Correlation between Front and Rear spring rates

[lagoon]

This probably isn't going to get too much traffic here but I thought I'd post up anyways.

What is the correlation between the front and rear spring rates? I know the rears are stiffer, but is there a percentage or relationship between the two? Just wanted to bring up this discussion to help me and other people pick custom spring rates.

Now I know I could just opt for a standard kit OR even pick arbitrary numbers with the rear being stiffer- but once in a while it's nice to know wtf is going on. It's obviously going to be different for cars with different engines/weights etc, but just wanted to know sort of a general rule.

Thanks

[mtechnik]

Subscribed.

[jlevie]

Seems to me that the basic relationship is determined by how the weight of the car is distributed between the front and rear spring pickup points. After that you can fine tune weight distribution by shifting weight to the front or rear by fiddling with spring rates.

[Jean]

I believe front and rear spring rates aren't the same (front strut vs. rear trailing arms), so the wheel rates are different.

Maybe I am wrong though...


subscribed.

[xLibelle]

Ive asked this question on other boards, thinking i'd get an answer from someone that sets up race cars, etc, and would have an actual answer, and the feedback has been a downer.

Lets get past spring rates and wheel rates first. Spring rates are obvious, its the compression value of the spring its self. Then theres wheel rate, where the geometry of the suspension is involved. Best example is the trailing arm on our cars: the spring is located almost exactly half-way between the hinge point and the tire's contact patch. That means your 400 lb/in spring translates into a 200 lb/in wheel rate.
Here is the extended version of what i just said: http://e30m3performance.com/tech_art...e/eff_rate.htm

Examples:
Spring - Wheel - Bias
140/250 - 132/168 - 78%
172/300 - 162/201 - 81%
315/570 - 296/381 - 78%
185/380 - 174/254 - 69%
200/285 - 188/191 - 98%
102/277 - 96/186 - 52%
160/445 - 150/298 - 50%
225/360 - 212/241 - 88%
315/570 - 296/382 - 78%

Considering Gustave's measurements are correct, then my math and these common spring rates are a basic example of the difference between front and rear wheel rates. The average front bias is at 75% meaning the rear is that much stiffer. I would have thought that, because the front of our car's are slightly more nose heavy, that the wheel rate would be more forward bias but these calculations provide otherwise.

If anyone has more knowledge on this topic, please tell.

[whiltebeitel]

https://www.corner-carvers.com/forums

These guys have more tech than you can handle. I'll try and see if they have any e30-specific threads. **WARNING** They do not suffer fools. Dot your i's and cross your t's, double check your sources. They will rip you a new one if you misinform.

[lagoon]

xLibelle, that's an interesting site you linked. Hope this thread makes a decent reference for the future.

Thanks for posting that site as well whiltebeitel, that one will take a little more time to view. I think I'll do some poking around that site tonight.

[PaintPro21]

The rear inboard springs are effectively softer due to their location, when you compare it to a true coil over shock setup. I believe the effective rate of the rears is 44%. I was discussing this with someone else, but I forgot the figures. If you do a true coilover in the rear, then your spring rates will be easily half the rate they were before.