rear shock spring rate help!!!

[WeberKid]

So on my project, I've basically been shooting from the hip and hoping for the best in regard to suspension.  I have my project mostly together, but the rear shock is a touch too firm.  and by a touch I mean you sit on it and there is no compression unless you sit way back on the thing, put all your weight and push down as hard as you can.  I know its rated at 600lbs per inch.  I know I can get anything from race tech down to 100lbs per inch if I wanted.  The question is, what do I get?
Here is a picture of my linkage situation:

any advice would be appreciated!

[ktriple]

dont they have an online calculator for this purpose?  plug in rider wieght, bike weight, riding style, etc...

EDIT:
my bad, wont work for mono-shocking.  call them.  supposedly great customer service

[WeberKid]

i called, but the guy basically said "I can get whatever you want, but you need to tell me what you want" - so I'm wondering if anyone has experience first hand.  The bike would be stiff, but definitely ridable as is - it just doesn't have that soft settle when you sit on it.

[WeberKid]

I took a gamble with a 400lb spring.  We'll have to hope it works out.

[ktriple]

would it make any sense to take the rate of the recommended dual shock rate and double it? 

[WeberKid]

would it make any sense to take the rate of the recommended dual shock rate and double it? 

well its a CBR600rr shock mounted to a 1970 triumph frame with a CRF450r swing arm and linkage.  I'm not really sure who's recommendation I'd follow, but I'm all ears.

[ktriple]

maybe use the online calculator rate for the dual shock setup of an approximately the same weight bike, then double it?  Just a far fetched idea.  I guess if you already have it, hook it up and ride it.  you'll know immediately if its way out. 
I just had another thought... how much does the CBR600 weigh and how much will your bike weigh, approximately?  This may get you close

[Roc City Cafe]

you're going to have to figure out the velocity ratio, i.e. the ratio of a unit of rear wheel up and down travel to the unit of shock up and down travel, then figure out what the original or desired ratio is (usually performance bikes are tunes with a resonance frequency (yeah you need to figure that out based on bike mass and f/r distribution) of about 2hz, if i remember right... so yeah, do the math... sorry but modifying bikes often requires the basic algebra...) =]

[Roc City Cafe]

oh yeah, and since you've got a complicated linkage there, the velocity ratio is not constant... but you can probably take the spring off and measure the distance the shock linkage travels vs the distance you move the rear axle, at least that'll give you a ballpark idea for around a neutral riding load

[WeberKid]

you're going to have to figure out the velocity ratio, i.e. the ratio of a unit of rear wheel up and down travel to the unit of shock up and down travel, then figure out what the original or desired ratio is (usually performance bikes are tunes with a resonance frequency (yeah you need to figure that out based on bike mass and f/r distribution) of about 2hz, if i remember right... so yeah, do the math... sorry but modifying bikes often requires the basic algebra...) =]

math?  yeah right!  that's why I went to law school.  I took a lot of short cuts to avoid math.  For instance, I know a street bike shock doesn't travel very far, but dirtbikes do.  I used the CRF linkage, but set it at a point in rotation to correspond to the stiffer street shock.  I suppose I have no place looking for advice when guestimation has been my weapon of choice thus far.  I'm actually feeling pretty good about the 400lb spring - it should be in next week.  If I'm wrong, I'm the first to admit it.

also - I can't even wager a guess as to what "2hz" even means.