Spring rate theory and application regarding our abnormal chassis.

[Mystikal]

I prefer something near a 2:3 ratio.

400/600
500/750
600/900

But there's a whole load of other factors at play here. Talking about springs alone is dangerous.

[BenFenner]

The question that seems to be going unanswered then, is why are people going with such higher wheel rates in the front than the back on a ~50/50 weight distribution?
What is the high rate up front fixing or helping with? What is deficient in the factory and Dinan (and others) setups that have mostly equal wheel rates front-to-back?

The only thing I can think of is that you might want to reduce dive during braking (fore/aft g-load under braking is typically higher magnitude than during acceleration), and probably can benefit from some squat during acceleration. That would dictate slightly higher rates up front and slightly lower rates out back which makes for a good argument to have a good bit higher wheel rate up front than in the back. The 2:3 spring rate ratio mentioned above sounds like an attempt at that style of chassis tuning. However, in my opinion it still favors the front too much. What other successful ~50/50 RWD chassis gets setup that way? I'd wager none? I know the swing arm geometry makes comparisons to other RWD chassis difficult, but does it account for that much difference?

Maybe since we sit on top of the rear axle, the front rates being high don't bother us much, but if the rear were just as stiff as the front, we'd have our fillings rattled out? That's no reason to go stiff up front.

Those spring rates above translated to wheel rates:

352/270
440/338
528/405

[albrazzi]

The question that seems to be going unanswered then, is why are people going with such higher wheel rates in the front than the back on a ~50/50 weight distribution?
What is the high rate up front fixing or helping with? What is deficient in the factory and Dinan (and others) setups that have mostly equal wheel rates front-to-back?

The only thing I can think of is that you might want to reduce dive during braking (fore/aft g-load under braking is typically higher magnitude than during acceleration), and probably can benefit from some squat during acceleration. That would dictate slightly higher rates up front and slightly lower rates out back which makes for a good argument to have a good bit higher wheel rate up front than in the back. The 2:3 spring rate ratio mentioned above sounds like an attempt at that style of chassis tuning. However, in my opinion it still favors the front too much. What other successful ~50/50 RWD chassis gets setup that way? I'd wager none? I know the swing arm geometry makes comparisons to other RWD chassis difficult, but does it account for that much difference?

Maybe since we sit on top of the rear axle, the front rates being high don't bother us much, but if the rear were just as stiff as the front, we'd have our fillings rattled out? That's no reason to go stiff up front.

Those spring rates above translated to wheel rates:

352/270
440/338
528/405

I was trying to address that in my last post, simply stiffening the front improves the Factory understeer, I don't know if ride frequency is thrown under the bus in that pursuit, but it is generally accepted in Racing circles that a very stiff Front spring and roll stiffness along with 3-4 degrees of Camber creates turn in not available with OEM springs (and aftermarket springs with the same bias). The fact is that it is counterintuitive that stiffening everything in the Front increases grip where generally it wouldn't. I think its more the way you have to approach this hybrid chassis swing arm design to achieve neutral handling. I was corrected on my Math approach but the conclusion was valid. I do get lucky sometimes.

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Something is off on your setup. I've run every spring rate front (from 400-700) and rear (500 to 800) with varying settings on bars, no bar rear, rebound/compression settings and tire pressures in search of the best different setups for asphalt and concrete in dry and wet. Much of the testing started with the discussion of motion rates such as outlined here, considerations for additional bar rate (Josh's comments) and chassis flex. For the record, you can buy used GC, Hyperco, or Swift springs in pairs for $60-70 shipped. I have friends with libraries of springs 5"-6" in length. I know some believe in the 100# split, and others discount the adjustment for rear chassis motion rates. Since I race on street tires, I tend to have a "softer" rear to get more rear end squat grip while giving me a car with crisp transitions. My car is pretty comfortable on the highway and I still take it on trips to cities without concern while running right now on 575/700 with H&R bar on soft up front and rear OE bar attached.

Allow me to clarify, smooth Highway cruising is fine its the broken roads that are uncomfortable just not as much compliance as with softer setups, I will not rule out shock settings needing some dialing since its hard to get the seat time I need consistently to play with it. You mention DA shocks, that may put you at an advantage right away since the high speed damping will be set and you can play with the low speed settings which is dialed in to achieve "Handling".
I'm open for suggestions of course but don't want to hijack the discussion I think it still has some legs.

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Trying to keep things spring related here.
If I wanted to try some much stiffer springs in the rear, I would obviously want to get a damper with appropriately stronger rebound damping. rf900rkw, you're saying that needs to be stronger high speed rebound specifically, correct? That makes sense.
And the adjustment for the single adjustable Koni is for compression anyway? And low-speed at that?

So with my single adjustable Konis, I'm not compensating for spring rate with those adjustments... I'm controlling low-speed compression. Which is basically a way of adding yaw/roll control while trying to maintain ride quality. That is not how I prefer to tune a chassis, that's for sure.

It might be time for me to get a proper setup on my car, versus living with what the previous owner tossed on there.

I thought SA adjusted rebound.

