'Soft' versus 'Hard'

[Tanky]

We know how the Missus likes it but.............

I am having a debate about wether or not my car is too softly sprung with a friend. He believes that it should be more like kart - ie: no suspension, and says the stiffer the better but through past experiances with my car I have found that I can get better times with the car at its softest settings.

Now Im sure there are a million answers to this and that it really comes down to testing and seee which works best, but it does seem my car has none or very little damping with the setup I prefer.

Consider this assuming there are no wings front and rear. Car has independant suspension and a suspension mechanical leverage of around 1.25:1.

Situation 1:

The car weights a meesley 300kgs. Front springs are 500Lb, rears are 320Lb on 1 way shocks (bump/rebound)
There are no anti roll bars, steering geometry and tyre pessures are to a ideal seting. However allthough transient handling (turn in) is good there is terrible undertseer on steadystate cornering (mid to exit) on BOTH slow, medium and fast corners. Making changes to bump/rebound are to no positive effect (pusing the car down by hand is dificult and will only move a few mm)

Situation 2

Front springs are now 225Lb and rears are 325Lb and again an optimum basic setting used along with a raised ride height to avoid bottoming. This time the car feels balanced and there is no underteer in any situation. Dampers are at 0 bump/rebound (you can push the front of the car down with your hands 1.5" very easily as if there is no damping, but it does not oscillate erraticly). On the faster corners it was found that going up 5 clicks on the front shocks improved stability. Also worth noting is the roll angle is nearly lifting the inside front wheel.

Why was situation 2 better? Why is it not the other way round? Surely with the harder setup there would have been more weight on the outside tyres during cornering?

or

does the softer situation 2 just have more 'bite'.

and finally - would progressive springs be of good use with situation 2?

[phoenix]

There are many factors that you haven't told us that preclude a detailed answer to this question, for example what surface you are running on, front/rear SPRUNG weight distribution, unsprung weight, damper/coilover inclination, motion ratio (i.e. is it the same 1.25 leverage on both front and rear suspension) and a few mor things besides.

However, guessing at a 50% rear and 50% front weight distribution, putting in an guestimate for unspring weight and taking the motion ratio as 1:25 front and rear and damper inclination of no more than 20 degrees from the vertical, I would say that the car is 'hard' with both setups - particularly if you are running on anything other than ultra smooth tarmac.

Regardless of that, the difference in grip and resulting corner speed comes from the front/rear distribution of weight transfer. If you transfer more weight onto the outside front tyre than the rear outside tyre the car will tend to understeer. If you transfer more weight onto the outside rear tyre than the outside front tyre the car will tend to oversteer.

The a stiffer springs will not transfer more weight than a soft springs, if the front and rear springs are made stiffer or softer in proportion to one another. If you soften the front on it's own then you will be transferring less weight onto the outside front tyre and more onto the outside rear. The same total amount of weight will be transferred, only the distibution will have changed. i.e. your car travelling round a fixed radius curve at a steady speed will tranfer the same TOTAL weight towards the outside PAIR of tyres whatever springs you are running.

What you have experienced, by reducing the front stiffeness, is a reduction of load transferred onto the outside front tyre, which in your previous setup was carrying more load than it had grip, hence the understeer. By softening it more load is now carried by the outside rear tyre and if there is no sign of oversteer you could continue along the same path by either softening the front further or stiffening the rear. At some point the nature of the car will change (in steady state cornering) from an understeering car to an oversteering car.

There is another consequence of having stiffer suspension that you have also experienced. With stiffer springs the RATE at which load is transferred onto the outside tyres is increased. i.e. load transfer happens quicker with stiffer springs beacuse they take less time to compress to the new loaded position. This makes the car feel more responsive.

As to picking up the inside front wheel, if it is not a driven wheel then I wouldn't worry about it too much. Without changing suspension geometry, roll centre height, centre of gravity - or a combination of those things - if you were able to corner at the same speed with the harder front springs as you can with the soft (which you cannot at present becaue of the understeer) you would find that the inside wheel would still come off the ground, possibly even quicker as you will have less droop in the suspension with the harder, shorter travel springs.

There are a lot of good books you can read on this subject but I would also make two further observations.

As your car i very light I would imagine that the stiffness of the chassis is not enormous. It is not beyond the realms of possibility that with the 500 lb springs fitted most of the energy put into the suspension by cornering or hitting a bump is not absorbed but the spring at all but by the chassis - i.e. the chassis is twisting because it offers the path of least resistance (see the thread about two springs fitted in series). If this is the case your dampers will not be moving, and if dampers don't move, they can't work!

Secondly, if indeed the chassis is stiff enough and the springs are compressing, then the actual damper piston speeds are going to be very low and the movement minimal. With some rough guesses, I reckon that you 500 lb front springs compress 14 mm or less when the car is put on it's wheels. In rebound this would give and undamped piston speed of about 0.12 metres per second and you would need just over 200Nm of damping force to prevent oscillation. Not many dampers are capable of such levels of damping at such low piston speeds. So, unless you have racing Koni, Penske, Ohlins or similar dampers, it is likely that they will have valving that will work with your hard springs.

Of course all that I have written relates to 500lb springs on a car with 225 lb front corner weight - so those of you with cars weighing 3 times as much or more, or running with wings or ground effect, please don't jump on me!

[Red Dog]

Thats a good reply Pheonix.

Tanky, I have a space frame car weighing about 350kg. The front/rear split is 47/53% with me in it. I have a very soft front arb and no rear bar. My dampers are single adjustables and laughably ancient. I run springs of 225lbs front and 120lbs rear but this does not mean much unless you calculate the wheel rate. There is a formula you can use on the Eibach website. My wheel rates are 125lbs front and 105lb rear. BTW halving or doubling your spring rate at one end of the car is too big a change to make IMO I feel you should go up/down by about 15% max and you need to do both ends of the car, if it is reasonably balanced.


I would say that you should not be able to push down the front of your car 1.5" - when I stand on the front of mine it goes down about 1/2" - I think it is still a bit soft and I will be stiffening the springs again.

Pheonix may have got it right about the chassis stiffness and if yours is flexing then you will benefit from making it stiffer. My chassis is fairly stiff. You say your tyres and steering are ideally set but these things are not set in stone so be prepared to change them as you alter your spring and damper settings.

One thing I found when I was learning how to set my car up was that with soft spring and damper settings it was easy to feel what was happening to the car and relatively easy to correct it. This is good for driver confidence but not necessarily quick. As I put stiffer and stiffer springs on the car reacted faster and was harder to catch but I soon got used to it and my lap times fell.

One other thing not mentioned is corner weights - these are important and you need to get them within a very few kg left to right to have consistency, especially important when you are braking. You also need to get the ride height and rake right from front to rear.