Calculating Fork Length

Hey Guys,
I have just bought a Carolina Custom rigid frame. It has 41 degree
rake, 6" up, and 6" out. I am thinking of an old school look by using a
springer front end. Can someone tell me the correct way to calculate
how long a front end I will need to achieve the correct trail of 4"?

 

a springer front end. Can someone tell me the correct way to calculate
how long a front end I will need to achieve the correct trail of 4"?

Do you do trigonometry? I don't use it much, so if I'm wrong, go ahead
and laugh at me, but the trail result I get from calculating a 41
degree rake seem to agree with what I remember...

To find the required lenghth of your fork, you'll have to subtract one
triangle's hypotenuse from a larger triangle's hyspotenuse...

Remember from trigonomtry, the sum of all the angles in a triangle is
180 degrees. One of the angles in a right triangle is always 90
degrees. Your design fork angle is 41 degrees, so

180 minus ( 90 + 41 ) = 49 degrees

You're probably going to use a 3.00 x 21 front tire with a wheel radius
of 21 divided by 2 = 10.5 + 3 = 13.50 inches

The front axle will be 13.50 inches above the ground. that's the only
known side of our first triangle (call it "z", )

sin 49 degrees = 13.50 inches divided by unknown hypotenuse (call it
"x"), so

x = z divided by sin 49 degrees
x = 17.88 inches ( long side, or hypotenuse of your smaller triangle)

Now we want the trail, the second unknown side in our first triangle
(call it "y")

y = z divided by tan 49
y = 11.74 inches (that's a bunch of trail, but keep reading)

So there we have the dimensions of our first triangle...

Find a nice flat surface so you can get reasonably accurate
measurements. Set your frame on blocks so the slots that the rear axle
is going to go through are at the height of exactly half the diameter
of your rear wheel and tire assembly...

Using the biggest protractor you can get, adjust the steering head
angle by putting blocks under the frame until you get exactly 41
degrees and then measure the height of the top of the steering neck,
plus whatever you think the thickness of the upper triple clamp is
going to be...

That will be the known side of the second, larger triangle...

Then proceed to calculate the hypotenuse of your second triangle, the
larger one, just like we did above, and subtract the length of the
first hypotenuse from the second hypotenuse. That's the length of your
fork leg if it was a solid strut...

I know, you said you wanted to keep your trail at about 4 inches, and
use a *springer* front end. The length of the lower link from the pivot
point to the center of the front axle will have to be subtracted from
the trail I calculated in the first triangle, and the triple clamp off
set will have to be subtracted too.

In order to get your trail at 4 inches, the total offset for the front
link plus the triple clamps would have to be
11.74 minus 4 = 7.74 inches...

 

Wow. Thanks for the lesson !!!!!

 

See this? This is a snipped posting ;-)