Questions on Roll stiffness, understeer, and Nismo Springs
 

I'm trying to learn a little more about suspension and I am looking to get some input on what I think is increased understeer with the Nismo suspension.

I installed new Nismo springs and shocks in an 07 Z. I wanted to keep OEM ride height for the roads, speed bumps, and driveways in my area but was looking to replace the ageing original 90K suspension and figured I might as well upgrade. My initial though was that it should decrease or keep understeer about the same. With a little time driving on the new suspension with the reasonable speeds I can do on streets and on/off ramps it feels like the car has more understeer than before. The solution would be to get Hotchkis sway bars but for now I am just trying to get a better idea of why this happened.

Oem Spring rates
Front:314 Rear:427

Nismo Trim level (07-08) spring rates
Front: 492 Rear:336-672

So I think the front is pretty simple the spring rate is about 56% higher and therefore front roll stiffness should have increased causing more understeer if the rear springs stayed the same.

The rear is where my initial thinking probably went wrong. The final spring rate in the rear is a similar increase 57% from stock but the initial rate is 20% lower than stock. The upper tighter coils in the spring look to be bound on flat ground so I was initially thinking the rears would act mostly like they are 672 Lbs/in springs. This would keep the front and rear spring increases about the same and I would think keep understeer about the same.

Well it doesn't feel like that happened so backing up I probably make some bad assumptions with the first thought process (and might again). So now I am thinking the rears are acting like something in between 336 lbs/in and 672 lbs/in on flat ground and(or?) in a corner the outside spring is compressed to closer to a 672 lbs/in spring and the inside spring is slightly extended and closer to 336 lbs/in. Both of these should reduce roll stiffness and might the cause for the increase in understeer.

So am I thinking about this correctly or just way off in the weeds?

Never got a chance to drive a Nismo on track, but thinking about it, I would expect the car to become more neutral as it’s pushed hard once throttle is added with the rear fully loaded.

I believe the sway bars are also slightly different on the Nismo and will also change handling

LONGISH (but hopefully helpful to someone) POST, YOU ARE WARNED.

You didn't specify... are you talking about Nismo suspension from a 350Z Nismo or the over-the-counter Nismo S-Tune suspension? Very different animals.

But no matter as I'm going to describe in generalities, not which specific setup.

Couple of things on this...

Warning: This gets confusing. Hahahaha.

Steering attitude (over, under, neutral steer) is determined for a given car based upon roll stiffness as just ONE factor. (But since we're talking just increases in spring rates, we'll limit it to that. That is, not including lateral force transfer via sway bars.)

The steering attitude of a car is based on the DELTA between front and rear roll rates, not the front nor rear independently.

Most cars from the factory, even highly tuned ones like a Z33 are engineered with understeer as the default. This means that in general, FRONT roll stiffness will be higher in front and softer in the rear. Rarely are the spring and shock rates the same F-to-R simply due to weight considerations and because the front wheels are steering the car. That much is fairly obvious.

Keep in mind the old adage:

To increase oversteer (decrease understeer): Increased roll stiffness in the REAR

To increase understeer (decrease oversteer): Increased roll stiffness in the FRONT

Without getting into the science of that, suffice to say that the more roll stiffness, the quicker the tires on that end of the car (vis a vis the opposite end) reach their maximum slip angles and are at their limit of adhesion, nearly and sometimes over. In other words, that end could/will lose traction first.

But what does changing shocks and springs do to this delta?

Well, specifically, if you think about the numbers you quoted, a 56% increase in spring rate of an already stiffer (than rear) front spring (in absolute spring rate) is going to increase F-R delta even with the commensurate (and compensating) 57% increase of the rear's lower spring rate. Follow?

I thought not.... that's counterintuitive. Laff. :icon17:

For a purely hypothetical example (highly oversimplified; the math is easier, laff), say you have a front spring of 1000lbs/in deflection and you increase it to 1560lbs/in (56%) and...

You have a rear spring of 800lbs/in and you increase it 57% to 1,256lbs/in, what does this do to the delta?

