Spring rates and understeer

[Fishy Dave]

Following a weeks fun at Dads Day Out, Curborough and the handling day I've become very aware of how much my car understeers. Now much of that is down to the awful tyres, however, the photos and other drivers have revealed how soft the current set up is, at both ends of the car. Having spent a bit of time trawling through old posts, particularly from Dave Jackson, plus Angus website, it looks like many have settled on a rate of 225lb front and 150lb rear. I remember BOSS saying that he changed the springs for a softer road set up many years back, and I attempted to check the current springs using the spreadsheet, but the diameter seems to be between sizes and just 1mm variation gives a big difference in the stiffness.

 

I posted a wanted ad, and have been kindly offered a set of springs which my race car would have come with originally. These are estimated at being between 250 and 300lb front and 200lb rear. I know these are rather on the firm side, question is would they be too firm? Are they likely to help the understeer situation or hinder due to the stiffness of these front springs?

 

I use the car for a bit of everything - road driving two up, hooning/trackdays and occasional racing using slicks. The dampers are original Bilsteins, narrow track front, 2.75 degrees front camber, 18 - 20mm rake, red front ARB and thin rear ARB on second stiffest hole. Engine is a 1600SS K, total weight with half tank of fuel is 560kg, flat floored by me.

 

I'd love to go wide track and chuck on a new set of dampers and springs but budget as always stops that.

 

Advice gratefully received. Thanks, Dave

 

 

[Mick Day]

I'm running more or less the same geometry/ARB as you with wide-track. 1994 chassis.

 

I have 250 ft/lbs on the front & 170 ft/lbs at the rear & this gives a nice neutral set-up.

 

Don't bother with wide-track unless you need new wishbones.

 

If you do the maths 250/170 gives the same (ish) vertical component of spring force at all 4 corners (the front springs being canted at about 45 deg ish).

 

From then on its playing with tyres, pressures, rake & damper adjustment.

 

On the road it is firm but my wife & I do a lot of touring & she doesn't complain.

[David Lynch]

Dave

 

If you are worried about understeer I'd certainly avoid my springs . Only under race conditions on proper surfaces do I think they maywork. On the road they made little sense (apart from teaching me to drive a RWD car that ploughed on and value of teeth and spine) but then I didn't understand interactions of Rake, Anti Roll Bars, LSD etc. on a half tonne car. (I never will fully, just know what feels useable)

Even now I still have rear overslung ARB on stiffest setting (that will get backed off a notch before next sprint event) and 15mm rake set/flat floored using most basic of methods.

 

Cheers

 

Dave

[Pendennis]

Hi Dave

 

Problem is, if you find the right setup (spring, ride height, rake & tyre) this may all change when the tyres wear or when you replace especially if not like for like. You really want a set up that maintains equal balance as tyres wear, so when your times detiorate or car goes into a four wheel drift, that's when you know your tyres are shagged.

 

My recommendation, a bit dramatic maybe, but get rid of your resistant rubber CC front bushes and replace with an equivalent that would reduce the stiction, something like Powerflex.

 

I use 400lb on the front with a 6.5" tyre, rear has 200lb with 9" tyre. I use the stiffest front ARB without an ARB on the rear, my Cat has no rake. My car tends to oversteer as the tyres wear, but generally very well behaved.

 

Ian

 

[Fishy Dave]

Thank you all, I have a good idea what I'm looking for now.

 

Appreciate the e-mails Dave

[oldbutnotslow]

 

There is an amazing amount of info around handling, much of which is so technical as to be incomprehensible to most of us, Roll centres Slip and Ackerman angles and such like, either that or the kit needed to make the measurements cost as much as your car!

 

However , you can really only deal with the basics as much of what is discussed is not possible to adjust on a standard set up 7.

 

The basic axiom of "what you do to one end has the opposite effect on the other" holds true. ie a stiff arb at the front increases grip to the rear and thus will have the tendency to induce more understeer.

 

The old joke is true when you start looking at suspension "if I were going to there, I wouldn't start from here!"

 

In other words dont try to overcome a problem that you have caused , by doing something else. ie having fitted a stiffer front arb you then increase the stiffness of the rear arb in an attempt to overcome the understeer that you have just dialled in by fitting the stiffer front arb!

 

Taking a step back for a moment.

 

Before you can decide upon a course of action you need to define the problem.

 

How and when is the car understeering?

* Mid corner

* Entry or

* Exit

* Is the corner entry after hard braking?

* Does it occur at high speed or mid to low speeds or a combination of both?

* Does the car understeer in the same way with another driver (you could be half the problem!)

 

How is the car set up?

What caster set up are you using?

What camber setting are you using?

How is the tracking set..... toe out or toe in?

What tyre pressures are you using? Small pressure changes make a big difference ... a 3 psi change is a 15% difference at 20 psi

Are all the suspension mounting rubbers/ bushes in good condition?

Are all the dampers working correctly?

What ride height are you using?

How much rake do you have set?

What front arb thickness are you using.

What position is the rear arb on.

Is the rear arb balanced ie drop links set correctly?

Has the flat floor been set correctly?

 

 

Get the car back to a neutral position as a starting point. In the end its going to be a compromise if you want something that will be reasonably civilised on the road and yet will perform reasonably on track.

 

My 2d worth

Track stiffness springs make for a very uncomfortable ride on the road

Get your tyres sorted before you start!!

 

I will happily give a few general pointers to some of the questions that I have raised if you want but in the end and with the absence of measuring devices much of what you will do will by trial and error but be careful if you attempt to do this on public roads. A TD may well be the best place.!!

 

[Richard Price]

Dave,

Be careful fitting stiff springs to standard dampers.

Remember, the job of the damper is to control the spring.

If stiffer springs are fitted, then stiffer dampers are required to control those springs.

 

When the Caterham race cars use 250 front and 200lbs/in rear springs, they are used with the stiffer "M1" dampers (which look identical to the standard "M0" ones).

 

 

[NoLimits]

I would completely concur with Richard. Even the M1's are not strong enough for those springs IMHO.