Spring rate Question

[LS1-IROC]

Should I have to increase spring rate on live axle when adding a fair amount of unsprung weight? I currently have 150lbs springs on the rear of my IROC. I have added heavier wheels/tires and a ford 9 inch axle. I suspect I have added over 100lbs to the rear of the car’s unsprung mass. It seems as though the car rides too soft with those springs but I am uncertain if it’s more of a limitation on my shocks. Currently I have single adjustable Koni’s on it. If I need to increase spring rate, is there a way to rough guess how much increase will be needed or does this need to be a trial and error deal?

[Josh@Ridetech]

Are you running a coilover setup or coil spring?

Here's our spring rate calculator we use for setting our coilovers up with their correct rates - http://www.ridetech.com/tech/spring-rate-calculator/

[LS1-IROC]

Are you running a coilover setup or coil spring?

Here's our spring rate calculator we use for setting our coilovers up with their correct rates - http://www.ridetech.com/tech/spring-rate-calculator/

I'm using a QA1 2.5" diameter coil-over type spring on adjustable spring perches. The shock sits right next to the coil spring.

[CA B4C]

The spring rate of the coils springs only effects the sprung rate. With a heavier rear axle you can increase the compression on the rear shocks, however, I don't think the Koni's you are running allow this adjustment. If you lowered your Panhard Bar with the new rear axle you should increase the rear spring rate. I'm running 300# springs with the Panhard Bar about 2" below the axle centerline. Hope this helps.

[LS1-IROC]

Thanks for the info. My koni's are only rebound adjustable. Sounds like I either need a better shock in the rear or a heavier spring. I'm leaning towards a heavier spring right now (due to cost). I'm hoping maybe a 200lbs spring will help without making the ride too harsh. This is a 99% street car so comfort is key. Are you running 300lbs springs in a 3rd gen? That seems really high?

[MIKE67]

Josh:

The ridetech spring calculator page has a link to download an excel sheet that doesn't work. Please get that back up.

Thanks!!

[SLO_Z28]

Thanks for the info. My koni's are only rebound adjustable. Sounds like I either need a better shock in the rear or a heavier spring. I'm leaning towards a heavier spring right now (due to cost). I'm hoping maybe a 200lbs spring will help without making the ride too harsh. This is a 99% street car so comfort is key. Are you running 300lbs springs in a 3rd gen? That seems really high?

Yes he is and having driven his car it rides great. He has like 750#+ fronts too. Changing the panhar d bar has a huge effect. I have 500lb springs in the back of my under 2000lb Honda and I drive that thing 2500 miles a month, and it rides great.

[LS1-IROC]

Interesting...So what's the ideal position for the panhard bar?

[BillyShope]

The spring rate of the coils springs only effects the sprung rate. With a heavier rear axle you can increase the compression on the rear shocks, however, I don't think the Koni's you are running allow this adjustment. If you lowered your Panhard Bar with the new rear axle you should increase the rear spring rate. I'm running 300# springs with the Panhard Bar about 2" below the axle centerline. Hope this helps.

"On the money" answer! Don't fool with your springs! Chances are, you don't have to do anything. If you really feel a change is necessary, save your pennies for adjustable shocks. Don't waste them on other springs.
http://www.shopeshop.org

[LS1-IROC]

"On the money" answer! Don't fool with your springs! Chances are, you don't have to do anything. If you really feel a change is necessary, save your pennies for adjustable shocks. Don't waste them on other springs.
http://www.shopeshop.org

Chances are...I HAVE to do something.....the car rides too soft in the rear. So by saying I won't have to do anything, you mean get a better shock with compression adjustablility?

What i don't understand is the car rode fine before all the unsprung weight was added to it. That's why i thought I needed a new spring...nothing else changed.

[Josh@Ridetech]

Thanks for letting me know, I'll get that information passed along!


and sorry OP, I completely misread your initial question

[CA B4C]

Just got back into town from 'Vegas, sorry for the delayed response. First check to make sure you are not bottoming out the rear suspension. Then before buying new springs you can try some spring inserts which helps to increase spring rate. The difference between the Panhard Rod (or roll center) height and the center of gravity at the rear of the car establishes the length of a moment arm. The lower the Panhard Rod the longer the moment arm, thus more leverage for the car to roll. The goal is to increase rear bite thru better lateral weight transfer, and increasing spring rate to control roll.

I've found the most annoying part of stiffer rear springs is the 'launch' in the rear after hitting a bump. More rebound will help. The down side I've found with the Koni single adjustables, when moved off of soft, is the increased rebound will hold the rear axle compressed too long during rapid direction changes on an autocross. Based on your first post I don't think this will be a problem and worth a try. Hope this helps.