Rover's Vikingship The SD1's Suspension
The basics........
Rover's Vikingship

During the introduction of the SD1 in 1976 lots of people were disappointed on some aspects of the all new SD1. For instance the loss of all that wood and leather, Another thing...... the suspension!!

The good old Rover P6 always received high praise for its superb suspension system which was still very advanced at the time. BMW and Mercedes, the two Teutonians, were just getting around building decent IRS systems (Independent Rear Suspension). With a very complicated front suspension system and a De Dion rear axle the P6 owner was seriously spoilt. And what did the poor SD1 owner get in 1976 ??

Mc Pherson at the front
A live axle at the back


This certainly doesn't sound like the specification of a supercar....but then is the ride of an SD1 really bad?? Fortunately it isn't. Actually it is quite good!! In this corner of our site we will look somewhat deeper into the SD1's suspension. To start with the basics......

Why do we have suspension on our car??

Well if the roads were perfect we wouldn't need any form of s uspension. Unfortunately..... roads are far from perfect. To keep the shocks and bumps away from the passengers engineers have developed all kind of mechanism's to keep the shocks and bumps away.

Basically all mechanism's use something which can deform to provide a flexible medium between the road and the car itself. This flexible medium can be all kind of things i.e.

  • Torsion bars
  • Coil springs
  • Rubber
  • Hydro pneumatic elements

Even the wheel itself can be used as a kind of suspension....(In go-karts for example) but more often than not, just the wheel is not enough.

When a wheel hits a bump it compresses the suspension (i.e. energy is stored in the suspension) after the bump the stored energy wants to flow back....As a result when you hit a bump without shock absorbers the suspension would continue to go up and down. The shock absorber absorbs the energy generated by hitting the bump and prevents the suspension to continue its up and down movement.

Well that is the basic part of the suspension.....not to difficult hey??..... now to let a wheel move up and down is another thing....What functions does the suspension perform?

  • It has to carry the car and its weight......big deal!!
  • It has to keep the wheels perpendicular to the road for maximum grip
  • It has to take the forces for accelerating or braking the car
  • It has to take the torque created from the driveline
  • It has to take the forces involved when cornering the car
Wheel & suspension forces  (6,3 kB)

Oops....doesn't sound easy now does it?? So back to the basics once more...

Panorama applet



To let the wheel bring all those forces and torque on the road it has to be kept in constant contact with the easy as that. This means that under ideal circumstances a wheel is always perpendicular to the road. Tilting in corners or on bumps minimises the area of the wheel in contact with the road so this is not desired. This tilting of the wheel is called camber. In an ideal situation the camber angle of the wheel is always zero degrees. In reality the camber angle changes with the up and down movement of the suspension. Also body roll affects the camber angle. Often cars have a light Positive camber angle under no load conditions to make up for the compression of the suspension and rubber bushes. When normally loaded the camber angle becomes zero. More on camber in the understeer/oversteer section. This angle is one of the most important in the suspension system!

Camber  (5,0 kB)

Caster & Kingpin Inclination (KPI)

To offer enough feel to the steering and to get a self returning action the suspension can be set up to achieve this by carefully choosing the caster angle and kingpin inclination angle.

You can get a very strong self centring effect if the tyre footprint trails the steering axes. As shown here in the wheel of a handcart and the same principle as used in our car. The angle between the steering axis and a line from wheel center to the ground is called the caster angle. The bigger the angle the stronger the self centring action. If the angle is negative the steering is very light and very very nervous.

Another way to get a selfcentring action is by creating a steering offset. (3. In the figure to the right). This can be created if the projected KPI angle doesn't align with the footprint center of the tyre. The wheelforces will try to pull the center of contact patch of the front wheels forward, thus the wheel will rotate about the point of the kingpin axle projected to the ground. The steer momentum is the product of distance 3. and the wheelforce. Increasing the inclination angle will decrease the self centring steering effect. Also notice that the steering effort here is changing with the amount of power supplied to the wheels.... So for front wheel drives reducing steering effort by altering the KPI is not a very elegant solution. So why not increase the KPI so that distance 3. is zero.....?? Well with increasing KPI also the lateral forces on the cars increase thus making it more receptive to roll and instability. With McPherson an increase of KPI also means that the turrets protrude more deeply into the engine bay.

