Topic: Optimizing Spring Rates

[Ag_Sky]

I'm looking for input from others that have been experimenting with their spring rates on their Kappa cars. My goal is to maximize grip for autocrossing.

I have built a set of coilovers that allow me to change my springs to whatever rate I choose.
[attachimg=1]

My car is the 2.4 L Sky with Hankook Ventus RS-3 245/40R18 tires. My car class is STR.

I started with natural frequencies of 2.11 Hz (700 lb/in) in the front and 2.27 Hz (900 lb/in) in the rear. I drove these on the street for a week and ran one autocross event with them. This set up was too loose for my liking so I changed the set up to 1.90 Hz (550 lb/in) and 2.04 Hz (700 lb/in) for the next autocross event last weekend. My times were better with the 700/900 set up, but the car felt very slippery both front and rear. The car feels much better with the 550/700 set up, but my times for the last event were slower. I've got another event next weekend so I'm going to test the 550/700 springs again to make sure it's not the driver.

 

[cbr6]

why such high rates? the Z0K rates are much lower. I'm not criticizing just trying to learn.

[Critterman]

higher rates are going to dance around on you.

[Ag_Sky]

why such high rates? the Z0K rates are much lower. I'm not criticizing just trying to learn.

Good question. I selected springs rates that relate to natural frequencies of 2.0 to 2.5 Hz. This is based on the idea that non aero race cars use natural frequency in this range. For comparison passenger cars are designed for natural frequencies of .9 to 1.0 Hz. The ZOK suspension has a natural frequency of 1.23 Hz in the front and 1.36 Hz in the rear.

The idea is to minimize chassis roll, pitch and dive which in turn keeps the tires evenly loaded maximizing grip.

[Ag_Sky]

higher rates are going to dance around on you.

That's what I expected, but what I found so far is that it handles pretty good until I reach the limits of traction and then if I'm not on top of it the rear end starts coming around.

[wspohn]

I think you are way too high.

I run 7 kg/mm on both ends and that equates to about 390 lbs/in and have been very happy with that set up.

You normally only run as high as you do (I run 900 fronts on an old MG race car) when the specific platform responds to that, and the Kappa does not.

[2kwk4u]

I switched this summer to KW V3's w/ 6kg/mm springs (~340 Lb/in) and was happy with them in the limited bit of driving I got to do.  Didn't get to autocross it, though, before I had to put the car back in storage.  It seemed to be quite neutral.

With yours having a tendancy to oversteer, you might want to soften up the rears.  Try running the same rate front and rear to see if that settles things down for you.

[GXP_Matt]

Look up some of the threads where the guys that used to run ASP Redlines and GXPs talk about their spring rates, IIRC they were at 600ish front and 500 rear.  For autocross you really want to run lower spring rates in the rear.  For street driving a higher rear frequency gives a better ride characteristic but as you experience it also makes for significant oversteer.  I autocross my GXP in BS all the time and the miata guys in STR I am friends with are running 700 front and 550 rear.  Try swapping your front springs to the rear and the rears to the front and see how you like it. 

Good work figuring out the spring rates from the natural frequency, did you measure the motion ratios and stuff?  Nice coilover setup too, looks like the Koni sleeves?  2.5" x 7" springs?

[Ag_Sky]

Good work figuring out the spring rates from the natural frequency, did you measure the motion ratios and stuff?

Thanks, yes. Here's a link to the calculations. http://lasanguine.com/suspension.html It's not complete, I'm updating it as I figure things out.

...looks like the Koni sleeves?  2.5" x 7" springs?

Yep. 2.5" 7" springs in the front 6" springs in the rear.

[DrJones]

re-posting what I had on the thread back on SF.

your spring rates seem pretty high, not sure your NF are calculated correctly.
450f/550r on my car.

The wheel rates/natural frequencies that you use with one kind of tire might not work for another tyre. We changed our wheel rates a bit when switching to the AVON slicks since they prefer a higher wheel rate.

Being that we don't run HIGH down force aero or aero that's sensitive to it's position to the ground (e.g. keeping it level) we don't need to run really high rates.

[elff]

I'd listen to DrJones on this subject.

The man is one of the Elite Auto-Crossers in this country.

Personally I have found, heavier in the back works better for me in AutoCross.  So I agree with DrJones setup.   
 If the car understeers, it will push through a corner.  I'd prefer to have that not happen and the rear end come around quickly.  The balance point is you want to avoid both pushing and drifting as you lose time whenever you skid in AutoX.   

