I'm currently running a set of HKS Hipermax 3 coilovers, which use a 9kg/mm spring on all 4 corners. I want to go up in spring rate in the front, thinking of using an 11kg/mm spring. Has anyone here had these on a shock dyno or has enough experience to say with some degree of certainty that the shocks will be able to handle a ~20% increase in spring rate?

I really like these and want to run them for some time still but don't want to risk significantly reducing their life expectancy and that's my concern here. Is it a valid one?

 

Depending on your location you could bring them to Feal suspension and have them checked , or just send them for a proper revalve. Why would you want to make your car understeer more ? If you do the front to 11k , you should also do the rear. Feal also sells Swift springs, everything can be done thru them.

 

Your concern is very valid, what you should do is call HKS and find out what their spring rate maximum is for their dampers on the hypermax 3s, if they dont want to give it to you, Im pretty sure you should be able to go up to 11kg no problem, I think the Hipermax sports and the 3s share the same dampers.

 

I'm actually going for the opposite effect. Plan is to add spring and reduce roll bar, which is currently at medium stiffness up front (Hotchkiss). The car pushes in low speed corners more than I would like, so I want to go down in roll bar stiffness up front, but maintain the same body roll control, hence up in spring rate.

 

You actually might be able to just tune it out with the sway bars. If you go down one setting and stay the same or go up one setting in the rear that should help the pushing in those low speed corners

 

I will give you a better answer when i get home from work and have 10 to 15 minutes to type everything out. Short answer, will not work.

 

cool man, looking forward to it, might save me some trouble attempting this experiment

edit: Just looked up motion rates for the Z and going to an 11 would put my F/R motion ratio at 1.37 from what it is now (1.12).....hmmm. I might actually go to a stiffer rear spring.

 

Motion ratio stays the same unless your changing from oem style to true coilover in the rear. What you change with spring rate is frequency HZ.

Right now you are at 1.75hz front and 1.76hz rear. This setup with a non staggered tire combo should be semi neutral depending on your sway bar setup.

Your problem, most likely swaybar adjustment. I don't know what bars you have, i base my stuff off 4 hole adjustable hotchkis front bar and 3 hole rear. http://www.hotchkis.net/nissan_350z_...sway_bars.html for the published rates. If you notice when nissan raised the rear spring rate on the updated suspension, they also raised the front swaybar to bring it back to an understeering car. 06 G35 front, will be the same of very close to OEM late model Z.

When braking, you are only using your springs. As you start turning, the sway bar becomes effective and starts to add spring rate. Soft swaybar and stiff spring , car have the same wheel rate (with the sway bar included ) as a stiff spring and soft sway bar setup. Weight transfers will be different under braking, and will cause a difference in initial turn in as your coming off brakes.

What i suggest for now, don't spend any money. Play with your sway bars, set them to the softest settings and work from there. Look at the Hotchkis chart and see what they do, the stiffer the bar, the more traction you remove. Softest setting in the rear is 80% stiffer, if you want to reduce understeer, you front bar needs to be set a a rate less than 80% stiffer than stock. My front bar is at hole 2 on one side and hole 3 on the other giving me a 2057 lbs/in front bar, about 60% stiffer than stock. Rear set to full soft, i run basically 13k springs all around with a square tire setup.

Now if your running a staggered tire combo, you will need even less front bar , or more rear bar to get rid of understeer. The softest suspension setting possible is usually the fastest. I have reason for my spring combo, i run 285's front and rear, plenty of static camber , and need the pitch control for the aero on the car.

Last thing for balancing out understeer, camber. More camber in needed up front then the rear. There are ups and downs to camber, it effects traction under braking , acceleration , and cornering. YOu have to figure out where you want it set. My experiance, front camber settings of over -3 start to give noticably less front end grip on the brakes, but corner grip is great, i run -3.2 up front. Rear camber, with my LSD and tire combo, the -2.1 to -2.5 area give me the best accell out of a corner while still giving me plenty of mid corner grip.

its a lot to digest. Also do some reading in http://farnorthracing.com/autocross_secrets.html its is for autocross, but a lot of it transfers over to the roadcourse.

 

OP this ^

 

Thanks for the response and no, not at all too much to digest, good stuff. Anyways you are correct in that motion ratios don't change, I misworded that initially. I don't know what my corner weights are as my car was never corner balanced, so I am simply referring to the wheel rate ratio front to back.

My front wheel rate is at 4.26 kg/mm and rear wheel rate is at 3.79 kg/mm assuming motion ratios of .688 and .649 respectively the front/rear ratio is biased to the front at 1.12 (4.26/3.79 = 1.12)...but again this bias doesn't really mean much as i don't know what the actual corner weights are and thus don't know the natural frequency.

However I did a quick search and found some info from other users/members and it looks like the front corner balance for a G35 is ~975lbs per corner and rear is ~825lbs, so I ran some numbers and calculating for frequency. Unfortunately the 11kg spring up front would prob put me way too far in the wrong direction and if I wanted to make it work I would probably need to turn the sway bar way way down and turn the rear up quite a bit...not sure I want to experiment with it...might b an interesting thing to do, but again as you said spending money on it right is prob not the best option.

Here's a plot of the natural frequency for the 9kg front, 9kg rear, and 11kg front springs:



So anyways as of right now assuming those corner weights my car is at 1.54Hz front and 1.58Hz rear. As for the sway bars, they are 4 hole hotchkis up front set to hole 3 and 3 hole in the rear set to 2nd hole (same sways as yours). The tire set up is 265 f / 275 r. This all in all places my car at close to neutral/slightly pushing at low speeds.

I think at this point I'll play with the front sway bar, keeping it at hole 3 might be too much. Initially I did that to control roll as I was seeing a lot of wear on the outside of the front tires, but I've since improved my driving a bit, and made some other changes (run more camber+adjuster roll center). I think I'll try running your front bar setups first (2nd hole one side, 3rd on the other) and if its still pushing I'll tr 2nd hole on both sides. Nonetheless as mentioned before looking at the frequencies I'm not sure I want to experiment with the stiffer spring up front anymore even though I like the idea of relying on springs for grip and roll control. My general understanding of suspension theory is a relying too much on a roll bar will ultimately reduce overall grip of the axle as you are overloading one side and removing grip from the other.

 

I'll see what i can do tonight to run better numbers. The ones i ran were for a Z that weights 3350 pounds with driver and 51% of the weight up front. My car. I'll see what i can do for adjusting the wheelbase for a G35, but i don't know the weight distro or car weight without driver.

BTW you have not mentioned tires. Stagger makes a F/R car understeer, very hard to overcome.

 

i did, its somewhere up there in my post.

The setup i run now is new for me this year: 265/35/19 , 275/35/19 Advan AD08 tires. I could go with 18s, but f-it, i like to look good too.

I did learn about the stagger understeer thing the hard way a while back; when i did my first track day i had a 255/285 setup and boy did that thing squeal like a pig. Unfortunately didn't know any better at the time and listened to the forum racers....should have gone with a square or near square setup from the beginning....so much better.