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Old 05-22-06, 12:18 PM   #1
Peek the Geek
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Fork length and handling

I posted this in the framebuilder forum, but got no response, so I thought I'd ask here:

I've got a full-rigid single speed mountain bike I'm building up that's spec'ed for 80mm of travel. For a rigid fork, the frame manufacturer suggests a 438mm fork, but I found a great deal on a 425mm fork.

I e-mailed both frame and fork manufacturers to ask if a 425 would be okay. Here's the response, in part, from the fork manufacturer: "The handling on the bike will be much twitchier and the BB will be lower and youll most likely clip more rocks with a shorter fork but you may actually like the more agressive position on the bike."

I know the 13mm shorter fork will change the handling, but will it be as extreme a difference as the quote above suggests? Much twitchier? Clip more rocks? Sounds a bit drastic for a difference of barely over a centimeter, but I've been wrong before.

What do you all think?
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Old 05-22-06, 12:26 PM   #2
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With some trigonometry you can figure out how much change that will make to the head tube angle. I suspect it would be quite a lot. If it was me I would not do that, but hey, it's your bike.

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Old 05-22-06, 12:44 PM   #3
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With some trigonometry you can figure out how much change that will make to the head tube angle. I suspect it would be quite a lot. If it was me I would not do that, but hey, it's your bike.

Al
Define "quite a lot." To tell you the truth, I can't remember all that sine/cosine/tangent stuff, so I'm not sure what the difference in head angle would be, so a little help here BF members. (and while you've got the calculator out, what would be the difference in BB height?)

As for the difference, it seems to me there are so many variables in mountain biking (tire size, PSI, terrain, fork sag, etc), well....
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Old 05-22-06, 01:47 PM   #4
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whelp, your bottom bracket is probably a little behind the midpoint on the bike, so dropping the front 13mm will probably give you a whopping 5-6mm of BB drop; moving up (or down, if you want to compensate) 2 teeth on your largest chainring would change your clearance by 8mm, to give you an idea. I'm no MTB expert, but it sounds like you're not an anal/technical enough rider to care. It is going to steer a little twitchier, and put you a bit more forward (though you can raise your bars if you don't like that). The other thing it's going to do is reduce the amount of "give" you perceive from a rigid fork, as it's going to be closer to vertical.
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Old 05-22-06, 02:06 PM   #5
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whelp, your bottom bracket is probably a little behind the midpoint on the bike, so dropping the front 13mm will probably give you a whopping 5-6mm of BB drop; moving up (or down, if you want to compensate) 2 teeth on your largest chainring would change your clearance by 8mm, to give you an idea. I'm no MTB expert, but it sounds like you're not an anal/technical enough rider to care. It is going to steer a little twitchier, and put you a bit more forward (though you can raise your bars if you don't like that). The other thing it's going to do is reduce the amount of "give" you perceive from a rigid fork, as it's going to be closer to vertical.
Now that I think about it, BB height is not going to be an issue. I'm used to riding my geared bike with a 44T big ring. The ss I'm building obviously will have more clearance with a 32T ring and bashguard. I didn't stop to think about that. Duh.

I'm still curious to know how many degrees steeper the head angle would be.

And really, that's all it is right now is curiosity. I've already got the fork. Once I get the bike built it'll just be a matter of riding it and seeing if I like it. If not, I sell the fork and pick up something else. No big deal.
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Old 05-22-06, 02:25 PM   #6
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Quote:
Originally Posted by Peek the Geek
Define "quite a lot."
That's a good question. To make the comparative calculation we'll need to know the rake of both forks. And after we make the calculations it's still going to be pretty subjective as to how different the handling will feel. I guess my previous answer should have been " I don't think I'd take a chance on what it does to the handling." The reduced clearance problem would be something less than 13mm. The change in HTA is probably less than 1/4 degree, but that could effect handling a lot.
Wish I could be more helpful.
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Old 05-22-06, 02:47 PM   #7
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Wish I could be more helpful.
Al
No, you've been very helpful. Thanks for the replies. As I mentioned, I'll find out for myself how the fork effects handling soon enough. I was just interested in hearing some other opinions on the matter.
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Old 05-22-06, 07:38 PM   #8
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The fork-length will affect the head-angle which affects steering quickness. However that's 1/2 of the equation. The other part is the fork-rake which then determines the trail. It's actually possible to end up with a situation where the shorter fork increases head-angle, yet its smaller rake results in more trail, thus giving more stability and more lazy steering. Or the shorter fork can have a larger rake and give you less trail, resulting in more twitchy and quick steering.

What's fork-rake before and after?
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Old 05-22-06, 09:47 PM   #9
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The fork-length will affect the head-angle which affects steering quickness. However that's 1/2 of the equation. The other part is the fork-rake which then determines the trail. It's actually possible to end up with a situation where the shorter fork increases head-angle, yet its smaller rake results in more trail, thus giving more stability and more lazy steering. Or the shorter fork can have a larger rake and give you less trail, resulting in more twitchy and quick steering.

What's fork-rake before and after?
Both forks are Vicious Cycles rigids with 1.5" rake.
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Old 05-23-06, 10:01 AM   #10
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Both forks are Vicious Cycles rigids with 1.5" rake.
That's 38.1mm. I'm use to working on road bikes with more than 40mm rake. I get 0.15 degree change in effective head tube angle (steeper). SIN 38.1/438 - SIN 38.1/435 = 0.15.
Doesn't sound like much but if you were changing from one fork to the other it would be noticeable.
I'm beginning to think you may be fine with that. Personally I'd feel better if the change was in the other direction.
If you can give me the frame's head tube angle we can calculate steering trail for the two forks. I assume that the wheels are 26 inch?.

Let us know how it turns out.

Al

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Old 05-23-06, 02:49 PM   #11
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I calculate that 0.13/wheelbase(about 100cm) is the tan of about 0.7 degrees. This will make a noticable difference but not drastic. Do the change, and if the steering angle is too steep, put a slightly fatter tire on the front or a thinner one on the back.
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Old 05-23-06, 03:19 PM   #12
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Both forks are Vicious Cycles rigids with 1.5" rake.
I doubt it's a significant difference either way. Go for it!
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Old 05-23-06, 03:33 PM   #13
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I calculate that 0.13/wheelbase(about 100cm) is the tan of about 0.7 degrees. This will make a noticable difference but not drastic. Do the change, and if the steering angle is too steep, put a slightly fatter tire on the front or a thinner one on the back.
Good point. As it's a rigid, plans are to run a larger tire up front anyway.
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Old 05-24-06, 08:06 AM   #14
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Quote:
Originally Posted by AndrewP
I calculate that 0.13/wheelbase(about 100cm) is the tan of about 0.7 degrees. This will make a noticable difference but not drastic. Do the change, and if the steering angle is too steep, put a slightly fatter tire on the front or a thinner one on the back.
I don't know how you arrived at .7 degrees, but if the HTA angle change was that much it whould make a huge difference in handling.
Handling is mostly about trail which is a function of head tube angle, fork rake, and wheel size. Changing fork length has some effect on HTA.

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