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Old 07-13-04, 10:30 AM   #1
531Aussie
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Steering is too twitchy, what can I do?

Will a longer raked fork make a big difference?

How long do rakes get (or "trail", or whatever it's called)?
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Old 07-13-04, 12:31 PM   #2
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Take a look at this site :
http://www.phred.org/~josh/bike/trail.html
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Old 07-13-04, 12:37 PM   #3
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Quote:
Originally Posted by 531Aussie
Will a longer raked fork make a big difference?

How long do rakes get (or "trail", or whatever it's called)?
I believe that if you want to increase stability, you want to *decrease* rake, not increase it. Actually, you want to increase the trail, but if you consider head tube angle and tire size fixed, decreasing rake is the only way to do this.
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Old 07-13-04, 04:09 PM   #4
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Quote:
Originally Posted by 531Aussie
Will a longer raked fork make a big difference?

How long do rakes get (or "trail", or whatever it's called)?
Before you do anything, check your headset adjustment. The handlebar should flop from side to side by itself if you lean your bike over. A too tight headset will handle very twitchy.

If that's not it, then you need to get a "straighter" fork to relax your steering.
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Old 07-13-04, 04:40 PM   #5
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What is the head angle of the bike? It is impossible to determine if a shorter or longer fork will make your bike less "twitchy" without considering the head angle and geometry of the bike.

Of course it could be a mechanical problem, and you should eliminate those variables first.

Last edited by don d.; 07-13-04 at 09:57 PM.
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Old 07-13-04, 09:12 PM   #6
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Stable equals low BB height there for you need less rake or you should work on your riding position.
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Old 07-13-04, 10:52 PM   #7
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Quote:
Originally Posted by don d.
What is the head angle of the bike? It is impossible to determine if a shorter or longer fork will make your bike less "twitchy" without considering the head angle and geometry of the bike.

Of course it could be a mechanical problem, and you should eliminate those variables first.
Not sure about the head tube angle. Someone's looking at it today
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Old 07-13-04, 11:33 PM   #8
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Is this a new bike? If so, maybe you need more time to get used to it. I found my road bike too twitchy after I first got it, but over the course of a few hundred miles I am now finding it perfect.
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Old 07-13-04, 11:43 PM   #9
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Originally Posted by bg4533
Is this a new bike? If so, maybe you need more time to get used to it. I found my road bike too twitchy after I first got it, but over the course of a few hundred miles I am now finding it perfect.
Yeah, it's new, but I bought a frame only -- no fork, so I'm not sure what fork is the ideal match.
I've been riding it for a week, and it still feels a bit unstable, especially if I relax my hands on the bars, or take one hand off.
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Old 07-14-04, 08:28 AM   #10
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What if I put on an Ouzo Pro with a 50mm rake?
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Old 07-14-04, 07:31 PM   #11
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Originally Posted by 531Aussie
What if I put on an Ouzo Pro with a 50mm rake?
Suggest you do a google search for rake and trail. Then you'll know "what" and you won't have to guess. 50 mm sounds pretty extreme and very unstable. Try 43 mm for a 72 to 73 degree head tube.

Al
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Old 07-14-04, 09:34 PM   #12
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Originally Posted by 531Aussie
What if I put on an Ouzo Pro with a 50mm rake?
To obtain what is called "neutral" steering, a 50mm fork would work well with a 72 deg head angle. Neutral steering means that the bike will respond to deliberate input you put into the front end instead of having a tendency to oversteer when you make subtle changes in your position like reach for a water bottle or take your hands off the bars. Some people consider neutral steering "slow" because the bike only does what you tell it to do.

A bike with a 73 deg head angle achieves neutral with a 45 mm fork rake, and the steeper the head angle, the shorter the rake as a rule, so a 74 likes a 40, so on and so on. Of course a few millimeters may not have a huge difference on handling, but the difference between a 40 and a 50 mm fork can be quite pronounced on a bike.

In general, the shorter you make a fork in relation to the head angle, the longer the trail. The longer the trail, the less likely a bike is to oversteer, or be twitchy. If you make the trail too long, the bike will be slow to turn at low speed, stable at high speed, but not very quick handling at high speed. If you make the trail too short, it will be quicker handling at low speed and very sensitive to rider input at high speed(twitchy). So if you put a long rake(50mm) on a steep head angle(74), you will shorten the trail and you will get a twitchy front end.

The other measurement to consider is the measurement from the fork crown race seat to the front axle center. A bike is designed to sit at it's correct head angle with a specific fork crown race/axle center dimension. If you buy a fork that is 5mm longer than what a frame is designed to accomodate, the front end will sit higher, which effectively reduces the head angle some. If you buy a fork that is 5mm shorter, the front end will sit lower, which effectively increases the head angle some.

The Reynolds Ouzo Pro with a 50mm rake has a 372mm fork crown race to axle center dimension. You can measure the fork that came with your bike and see what it is to see if it is the same.

Notice the dimension "F" on this frame specification chart. This dimension is the fork length. That is the dimension I'm referring to. As you can see, these frames were designed to take a fork 367mm long to maintain the designed head angle.

http://cbike.com/colnagospecs.htm

Just a word of caution: 9 out of 10 bike mechanics will look at you like you are speaking Inuit if you ask them these questions. Be prepared to do the measurements or research them yourself.
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Old 07-14-04, 10:37 PM   #13
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Quote:
Originally Posted by don d.
The other measurement to consider is the measurement from the fork crown race seat to the front axle center. A bike is designed to sit at it's correct head angle with a specific fork crown race/axle center dimension. If you buy a fork that is 5mm longer than what a frame is designed to accomodate, the front end will sit higher, which effectively reduces the head angle some. If you buy a fork that is 5mm shorter, the front end will sit lower, which effectively increases the head angle some.

The Reynolds Ouzo Pro with a 50mm rake has a 372mm fork crown race to axle center dimension. You can measure the fork that came with your bike and see what it is to see if it is the same.

Notice the dimension "F" on this frame specification chart. This dimension is the fork length. That is the dimension I'm referring to. As you can see, these frames were designed to take a fork 367mm long to maintain the designed head angle.

http://cbike.com/colnagospecs.htm

Just a word of caution: 9 out of 10 bike mechanics will look at you like you are speaking Inuit if you ask them these questions. Be prepared to do the measurements or research them yourself.

Seeing as how we're picking nits here regarding fork lengths and such...Reynolds, along with most other fork manufactuers, measure fork length parallel to the steerer tube. This is different than Colnago's "F" dimension which discribes fork length from the crown to the axle. A Colnago fork is actually 364.5 mm when measured the same way Reynolds does.

Sorry to pick, the post is very good overall. Fork length is one of my pet peeves and I've done a lot of study on the lengths of various aftermarket forks.

Ed
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Old 07-14-04, 11:08 PM   #14
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Originally Posted by Nessism
Seeing as how we're picking nits here regarding fork lengths and such...Reynolds, along with most other fork manufactuers, measure fork length parallel to the steerer tube. This is different than Colnago's "F" dimension which discribes fork length from the crown to the axle. A Colnago fork is actually 364.5 mm when measured the same way Reynolds does.

Sorry to pick, the post is very good overall. Fork length is one of my pet peeves and I've done a lot of study on the lengths of various aftermarket forks.

Ed
This has always been a pet peeve of mine as well. The whole fork as component concept leaves me cold. It just has to much to do with handling and ride for me to consider it something other than an integrated part of the frame.

But when you say that Reynolds measures parallel to the steerer tube, do you mean the center/axis of the steerer tube? If so, this would put the measurement to an imaginary line drawn from the ctr of the frt axle at what angle? 90 deg to the fork blade ctr? or 90 deg to the steerer axis? Just curious because I contacted Reynolds recently and asked them for the dimension from the fork crown race seat to the frt axle ctr in a straight line on their 50mm fork and they told me it was 372mm.
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Old 07-14-04, 11:36 PM   #15
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Originally Posted by don d.
This has always been a pet peeve of mine as well. The whole fork as component concept leaves me cold. It just has to much to do with handling and ride for me to consider it something other than an integrated part of the frame.

