Spring rates basics

[sandy309]

Choosing spring rates should be an informed process. Simply experimenting without a rough idea of what you want will be exhausting and wasteful.

My first rule of suspension is don't be sucked in by what others are doing or using, especially if they appear to be using something alot more extreme than you. If you use the softest springs that can resist bottoming out and serious geometry changes, body roll in itself is not going to lose you grip. Softer springs promote good tyre contact in principle. If the leading car is using 400lb/inch springs, don't assume it's handling or gripping well, it most likely isn't the springs that are the edge.

Also, when matching springs front to rear, you have to use the natural frequency as a guide. If your car has perfect 50/50 weight distribution, then the same rates front and rear in theory work best, but in fact you need to run the front slightly softer to avoid resonant oscillation, which creates a front to back rocking bounce!
Obviously the 106 is much lighter at the back, so to match out the natural frequencies, the rears will be much softer, ie 300lb/inch fronts do not match 300lb/inch rears, more like this:
(Example, not actual weights!)

Front sprung weight per side ~260kg
Rear sprung weight per side ~120kg

Front selected spring rate 250lb/inch, natural frequency approximately 124CPM (cycles per minute)
For a matching rear frequency (slightly higher) of 140CPM, the spring rate would need to be
147 lb/inch.

But as you can see, that wouldn't fall exactly on an available bar size, so the front spring choice really ought to be worked backwards from the chosen torsion bar diameter. Don't take my approximate torsion bar rates as gospel, you can check your wheel rates fairly accurately with some bathroom scales, a tape measure and a scissor jack.
*Remove a rear wheel and disconnect the rear ARB end plate from the arm.
*Put the scales directly under the rear hub and set up the scissor jack between the hub and the scales, so as you wind up the jack, the arm spring load depresses the scales.
*Preload the scales by about 50lb and then measuring from the arch to the hub, jack the hub one inch.
*Note the increase in weight on the scales from the preload and you have the approximate wheel rate!
Be careful the jack is secure btw, if it slips it could cause serious injury or maybe even death! Considering this, I advise you don't don't it ever etc.

You can increase the accuracy of the test by calibrating you scales before hand with varying amounts of water (1 litre is 1 kilo of fresh water, allow for the container) or by loading up the car a bit and increasing the test from 1 inch to 2 or even three if you're brave.

Approximate torsion bar rates at the wheel are:

18mm 90lb/inch
19mm 110lb/inch
20mm 135lb/inch
21mm 165lb/inch
22mm 195lb/inch
23mm 235lb/inch
24mm 280lb/inch
25mm 330lb/inch

Remember, you can tweak the handling balance with different anti roll bars (ARB's). Similar rules apply, stiffening the front increases understeer and reduces traction, stiffening the rear has the opposite effect. The reason for this it weight transfer and tyre loading.

Tyres don't grip totally when cornering, in fact they slip slightly according to the load they carry. That load is composed of three basic elements;
*The vertical load of static weight and weight transfer from the other side of the car, or other end. When you corner, the weight of the car doesn't sit equally on all the tyres, but moves to the outside tyres. By the same token, when braking or accelerating , the same transfer occurs front to back.
*The transverse load on the tyre is from the car trying to move "sideways" as you turn. in fact the car is wanting to go straight on, but you've turned the car to an angle against the direction of travel!
*The rotational load is from braking or acceleration.
These three loads obviously place very different stresses on the tyre's structure. Different makes and types of tyres have very different structures, so i'm sure you can imagine why they vary so much, without even considering the "rubber" compound.
With all these loads acting on the tyre, it slips slightly relative to the direction of travel of the car. The difference between the angle of the tyre and the direction it's travelling in is called Slip Angle.
The way slip angle increases as the tyre is loaded dictates the "grip" of the tyre. As you put more vertical load on the tyre, it can resist a higher sideways load, but only to a point, then grip reduces rapidly. Also if you apply too much steering lock or throw the car sideways to the direction of travel, you'll be getting much less grip than if the tyre is slipping gently at about 12 degrees of Slip Angle, typically. You'll also generate alot more heat in the tyres and scrub speed in the process. This is why you see F1 cars driving so smoothly all the time and why Rally cars are going the same way as the drivers seek to maximise grip and precision, wasting less energy.

The important thing i'm getting at, is that four tyres sharing the load will grip better than two or three heavily loaded tyres. The way to achieve this is to reduce weight transfer. If you're competing on a perfectly flat track, then very high spring rates will stop the car leaning and minimises transferring weight without any geometry changes. The trouble is, without enough compliance to cope with even minor bumps, the car will loose grip and traction; so a compromise has to be found.
If you accept that the suspension has to be compliant to cope with bumps and surface changes, then you have to consider how the weight of the car will be transferred from side to side and front to back. This is a function of the suspension geometry as much as the spring rates, but since the change in geometry of the 106 is pretty basic and predictable, then basic rules of spring choice apply, ie as soft as is practical to cope with bumps and promote traction, with good front to rear natural frequency matching.

