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View Full Version : Ridetech's coilover spring calculator



chevynut
06-03-2016, 09:56 AM
I saw this on Ridetech's site and compared it to my own coilover spring rate calculator with different results. I can't find the formulas they use, but I don't think theirs is correct. As you change spring angle or the car's weight, the preload should change but it doesn't. The spring rate changes, but only in certain increments which makes some sense as they probably round up or down.

http://www.ridetech.com/tech/spring-rate-calculator/

I also downloaded the spreadsheet and it's worse imo. For some reason they want you to measure the wheel rate first, then calculate spring rate. That makes no sense to me and I don't understand how a guy with a custom application is supposed to use it.

www.ridetech.com/info/wp-content/uploads/2016/02/spring-rate-calculator.xls (http://www.ridetech.com/info/wp-content/uploads/2016/02/spring-rate-calculator.xls)

It's not that hard to do the calculations to arrive at a spring rate. One thing I did notice is that in their spreadsheet they square the motion ratio but NOT the shock angle factor. I understood you have to square both of them and my spreadsheet does that. The difference isn't that significant at low angles.

My spreadsheet calculates the exact rate needed (not increments of 50) using the car's weight, unsprung weight, shock angle, motion ratio, and shock parameters, as well as the desired adjuster nut height. I round up or down depending on whether I want to go softer or stiffer. I will be trying to gather feedback from customers who I've used the spreadsheet for.

Ridetech is right about one thing...most guys over-spring their cars and then complain about how bad the ride quality is with coilovers. I try to use the softest spring possible since coilovers have a pretty limited stroke.

NickP
06-03-2016, 05:17 PM
My formulae for this is: C=WR/(MR)^2 (AFC) which is the same as Eibach.

chevynut
06-06-2016, 05:59 PM
Mind explaining what those terms all mean? I assume WR means wheel rate and MR means motion ratio. What is "C" and"AFC"?

So why would you start with wheel rate when imo it's the last thing you get AFTER you picked your spring?

Also, you need to factor in the shock length and preload to get a spring rate.

The way I do it is figure out the corner weight and deduct the unsprung weight to get a sprung corner weight. Then determine the motion ratio and the shock angle via direct measurements. Decide the length of the shock and how much preload you want on it, while assuming it will be about 50% compressed at ride height. With all of this you can determine the axial load on the spring and calculate the rate required to hold the car at a given height (by holding the shock at a given length). It's not that difficult, and I don't understand the concept of using wheel rate as it makes no sense to me.

When you build a car how do you know what the wheel rate is going to be?

NickP
06-07-2016, 05:35 AM
Reference: http://eibach.com/america/en/motorsport/products/suspension-worksheet

CONTACT INFORMATION If you need technical help -- Call them! - They are there to answer all your questions. 1-800-507-2338
Eibach Springs, Inc.
264 Mariah Circle
Corona, CA 92879



I accept no responsibility for the accuracy of the data contained within the site pages above.

chevynut
06-07-2016, 07:38 AM
Nick, I see no practical way to use Eibach's info to select a spring rate for a car from scratch. You have to first determine the wheel rate you want and how is that done? You said you use that formula so please explain how you use it to calculate a required spring rate for a given car.

How does a customer use that formula to determine which springs he needs to buy? Using their formulas to need wheel rate to determine spring rate, or you need spring rate to determine wheel rate. I guess I don't get it. :confused: