ol' paddy

does anyone know the mathmatical formula for calculating spring stiffness?
i bought my car with weight jacks on it, and i've really been wanting to know the stiffness of my front springs. the rears are 275 lbs/in2, but the fronts don't have tags on them.

i hate to take up a thread with a dumb *** question but i couldn't find a simple one for car coil springs anywhere, so if anyone knows it, that would be greatly appreciated.

sofakingdom

iTrader: (1)

Measure ride height from a CHASSIS feature such as a "frame" part, NOT a body one such as a fender, to the ground.

Add measured weight over that axle.

Re-measure the same height.

Divide the weight you added by the distance it moved, then divide that in half (2 springs); that's your spring rate, at ride height. E.g. you add 200 lbs and it moved .375", your spring rate is 200 / .375 * .5 = 266.67 lbs / in. Obviously there's some possible error in there, such as friction, so you might want to gently bounce the car for both measurements and make sure it's settled at the right spot.

//<86TA>\\

iTrader: (14)

or use this
http://www.pontiacracing.net/js_coil_spring_rate.htm

ol' paddy

okay, that worked fine, thanks for the link.
my front springs are 775#. is that too soft for having 275# in the rear? i'd like to get the car as stiff as possible with a good balance.

//<86TA>\\

iTrader: (14)

better question, why is the rear so stiff? what has been done to the car? Unless the rear roll height has been changed by lowering the panhard bar, i cant see needing a spring that heavy.

for a v8 car, 800-1100 seems to be quite common for a front spring.

sofakingdom

iTrader: (1)

775 is roughly stock replacement IROC springs.

275 is about 50% more than any stock replacement.

I'd be inclined to suspect that the car will be just EVIL loose like that. Doesn't sound good at all.

Spring rates aren't in pounds per inch squared; just pounds per inch. Put however many pounds on them to make them compress 1".

//<86TA>\\

iTrader: (14)

stock iroc springs are just a tad over 100lbs, so a 275 is a lot stiffer, over twice as stiff

sofakingdom

iTrader: (1)

Yeah the stiffest replacement ones I've seen are around 165 or something... I was giving it the benefit of the doubt...

hellz_wings

iTrader: (2)

The stiff rear spring will generate tons of oversteer as Sofakingdom mentioned. I have stock MOOG Iroc springs and find they are decent for the street (SBC V8 with alot of weight reduction over the front of the car). The rears are just right for me and they are around 100lbs (stock IROC springs). If you lower the rear PHB mount then the rear roll center gets lowered and then you would need a stiffer spring OR bigger sway bar (or a combination of both, I would get a spring that was stiff but just comfortable enough, and then play with swaybar sizes until you get it right, meaning balanced understeer/oversteer. I personally would opt for slightly more oversteer because it's easier to control a car with more oversteer rather than more understeer.)

Pablo

iTrader: (1)

275 with a 775 up front.. yikes. That sounds like a setup for doing some Formula D on the way to the grocery store.
Actually it might not oversteer if you are running a 36 mm sway bar up front and something tiny or nothing out back. You can calculate the added rate from the swaybar, there are calculators online for this.

That said, I'm not sure there is an off the shelf shock that is really adequate for controlling a 275 spring. I have been told that a 250 is about the upper limit for a koni yellow. Admittedly, I don't know how the added sway bar rate factors into that. I run an 830ish(?)/250 combo with 36/24 with a drastically lowered rear rc. I have the 24 clamps all the way inboard otherwise the oversteer is gnarly.

hellz_wings

iTrader: (2)

LOL, gnarly oversteer sounds like fun! How would weight affect the rear oversteer? ie. putting the battery over the rear as I have done, I imagine would increase oversteer, right?

ol' paddy

i laughed so hard at the bold..., the shocks are tokico illuminas, the panhard height is stock, and the sway bars are 21 in rear and 32 in front. i bought the car this way so i don't know what he was thinking.

what will lowering the roll center by lowering the panhard mount do for the car, cause i'm sure i could easily make a bracket to do so, if i needed to.

hellz_wings

iTrader: (2)

unbalanced engineering makes a set but its like 165$ i think. It lowers the body side and axle side so you can lower it up to 4 inches or just use it to straighten it out on a lowered car. Either way, it would help.

