Understanding Spring Rate

[KlassicMotion]

I just came across this info while researching what the best spring rate and length would be for my 510.  Yes, I have read most every post on here about what everyone is using for their coilover setups (most say 200lb-250lbs, but I like the comfort of my stock style ST sport springs which are rated at 136lbs).

 

This first link explains the theory and calculations:  (Its a very good read, and explains why the following calculator is the best method to use)

 

http://www.pirate4x4.com/tech/billavista/coilovers/Part_1/#SpringTheory

 

 

Second, here is a calculator for determining the proper spring rate:

 

http://www.hypercoils.com/spring-rate-calculator/

 

 

I made some actual measurements, and some estimates when using the calculator, but by adjusting my calculations up and down, I get anywhere from 146 lb to 173 lb springs. 

 

Input calculations:

 

Corner weight: The only real way to determine corner weight is to use scales under all four corners, I don't have these...  I used 500 to 550.  This estimate was based on their approximation that a 2500lb street rod has a corner weight of 625 lbs.  If someone has corner scales and can weigh a street car, I'd be curious what it weighs out to.

 

Unsprung weight:  I calculate 80 lbs for one side.  14" 280zx 6 spoke wheels/ 185/65 tires (25lbs), zx struts/brakes (56lbs).  I didn't weigh the hanging weight of the control arm, but then again, your supposed to only use half the weight of the shock and springs....  equals out?... close enough...

 

Dimension A:  I measured about 12"

 

Dimension B:  Again 12"

 

Spring Angle:  I estimated about 70-80 degrees

 

Shock Ride Height:  I'm using the KYB AGX strut inserts for MR2 fronts.  They have 6.3" of travel, giving 2.85"  compression ride height.

 

 

If your wondering, I'm looking at purchasing 150 lb or 175 lb springs....  As cheap as they are, I will probably buy both and try them out.

 

On the length; I still haven't determined the best length, but I'm considering 10-12" springs...

[Ooph!]

" If someone has corner scales and can weigh a street car, I'd be curious what it weighs out to. "

 

If there is a race shop near you either road racing of some form or a dirt car shop they will have scales and usually charge a nominal fee $ to scale and setup a car.

[MikeRL411]

" If someone has corner scales and can weigh a street car, I'd be curious what it weighs out to. "

 

If there is a race shop near you either road racing of some form or a dirt car shop they will have scales and usually charge a nominal fee $ to scale and setup a car.

 

Or pull into a truck stop weigh scale when they are not too busy and bull shit with the attendant for a while.  He or she just might let you on to the scale off center, one wheel at a time. 

[wayno]

" If someone has corner scales and can weigh a street car, I'd be curious what it weighs out to. "

 

If there is a race shop near you either road racing of some form or a dirt car shop they will have scales and usually charge a nominal fee $ to scale and setup a car.

 

Someone should ask Bob3 what he uses, he weighs every corner of his race cars/trucks. ")

[Ooph!]

I use an older version of these, I would bet that Bob3 has something similar.

 

[datzenmike]

11,250,000 x wire thickness in inches⁴

^{__________________________________________}

8 x number of free coils x coil diameter³

 

 

My stock 710 front coils were exactly 5.0625" across the coil center to center and the wire was 0.500" thick. There were 6.5 active coils. Active coils are counted from where the coil leaves touching the lower spring perch up to where they touch the top hat. These are coils that can actually move when compressed.

 

Using the above formula you can easily calculate a spring rate of 104.2 pounds per inch. There are no variables. As long as you carefully and accurately measure those three things... coil width, coil wire thickness and number of coils you will get one answer.

 

This is great for figuring out the increased spring rate for cutting a coil or part coil will do.

 

For reference, stock 510 front coils are 89 lb/in. 250-300 lb/in have been used but this is more for racing or if you are young and can put up with the discomfort for the sake of being 'kool'. For street cars 150-200 is probably plenty, and the majority of drivers will find  themselves somewhere in this range. My 710 is currently at 150 lb/in and if and when I remove my struts next time I will remove enough coil to try 175. (this puts me in the above 55 age group)

 

510/610 owners... don't forget the rears have to roughly match the fronts. If running coil overs in the shock absorber location at the axle, then match the front rate. If retaining the stock spring location multiply everything by 3.8. For example a stock rear spring is about 340 lb/in. This is due to the cantilever effect of the spring's location forward of the actual wheel location. At the wheel it feels like 90. A rear racing spring can exceed 1,200 lb/in.

[KlassicMotion]

I ordered my coilovers today from T3......  I called them up on the phone and gave them the run down of my car and equipment....  It seems Gabe knows his shit.  I initially told him I don't want a stiff setup, and that I wanted 175lb 10" springs, but after a 20 minute conversation they sold me 200lb, 8" springs....   I gave in for multiple reasons.  One, they don't have 10" 175lb springs, only 8", and an 8" would reach spring bind to early for my application. The 8" length is important for wheel clearance and to keep the spring perch high on the collar with plenty of adjust-ability.  Also, with my KYG AGX, I was informed (in their experience) those inserts are better suited for 350lb springs or greater, so he thought I might actually need 250lb springs to match the struts....  But here is the deal, If I don't put too many miles on the springs, and I'm not happy with them, they will swap them out for me for another spring rate, even if it takes me a year to get everything put together.  That's great customer service!  So I will try them, and report back how they feel.....