But when you say that Reynolds measures parallel to the steerer tube, do you mean the center/axis of the steerer tube? If so, this would put the measurement to an imaginary line drawn from the ctr of the frt axle at what angle? 90 deg to the fork blade ctr? or 90 deg to the steerer axis? Just curious because I contacted Reynolds recently and asked them for the dimension from the fork crown race seat to the frt axle ctr in a straight line on their 50mm fork and they told me it was 372mm.

Measurement is per the diagram shown below.

For some good info regarding how fork length affects head angle, check out the following link. http://www.sheldonbrown.com/rinard/forklengths.htm
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Old 07-15-04, 06:29 AM   #16
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this is it!

The old Kinesis fork is the third fork in a week. I've also tried the Profile-Design BSC and the locally made alu.

Today I put in a heavier stem (12cm 3ttt) and bars (old ITM Pro 260) which I think has made more difference than anything.
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Old 07-15-04, 08:02 AM   #17
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Does the BSC fork have very little rake, or is it just a bad photo?
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Old 07-15-04, 08:12 AM   #18
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Originally Posted by Fat Hack
Does the BSC fork have very little rake, or is it just a bad photo?

Profile forks such as the BSC and BRC have 43 mm of rake, or offset if you prefer.
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Old 07-15-04, 08:15 AM   #19
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Originally Posted by Nessism
Measurement is per the diagram shown below.

For some good info regarding how fork length affects head angle, check out the following link. http://www.sheldonbrown.com/rinard/forklengths.htm
Ok. Actually this measurement is a little unwieldy for the LBS or the home mechanic. You need to mount the fork in a fixture, ideally on an alignment table to be sure it's level, then locate a vertical line off the fork crown race seat and off the front axle using machinist squares, then measure that . This is good in the frame shop if you're prefitting unbrazed blades to a crown in a fixture and trimming at the top for correct length.

I think Colnago's method is much more end user friendly, although it may not be the industry standard. But then, when did the industry do anything just because it made things more end user friendly?

Thanks for the link.
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Old 07-15-04, 10:54 AM   #20
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Originally Posted by don d.
To obtain what is called "neutral" steering, ...(snip)...yourself.
Great explanation, Don D - thanks!
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Old 07-15-04, 12:26 PM   #21
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Originally Posted by Nessism
Profile forks such as the BSC and BRC have 43 mm of rake, or offset if you prefer.
It's disappointing that the Profile-Design web-site does have rakes (well, I couldn't find them).
In my opinion, a lot of these companies don't appreciate what total "gear geeks" we are. I cannot get enough information on something I wanna buy.

Infact, I think most companies are of the opinion that we're more impressed with 'dazzle and bull' than cold hard facts; you've just gotta check the Scapin site (http://www.scapin.com). All those jazzy pictures take ages to load....blah blah........what was I talking about? Oh yeah, light steering. Hmmm.
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Old 07-15-04, 02:11 PM   #22
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Get a larger tire with a round profile.
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Old 07-15-04, 11:13 PM   #23
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I just want my thread back up the top of the list
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Old 07-16-04, 12:57 AM   #24
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Quote:
Originally Posted by 531Aussie
Will a longer raked fork make a big difference?

How long do rakes get (or "trail", or whatever it's called)?

It's your bars. Youve got them at a bizzare angle.
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Old 07-16-04, 01:20 AM   #25
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Originally Posted by meatwad
It's your bars. Youve got them at a bizzare angle.
It's not a good photo, and the wheel is slightly turned; the bottoms of the drops are 'dead' parallel with the top tube

Last edited by 531Aussie; 07-16-04 at 01:36 AM.
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