I have, for quite some time, been wondering the differences between hard and soft suspensions, the pros and cons of both. As far as I know, hard suspension is used for flat tracks (less bumpy) due to the limited travel of the springs while the soft suspension is used otherwise. Is this right? I appreciate your comments and welcome your replies. =D>
Well of course it is. But we're going to be talking in vast generalizations. As any other forum member, and JT no doubt would as well, tell you, you can't really view one part (eg suspension) in isolation. The thing works as one whole package, even as a setup. What it takes on one hand it gives away in the other. Even then, suspensions are made of many parts. eg. it could maybe have a soft anti-roll bar, and stiff damper, etc. It's not quite as cut out as "stiff" vs "soft". Generally speaking though softer would give you more mechanical grip.
Because the suspension is doing a few jobs it typically cannot do them all perfectly. To answer the (very general) topic question, you need to understand the roles of the suspension. I.e, what jobs is it controlling?
First off, the whole point of inventing suspension was to allow movement over uneven ground. To do this you need the wheels to be able to move vertically and the best (and cheapest) way to control this is with a spring. The stiffer the sping, the less movement you can do. So here softer is better.
Second. By introducing sprung corners of the car, you have "decoupled" (though not completely) the movement "light" mass of the wheels from the "heavy" mass of the chassis. This allows the movement over uneven ground without too much body movement which is a plus for comfort. In a racecar, there is also a positive in that the light wheels can follow the road surface without large variations in vertical load which kills grip. However, the siffer the suspension, the more "coupled" it becomes again.
An example? If you jack a car up and pull the spring out you can move the suspenion up and down without disturbing the body at all and its very easiy. Thats completely decoupled. If you put in a very soft spring spring, you could still move the wheel up and down but the body will slightly respond by bobbing up and down and its a little harder to move. Its now slightly coupled. If you increase the spring stiffness to the point its almost solid, when you move the wheel the body will move by exactly the same amount. ...And it very hard to move. That completely coupled. The effort required to move the wheel is related to the contact patch force which the tyre will see going over a bump. To minimise these, softer is better in this case.
Third. Now that we have introduced vertical movements of the wheels, the chassis will now pitch and roll. This is not delibrate, and is often not wanted, but you can't design it out without stuffing up the rest of the suspension so you have to control it. Roll and pitch in itself is not bad, but its the secondary effects of roll and pitch which cause problems such as;
Roll camber (always goes in the wrong direction)
Pitch camber (generally unwanted in any case)
Bump steer (more roll = more bumpsteer)
Delayed response of body movement which means delayed build up of steady state forces in the tyres (not necesarily bad but necessarily complicated)
So, to get rid of these effects siffer is better. Now were stuffed because we have to compromise with the first two points.
Fourth. As mentioned, aerodynamic performance is often critically affected by ride height. As JT said, there is no hard and fast rule but generally roll and pitch stuff up your elements working in ground effect so you can get better aero performance by reducing them. This is the reason F1 run insanely high spring rates. Here again stiffer is better.
Fifth. Another effect of a rolling body is that is allows the rolling forces to be transmitted in a split fashion to the wheels. Because of the non linear characteristics of the tyres, the more you load it up, the less capable it is of moving that additional load (massive oversimplifaction, look up tyre load sensitivity). Stiffen the rear springs, it gives them more rolling load and therefore less grip. Ideally, you want the rolling forces balanced so both the front and rear break away at the same time. The optimum balance is a function of many things, one of them being corner radius. A tight track requires a different roll dist than an open track. Here, stiffer or softer overall doesnt matter, rather its the balance between the front and rear.
Sixth. This is where is all turns to s*** because again there is no clear answer. You can add extra springs (third springs) which operate only in axle heave (both wheels up and down together) to let you run stiff and low to the ground for aero reasons (point 4) while still running soft to minimise contact patch load variations (point 2). But now the car is not so decoupled, but only in the axle heave movement, so you soften your ride (corner springs) to regain mechanical grip (point 1). But now the car will rolls too much (point 3). So you add an anti roll bar to stiffen up that mode while still allowing soft single wheel bump (point 2). Adding the ARB now couples the left and right sides together slightly. This means going over a bump on the left wheel, will put vertical disturbance forces onto the right wheel and hurt its grip (point 2). Now you've stiffened everything up and the wheels no longer move anymore so the car wont go over a bump and youve defeated the purpose of the suspenion in the first place (point 1).
