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Old 09-13-2010, 11:41 PM   #1
oxcintron
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Default Fork length calculations.

OK, first off, I've done the necessary searches, both here and elsewhere regarding front end geometry. The issue I have is not so much with rake or trail, but conflicting information regarding what length front end to run on any given setup. I've seen differences of as much as 3-4 inches in the lengths for the same setup that some people run.

Not trusting the online trail calculators on the internet, I used a CAD program to calculate a large number of setups, taking into account downtube stretch, neck rake and triple tree rake, and crunching the numbers for trail and fork length for every possible combination.

In order to do this, I operated under the following assumptions:

1. The distance from the bottom of the neck to the ground is 24.25". This is based on a rigid wishbone frame I took the measurements off of.

2. Ground clearance is set at 4.5", and frame rails will be level with respect to the ground.

3. Triple tree offset (distance between centerline of neck and centerline of fork tubes) is 2.45".

4. Front tire is a 21" with a diameter of 27.6"

With these givens established, I drew up rake and trail diagrams for a frame with either 38 or 40 degree rake, downtube stretches of 2, 4, and 6 inches, and triple trees with with a rake of 0-8 degrees. I then measured the appropriate trail and fork length figures. Here are three such drawings chosen in no particular order:

2 up, 38* neck rake, 5* triple trees:


4 up, 38* neck rake, 6* triple trees:


6 up, 40* neck rake, 7* triple trees:


I did 32 such diagrams, and recorded the figures in a spreadsheet:



The chart is pretty much self explanatory. The second column from the right lists whether that particular setup is compatible with a set of Exile triple trees I am looking at (they only come in 0, 3, 6, and 8 degrees). The figures in the "Trail" column are colored green if they are close to the 2"-4" ideal, and red if they lie outside it. The last column takes the Fork Length and subtracts 26" from it, and rounds it to the nearest tenth.

Here's where I'm having problems. I thought most front ends where offered in 2" increments (short of cutting down tubes). I am seeing a lot of setups that (according to this chart) requires forks that are an odd number of inches over.

For those of you who made it this far, am I calculating these numbers right? Or am I subtracting the wrong length from the total fork length to find the "X inches over"? Or is it okay to run an inch longer or shorter (to get a nice even number of inches over?
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Old 09-14-2010, 12:11 AM   #2
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Default Re: Fork length calculations.

What do you mean by trail? Be specific, is it mechanical trail, or pneumatic trail? I'm also confused about what youre triple tree rake is???

If you are trying to calculate the pneumatic trail or mechanical trail. You might have to draw a straight line from the frame neck to through the center of the wheel. Then measure that distance where it hits the ground to the center of the tire contact patch to get that measurement.

I haven't read RCVD in a while, and im not at home so some of my nomenclature may be off and a little confusing, but i will get back to you later.
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Old 09-14-2010, 05:19 AM   #3
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Default Re: Fork length calculations.

Great chart, very useful. Thanks
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Old 09-14-2010, 12:05 PM   #4
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Default Re: Fork length calculations.

When they say a frame downtube has been stretched a certain amount (say 2" up) do they mean 2" up in a vertical plane? Because I find it hard to imagine that it would be two inches along the angle of the downtubes (which would be affected by goosenecks etc.).

Also I've read that (obviously) you have to account for 1.5"-2" of additional fork length for the bike to settle once it has the drivetrain and rider weight on it. So assuming my diagrams are even correct, do I simply take the total fork length off the chart and add 2" to it so that it settles to the correct trail number once laden with weight?
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Old 09-14-2010, 12:17 PM   #5
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Default Re: Fork length calculations.

I can understand planning out a build,,but Dude..


Personally seems your pissin on a flat rock in the middle of a thunderstorm here.

Put away the computer and all the #'s Mock something together,,see what works ,,what hits or interferes etc. Your really over thinking the whole thing


Also wheel size and width play a factor and also how do you want it to handle ?
Quick? Laid back ? etc
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Old 09-14-2010, 12:24 PM   #6
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Default Re: Fork length calculations.

Hey I'm only trying to save myself from losing more money like the $500 I lost on the Panhead frame I just sold. I understand those of you out there who say I need to just "dig in" and start buying up parts, but these calculations are going to affect whether I buy a 2" up frame, or a 4" up frame, and whether I will be able to run a set of Exile triples trees which only come in specific degrees. I don't wanna buy the parts and throw it all together just to find out I have 8 inches of trail, or negative trail, or whatever.

I really am still trying to keep it simple build-wise (no more Panhead, kickstart, etc.), but I read through countless threads asking "what length front end should I run with my chopper" and hearing a million different responses with no general consensus. Just trying to figure it out on my own and with the math to prove it.
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Old 09-14-2010, 12:35 PM   #7
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Default Re: Fork length calculations.

Quote:
Originally Posted by oxcintron View Post
Hey I'm only trying to save myself from losing more money like the $500 I lost on the Panhead frame I just sold. I understand those of you out there who say I need to just "dig in" and start buying up parts, but these calculations are going to affect whether I buy a 2" up frame, or a 4" up frame, and whether I will be able to run a set of Exile triples trees which only come in specific degrees. I don't wanna buy the parts and throw it all together just to find out I have 8 inches of trail, or negative trail, or whatever.

I really am still trying to keep it simple build-wise (no more Panhead, kickstart, etc.), but I read through countless threads asking "what length front end should I run with my chopper" and hearing a million different responses with no general consensus. Just trying to figure it out on my own and with the math to prove it.
Not at all saying ya need to go out and buy parts,,and Maybe you did'nt need to sell you frame. Not every bike is perfect spot on geometry wise . Yes something will affect but others not so much

And a kick pan,,then go stock pan frame and forks,,It's actually a nice ride
Your fishing parts from to many holes,,and that may be whats complicating things.

