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Andy Dawson

Dawson, A.
Handling : General principles / by Andy Dawson. - Cars & Car Conversions 1975, March (?). - p.48,49-51

 

Handling
1. General principles

PLENTY has been written about making cars go faster in a straight line, and as long as you accept what so called experts say, your car will hold the road very well. But, does your car handle in the way that you want it to? Perhaps you like understeer, perhaps you like the tail hanging out every where you go - anything is possible as long as you understand how to make your suspension do what you want it to do.

The idea of this set of articles is to explain why cars do what they do, and how they can be improved. The theories that I am going to discuss in this first article are relevant to pure single seat racing cars as well as everyday road cars.

I am going to follow the theory by putting what I preach into practice with a number of different cars, starting with an Imp, because of all the cars on the road today this has the simplest suspension and yet is one of the least well understood.

Can I also say, at this point, that in researching the article I have learned many things and even now I wonder whether I know all the answers!

Let me start with my definitions of the workings of a car's suspension.

Roadholding is how well a car goes round a bend.

Handling is the ease with which a car goes round a bend.

Controllability is the ease with which the driver can get the car round the bend.

Ride is how well the car copes with uneven road surfaces.

Thus a Formula 1 McLaren has better roadholding than an Escort, but the Escort might have better handling than the McLaren.
Ride, controllability and handling are functions of springs and shock absorbers, but roadholding is a function of tyres, wheel angles and the height of the car's centre of gravity.

    slip angle, made by the line of the tyre stepping to the side
Fig 1. Exaggerated diagram of tyre walking
Showing the slip angle developed in response to a sideways force

 

 Car roll, more weight on the outside wheel
Fig 2. Car rolling in a corner
Centrifugal force is acting effectively on the centre of gravity, causing roll and resulting in a greater weight on the outside wheel than on the inside wheel.

 

 Ackermann angle
Fig 3. Ackermann angle is the amount by which the inside wheel turns through a larger angle than the outside wheel, in order to give perfect steering geometry.

 

 steering axis inclination, camber angle and scrub radius
Fig 4. A much exaggerated view from the front of the car showing: steering axis inclination, camber angle and scrub radius.

 

 Castor angle
Fig 5. Castor angle is the angle of the centre line of the kingpin with the vertical, viewed from the side of the car. The angle is what gives feel to the steering.

 

 roll centre, with swing axle suspension
Fig 6. Roll centres: with simple swing axle suspension, the roll centre is midway between the swing arm mountings.

 

 roll centre, with twin wishbone suspension
Fig 7. With twin wishbone suspension, the roll centre is on the car centre line at a point found by taking the point where the wishbones on one side would meet (i.e., the effective sw ing axle centre) and projecting sideways to the opposite tyre centre.

 

 roll centre, with MacPherson or Chapman struts
Fig 8. With MacPherson or Chapman struts, a line is drawn at right angles from the top of the strut to cross a line projecting from the bottom arm. A line from the point where they cross is taken back to the opposite tyre centre and the roll centre falls on this line at the car's centre point.

 

 roll centre, with cart springs
Fig 9. With cart springs, it's much simpler to find the roll centre. It lies on the centre point of the car, midway between the spring hanger height and the axle mounting height.

 

 roll centre, with  Panhard Rod, Watts Linkage or A bracket
Fig 10. (above) Live rear axle (cart sprung or coils) with lateral location by Panhard Rod, Watts Linkage or A bracket. Here the roll centre is at the point where the lateral location meets the car centre line.

 

 roll centre,  semi-trailing arm rear suspension
Fig 11. (right) With semi-trailing arm rear suspension, the roll centre is found by projecting a line through the wishbone mountings to the wheel axis (as seen from above and from the front). The crossing point is then projected back (on the front view) to the opposite tyre centre and the roll centre is on this line at the centre point of the car.

 

 force needed to compress or extend shock absorbers
Fig 12. Graph showing the difference in force needed to compress or extend dampers for various applications at a set speed. Distance A:B is at ratio of 3:1. Distance B:C is 2:1.

 

Going back to first principles and the mechanics of a car going round in a circle, we can imagine the car being tied to a central stake by a piece of rope. The rope joins the car at its centre of gravity, which is the point through which all forces can be assumed to act, as it is the centre of the cumulative mass of the car. The rope will exert a force on the car opposing the centrifugal force, and if the rope is cut then the car will continue in a straight line as there will be no force acting on it.

The object of a car's suspension is twofold:

The springs in the suspension will cause the car to go up and down, like the weight on the end of a spring, unless there is something to stop it and it is for this reason that cars have dampers. Let us assume for now that the dampers are perfect and only stop the oscillations of the car.

Slip angle

The only contact between a car and the road is via the tyres. This is where we must begin our search for the answers as to why a car handles in the way it does.

