This is an article I wrote for another forum about extended swingers and spring rates. It applies to extended arms also, as long as mounting points remain in the stock locations to maintain stock leverage geometry.
It is a very basic simplified version.
Hope it helps.
Spring Rate and Extended Swingers 101
Stock swinger = 20"
New swinger = 24"
4" is 20% of 20"
you increased your leverage 4" or 20% over the stock swinger.
Now you need to increase your spring rate 20% to match.
And yes, the cushion lever will effect the geometry of all this,
but not enough for our practical purposes here of installing an extended swinger.
The rebound and compression dampening of the stock shock, in good condition,
cleaned up good, and new slightly heavier oil, may not be perfect, but will be OK.
Here is a shade tree mechanic method to figure out your spring rate.
1) Put your stock spring on a regular bathroom scale.
2) Use a small square piece of plywood on the scale under the spring
and another piece on the top of the spring.
3) Put this scale/spring setup on your drill press table.
4) using the the drill press for leverage, compress the spring exactly 1"
read the weight on the scale, that is your 'spring rate'.
So just for an example:
If your spring is 100# @ 1" of compression,
20%(your increased leverage from +4" extended swinger)of 100 is 20
100+20=120
You now need a 120# spring for your 4" extended swinger
to maintain the same suspension compliance as stock swinger
Call Elka, tell them what spring rate you need.
They sell rear springs for $60 last time I got one.
ymmv