That'll do for now!

[Robinson]

Edit: Ignore me.

You would have to be talking front wheel rate for the frequency matching calculations, meaning your 250lb at wheel rare is only around 200lb/in spring rate. So a 200lb front spring rate, matches roughly 150lb rear wheel rate. By those rough weights(not that far off) and calculations, with a 30% rearwards bias of frequency, you would want 220-230lb spring rate at front, to match 21mm torsion bars? Then use anti roll bars to adjust the balance(smaller front at a guess)

[Aleksandr]

Sandy, what torsion bars would you suggest to use with Pugsport Tarmac dampers and 300lb springs? 23mm seems to be best suited.

[sandy309]

One of my customers is running about 350lb/inch front with 23mm bars, maybe a touch soft on the back, but it's working well currently.

[craig]

There is one more point to add to this, it is driver preference. The theory and math are important of course but testing will determine the best set-up for any given driver. This i have found from 15 years of competing myself, many works race/rally teams run slightly different set-ups for different drivers. But testing costs money of course

regards

craig

Dyno Developments

[Aleksandr]

yeah I realise that, got to start somewhere though

[sandy309]

It also takes no account of camber/castor/toe/track geometry; simply meant as a a rough guide to get you in the right ball park for trial and error.

[eggbutt]

where do you buy your springs with specific ratings?

[dunc]

www.dfaulknersprings.co.uk

[eggbutt]

thanks put the link doesnt work

[Cyb3r]

Randall motorsport i used.

[handsometin]

It's http://www.dfaulknersprings.com rather than .co.uk.

[scottmmw]

If you we're to use 350lb front springs would you match that to 23mm bars? Im indecisive as to go 350-400lb front but want the best possible match to the rear, all advise would be great! Thankyou

[rajjinator]

I'd say you want to go a little stiffer at the rear with 350-400 lbs... i'm running 325lb front with 18mm front ARB and 23mm rear with 24mm Rarb and it feels pretty good..., tiny bit of understeer but that will probably be solved by 25mm Rarb...

[Garry.M]

I wouldn't be going as hard as 400lbs on 888's ,,,,I wouldn't be even going as high as 350 but it all depends how you like your car,,,

[Saxosteve]

Depends how wide your 888s are though Mr Muir

[Dave P]

Go for 300lb for a start., 400lb is too stiff for R888's grip level if you ask me.

[scottmmw]

Cheers lads, I know alot is going to be trial end error but I don't wnt to be to far away from the end result and spending excessive amounts getting there .

[S2Jonny]

Need help please right well.....i have a 106 S2 rallye, i have bought some 40mm lowering springs, what are the measure meants ect for the torsion bar ? how many clicks....blah blah blah, have read the guides but still not quite getting it, thanks (y)

[rajjinator]

The "clicks" method is for pikeys. get both rear wheels off the ground and measure the droop before and after. it's that simple.

[MJRallying]

Im currently using Peugeot Sport Tarmac Front Coilovers which i believe are around 300lb, with Standard rear bars and Bilstein GRP N rear dampers, everything seems good from a day at Castle Combe, in fairness i havent got it out on the stages yet to see how it will handle under rally conditions...

We will see.

[matty_d]

Great read Sandy, followed a lot of your work through Nick c's racing. Lives on my road and I'm a Combe regular.

Hats off!

Also whats up with the new 206 on his drive....? Seen some prep happening on that.

[welshpug]

Silver one is his road car, if theres one being worked on uts a friend of his i believe, he was doing some gearbox work or something on it.

Rides his bike more these days!

[denny]

Great read.
so if i understand it correctly and do the math for it this would mean a minimum of 300lbs on the front when using 22mm torsionbars?
is it normal to run such high frequencies? Normal for a trackcar would be something in de 2hz region.

I am driving a citroen ax wich is a bit lighter than the 106 so my frequencies would even be worse.
is that a problem?
Momentarely i am using 225lbs on the front axle and using 22mm torsion on the back. Not in balance, midcorner it tends to want to oversteer.
What front lbs would you all advice?

22mm at 120 kilos would suggest a frequency of 2,65hz on the back. Doing the numbers 300 lbs on front would bring me in de 2,3 hz range.

[TarkMalbot]

How do you find the natural frequency of a given spring weight?

And in the example above 250lb front and 147lb rear spring rates is this a percentage that can be used for other spring rates? I.e. 147 to 250 is 70% so if running a 19mm torsion bar with 110lb spring rate, 70% stiffer spring for the front would be 190lb?