Where you originally had a F/R RATIO of 1000/800 or 20% difference F-to-R (or 25% rear-to-front), you now have a 1560/1256 F/R ratio or 19% ratio front-to-rear (24% rear-to-front)

Splitting hairs? Minimal? Yes, but the fact remains that you've decreased the delta by an amount that nets out to the rear still being (even) softer than the front and even though it's only 1% - in this example - it's enough to maintain the original understeering attitude and theoretically, actually increasing understeer by this amount; however you want to quantify that. So, it may not be your imagination that the car "understeers more".

Point is, all things equal (tires, bars, alignment), while the overall handling of the car has increased capability, due to uniformly stiffer shocks/springs that are now better able to maintain the tire contact with the road, the steering attitude of the car remains largely unchanged from its original state.

IOW, if it understeered before, it will continue to do so with the spring rate change in those particular percentages that you quoted (56 and 57% increase). The math might be skewed and not realistic because these are hypothetical, made-up numbers, not the actual ones you mentioned. But they serve as an example of how changes in spring rates front and rear affect the car's steering attitude.

Bottom line, when talking about altering roll stiffness, one must pay heed to the DELTA between F & R IF your goal is to change steering attitude. If you want to modify how it behaves, well, the choice of springs is very broad. (Or you can opt for coilovers to adjust.) So, unless you want to do the in depth research to find a set that meets your exact requirements without going to sway bars, have at it; but I can tell you right now that what you'll find is that a good set of engineered springs - even if slightly off optimal - are "good enough" and then use angle of attack adjustable sway bars for that broader range of adjustability.

No, I am not a sway bar dealer nor manufacturer. Hahahaha.

Hi rancor, I would like to add a comment and more info..

I am guessing (but suspect with some certainty based upon the spring rates you posted) you installed the Nismo springs (and not the S-Tune Nismo Springs) because you stated that you wanted to maintain the stock height (S-Tune Nismo springs drop the car about 0.5”, and the Nismo suspension maintains the stock height).

Am I correct?

I installed Tein S-Tech Springs (Spring Rate (lbs/in): Front: 386 - Rear: 402). Although the spring rate is close front-to-rear, the understeer I experienced was noticeable. But, I was running 245/40-18 tires on the front and 275/40-18 on the rear. This is complicated because there are so many variables (wheels and offset, camber, tire sizing, tire composition, springs, shocks, sway bars, and even more).

I installed Nismo JDM Front and rear Sway Bars (Stabilizer Bars), Part Number: 54600-RSZ30US to reduce the understeer. That worked (reduced the understeer). I should mention that the Nismo sway bars are not adjustable. There are better sway bars that cost less and are adjustable.

You can see why the purists want coil-overs and adjustable sway bars.

Not sure how you define "purist", but despite almost 30 years in road racing, I really don't want coilovers or adjustable sways on my street car. Why? All that adjustability is only of value if you know how to adjust them properly. That includes resetting toe and camber (at the racetrack) to take advantage of raising/lowering ride height or roll stiffness. I'd rather find a high quality set of shocks/dampers with rebound adjustment (single adjust) and a set of swaybars that increase front and rear roll stiffness in a balanced manner. That way, once the suspension is set and a careful alignment done, there's no need to redo it every time an adjustment is made.

You describe why I did this 350z Street Suspension Upgrade:

https://my350z.com/forum/brakes-and-...coil-over.html

My “solution” won’t satisfy a purist (someone demanding full adjustability without constantly adding components), but will be useful for many (the majority?) owners who want to upgrade their suspension without spending a great deal of $’s and entering a very technical arena of suspension parameters and requirements.

I would imagine what you are observing in practice, supports what the calcs tell you in theory...by the numbers you are actually decreasing the F to R ride frequency ratio slightly (assuming 670 rear peak rate under compression). Hard to know exactly what rates you are seeing though without testing.

Thanks everyone.

You are correct. I thought I had clarified that I must have missed it. Nismo trim level (07-08) springs not the s-tune.