For the street, I really like the ZOK springs, but, the FE3 shocks are lacking for me. I did not like the Koni's on my car for the street because of what Critterman stated.  But the street is not your goal.   so these last 2 comments can be ignored for your purpose

[Black Magic]

I'm looking for input from others that have been experimenting with their spring rates on their Kappa cars. My goal is to maximize grip for autocrossing.

I have built a set of coilovers that allow me to change my springs to whatever rate I choose.
[attachimg=1]

My car is the 2.4 L Sky with Hankook Ventus RS-3 245/40R18 tires. My car class is STR.

I started with natural frequencies of 2.11 Hz (700 lb/in) in the front and 2.27 Hz (900 lb/in) in the rear. I drove these on the street for a week and ran one autocross event with them. This set up was too loose for my liking so I changed the set up to 1.90 Hz (550 lb/in) and 2.04 Hz (700 lb/in) for the next autocross event last weekend. My times were better with the 700/900 set up, but the car felt very slippery both front and rear. The car feels much better with the 550/700 set up, but my times for the last event were slower. I've got another event next weekend so I'm going to test the 550/700 springs again to make sure it's not the driver.

We will be installing new springs in a Solstice and am happy to find your post and am interested in what you learn from your spring/damper combination.  Noted your 700/900 combination produced a loose condition.  We have a lot of experience with Dual Adjustable Penske's to control the soft ZOK springs in C/Stock.  Our Penske DA dampers are valved with very high numbers at inch per second rates to act like much stiffer than stock spring rates.

Assuming your calculations for the 700/900 spring rates are correct I am sure you will be able to tune out under or over steer conditions by tuning the entire suspension setup.  Looks like you have rebound only Koni Sports so you will be somewhat limited in damper adjustments because only your rebound can be adjusted.  That said there are several ways to deal with the loose condition you experienced.

1.  Adjust your Koni's for more front rebound and/or less rear rebound.
2.  More rear tire pressure/less front tire pressure from the pressures you ran when the car was loose.
3.  Stiffer front sway bar.  Drill another end link attachment hole further from the bar end in the FSB you have.
4.  Less front toe out and/or more rear toe in.
5.  Reduce the amount of rake to the front. 

With our alignment dialed in we are able to balance our Solstice with ZOK springs to go from push to loose with just shock and air pressure adjustments.   I don't think the 700/900 spring rate alone is what made your car exhibit looseness.  You need to address the other adjustments that go with your spring rates to find the balance you like.  Good luck.  I will be following your progress and Thank You for the information you have provided about spring rates.

[Sly Bob]

Black Magic, welcome to the forum. What are you running for front and rear bars?

[Ag_Sky]

We will be installing new springs in a Solstice and am happy to find your post and am interested in what you learn from your spring/damper combination.  Noted your 700/900 combination produced a loose condition.  We have a lot of experience with Dual Adjustable Penske's to control the soft ZOK springs in C/Stock.  Our Penske DA dampers are valved with very high numbers at inch per second rates to act like much stiffer than stock spring rates.

Assuming your calculations for the 700/900 spring rates are correct I am sure you will be able to tune out under or over steer conditions by tuning the entire suspension setup.  Looks like you have rebound only Koni Sports so you will be somewhat limited in damper adjustments because only your rebound can be adjusted.  That said there are several ways to deal with the loose condition you experienced.

1.  Adjust your Koni's for more front rebound and/or less rear rebound.
2.  More rear tire pressure/less front tire pressure from the pressures you ran when the car was loose.
3.  Stiffer front sway bar.  Drill another end link attachment hole further from the bar end in the FSB you have.
4.  Less front toe out and/or more rear toe in.
5.  Reduce the amount of rake to the front. 

With our alignment dialed in we are able to balance our Solstice with ZOK springs to go from push to loose with just shock and air pressure adjustments.   I don't think the 700/900 spring rate alone is what made your car exhibit looseness.  You need to address the other adjustments that go with your spring rates to find the balance you like.  Good luck.  I will be following your progress and Thank You for the information you have provided about spring rates.

Welcome to the fray. Thanks for your input. I had the ZOK GXP springs on my car before I started experimenting with the higher spring rates. I was quite happy with the balance of the car. I used the ZOK natural frequency (NF) difference between the front to rear as a guide when I selected the 700/900 rate springs. The 550/700 rates are actually closer to the ZOK NF delta (0.0064). But as others have stated the front to rear split is more about ride comfort than performance.