The centrifugal force on the car in a corner acts through the tyre treads, which are pliable and therefore 'walk' across the road. The angle between the direction of travel of the tyre and the direction in which it is pointing is known as the slip angle (fig 1). As the centrifugal force increases so the slip angle increases to the point where the tyre cannot cope any more and 'break away' occurs.

For a car to have good roadholding the slip angles should be as low as possible for a given centrifugal force.

The ratio between front and rear slip angles determines how the car handles.

Understeer is when the front slip angle is greater than the rear and Oversteer is when the rear slip angle is greater than the front.

The slip angles can be reduced in a number of ways:

Centre of gravity

As a car's centre of gravity is above road level (usually about 20 inches on a saloon), then the centrifugal force acting on the centre of gravity will transfer weight from the inside wheel to the outside wheel (fig 2). The higher the centre of gravity the greater the weight transfer and thus the higher the slip angle of the outside tyres. For maximum roadholding the centre of gravity should be as low as possible.

If we think of a kart with a rigid chassis, vertical wheels, tyres of the same section and pressure front and rear, then if the centre of gravity coincides with the centre of the kart it will handle neutrally, as the weight transfer will be the same front and rear. If we now imagine a front engined kart with the centre of gravity well forward then the weight transfer will be greater at the front and the kart will understeer. A rear engined kart will oversteer for the same reason, but it can be made neutral again by either increasing the rear tyre pressure or their section to reduce the slip angle.

Weight transfer

Weight transfer is probably the most important factor in suspension design and it is governed by:

Roll centre

The roll centre is the point about which the car rolls (figs 6 to 11) and by joining the front and rear roll centres we get the roll axis about which the car rolls. The roll centre for a particular suspension is dependent upon the design, but the lower it is, the greater the car will roll - as the roll moment (twisting force) is the product of the centrifugal force and the distance between the centre of gravity and the roll axis.
It is usual for roll centres to be between ground level and 18 inches above, but for independent suspension systems it will move as the car rolls.

Roll stiffness

The roll stiffness of a car is dependent upon the spring rates and any anti-roll bars that may be fitted. The spring rate is such that the suspension frequency (how fast the car would go up and down were it not fitted with dampers) is usually about 80 cycles per minute for saloon cars at the front, and 20% higher at the rear, to stop pitching. Some luxury cars have suspension frequencies as low as 50 cpm but for competition we want it to be in the 140 cpm range, to give good controllability.
If the spring rate is such that the roll stiffness is too low, then we can increase it by adding an anti-roll bar that is twisted when the car rolls. If the stiffness is too high, then we can lower the spring rates and add a Z bar which is twisted in bump but not roll and increases the frequency but not roll stiffness.

When considering a suspension system and how a car is handling we must take the car as a whole and consider the total roll stiffness, but look at the relative roll stiffnesses of front to back to compare weight transfer. For instance: all a car's roll stiffness might be at the back and thus all the weight transfer will be at the back. We can alter the weight transfer either by altering the roll stiffness at that end or by changing the height of the roll centre at the other end, this decision being made for us by the value of the jacking effect.

The jacking effect

Jacking is the term given to the height change due to the vertical forces put into the chassis because of the cornering forces. The higher the roll centre and the shorter the effective swing axle length the greater the jacking force.

Steering geometry

The other variables to give a car the handling required are steering geometry and shock absorbers.
Steering variables are

Toe-in

Toe-in is only a means of taking up the slack in a steering system to make the tyres run in a straight line. In a corner, however, the inner tyre has to go round a tighter turn than the outer tyre, and thus as the steering wheel is turned, so the wheels should toe-out. The is known as Ackermann geometry (fig 3). In practice the inner wheel has a much lower slip angle than the outer wheel. So as the car rolls, it is usual for the wheels to toe-in, which gives anti-Ackermann geometry.

Steering axis inclination

Steering axis inclination is the angle with the vertical line that the wheel steers through, ie. the angle through the ball joints or the kinq pin (fig 4 ).
In the fore and aft plane the steering axis inclination is known as castor angle, and it is this angle that determines the amount of feed back that there is through the steering to the driver. The usual values are from 1 to 5 degrees, and personally I like it to be on the high side as it improves controllability (fig 5).

Scrub radius

The scrub radius is the distance between the steering axis and the centre of the tyre contact patch (fig 4). This can be changed by altering the offset of the wheel or by changing the wheel diameter. But it always should be positive, ie. with the tyre contact patch outside the steering axis. The more positive the scrub radius is then the greater the car's stability over surface irregularities.

Camber

Camber is the angle that the wheel is at, compared to the vertical. Negative camber being when the top of the wheel is leaning into the car. As said before a small amount of negative camber is beneficial for road-holding, but to reduce the grip of the front wheels (increase the slip angle) many designers set front wheels with positive camber. To bring the camber back to negative usually means that it is also necessary to increase the steering axis inclination and thus make the steering heavier.

