Sachs PP Shock & Eibach Spring Combo - Roll Center Correction?

[x_hawker]

Originally Posted by churchx

x_hawker: worth noting, lower (or higher) spring rate where, at front or at rear? Usually it's about changing it on one end to change grip bias, no?

Hi Church, I have little knowledge on the effects of spring rates on grip bias. Do you have any insight on how that usually works?

Per another forum post by "Moto-P": Start Quote:

Now having said this, installing shorter stroke, stiffer set of suspension will easily overload the stock tires, and the result will be a car that is really difficult to rotate well, and more prone to initial under-steer, making it less entertaining to drive, and a bit more risky.

My advice is therefore, to purchase a proper set of higher grip tire if upgrading the suspension all at the same time, and to choose a spring rate that is mild and soft, so that the agility of the car is not lost due to unwilling suspension, that might be too stiff. Set your shocks very soft if it's adjustable and increase it only if you can drive it without much "push" or initial under-steer. The FRS and BRZ needs all the suspension travel and shifting of the weight to flick vectors under braking (dive), and depriving it will reduce the fun factor and increase the efforts for the driver to drive it faster.

Stiff, low, suspension and stock tire is about the worst combination you can have in the FRS. Soft compliant suspension and mild drop and healthy set of fairly grippy tires is the only way to NOT ruin a great car, if this is a mild build.

End Quote


Perhaps I am interpreting what he is saying incorrectly?

[Tristor]

Originally Posted by x_hawker

Noted. Whiteline bushings are also at the top of my list, and I have 102k miles on my current shocks, but they don't feel like they're out yet (I probably can't even tell between worn and not unworn).

Hope I'm not asking too much, but is there an explanation as to why a lower spring rate would be geared towards oversteer, and vice versa, why a higher spring rate would induce more grip? Or is that just a fallacy?

WRT to spring rates, that's not really accurate. Fully explaining how spring rates work is a lot more involved than it sounds like, because the effective spring rate isn't the same as the spring rate of the springs on your struts due to motion ratio, angle correction factor, and other variables. There's a few ways to approach choosing the right spring rate, and I think maybe RCE could do a better job explaining that than I can.

Suffice to say though, this platform has been pretty heavily explored and for stickier (200tw) street tires you want to be in the ballpark of 5k-8k spring rate, and you want either identical rates or a heavier rate in the rear, as the effective rate will be lower in the rear due to the motion ratio.

Generally speaking, softer spring rates are better than heavier spring rates, as they provide more compliance which can help keep tires planted, but the trade-off is softer spring rates require the damper to do more work and are limited in the amount of dynamic loading they can handle (due to something like aero, for instance).

If you're just getting lowering springs, you pretty much don't need to worry about spring rates if you buy quality products as this is already figured out for you by the companies making the lowering springs to match the valving of the factory dampers. If you are getting coilovers, then you should probably consult with a suspension engineer about your specific vehicle configuration (tire, weight, aero, target alignment, target ride height) to find out the appropriate spring rate.

[churchx]

x_hawker: hmm, for example here (read all three pages including small table at end of 3rd) & here? I prefer to get wished grip balance via camber though during alignment and once dialed, stick with it, changing only for seasonal change pre/post winter, when lowering negative camber overall.

[Tristor]

Originally Posted by churchx

x_hawker: hmm, for example here (read all three pages including small table at end of 3rd) & here? I prefer to get wished grip balance via camber though during alignment and once dialed, stick with it, changing only for seasonal change pre/post winter, when lowering negative camber overall.

That second link almost looks like a direct copy of the material inside the excellent Carroll Smith's Engineer in Your Pocket. It's a great reference for tuning suspenion if you understand what the car is doing, but I think OP is not necessarily clear on what his car's behavior is right now that he wants to correct and is mostly interested in lowering. Given that, I stand by my advice that if he sticks with high quality lowering springs they are already the right spring rates for his dampers and chassis and he doesn't really need to think about this stuff yet.

[mitama]

I have a Prokit and PP strut combo, but eventually replace PP strut to Koni yellow one day.

[LancePower]

Originally Posted by mitama

I have a Prokit and PP strut combo, but eventually replace PP strut to Koni yellow one day.

How is it?

Did you ever have the Prokit with base struts?

[Racecomp Engineering]

Originally Posted by x_hawker

Noted. Whiteline bushings are also at the top of my list, and I have 102k miles on my current shocks, but they don't feel like they're out yet (I probably can't even tell between worn and not unworn).

Hope I'm not asking too much, but is there an explanation as to why a lower spring rate would be geared towards oversteer, and vice versa, why a higher spring rate would induce more grip? Or is that just a fallacy?

Higher overall spring rates means faster response and less roll so less geometry/alignment change.

Lower overall spring rates mean more compliance.

Softening spring rates at one end of the car means less weight transfer at that end of the car, meaning more evenly loaded tires, meaning more grip at that end of the car. So softer up front means more grip up front and thus more oversteer. BUT the other consequences (more goemetry change, potential bottoming, etc) may negatively impact grip, along with reduced response.

That's the short version.

- Andrew

[mitama]

Originally Posted by LancePower

How is it?

Did you ever have the Prokit with base struts?

Yes, I have prokit and PP stock dampers now

[LancePower]

Originally Posted by mitama

Yes, I have prokit and PP stock dampers now

Compare and contract the base vs PP dampers please

[x_hawker]

Originally Posted by mitama

I have a Prokit and PP strut combo, but eventually replace PP strut to Koni yellow one day.

I too am curious, how did you like the PP Sach's w/ pro-kit combo? How can we quantitatively compare the performance this combo with other combos other than anecdotal evidence?