Look at stock Pan blueprints get your ideas there to see what you can do
I believe you mentioned before you bought up a bunch of parts on impulse ? Well your kinda heading that way again. There is lots that can be done just swapping stock parts and simple aftermarket stuff. When you start goin after raked trees and all the new fangled goodies its gonna get $$ no matter what.

and there will be no "no general consensus" there are variables,

How your ride
Where you ride
Weight, etc etc

Exact math will only get you so far, how things work off paper can be far different
thats all.

Hell grab a simple honda and mess around with it, build a model so to speak,,then you can take that model once all the kinks are worked out,,and apply that to you build
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Old 09-14-2010, 12:49 PM   #8
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Default Re: Fork length calculations.

Thanks for the clarification. Are you implying that some people run setups with trail that might be out of the "2-4" norm just because of their riding style? I've crunched the numbers on some existing builds on here and once got 6-7 inches of trail on someone's bike who said their ride "handles and corners well".

And as far as my specifics, I'm 5'7", 125 lbs., and am looking to run maybe some FXR mid controls with 10" apes on 2" risers. Same basic setup as on my Sporty. Riding will be mostly local traffic where I want it to be able to ride aggressively, and maybe some highway pegs up front for the rare long distance trip.
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Old 12-14-2010, 10:14 AM   #9
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Default Re: Fork length calculations.

Hello to All Can anyone kindly suggest where a reliable front fork calculator can be found? I find myself addressing this very same dilemma and cannot yield anything consistent from the online calculators used. Thanx Very Much!
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Old 12-14-2010, 10:42 AM   #10
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Default Re: Fork length calculations.

here ya go
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Last edited by Scavenger; 03-11-2014 at 07:02 PM.
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Old 12-14-2010, 01:24 PM   #11
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Default Re: Fork length calculations.

Quote:
Originally Posted by oxcintron View Post
Thanks for the clarification. Are you implying that some people run setups with trail that might be out of the "2-4" norm just because of their riding style? I've crunched the numbers on some existing builds on here and once got 6-7 inches of trail on someone's bike who said their ride "handles and corners well".

And as far as my specifics, I'm 5'7", 125 lbs., and am looking to run maybe some FXR mid controls with 10" apes on 2" risers. Same basic setup as on my Sporty. Riding will be mostly local traffic where I want it to be able to ride aggressively, and maybe some highway pegs up front for the rare long distance trip.

I don't know where you got the 2"-4" norm for trail from? It's more like 3.25"- 5". I can't open any of your pics so I don't know if you are more concerned with looks or handling. If it's a rigid I don't see the point of having less than 4.75" of trail. Remember they bounce and need that kind of trail IMHO to feel right especially if you don't weigh a lot. Also, don't forget to take into account how your tires will flatten with weight and how your front suspension will sag with you on it. As long as your trail doesn't decrease to less than 3" under braking you will be ok.
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Old 12-14-2010, 03:51 PM   #12
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Default Re: Fork length calculations.

Quote:
Originally Posted by oxcintron View Post
Thanks for the clarification. Are you implying that some people run setups with trail that might be out of the "2-4" norm just because of their riding style? I've crunched the numbers on some existing builds on here and once got 6-7 inches of trail on someone's bike who said their ride "handles and corners well".

And as far as my specifics, I'm 5'7", 125 lbs., and am looking to run maybe some FXR mid controls with 10" apes on 2" risers. Same basic setup as on my Sporty. Riding will be mostly local traffic where I want it to be able to ride aggressively, and maybe some highway pegs up front for the rare long distance trip.
I ride a bike I built with 6-7 inches of trail and built it that way because I LIKE the way it handles. I really think you're over thinking this. You're not going to get in trouble with trail unless there's not enough of it. If you rake it and find the steering too heavy, throw a set of raked trees on it.
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Old 12-14-2010, 04:08 PM   #13
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Default Re: Fork length calculations.

Hello again Folks And, Thanx Very Much for those of you who have replied so far, it is indeed much appreciated. I weigh approximately 220 pounds and the frame is a softail type. The neck is raked at 42 degrees and I intend on using a 21 inch front tire. My measurements to date are 30.25 inches from the ground to the center-top of the steering neck and it appears as though I will need at least 4 inches of ground clearance. The forks I intend to purchase are fat inverted hydraulic type. Also, my rear wheel is an 18 inch wide style. From what I have been able to preliminarily assess, the triple trees will have a 6 degree rake and the offset is 2.3 inches. Please do allow me to volunteer that I in no way possess any extensive knowledge nor experience in this domain; I have had anywhere btween 4 to 8 inches of difference based on various calculators. I would sincerely appreciate any assistance in empirically figuring out what the proper fork length would be as well as what an "ideal" trail would be for this style of a ride. Cheers!
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Old 12-14-2010, 04:20 PM   #14
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Default Re: Fork length calculations.

Quote:
Originally Posted by Scavenger View Post
Put away the computer and all the #'s Mock something together,,see what works ,,what hits or interferes etc. Your really over thinking the whole thing
Yeah, engineering has no place in chopper building.
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Old 12-14-2010, 04:28 PM   #15
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Default Re: Fork length calculations.

I am far from an expert and my current ride is pretty good, 5.5 inches of trail...I rode a sugarbear front at 0 trail and it was just perfect..........not being an expert I would defer to the person manufacturing the forks for guidance.....there are just to many variables to throw a single number out there and have it be "right"..from what i have read anything beyond negative can be right for different configurations
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Old 12-14-2010, 11:34 PM   #16
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Default Re: Fork length calculations.

Surprised to see this old thread has been resurrected. I post elsewhere nowadays after starting a different motorcycle project, so you can spare yourselves the trouble of responding to any posts I made in the past. Thanks.
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