I think this question seems to belong here.

Determining natural frequency seems to be a complex task... Calculating the "wheel rate", "sprung weight", "motion ratio", etc..

Have any of you experts done this and come up with a general rule of thumb for our Evo's(VIII/IX)?

For example, if a stock evo has a natural frequency of 1.6hz(ex) in the front and you want to go to 1.9hz you would need to increase the spring rate by Xk.. If you wanted to go 2.2hz you would need to go to Xk..

So the general rule of thumb could be, for every .1hz the spring rate needs to increase Xk? Do spring rate increases affect the natural frequency linerally? My guess if this is possible is it would be a different rate for the front compared to the rear.

http://www.worksevo.com/Spring_Rates_1.pdf
http://farnorthracing.com/autocross_secrets5.html

Another related question? Has anyone measured the natural frequency of a stock Evo(VIII/IX), and willing to share the numbers?

 

Its not complex, its just a little time-consuming. Speaking of susension frequency and spring rates... IMO, the choice of spring rates should not be based on the spring rate itself - it should be based on what the spring rate represents, and that is the result of calculations based on a desired front and rear ride frequencies.

Since some very specific numbers are used to calculate ride frequency, it doesn't make sense to ask a coilover manufacturer "what spring rates should I buy?" but rather "how did you come up with the spring rates you offer?" IMO, the one who can give you a straight answer is the one that you should consider, whether or not you end up buying the product. Chances are, if they can tell you how they came up with the spring rates, they can also tell you why their valving is what it is.

 

Wow thats a lot to read! and alot of my favorite EvoM members and friends are here.

With what has been said and the amount of variation I would like like to ask a question about price versus performance on the coil overs. Also coil overs versus say re-valved bilstiens. So whats economical about racing

I have made some very ardent efforts to become a better driver this year (having a tight budget does that). This is my perceived order of coil overs and options for my STU car.

1) Ohlin's (doesnt matter flags or not) probably the best CO and the most expensive (3K+)
2) KW V3 7K/9k maybe with a robispec tune still looking at 2500 or so
3) Bilstien pss9's real close to the kw's in all matters (2500)

So are there other brands that maintain the best features of the above without out the big price tag. Or is that just wishful thinking that someone makes a very good coil over in the 1500 dollar range ?

I have the whiteline bars, cusco ucp, gtworks springs. For the most part my biggest problem is rapid compression. This has been pointed out to me on several occasions. I agree but im kinda tapped on the car fund. I am even considering getting some mr bilstiens and having them rebuilt and tuned. I hate throwing parts away when I am really happy with 90% of handling.

I am not nor do i ever expect to go to nationals. But I dont miss an autox. So whats in the middle ?

 

thank you for this thread. i had always thought the spring rate should be stiffer in the rear and could never understand why aftermarket companies for the most part sell their coilovers with softer springs in the rear. i brought this up once in a thread, and had people try to argue me down that it made sense, but i knew that wasn't right.

this thread has been very informative, and helps to reassure what i believed about suspension tuning and chassis dynamics.

 

Does running wheel spacers change the motion ratio (both front or back)?

l8r)

 

Wheel spacers don't affect motion ratio. Track width yes and probably a couple other obscure things.

 

bump.

 

Seems to me that the next incremental upgrade would be a set of Bilstein HDs (or used MRs) and Swift Spec-R springs. If you need more spring than that, then it's gonna have to be COs.

l8r)

 

^ I really don't think re-valved struts will help. You will still be limited to the quite soft aftermarket springs. You will still be camber limited. They will still be very heavy and odds are they will rub if you toss on wider tires.

 

^ I agree that stock struts with springs have some limitations, but that's why I stated that if he needed more spring rate than the Spec-R can give him (Spec-R is approximately equivalent to a 6.5K/7K setup), then he'd have no choice but to go with a CO. Also, unless you're looking at going with something like a 285 tire, rubbing should be a non-issue. Hasn't been an issue for my 265s on stock struts.

l8r)

 

What should straight race spring rates be for bc er series coilovers with a hotchkis rear sway bar?

 

Rates are going to be personal preference. Some like super stiff and some like compliant.

I've run 10k/8k, 8k/10, 12k/10k, 10k/12k, 9k/11k, and I'm about to pull my legs off and try 8k/9k.

As for the RSB.... I love mine. Some claim stock bar and stiff springs but I prefer a mild spring and stiff bar.

 

So, of all of the rates you've tried, which did you like best, so far?

 

im going 12k front 14k rear.

 

I am currently running 9k/11k. This feels excellent on the track but I still think 9k fronts are too stiff for rough roads.

This last weekend I was at the Milwaukee Mile oval.... it has a center road course... even with the 9k fronts I was seeing 5" of suspension compression going on and off the oval. I actually saw marks on the inner fender well from the brake line bracket. Anything softer than 9k probably would have bottomed out.

I want to try 8K/9K because I think that will be excellent on the street and on flat road courses.