Caster  (3,8 kB)

Camber & Kingpin inclination  (6,5 kB)
Toe-in & Toe-out

Toe-in and toe-out is the angle that the wheels deviate from the driving direction. It is used because the driving force and road resistance tends to squeeze the rubbers in the suspension. By giving the car a toe-in or toe-out setting the wheels are in a straight line when driving. The rear wheels seldom have toe-in or toe-out. The front wheels of a rear wheel driven car mostly have toe-in. Toe-out is used in front wheel driven cars because the driving force tends to turn the wheel inwards. The kingpin inclination also plays a (minor) role in how much toe-in or toe-out is needed. The less kingpin inclination the more steering momentum is generated pulling the wheel inwards thus more toe-out is also needed to compensate.

Toe-in  Toe-out  (2,7 kB)
Sprung and unsprung weight

Imagine a car of 2000 lbs with wheels of 200 lbs . Each time this car would hit a bump it would almost be launched!!.....but if the wheels were 20 lbs the car would barely notice the bump. This means that everything that goes up and down (i.e. the wheel and most part of the suspension) should be kept as light as possible to provide a comfortable ride. Also in the figure can be noticed that the time which the wheel loses contact with the road increases with the weight of the wheel and suspension. This can be reduced by stiffening the springs of the suspension but then.....this will not do any good to a smooth ride. The weight which goes up and down when a car hits a bump is called the unsprung weight....the car body itself the sprung weight. So the lighter the unsprung weight the better the comfort and the roadholding.

Sprung/Unsprung weight  (4,8 kB)
Over and Understeer

Over and understeer are responses of the car as a result from change in side forces. For instance while cornering. As a result from over or understeer the yaw angle of the car won't point in the direction of the car.

Oversteer means the rear tyres have a wider slip angle. The car makes a tighter turn than intended. The backside of the car tends to break away. This is a typical Rear wheel drive characteristic.

Understeer means the front tires run wider slip angles than the rear. It means the car makes a wider turn than the driver intended. The car takes a tendency to drift over its front wheels. Understear is a typical characteristic of Front wheel driven cars.

Over/Understeer  (5,7 kB)
Roll steer

Roll steer also has an effect on over and understeer. Roll steer or camber thrust is the effect the change of camber has due to body roll when the car is cornering. How does this happen?

Well while cornering the tyres will take a form like the base of a cone. The wheel wants to turn around the peak of that cone. This effect makes the wheel trying to steer away from the center of the car. In a straigh line the effect does not exist. If the car is cornering more weight is transferred to the outside wheels than the inside wheels. This means the outside wheel has more influence on the car. As the outside (positive cambered) wheel tries to steer the car to the outside of the corner, the car will be understeered. If the wheels have negative camber the car will oversteer. Engineers can use this effect to create the desired grade of under or oversteering. An understeered car feels more stable as it has a tendency to correct itself. Not so with an oversteered car.

Oversteer  (4,2 kB)

Here are a few general rules about how changes to your car can affect its handling.

Adjustment More Understeer More Oversteer

Front tire pressure Lower Higher
Rear tire pressure Higher Lower
Front tire section Smaller Larger
Rear tire section Larger Smaller
Front wheel width Narrower Wider
Rear wheel width Wider Narrower
Front wheel camber More positive More negative
Rear wheel camber More negative More positive
Front springs Stiffer Softer
Rear springs Softer Stiffer
Front sway bar Thicker/stiffer Thinner/softer
Rear sway bar Thinner/softer Thicker/stiffer
Weight distribution More forward More rearward
Front aerodynamics More downforce Less downforce
Rear aerodynamics Less downforce More downforce

Now we finally mastered the basics of the suspension we can have a look how Rover got around it with the SD1 suspension.

    SD1 front suspension....the Mc Pherson
    SD1 rear suspension......the dreaded live axle

mainpage © A3aan jan. 2001