[BenFenner]

Having the rate up front high for good turn-in makes sense, but it doesn't explain why you wouldn't also go similarly high in the rear...

You bring up a better point though. The undesirable stock understeer would explain the need to correct with even more rate up front.

I guess we're left trying to decide how much is enough, and how much could be too much.

I thought SA adjusted rebound.

That is what I assumed as well. And if you're going to make one aspect adjustable, you'd think [high-speed] rebound would be it.




(I have to say, I've never experienced this stock understeer. My car came with a TCK setup that is sublimely neutral on the track (and the street), even if it is lacking in other areas. I never understood what it meant to feel like you were absolutely dancing with a car on track until this chassis/setup. Maybe it is my history of FWD driving lines and tactics that have conditioned me to be mindful of front grip. Maybe someone else would experience understeer in my car when I don't.)

[rf900rkw]

A unique feature of the Z3 is the occupants butts are at the rear axle. Thus they don't feel the pounding these extreme front rates are dishing out. The same seating position relative to the wheelbase makes the turn-in feel much slower than it would in a cab-forward seating design. Stiffening the front is going to make it respond *right now*, but will cost in the ultimate grip area.

As many have said, you just can not really isolate the spring from the shocks, bars, even the tire pressures. They are work together (or against each other) to control how much weight is on each tire and the rate of change. It's these transitional states that make or break the setup. Anyone can hit a balanced ultimate grip setup. But just like ultimate top speed, you aren't there very often if at all.

[JoshS]

As many have said, you just can not really isolate the spring from the shocks, bars, even the tire pressures. They are work together (or against each other) to control how much weight is on each tire and the rate of change. It's these transitional states that make or break the setup. Anyone can hit a balanced ultimate grip setup. But just like ultimate top speed, you aren't there very often if at all.

Just quoting for truthiness (and posterity!) Well said. Or as the twitterers say, "+1".

[albrazzi]

Having the rate up front high for good turn-in makes sense, but it doesn't explain why you wouldn't also go similarly high in the rear...

You bring up a better point though. The undesirable stock understeer would explain the need to correct with even more rate up front.

I guess we're left trying to decide how much is enough, and how much could be too much.


That is what I assumed as well. And if you're going to make one aspect adjustable, you'd think [high-speed] rebound would be it.




(I have to say, I've never experienced this stock understeer. My car came with a TCK setup that is sublimely neutral on the track (and the street), even if it is lacking in other areas. I never understood what it meant to feel like you were absolutely dancing with a car on track until this chassis/setup. Maybe it is my history of FWD driving lines and tactics that have conditioned me to be mindful of front grip. Maybe someone else would experience understeer in my car when I don't.)

If you have driven VIR I recall a point where coming out of Nascar at the right speed would require a slight lift to reset the Front end and not eat dust, more setup notes would be nice now but I have never been much of a record keeper. I didn't do much performance driving until modifications were well under way. Keep in mind the basic chassis setup was utilized well before electronic nannies were commonplace, more by Detroit than Munich but its still there. All you need to do to start changing things is add camber where none was available from the Factory. If you really considered what action created the application of ASC or DSC then understeer would be in there somewhere

[The ZED]

A unique feature of the Z3 is the occupants butts are at the rear axle. Thus they don't feel the pounding these extreme front rates are dishing out. The same seating position relative to the wheelbase makes the turn-in feel much slower than it would in a cab-forward seating design. Stiffening the front is going to make it respond *right now*, but will cost in the ultimate grip area.

As many have said, you just can not really isolate the spring from the shocks, bars, even the tire pressures. They are work together (or against each other) to control how much weight is on each tire and the rate of change. It's these transitional states that make or break the setup. Anyone can hit a balanced ultimate grip setup. But just like ultimate top speed, you aren't there very often if at all.

Key word there is FEEL, not reality.

But we still have to pick something. Just saying "Oh it's too hard to isolate so we shouldn't" doesn't get you any answer. Is it dependent on other systems in the car, yes. But like anything in physics we can isolate that variable and discover it's effects on a system. Now for the equation bit..

[kornfeld]

Bumping this old thread up....I read, and I read, and I read...and I decided to go with the TC Kline recommended 400F/400R when I placed my order today. We'll see how it feels!

[rf900rkw]

I find it funny that "they" will tell you that those totally srewy suspension frequencies are necessary, if you go down towards sane in the front the car will oversteer badly. Yet in the next sentence they will tell you that you can't run decent sized tires in the rear or the car will understeer badly.

[328 Power 04]

Did you ever get a chance to do 600# rears Randy? Since I popped mine in (along with sway bar disconnect) it cured snap oversteer, and car feels 'better'. Car was on PO's H&R/Bilstein infamous combo.
But I do like this conversation.

[SunzOut]

Bumping this old thread up....I read, and I read, and I read...and I decided to go with the TC Kline recommended 400F/400R when I placed my order today. We'll see how it feels!

SA or DA or something else....

Did you asks for a "street" setup or track/autocross.

I think these days TC is just picking what they have in stock, I think my first set was 400/500 second set were 450/500

But I'm now looking to try 450/600 to try and stop the back end bouncing under cornering load.

