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Changing fork to make my bike less twitchy

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Changing fork to make my bike less twitchy

Old 04-28-12, 07:12 PM
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theoretical
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Changing fork to make my bike less twitchy

Hi,

I'm exploring the idea of changing the fork on my IRO Mark V to make it less twitchy at low speeds. I love this bike, but on almost any other bike I've ridden I have no problem riding hands-free. The IRO just won't cooperate, and since I'm not racing or anything of the sort, would like slacken things up a bit. I'd just like it to be less twitchy when cruising around the city. I really don't have a head for geometry, and from what I've seen, most threads online about fork rake devolve into arguments about geometry between people who have much better math skills than me. Fork rake is apparently among the most controversial topics one can raise. So, I'd like to present some very specific data, and ask for feedback.

MY BIKE:
IRO Mark V 53cm
headtube angle 72 degrees
current fork: 43 degree rake
current stem: 90mm
current tires: 25/700
The entire geometry of this frame can be found here: http://www.irocycle.com/markvframeset.aspx

POSSIBLE NEW FORKS
1. Surly Pacer Fork (http://surlybikes.com/parts/pacer_fork)
Rake 45mm

2. Surly Steamroller Fork (http://surlybikes.com/parts/steamroller_fork)
Rake 38mm

Surly forks fit IRO bikes, and these are the two with the biggest difference in rake from each other and from the IRO fork. Now the specific question is: Would one of these forks help my IRO be more stable/less twitchy? Which one?

You will have the sincere thanks of the geometry challenged for a clear and simple answer.
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Old 04-28-12, 07:33 PM
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agray2
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38 will make things more twitchy, a 45 will help lengthen the geometry out a little. If you have skinny high pressure tires on now, try a wider tire and less pressure.
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Old 04-28-12, 07:48 PM
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well biked 
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Originally Posted by agray2 View Post
38 will make things more twitchy......
The opposite is true, actually. Less rake creates more trail. The more trail, the more stable the bike. So the fork with 38mm of rake will make the bike more stable, and the fork with 43mm of rake will make it less stable.
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Old 04-28-12, 08:29 PM
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hueyhoolihan
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looks like you are off to a good start with these responses . good luck.
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Old 04-28-12, 08:44 PM
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Shimagnolo
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Originally Posted by agray2 View Post
38 will make things more twitchy, a 45 will help lengthen the geometry out a little.
Wrong.
In fact I have a bike on which I replaced a 45mm rake fork with a custom-made 38mm in order to get rid of the high-speed shimmy that plagued a tall frame with bad geometry.

One of Moulton's articles on the topic: http://davesbikeblog.squarespace.com...f-history.html
(Other articles are listed in the righthand margin)

Last edited by Shimagnolo; 04-28-12 at 08:52 PM.
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Old 04-28-12, 08:58 PM
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Originally Posted by hueyhoolihan View Post
looks like you are off to a good start with these responses . good luck.
Except well biked is correct. The measurement that best defines bicycle steering responsiveness is "trail" and more trail produces less twitchiness. Trail is determined by wheel radius, headtube angle and rake. Trail increases with a larger radius wheel, a more slack head tube and LESS rake. So for a given bike, fitting a fork with less rake will produce more trail and therefore it will be be less twitchy and more straight-line stable.
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Old 04-28-12, 08:59 PM
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Yes, this is exactly why I couldn't figure out the answer for myself by reading the 6,785,908 threads on fork rake already on the internet. I was leaning towards the 38mm based upon my research, but every time I think i settle on that I see another thread that directly contradicts it...

