corner weights v. spring rates?

[970subaru]

what is the correct method of determining the right spring rates to use? I'm assuming you use the corner weights combined with how you want the car to act?
are there specific equations or is it trial and error? any innnput is much appreciated.

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[BriDrive]

This is such a "It Just Depends" question that its hard to answer.

Many folks spring rates are selected to try an accomplish certain dynamics as they race their cars through quick turn auto-x courses or road racing circuits or just for rideheight and aesthetics.

Most manufacturers' equip their cars with spring rates geared toward an overall road going strategy and how those spring rates work together with the particular struts, control arms, stock ride height, tires and a whole list of intangibles that came together through their own R&D.

The only thing I have been able to determine by looking at various different cars from Porsche and Ferrari to Subaru, etc is this: Almost all spring rates listed shared a front to rear ratio that was within 2 - 5 % of their actual front to rear car weights.

For example: the 2002 WRX sedan came with 163 lbs/inch springs in front and 119 lbs/inch rears...this is a (+/-) 57% / 43% ratio. The car in stock trim and ride height has an approximate (+/-) 60/40 front to rear ratio......

It seems quite reasonable to maintain these ratios no matter what the spring rate, right? Well, yes and no, especially as the spring rates get to the really stiff end of the spectrum.

Ultimately, frankly, it does become trial and error and VERY subjective....

Another example: Some of the top rung STX auto-x'ers run much higher spring rates in the rear. I've seen many folks running 8K front 10k rear setups on the WRX. Thats a spring rate ratio of 44%/56% front rear to handle a properly corner weighted car that is still approximately carrying 59% of its overall weight over the front wheels.

I've seen full on road racing WRX's quoted as using (Sheehan) 10k/10k to 12k/12k evenly front/rear sprung packages depending upon circuit......

Successful spring rate and valving options are also quite dependent upon tire choices. The way they react to a full on set of premium race rubber is quite different the what you could get out of it on a "good" DOT approved street tire.

Can't argue with success, Soooooo, what's the formula? There is no formula...you have to determine what you are going to use your car for, if its racing, look around at people who are successfully racing your brand of car an ask 'em what they are running.....This might be a good starting point for you.

If you are not racing and just want a stiffer ride with rideheight adjustable gear....stick close to the stock front to rear ratios.

Realize that the spring rate only works properly with an appropriately valved strut to handle it, and ride comfort (subjective) will suffer as you increase the rate. If the car is pretty softly sprung, a good start point might be just doubling the spring rates. For example a good / conservative all arounder package for the WRX is a 7k/5k package which maintains an approximate 58%/42% front to rear ratio an just slightly higher than 2x the stock spring rate......

BriDrive

[bullseye17]

BriDrive wins best poster of the year award.

[KC]

Originally posted by bullseye17
BriDrive wins best poster of the year award.

Agreed.

[DrBiggly]

Wonderfully intelligent post. Reminds me of the old days when the board was populated by folks who A) Gave a damn and B) Had a clue

[jmott]

Id go with more rear stiffness than the car had stock. loose is fast

[Storm]

The book "How to make your car handle" has a very informative section on choosing spring rates. It comes down to weights, measurements, and frequencies.


Jay Storm
There's a fine line between "hobby" and "mental illness"......

[BriDrive]

Originally posted by jmott
Id go with more rear stiffness than the car had stock. loose is fast

Thanks Harry Hogge ...... that's instructive

[970subaru]

thanks for the great reply bridrive!
I have a 1991 legacy wagon. Its stock springs are 124# front and 190# rear(most likely for load capacity). It is rather tail happy right now which I like. I havent corner weighted it yet, but I think I will probably get something more like 250/275, wont know for certain until I get those weights. I will be getting ground controls and agx's. tires are 215/45/16 712's. will probably staywith the 18/20 mm sways I have now.

[makofoto]

vis a vis your Legacy having heavier springs in the back, possible because of the potential load that you could carry in a wagon ... WRX Wagon has softer springs in the back ... go figure? The Tein Wagon Coil Overs come with 280/168 springs ...

[DrBiggly]

Originally posted by makofoto
WRX Wagon has softer springs in the back ... go figure? The Tein Wagon Coil Overs come with 280/168 springs ...

That's too soft IMO. Why buy coilovers with rates that soft?

[makofoto]

Bought those (used) before I started AX ... I'm running the new Tein Flex's now ... 550/450 ... before I go heavier in the rear, I want to see what my Whiteline adjustable lateral links, STI trailing arm, Subframe bolts and in front, bushings ... do to the handling ...

[kfoote]

Originally posted by BriDrive
...
Most manufacturers' equip their cars with spring rates geared toward an overall road going strategy and how those spring rates work together with the particular struts, control arms, stock ride height, tires and a whole list of intangibles that came together through their own R&D.

Very nice post, but i have to throw in a little caveat with this particular issue. This would be correct if you take the wheel rate (the effective spring rate of the car that the wheel sees, determined by spring rate and the motion ratio of the spring on that particular corner of the car). For the Subarus and 1990+ Porsches, the motion ratios front to rear are about the same, but there are cases (any BMW 3-series from 1983-present comes to mind) where the actual spring rates may be a different ratio than the weight distribution (in the case of the BMW's, the actual spring rates are higher in the rear proportional to weight distribution). This slightly more broad generalization also accounts for other types of springs as well (leaf springs, torsion bars, etc)