[ZoO]

i had 375 fronts 475 rears, went to 525 rears

i have 500/600 springs waiting to go in.

[Msilvia]

FWIW, I changed my rear shorties from 600 down to 500. Fronts stayed the same at 400.

Car drives worlds better through my socal canyon butt dyno analysis.

[kornfeld]

SA or DA or something else....

Did you asks for a "street" setup or track/autocross.

I think these days TC is just picking what they have in stock, I think my first set was 400/500 second set were 450/500

But I'm now looking to try 450/600 to try and stop the back end bouncing under cornering load.

TC Kline SA system. I asked for street setup. I do my track time in other much less valuable cars. However, I do enjoy some spirited driving.

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I find it funny that "they" will tell you that those totally srewy suspension frequencies are necessary, if you go down towards sane in the front the car will oversteer badly. Yet in the next sentence they will tell you that you can't run decent sized tires in the rear or the car will understeer badly.

You were one of the original people that helped generate this thread, so I'm curious how your setup has developed over the last few years....what spring rates have you settled on? And what tire widths?

[rf900rkw]

Nothing much has changed on my end as far as actual development work. I'm still running the H&R front (375ish) with 550 rears. The only change is the tires went from 235/275 to 255/315. I also added a coupe with KW V1 (progressive rates) and it now has the 235/275 setup. Since the local club made a rule that you had to be there the entire day (no arrive late or leave early) and I could not dedicate entirety of my one day off to sitting in the wind and rain, I went a different direction with the cars; neither has been trashed on the autocross course in several years.

[kornfeld]

Cool, thanks for the info. I'm most likely going to be going with a square set up (255s all around), and based on what you're saying, that may work well (or well enough) with equal spring stiffness front and rear. I guess springs are cheap enough that it wouldn't be the end of the world if I ended up moving to a stiffer spring in the future if it feels necessary.

[jbrannon7]

Here is a chart I made a while ago when I was thinking about springs.

Attachment 585635

[edx1]

Just noticed this thread, I'm looking at TC/Kline SA's for a 3.0 coupe that gets some HPDE track use. I was told 400/400 by TC and was surprised as I was leaning towards 400f/500r. I'm on the 225/245 bbs optioned wheels for now. Is 400f/500r not a good choice anymore?

[bionicbelly]

My ground control kit came with 525 fronts, and 675 rears. (i had them send me what they thought would be best for autocross) I have since gone to 725 rears, and I think I might need a touch more in the back. Sway bars are stock. Alos, my struts have the mount to use the M end links, but when I do, it adds way to much spring to the front, and the car pushes like mad. I went back to the control arm mounts.

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Here is a chart I made a while ago when I was thinking about springs.

Attachment 585635

Wow, according to this, I really need to bump it up in the rear.

[rf900rkw]

That chart shows the installed spring rate, which is spring rate times suspension motion ratio. The wheel rate, the forces as seen at the tire, is spring rate times the square of the motion ratio. This means that the 400/400 example would net a wheel rate of 350/180. While a RWD setup typically works best with a slightly lower wheel rate in the rear, that is ridiculous.

[jbrannon7]

I am currently running 550 front and 650 rear, which is what was recommended by Vorshlag back when I bought the shocks and springs.

I had run 650 front and 800 rear for a long time but in quick transitions like a slalom and especially an offset slalom or offset gates the back would move around too much, however the car would rotate well in a pin turn or sustained sweeper. With the stiffer springs in the car it felt stiff at both ends going over bumps. I changed the rear springs to a softer rate because I was going to a venue with known low grip. However now that I have the softer rear springs installed the car is very well balance in most condition except when I am adding throttle in a turn, then it will understeer more than it did with stiffer rear springs. The rear of the car is noticeably softer now going over bumps than it was with the 800 springs.

I think I may soften the front sway bar a little to remove some of the throttle on push. I am running the Bimmerworld Ground Control E36 tubular front sway bar. I am running 255/40-17 Hankooks all around now. I am running AST 4100s set full soft in the rear and 3 clicks from soft in the front. I suppose I could tighten the rear shocks some to help with rotation but that would not help in steady state turns, only in transitions. It is the springs, tire pressures, and sway bar that would effect steady state I believe.

[JoshS]

TC Kline himself helped me set up my road race car. Keep in mind that it's possibly lighter than some of yours due to the removal of all of the interior components. But we settled on 550/600. He told me (and testing proved he was right) that it wouldn't FEEL as fast, but it would be faster, with these softer rates. Also no rear sway bar. So that's a lot of softness in the rear vis-a-vis the front, but the car was SO easy to drive fast.

[Bobkid]

Most of this thread is waaaay above my head, but now it's got me questioning the F550 / R225 spring rates on my GAZ coilovers. Car is extremely well balanced with almost no nosedive on hard braking, and it loves the gas midcorner out.

[jbrannon7]

Most of this thread is waaaay above my head, but now it's got me questioning the F550 / R225 spring rates on my GAZ coilovers. Car is extremely well balanced with almost no nosedive on hard braking, and it loves the gas midcorner out.

If you have coilovers on the rear you cannot go by our rear settings.