The idea for increasing the spring rates is from reading Dennis Grant's Far North Racing site (http://farnorthracing.com/autocross_secrets.html). I'm beginning to believe that the R-compound tires benefit more from higher spring rates, but I'm still open to further investigation. It's hard to quantify the results other than going by how the car feels and I don't really want gauge any gains by feel alone. The only way I know how to consistently measure any gains would be with some kind of data acquisition system.

I think your spot on with your with your adjustments on your Penske shocks. It sounds like you've got the handle on your set up. Unfortunately my Koni SA shocks need to be set at their stiffest setting for the higher spring rates.

For the anit-roll bars, I have the FE3 bar on the front and the FE3 ZOK bar on the rear. These worked well with the ZOK springs so I've left them alone for now.

Again thanks for the input.

Go fast - don't hit any cones!
 

[Ag_Sky]

... not sure your NF are calculated correctly.
450f/550r on my car....

I'm curious what NF values you calculated for the 450/550 springs.
Thanks for your input!

[elff]

AG_Sky
I run my sway bars the same way.

[Black Magic]

Black Magic, welcome to the forum. What are you running for front and rear bars?

We have a stock ZOK front bar with a third  hole drilled to make the bar stiffer than ZOK stiffness if needed to help fix a loose condition.  The rear bar is a stock ZOK.

[Black Magic]

I'm looking for input from others that have been experimenting with their spring rates on their Kappa cars. My goal is to maximize grip for autocrossing.

I have built a set of coilovers that allow me to change my springs to whatever rate I choose.
[attachimg=1]

My car is the 2.4 L Sky with Hankook Ventus RS-3 245/40R18 tires. My car class is STR.

I started with natural frequencies of 2.11 Hz (700 lb/in) in the front and 2.27 Hz (900 lb/in) in the rear. I drove these on the street for a week and ran one autocross event with them. This set up was too loose for my liking so I changed the set up to 1.90 Hz (550 lb/in) and 2.04 Hz (700 lb/in) for the next autocross event last weekend. My times were better with the 700/900 set up, but the car felt very slippery both front and rear. The car feels much better with the 550/700 set up, but my times for the last event were slower. I've got another event next weekend so I'm going to test the 550/700 springs again to make sure it's not the driver.

 

Thanks again for the information you have shared.  I am wondering how your event went this weekend with the 550/700 combination. 

[Ag_Sky]

Thanks again for the information you have shared.  I am wondering how your event went this weekend with the 550/700 combination. 

I didn't make it. It was too cold for my tires and I needed to work on my SUV to drive this winter. So it will have to wait until next spring. 

[rlhammon]

I posted on your SF post and suggested you come here... glad to see the discussion moving forward.

I still think (as DrJones said) that your still too high on the spring rate.  I get what you are going for based on NF... but I think you'll be happy to try lower.  DrJones, SnaponBob, and I all settled about a 450/550 combination.

I'd have to dig out my old calculations, but I believe your motion ratios are off... I was given some data from GM on the suspension shortly after I started racing the car (510 miles after I bought it), which I've used to tune mine.  I originally ran 425 on all four corners, and the added spring rate in the back (along with balancing the car with the bars) was the key.

Perhaps you can get some other springs to try out at a spring test and tune.  As you know swapping springs isn't a hard process, so run the car 3 or 4 runs until you are consistent on time, swap springs, run again.

[Ag_Sky]

I posted on your SF post and suggested you come here... glad to see the discussion moving forward.

Me too! I appreciate every ones input. It's way better than working in a vacuum!

...I still think (as DrJones said) that your still too high on the spring rate.  I get what you are going for based on NF... but I think you'll be happy to try lower.  DrJones, SnaponBob, and I all settled about a 450/550 combination...

Next spring I'm going to try a 425/550 set. This will maintains the delta between the front and rear of the ZOK set.

...I'd have to dig out my old calculations, but I believe your motion ratios are off... I was given some data from GM on the suspension shortly after I started racing the car (510 miles after I bought it), which I've used to tune mine....

Interesting. I'd like to see that. Here's a link to how I determine mine. http://lasanguine.com/suspension.html

...Perhaps you can get some other springs to try out at a spring test and tune.  As you know swapping springs isn't a hard process, so run the car 3 or 4 runs until you are consistent on time, swap springs, run again....