Shock dampers

Last and by no means least, although we have been ignoring them until now, are shock absorbers or. to be more correct. we should call them dampers. The ideal damper will resist motion just enough to stop the suspension from completing more than one cycle following an input, ie, when the suspension is pushed down and released, the car will return to its original position and no more.

Transient state

In practice the damper is used to make the car handle as we want it to in the transient state. This is the period between the car being stable with one steering input and stable with another different steering input. In other words the time at which the dampers are being extended or compressed.

In the transient state the dampers will cause weight transfer, as the inside damper is applying an upward force on the suspension and the outside damper is applying a downward force. By increasing the bump resistance, we can increase the effective weight transfer in the transient state and thus deaden the steering response at that end.
On road cars it is usual for the bump to rebound ratio to be about 1:3 and the same at both ends.
On racing cars the front bump setting is usually higher than the rear to give some stability to the steering. But the bump-rebound ratio is still about 1:3.
For rallying the bump setting is used to supplement the springs to stop the car bottoming out without having very high suspension frequencies. Also to get twitchiness the rear bump settings should be higher than the front. The bump/rebound ratio usually ends up at about 1:2 (fig 12). I set my cars up to just oversteer and as long as the damper setting brings the car straight into the understeer or oversteer attitude that the springs give, I am happy.

The basic rule when setting shock absorbers is to set them as soft as possible. There are two reasons for this

  1. to allow the tyre to follow the road as well as possible and
  2. the stiffer the damper, the more it will fade as the oil gets warm.

Gas filled dampers

Gas filled dampers of the Bilstein or new Girling type fade far less than conventional units, as the oil is under pressure and will not aerate as easily. For this reason a gas filled damper will seem much softer until it is used in action, as the conventional units have to be set slightly harder than needed, to allow for fade.

One point here is that gas filled units are fitted with the body uppermost and not in the conventional manner.

Adjustable dampers

Of the adjustable types available (gas filled are not adjustable yet) the best known and probably the best for road use is the Koni but this is only adjustable in rebound, where as the Armstrong and Spax units are adjustable on both bump and rebound with the ratio fixed. Koni make a very special damper for racing that is adjustable on bump and rebound independently. Next month we will be trying some of these dampers.

Improve your car

So what can we do to improve our car? There are four variables, and we must determine which is most important for our use.

For a road car

  1. my first priority is a decent ride, followed by
  2. handling and
  3. controllability and
  4. at the bottom of the list roadholding.

My current road car, an Alfa Romeo GT is super - all I have done is

For a competition car my priorities are very different.
On a race track:

  1. Roadholding is the prime importance
  2. with handling and
  3. controllability next

But for rallying the order is

  1. controllability,
  2. handling and
  3. roadholding.

Ride is almost totally unimportant in a competition car. Although for rallying the car must be able to pass over rough roads without either breaking up or flying off the road.

One point that I should have mentioned earlier but it would have confused the issue, is the one of weight distribution, and polar moment of inertia.
If we try to spin a dumbell about the axis along the shaft, it will spin much more easily than about an axis at right angles to the shaft. This is because the polar moment of inertia is much lower about the former axis than the latter, due to the weight being close to the centre of gravity instead of a long way away.

Thus a car with all its major components close to the centre of gravity will respond to a steering input much faster than one with the major components at opposite ends of the chassis.

To get the best roadholding

We must also look at the other possibilities:

Tyre pressure

Tyre pressure is simple.
Increase it until the tyre begins to skip, due to being too stiff to accommodate the unevenness of the road surface.

Wider tyres

Usually wider tyres are simple, as long as we can keep them inside the bodywork. But we must remember to keep the width of the wheel correct for the tyre. Tyres that are too wide will slow a car on a straight due to their drag.

For rallying, tyres that are too wide will skitter across the surface on gravel roads, instead of biting into the hard surface below the top loose.

Wheel angles

Wheel angles can be easy to change. But on mass produced cars, it is usually a case of remounting the suspension. And when we are doing this, we can alter the roll centres as well, to give us the characteristics that we are looking for.

On single seaters with very wide tyres (sometimes as much as 18 inches), the tyre angle is by far the most critical variable and to ensure that it is correct, tyre technicians measure the temperature at each edge and in the centre. The higher the temperature the more work that part of the tyre is doing. Thus if a tyre is overinflated and has too much positive camber, the centre and outside temps will be higher than the inside.
For us mere mortals such techniques are going a bit far and the feel of the car will have to suffice.

 

oo - 00 - OO - 00 - oo

Next month I propose trying to improve an Imp (really Dawson, this is a quality product! - Ed.), and via a series of tests show what each different modification does to the car.

Meanwhile, try to understand the theory, you will find that to grasp why a car is doing what it is doing, will improve your driving.



 

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Version of: 26 Jan. 2014
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