Seventh. Tyre temperature is a critical factor in a racing tyre and you can increase it by putting more vertical load through the tyres (though there are other ways). This increase in temperature MIGHT increase grip (which means its going against point 2) or it might cook the tyre and you loose grip (which means its working WITH point 2) Then your benefit of better temperature could then be lost because the car is now rolling again to much (point 3 and 4), or you lost your balance (point 5) because the mechanical grip has changed on one axle (point 2 mixed with point 5)
Eighth. Then you get real world influences on your choice. I.e. your dampers may not damp well at high stiffnesses so the body movements are not controlled (point 1) and oscillations can cause contact patch force load variations killing your grip (point 2), possibly only on one axle too. So changing the stiffness means your optimum balance is stuffed (point 5) So in this case you will HAVE to reduce your stiffness. Another? The track profile could be exciting natural frequencies in the suspension again killing your grip due to load variations in the contact patch (point 2). You need to change either the siffness or the damping here. You can go stiffer and stuff up your balance (point 5) and reduce overall mechanical grip (point 2), but gain somewhat from an aero point of view (point 4) until you realise your car is a sedan that has no aero so you go softer. This should help mechanical grip (point 2) without the trade off in aero (point 4) but now the car is rolling so much the outside wheels are all pointing in the wrong direction (point 3) and your tyres are stuffed becuase part of them are running too hot (point 7). Also, now you have fitted one million springs to the car to stiffen up all of your troublesome modes (point 7) you now cannot change anything in isolation because everything is now coupled to everything and everything you touch will probably violate points 1-8.
So now, you tell me. What better pull-rod or push rod?
apart from the aero vs. mechanical setup/grip compromise we have mainly in F1
there is another compromise to consider, which is a consideration in Touring Car and GT racing.
Tim has mentioned in his 3rd point.
Let´s call it the response vs. absolute grip compromise.
This is a point where your driver plays a large part, if we see smallest variation in CPL (Contact Patch Load) as desireble to increase "grip" it requires a level of anticipation from the driver, to drive a "softly sprung/damped" car quick.
Some drivers are better at this then others, and some will refuse to driver a very "softly" sprung/damped car.
At times (depending on your driver and the situation e.g. qualifying vs. race setup), it can be benefical to go for a slightly stiffer setup, if it helps the driver to gain confidence and push more --> resulting in a better laptime, even if this sacrifice "theoretical optimum grip" from a tire PoV.
There is no hard and fast rule.
On the other hand, I have worked with drivers (Touring Car) who where happy to drive a soft(er) setup in Qualifying (where they can aniticipate and choose their line and braking points) but have ask for a "stiffer" setup under race conditions, where they needed the "feedback" and faster response to have the confidence for close wheel to wheel (door to door) racing and overtakes "off line", and compromised braking points/racing lines due to infights/need to defend position etc.
It´s "horses for courses" IMHO, their is more then "one right answer" to this question.
The characteristics of your tire (how forgiving they are to "abuse" - think blistering) has a lot to do with this compromise - IMHO.
Track and weather conditions (track temperature) will play into this as well.
Some of the "control tires" in Touring Car racing are very forgiving these days, and make you "pay less" for a "wrong" setup.
So you (as a race engineer) have more scope/leeway to make/keep your driver "happy" instead of fearing for the survival of your best friends (your tires).
"Make the suspension adjustable and they will adjust it wrong ......
look what they can do to a carburetor in just a few moments of stupidity with a screwdriver." - Colin Chapman
“Simplicity is the ultimate sophistication.” - Leonardo da Vinci
Great post Time and good point from 747 regarding grip vs. response in GT cars.
I'm down in Sebring this week and spoke to a driver I know who's gone from a relatively stiff GT car to a much softer one and he commented that he was overdriving the car because he didn't feel the response was there. When he slowed his inputs down the car ultimately built more grip due to the softer suspension.
As an aside even very good drivers confuse (or conflate) control moment, response and grip as we'd refer to them in engineering terms.
I know it has nothing to do, but to add to the response thing between soft and stiff cars may I say it happens in racing games or simulators. I really like to stiff it up till somehow I "feel" its jumping everywhere.
If you use standard cars with street tyres like in GT4 or GT5, its really hard to gain consistency in the anticipation of turn in. And that happens to me with my actual car while playing a little bit, too.
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna
This is a very old question and argument. There are knowledgeable people on both sides. Soft springs and big bars vs stiff springs and small bars...I think driver preference enters into it a lot...Mike it would seem would like the latter...Hakkinen liked it the other way around. Ferrari is traditionally hard and McLaren soft and compliant.
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss
