Spring rate help?

GSS138

I like this purple car.

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Current Mods: Vorpal Weapon +5.

Masterbeatty

Looks blue to me.

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2011 MB 6MT
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j-rho

Lincoln is too far from SD when there's no jacket at stake.

03threefiftyz

Troof.

03threefiftyz

I should have given up when I said "I give up"...

wstar

Ok so after all the other diversions, I still have a couple fundamentally-simple questions (one of which I already asked!):

1) Will I need to use shorter springs if I switch to a significantly stiffer spring and want to keep ride height and stroke about where it is today? Or should I basically plan on ordering the exact springs I have today, just in a different rate value? What would a normal person that knew what they were doing tend to do?

2) The rear sway: it's a given I at least need to cut back to the stock bar. What about just dumping the bar altogether? I know others have dumped the rear bar and been happy in 350Z's, but I suspect this was in combination with an upgrade to stiffer springs all around. What's the worst that could possibly go wrong* if I dump the rear bar and fail to do (insert some other unknown thing that needs to be done at the same time)? Would the car get unstable in some way that I'd really hate?

* - https://www.youtube.com/watch?v=20KJhBX9xtE

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j-rho

If you go with a stiffer spring of the same free length, you'll need to lower your lower spring perch (assuming you have that adjustability) to maintain the same ride height. If you would end up having to move the perch by more than 1", then it may make sense to get a 1" shorter spring, and move the perch less. As spring length gets shorter, you'll just need to remember to check to make sure you don't encounter a coil bind condition.
Removing the rear bar will soften the rear relative to the front, which will shift the balance towards understeer - just like stiffer front springs would.
The only thing I'd suggest to watch out for -which probably won't be an issue if you have decent rate rear springs - is to make sure the new softness in the rear suspension, doesn't mean that it is compressing so much further, that it's now hitting the bump stops or something.

wstar

Yeah... so just thinking of the rear bar as extra spring rate, I started looking that. I don't know the rear motion ratios yet (for the shock or the bar), but nevermind that factor for the moment, it's probably somewhere around 0.8 or something, -ish.

So, the Hotchkis rear bar has a mfg-spec bar spring rate of 930 lbs/in at it's weakest setting (where I've had it for a while). Assuming the bar's motion ratio at 0.8 that would be ~600 (rate times motion ratio squared), and then I assume I divide that in half for the contribution to spring rate in each rear corner. So this means that the bar is contributing 300 lbs/in to my overall spring rate in each rear corner.

Let's ignore that I wanted to up my spring rates in general, and just focus on the idea of removing the bar and leaving the total spring rate alone, in theory...

So... if I just wanted to pull the bar to get rid of its side-to-side affects and leave my overall spring-rate as is, I'd have to bump my rear springs from ~500 to ~800, is what this sounds like to me. That sounds like a bigger jump than I would have expected, as it puts the rear springs significantly stiffer than the current front springs (which are 650 springs, and then if you toss in the 1970 lb/in front sway contribution it's effectively ~1280). It seems a little odd that just to remove the rear bar and cancel out that effect on rate, I'd be moving my rear springs from 150 less than the fronts to 150 more than the fronts - I didn't think it was worth that much.

Is this a sane line of thinking? Do those numbers make basic sense, aside from the totally fake 0.8 bar motion ratio? (I can fix the math for that later after I measure and get real numbers, but still, the general idea isn't going to change a lot). Or am I thinking about this wrong?

EDIT: in converting bar rate to "virtual spring rate", I think I left out also converting back through the motion ratio of the shock absorber which would change things further. But I think, if anything, that would make the +300 lbs/in value slightly larger anyways, so it doesn't really change the overall point.

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Masterbeatty

Have you spoke with Z1 on what spring rates they use on kleemans car? I would just start with a popular track inspired setup then refine it from there.

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wstar

Well I've got a good idea on ballpark spring rates at this point (and a little confirmational math to do before I pull the trigger, which will be after I've dealt with the rear sway). Keeping in mind that I don't want to have to swap springs constantly at this stage for different tires and don't want to be too unstable on wets or r-comps, I'll probably shoot a little lower than the track specs listed in this thread, but still considerably higher than where I'm at now. Something in say the ballpark of 900 and 700 - which would be approx the same ratio of raw spring rates as what I have now, with both ends boosted by about 30%. I know there's more math to be done than that, but it's a starting point guestimate at this stage. I'm gonna go measure the suspension later today or tomorrow so I can make some easy conversion factors for F/R spring rate -> F/R frequency.

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clintfocus

good to see the Z track people putting enough 2 cents each to get a informative thread into page 4

Shamu

This is "loose"

Chris Forsberg Formula Drift Atlanta 2014 - GoPro POV - YouTube

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Grant

j-rho

Couple thoughts:
The swaybar only behaves like a spring in roll, but not in dive/squat. If you take off the bar and up the spring enough to achieve the same roll stiffness, the car will squat less under acceleration.

If you're somewhat ok with balance but want to try removing the rear bar, upping rear spring rate in conjunction makes sense. The exact amount to do so you won't really know without testing. There are a lot of things like bushings and a chassis that flex, to make actual swaybar effectiveness less than theoretical - not to mention a small mis-measurement in bar motion ratio, can have a large effect on its apparent contribution. The suggestion of someone above, to get lots of spring pairs, is a good one. Some racer groups even have spring pools people trade in and out of, to make stuff like this cheaper/easier. If you think 300 is too big a jump, try doing half that - a 30% increase (500->650) in spring rate should be readily noticeable but not shocking.

The side-to-side thing of bars is generally misunderstood. If you remove the rear bar and substitute a stiffer rear spring that achieves the exact same roll stiffness, the load on the inside rear wheel at a given lateral g, would be the same as it was before with the bar. People thing the bar is "holding up" the inside rear wheel, instead of thinking about it as a natural effect of lateral load transfer.

FWIW, the fast guys I know of with these cars, are running rates in the upper half of what Shamu provided - around 1200 front, 850 rear.

03threefiftyz

More than 850 out back with no/small bar and hoos....

03threefiftyz

j-rho, GSS is a lost cause. I've tried before.