What is the stock spring rate?

[InsightRacer]

In looking at all of the aftermarket springs and coilovers availabe for the CR-Z, I wondered how they compare to the stock spring rate.

So i did a little experiment. I took two 50lb bags of duck feed (my wife has ducks) (100lb or 45.5Kg) and placed them over the front and back wheels (separately and as directly over the wheels as possible) and measured the difference in suspension height.

The suspension height difference was apprxoimately 3/8" for both. That works out to a 267 lbs/in or 4.77 Kg/mm spring rate. Given that my measurements are subjuct to mesurement error of approximately +/- 1/16", the mesurements may be off by +/- 0.5 Kg/mm.

Assuming that springs are made in 0.5 Kg/mm increments just for standardization purposes, that suggests that the stock spring rate is between 4.5 and 5.0 Kg/mm.

This raises a number of questions:

1. What are the stock spring rates? I have searched the Helms shop manual and cannot find it.
2. Is my methodology correct? Based on a basic understanding of physics and springs it should be unless they are progressive springs (I doubt it).
3. Why are these spring rates the same? The wight distribution on the CR-Z 6MT is 58.8 front /41.2 rear. Wouldn't the spring rates follow this?
4. What does this say about aftermarket springs? I have seen various rates from 5 Kg/mm front/5 Kg/mm rear (Tein) to 6/4 (Cusco) and many other combinations.

[93_turboz6]

Regarding your methodology:

From reading your post, my understanding is that you put two 50-lb bags on the front (one on each front corner) measured the difference in height, and then on both rear corners at once, and measured the difference in height, correct? This would be a reasonably accurate way because you are canceling out the effect of the front sway bar, and any sway bar-like effects the beam rear axle has.

However, if this is what you did, then your calculation is wrong. You should be dividing 50 lb by 3/8" to get 133 lb/in. That would be the "wheel rate" at each front corner. The spring rate of the actual spring is wheel rate / motion ratio^2. So if the front motion ratio is, for example .95, which would be reasonable for a mac strut, the front spring would be 133/.95^2=147 lb/in

This could be fairly close to the true number, it just doesn't account for the additional resistance of friction in the suspension, and the fact that any damper with compressed gas in it creates an added spring force.

As to the reason the spring rates are similar front to rear, there's a couple possible reasons.
1. "Ride frequency" is the rate at which the car will bounce up and down after a bump. Its determined by wheel rate, and the weight on that particular corner. Rear ride frequency is normally a bit higher that front ride frequency because that has been found to be most comfortable for the occupants of the vehicle. By having similar wheel rates front-to-rear, along with a front-heavy weight distribution, the higher rear ride frequency is created.
2. Having more roll stiffness in the rear of a FWD vehicle than the front makes the car turn better and improves corner exit grip.

[JCRZ]

What? Lol I only know psi

[InsightRacer]

Originally Posted by 93_turboz6

Regarding your methodology:

From reading your post, my understanding is that you put two 50-lb bags on the front (one on each front corner) measured the difference in height, and then on both rear corners at once, and measured the difference in height, correct? ...

93_turboz6,

Thanks for that.

No, that is not what I did (I knew I would have trouble making it clear). I put both 50lb bags on one wheel at a time. So my math was probably right, but, as you pointed out my methodology was wrong. By only doing one wheel at a time, I effectively measured the combined spring and sway bar rate. That may be an interesting measurement, but it doesn't get me to the stock spring rate I was looking for.

There is also the issue with shocks. I assumed that the effective shock spring effect would be neglidable. I thought shocks only buffered the load temporarily and settle (loose their spring) relatively quickly. But I will admit to not being an expert at these things.

I will try what you suggest with 2 50lb bags on each side and see what the difference is. It should help to elminate the sway bar effects and get a better measurement. And I will continue to use 2 50lb bags per wheel as I want sufficent deflection to minimize measurement error.

I will also use your spring rate calculations considering motion ration. A question, however, what is the motion ration for the torsion beam rear geometry?

Thanks.

[Ecotune]

[93_turboz6]

Originally Posted by InsightRacer

93_turboz6,
I will also use your spring rate calculations considering motion ration. A question, however, what is the motion ration for the torsion beam rear geometry?

Thats a good question. I dont have a CRZ so I cant measure it. You could get an approximate number by measuring the horizontal distance from the suspension pick-up point (bushing) to the spring, and divide that by the horizontal distance from the suspension pick-up point to the hub. To get a really accurate answer you'd have to remove the springs with the car on jack stands and the put a jack under the axle. Then jack it up in small increments measuring the movement of both the hub and the spring perch.

[InsightRacer]

So I redid the experiment per 93_turboz6's suggestions.

This time, 4 50lb bags of feed, 2 (100lb) on each side over the wheels. Front was done separately from the back (so it was 100lb compression on each wheel). By doing both sides at the same, effects of the roll/torsion bar are eliminated. Adjustments were also made for wheel rate vs string rate (spring rate compensates for suspension geometry).

The results are:

Depression (in): Front 0.563 Rear 0.688
Wheel Rate lb/in: Front 178 Rear 145
Wheel Rate kg/mm: Front 3.18 Rear 2.60 (Ratio 1.22)
Motion Ratio: Front 0.95 Rear 0.93
Spring Rate lb/in: Front 197 Rear 168
Spring Rate kg/mm: Front 3.53 Rear 3.01 (Ratio 1.17)

That suggests a more reasonable front spring rate of 3.5 kg/mm and a rear of 3.0 kg/mm

This makes most of the aftermarket springs look more resonable. Most are slightly stiffer than factory and tend to be stiffer on the front (although some are equal and a few even softer on the front which doesn't make much sense).

[InsightRacer]

Originally Posted by clarkandrew45

What is the result you got on trying this one? Im a bit confused regarding your methodology. Maybe I will need to read it more to understand.

They were in the post from 10/30.

But the bottom line is it looks like the stock spring rates are 3.5 kg/mm front and 3.0 kg/mm rear.

[kmanatt]

Great work, thanks for doing this for us. K

[northbridge]

^agreed. thanks again for the info