles... take it all the way back and you get a little more trail, bring it all the way forward and you get less trail and have a more responsive handling feel.

A small change in the head tube angle or slight change in your forks can make for a very different riding bicycle.
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Old 09-04-10, 12:36 AM   #5
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I got to do some side side side comparison of a few bikes today as I went riding with my friend who has a Gardin road bike and I was riding my Raleigh Gran Sport which is a sport class bike with longer chain stays and slacker frame angles.

The Gardin was really twitchy at low speed but really settled down nicely once I got it dialled up... my friend was so impressed with the Raleigh that I am sure he will be hunting for one now and should note that the Gardin had it's fork replaced which may not be an exact match to it's factory fork and may have reduced the trail a little.
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Old 09-11-10, 08:10 PM   #6
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Quote:
Originally Posted by Sixty Fiver View Post
I got to do some side side side comparison of a few bikes today as I went riding with my friend who has a Gardin road bike and I was riding my Raleigh Gran Sport which is a sport class bike with longer chain stays and slacker frame angles.

The Gardin was really twitchy at low speed but really settled down nicely once I got it dialled up... my friend was so impressed with the Raleigh that I am sure he will be hunting for one now and should note that the Gardin had it's fork replaced which may not be an exact match to it's factory fork and may have reduced the trail a little.
After riding the Trek much more and finding that the headset lower caged bearings were installed upside down, I found the ride similar to what you describe with the Gardin, a little unpredictable at low speeds, but smoothing out at higher speeds.

I often see sellers including tube angles with lengths, and it has always been a mystery to me. I'd like to be able to determine the riding characteristics of a bike by understanding its' geometry. I know this is too big of a topic for this thread. Are there any articles, books, etc. on the subject that would explain such a complex subject?
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Old 09-12-10, 12:36 PM   #7
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One additional thought, check to see that your rear wheel is centered in the frame (proper dish). I experienced similar symptoms to what you describe with an improperly dished rear wheel.
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