Shimagnolo: I tend to lean towards your real-world experience on this. Thanks for the feedback.
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Old 04-28-12, 09:07 PM
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The concepts of rake and it's effect on trail are pretty straight forward but are often confused by those who don't understand the geometry. Here is Sheldon Brown's glossary page defining trail and how it is determined. Scroll down to "Trail", read his description and also follow the link to the bicycle geometry article.

http://www.sheldonbrown.com/gloss_tp-z.html#trail
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Old 04-28-12, 09:22 PM
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http://www.dclxvi.org/chunk/tech/trail/

Above is some reading on bike geometry. The bits on "trail" are most relevant. Now, allow me to muddy the water with some amateur ramblings:

The folks above are arguing about the 38mm versus 45mm rake of the proposed forks, and the effect they'll have on your frame, with it's 72degree head angle. I'm not trying to equivocate here, but in a way, they are both right. It is my understanding, based upon readings, empirical data, and general butt-scratching, that differing trail figures have different effects based on the speed. In short, it has been my experience that bikes with higher trail are twitchier at low speeds, and more stable at high speeds. The lower-trail bikes tend to be less twitchy at low-speeds, but less stable at high speeds. This is why fake-french neo-rando clowns are all gaga for low-trail set-ups; they tend to carry a lot of weight on the front of the bike. The low-trail's stability at low speeds helps compensate for swing-weight from frontal loads while they're shoving off from the cafe with a snotty grin and a handlebar-sack full of baguettes and pinot strapped to their decaleur; once they're up to speed, momentum takes over and they need worry even less about swing weight.

The terribly high-trail geometry of my erstwhile Hero Roadster is what caused this 90lb, mile-long relic to seem so twitchy at speeds under @10mph, but what made it so smoooooooooooooooth and confident when I finally got the thing chugging along. If only the cheap, Indian rod-brakes were up to the task of stopping an overly confident, utterly stable 2-wheeled locomotive....I'm glad I sold that thing.

The point is, you may wanna ask yourself if the IRO feels too twitchy when you're going slow, or when you're going fast.... if it's too twitchy all the time, well...I cannot help you. But, if you want to decrease low-speed twitchiness, get the Steamroller. If the high-speed twitchiness is what ails ya, get the crosscheck fork.

To confuse things even more, I'll add that "twitchiness" in general can be impacted by other variables, such as the length of your stem and even the style of handlebar you're using. The way these items impact handling will vary depending on other confounding geometric factors, not the least of which is trail. When my wife made me put a straightbar on her old beater roadbike and I did so without changing any other variable (aside from brake levers), it became unbearably twitchy. I've witnessed friends/acquaintances who had similar experiences after changing their stems to ridiculously short-n-stumpy ones.

And now, back to your regularly scheduled flamewar. I gotta say: I'm done with this thread. I'll read it, possibly while eating a bowl of popcorn, but no matter how much vitriol is spewed by angry geometry experts, trying to goad me, I am done posting on this one.

Hope this helped!
-rob
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Old 04-28-12, 09:39 PM
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theoretical
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Rob, yes it is the low-speed twitchiness I'd like to get rid of, so it seems like the 38mm would be best.

Thanks for all the info everyone. I know there are a lot of variables, which is why I tried to specify the key elements of my setup. It's amazing how even though you'd think this would be straightforward once you know the explanations, there are so many arguments to the contrary online. Enough to make you doubt your judgement if geometry is a foreign language...
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Old 04-28-12, 10:47 PM
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Originally Posted by theoretical View Post
Rob, yes it is the low-speed twitchiness I'd like to get rid of, so it seems like the 38mm would be best.

Thanks for all the info everyone. I know there are a lot of variables, which is why I tried to specify the key elements of my setup. It's amazing how even though you'd think this would be straightforward once you know the explanations, there are so many arguments to the contrary online. Enough to make you doubt your judgement if geometry is a foreign language...
72 X 43 is not exceptionally fast steering. A setback seat post with the saddle moved aft most likely will cure your problem.
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Old 04-29-12, 12:39 AM
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less "twitch" is more trail , and that is less fork offset/ rake,
if the Crown race seat to axle center distance, and the head angle
remain the same..

You can gain 'Pneumatic trail'..
with a tire that lays down a bigger footprint..

Last edited by fietsbob; 04-29-12 at 12:43 AM.
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