I'm considering attending one of the Evolution schools this spring to continue my testing.

Again, I appreciate every one's help.

Go fast -don't hit any cones!
 

[GXP_Matt]

We have a stock ZOK front bar with a third  hole drilled to make the bar stiffer than ZOK stiffness if needed to help fix a loose condition.  The rear bar is a stock ZOK.

To be clear, these are LE5 Z0k parts (aka FE3 LNF), not LNF Z0k, correct?  So 33.3 x 4.5mm wall FSB and 25.4 x 3mm rear? 

Interesting that you went with stiffer front instead of rear as is common around here.  I prefer the LNF Z0k rear bar to the FE3 for autocross since it improved corner entry so much without seeming to give up much on exit, but I guess with custom valved Penske's you can change the balance significantly... 

[Black Magic]

To be clear, these are LE5 Z0k parts (aka FE3 LNF), not LNF Z0k, correct?  So 33.3 x 4.5mm wall FSB and 25.4 x 3mm rear? 

Interesting that you went with stiffer front instead of rear as is common around here.  I prefer the LNF Z0k rear bar to the FE3 for autocross since it improved corner entry so much without seeming to give up much on exit, but I guess with custom valved Penske's you can change the balance significantly...

Sorry I don't know about our bar measurements Matt.  I assume both bars were stock ZOK NA Pontiac. The ZOK components were added to our stock NA Solstice to run SCCA Solo in C/S.  The third hole was drilled in the FSB because only the front bar can be changed.

Is the LNF ZOK bar softer or stiffer than the FE3 bar?  The stiffer front bar helps the loose condition that Ag-Sky reported when he ran the 700/900 spring rates that he calculated to be the natural frequencies.  I don't know anything about springs and rates and frequencies because we have been burdened with STOCK springs for the last 9 years.  We are experts at getting the most out of a soft spring situation.

I do know that  STR Miata's that have something like 120/90  lb stock springs (guessing because I don't remember the exact rates)  run between 700 and 800 lb springs in front and between 400 and 500 in the rear so it  seems that Ag-Sky's natural frequency calculations are certainly in the ballpark.  If the only complaint was a loose condition with the 700/900 setup I will say again that there several ways to address that.  I would bet that 2 adjustments that only take a minute would resolve the looseness he described.

I have noted posters here that feel the higher spring rates are too high for the Kappa.  I know one ZOK Solstice that ran in SCCA STR ran 600/600 with good results but he didn't know if those rates were optimal or not.  I still have a couple more resources to check with and still have one more event to run this season in RTR before moving forward on the spring situation.

Yes we can change the balance with the dual adjustable Penske's but shocks are very much a black art in my opinion.  The key thing about shocks is to know exactly what each adjustment from full still to full soft is in terms of shock forces.  Our shock guy in California dyno's the shocks and gives us print out that shows us the force value for every adjustment throughout the entire adjustment range.  Basically all 4 shocks are different and to have the same shock force numbers from side to side requires different adjustment settings if you follow what I am trying to explain.   

[rlhammon]

I'm considering attending one of the Evolution schools this spring to continue my testing.

 

You won't have time to do any changes to the car in an Evo school.  You most likely won't want to make changes either, because they are tossing so much stuff at you I doubt you'll ever be consistent throughout the day to make a comparison.  Best thing you can do is pick up some data collection setup and run that during the school.  You'll learn a ton from the Evo school (I saw a step change performance increase)... believe me, as much as you might want to work on the car (and it's a good thing), you're still not as capable as the car... work on you, stabilize the car performance, and you'll have a crap load of fun next season. 

[Ag_Sky]

Sorry I don't know about our bar measurements Matt.  I assume both bars were stock ZOK NA Pontiac. The ZOK components were added to our stock NA Solstice to run SCCA Solo in C/S....

Black Magic - Here's a link to the various spring, shocks and roll bars available from GM for the Kappa platform. If your bars still have the tags on them you'll be able to determine which bars you have.

http://www.solsticeforum.com/forum/f42/suspension-packages-usage-58501/

That's good to know someone has had success with the 600/600 lb/in springs. I've found it difficult to be objective about what gains I've made with the higher spring rates. I suspect I'm actually driving faster, but because the car feels different at the limits of traction its hard to correlate the feel to the softer springs.  The cooler temperatures have also affected the feel of the car. The Hankook Ventus RS-3 love the heat, but not so much the cooler temperatures.

Thanks for you contribution.