a friend of mine who does a lot of suspension work for FSAE/SCCA was telling me a softer spring rate is better for contact surface on street and basically any non perfectly flat road. i do plan to track this car ever so often. The spring rates on the S2K are about 219 lb/in FRONT and 291 lb/in REAR for a stock setup. stock frequency being 1.75 front 2.02 rear. Id like to get as close to 3hz in the front to about 2.8 in the rear without completely killing street driving. here is a excel spreadsheet i have with some numbers tested. Now im curious as to why my targeted values are so off from what is recommended in the forums for street and track use. Ive seen a lot of talk about Ohlins struts but they are also rated at 11kg/mm front and i think 9 kg/mm rear. How are these driving on the street? the stiffer the strut is the less contact surface ill have when hitting a bump or uneven surface, so i also wanted to know how these performed under average road conditions. theres also the differences in ideology of either stiffer in the front or rear. i was recommended a spring rate of 4-500 lb/in front and 600 lb/in rear. im new to the enigma that is suspension so id love some experienced input. thanks

 

What brought you to target 3.0 / 2.8Hz for your ride frequencies? Those are probably higher than necessary for a street car. What are the corresponding spring rates and how do they compare to the commonly suggested rates you mentioned?

Typically, aftermarket coilover spring rates are higher than are necessary/desirable for bounce motion to increase pitch and roll stiffness (and subsequently load transfer distribution). I haven't gone through your calculations, but it wouldn't surprise me if you came up with rates significantly lower than typically found on coilover kits. Basically, when developing their parts, coilover manufacturers only have control over the springs/dampers, and can only be so accurate when making assumptions about the configuration of their customers' cars (swaybars, alignments, wheels/tires, weights, ride heights, etc.). So that is to say: conclusions you come to aren't necessarily wrong!

[EDIT] I just remembered your question about biasing spring rates front / rear. Since you've already done the calculations, I'll assume you're not mistaking/interchanging spring rates and ride frequencies, meaning your rear ride frequency can be higher than your front, even if the spring rates are lower.

Typically, a higher rear ride frequency (relative to the front) is desirable for ride quality - satisfying one of the handful of "Olley criteria." With that said, on a car like the S2000, with a short wheelbase, rearward driver seating position, already high ride frequencies, and high damping, this might not be the case or might not be necessary.

Ride frequencies might be higher in the front for a reasons related to handling. There may be limitations on swaybar sizing (packaging, suspension compliance, more direct lateral coupling of the suspension) that prevent them from being used to set the desired roll stiffness/load transfer distribution. It can also be done to allow additional compliance at the rear to improve tractive capability.

 

I'm also wondering where you got the 3Hz from? And who is recommending the spring rates for what shock?

You need to consider the spring/shock as a unit. I am guessing you are basing these calculations on the stock shocks which are fairly linear in response and tend to be stiffer relative to the springs. But the recommended spring rates you mention (500/600) seem way too stiff for the stock shocks and would produce a very stiff and jarring ride at the minimum.

The Ohlin shocks by contrast are not linear in response and the 10/8 or 12/10 recommend spring rate are for the off-the-shelf valving. Varying outside of that, it is recommended to have the shocks re-valved to match the springs. Again, you want to consider the shocks and springs as a unit.

 

Your numbers don't quite match mine; I think I found 1.3Hz front and 1.5Hz rear (or so). I think yours match up if you happened to forget to square the motion ratio. At any rate, I consider Autocross to Win (DGs Autocross Secrets) - ATW Home Page to contain a solid 90%+ of what you could need to know about suspension tuning, so start there. Using their tips and calculator, I settled on 12kgf/mm spring rate front and 18kgf/mm rear (which worked out to around 2.05/2.25Hz) on some custom valved shocks with an enormous range of linear adjustability according to their dyno sheets and Karcepts sway bars at 4/4 on 0.188 wall front and full soft rear. I can say that it absorbs bumps amazingly well on rough roads (considering the spring rates), but it's definitely rough over speed bumps. With my alignment and tires I think it's pretty well balanced and more than willing to either oversteer or understeer, but I'm also a slow driver in a fast car, so take that with a grain of salt.

 

Agree with the above. You are not calculating frequencies correctly.

My rates are 750F/600R which should come up to about 2.4Hz/2.0Hz if you do the math. That is already getting a bit stiff for rougher streets.

 

After taking a look at the numbers in the original post, I have a couple more specific comments:

I would reconsider the value you're using for your tire spring rate - it is rather low, and it doesn't look like you'd be able to achieve a ride frequency of 3Hz with the axle weight and tire stiffness you have listed.

Keeping the tire rate in mind, I'm again questioning the need and practicality of 3Hz ride frequencies. Using a more realistic tire rate (I pulled 1800 lb/in out of thin air), you'll probably wind up with a wheel rate similar in magnitude to your tire rate, meaning your tire is being "worked" nearly as much as your suspension.

This post might also get more traction on the Brakes and Suspension subforum.

 

thanks for your reply, the 3 hz came from our FSAE autocross kart. That being it’s a go kart compared to a street vehicle, we knew going in that my natural frequencies wouldn’t be so high. But i did end up settling with about 2.00 in the front and 2.25 in the rear.

 

your response was very helpful, thank you. As far as custom valved shocks I have to look more into that but it’s a start!

 

I'm at 2.6hz in the front and 2.3hz in the rear for my racecar. Rear aero with no front aero and the car is perfectly balanced.

FSAE students have trouble relating autocross cart dynamics with real world cars. While a higher rear suspension frequency in better in theory and in an autocross cart, it is not that way in a real car on a real track. It is very hard to drive and the consequences are much greater.

 

There are many reasons for the differences you're seeing between typical recommendation and what this spreadsheet says. A stock S2000 is staggered tire setup with certain rollbars, while many modified S2000s are squared up with equal width tires in front (In general, higher grip = stiffer spring used to control body roll) and stiffer front ARB and softer/no RRB. For a car that is staggered, you can totally run a square spring setup and be fine.

It's easy to blinder yourself looking only at suspension frequencies but in reality there's a lot more fine tuning to be done. Spring Hz gets you in a ballpark of where you should be then you can make decisions from there based on what you use the car for and your tire/power/other suspension setup.

2.0 and 2.25 is still pretty high for a street car. 10k springs all around is just under 2.0 Hz and would probably serve you well, on either stock stagger or square.

Last thing -- with nice expensive shocks like the ohlins, you can run higher spring rates and still maintain the road contact.

I'm not so sure this is true, many OEMs go for a slightly higher rear frequency to reduce front/rear bucking and provide a better ride. It's somewhat common to see a slightly higher rear Hz. In agreement to your statement, it is not at all a necessity if you are focusing on the performance of the car and not comfort. I also agree that FSAE folks typically learn out of a book and the application to real world is.... lacking It's interesting to think about the fact that these are the folks who go to work at GM, VW, Ford etc. It kind of aligns with the trends of getting cars that handle better and better while missing out on the intangibles.