http://www.unbalancedengineering.com/Camaro/

(scroll down to about halfway, there are no pics of what they look like)

IIRC, lowering the rear roll center will induce plant the rear tires in the turns and reduce oversteer, but will induce more understeer, so to compensate for that you add stiffer rear springs and/or a bigger rear swaybar. It's like lowering the front with raised ball joints or dropped spindles will lower the front roll center but not the rear, so this would lower the rear roll center as well which should help handling tremendously. This is the mod I was going to do next, dropped spindles with lowering panhard bar in the rear, with cut rear IROC springs (when you cut springs it increases stiffness), and if there was still understeer I would install a bigger sway bar in the rear until I get it just right (for me that's a slight bit of oversteer because that's how I like it.. I've seen too many people crash in road racing because of understeer but with oversteer you atleast have a chance at getting out of a bad situation).

Pablo

iTrader: (1)

Lowering the panhard bar mount does two thing, first off, if you have lowered your car significantly, the panhard bar is no longer level. This is going to increase any jacking effects that might occur. In a right turn a bar that is angled up toward the axle, is going to want to angle up further, raising the rear to the body, and the opposite will happen in a left turn.

So your first goal is to be sure the bar is level. In practice, it will never be level while in a corner, but if you start off with level, you should wind up with about the same amount of change making a left turn as when you make a right.

Now that that's settled, lowering the level bar to reduce the roll axis inclination does a couple of things. I'm no expert, but I think a lot of people have a hard time seperate what the springs/roll stiffness is doing from what the roll axis is doing. If you were to lower the rear roll axis you are lengthening the imaginary arm that the body is levering against about the suspension. Just like a wrench, if you put a breaker bar on it, you generate more force. More force is going to require more roll stiffness on that end to compensate, otherwise it's the equivalent of putting on a softer spring.
Lets say you start off with a neutral handling car. When you lower a roll center on one end, you end up with more traction on that end because those wheels are not seeing as much of the load as the stiffer end.
But what if we adjust the roll stiffness so that the car is once again balanced? Well then the way I imagine it, is for example a box suspended at both ends, one point higher than the other relative to the ground. If you lean the box 2 degrees about its roll axis, the lower end is going to be closer to the ground at one corner than all the rest, and if you leaned it over far enough to where it would stay leaned over, the lower corner would be the heaviest.
So if you were to draw a line in your head where the direction of weight is traveling in this instance, it would be from the back corner that is raised, to the front corner that is lower. In a turn, with an inclined roll axis, this translates into weight being directed toward the outside front corner of the car. To me, this inertia equals understeer.

If you were to imagine the opposite, a low rear roll center vs the front, well now you have the body canting toward the back outiside corner of a turn which would contribute to oversteer.

I've moved forward with the philosophy that its best to try and distribute the load to the tires evenly and a closer to level roll axis seems to work for me.

Now why don't we want a perfectly level roll axis? That's a good question that I don't really know the answer to. Everything I have read suggests that for front engine cars, it's best to maintain some degree of inclination. I have not seen an explanation as to why this is, but the only thing I can figure is that if you were to go to 0 degrees, the spring rate in the back would have to be excessively high to compensate, which would make the back end very harsh and less compliant over bumps. The front end would do the opposite. Maybe the net effect of these spring changes also increases brake dive since the springs in the front are now relatively less stiff than the back. Conversely, rear weight transfer would be reduced under acceleration. I can tell you that from experience, having a lowered rear RC and raised front RC has required a spring combo that gives me a much harsher ride in the back of the car than it used to have.


Hellz_wings
more weight to the polar opposite end is going to increase inertia at that end so yes, it would make the car oversteer more. The flip side is you are going to get more traction at that end for braking and accelerating so maybe its a wash just entering a turn trail braking or accelerating out of the turn. At some point the force laterally becomes more or less than the force longitudinally. Mid turn, more over steer.. during the transition to or from that, there has to be some cross over point.

DISCLAIMER: The information above is written by a non-engineer lay person. So I might have used incorrect terminology, or I just may be wrong outright.
I welcome anyone with more knowledge on the topic to correct or clarify what I have written. I have also ignored the fact that the roll centers migrate which could be a very significant effect that should be considered in all of this.