Automotive suspension....The live axle

From live axle to De Dion and back....
Rover's rear suspension

F or years and years all Rovers had a solid well engineered rear suspension system consisting of a live rear axle with leaf springs. The basic setup as used in the P2/P3 was carried further in uprated form with the P4 and P5.

The live axle with leaf springs was simple, sturdy and could be produced at low cost. Very important for a small manufacturer like Rover.

Things changed considerably with the P6. The De Dion axle can be considered as a halfway house between a live axle and independent rear suspension (IRS). It gives non-independent wheels and always keeps them vertical to the road like a live axle but at the same time driving torque reactions are separated from the wheel-locating duties as is the case with IRS.

The rear suspension of the SD1 was quite a disappointment when the car was unveiled in 1976. The SD1 was equipped with a live axle and coils, a simple but well located setup. This in a time when many manufacturers (Notably Mercedes and BMW) were switching to independent rear suspensions. The SD1 could be considered as a step back in time to the days of the P5 with only the coil springs and torque tube as additional improvements.

In the automotive world basically the next rear suspension systems are used.

Multi link systems
De Dion systems
Double wishbones
Leading/trailing/semi-trailing link systems
Swing axles
live/dead axles

Like the front suspension, Multi link rear suspension systems can provide the best comfort and handling. Second best are the double wishbone and De Dion setups both with their own pro's and con's. Last.........you propably guessed it already... the live axle setup as used on our cherished Rovers.

The torque tube

A live axle with only radius arms and perhaps a Watt's linkage doesn't do much to control torque reactions in the axle housing. The French engineer Eugene Mathieu was the first one to find a solution in 1902 for this problem with the torque tube. It consisted of a torque tube bolted to the rear axle and two ball joints, free to swing up and down. The lateral forces were taken by semi-elleptic leaf springs.

Ford and Wills used the torque tube for the model N. It worked out fine and was also used in the T-ford! In Europe Fiat made good use of the torque tube. Then the torque tube slowly lost ground in the twenties to the Hotchkiss drive.

T he reason for Rover choosing a live axle was, again, costs. The De Dion setup of the P6 was a very expensive construction with lots of components from which many parts where especially produced for the P6 alone. A cheaper double wishbone or (semi) trailing link setup would have been possible but this required huge amounts of money for development. Unfortunately back in the early seventies Leylands bank account wasn't that good. So money was the reason Rover opted for a live axle setup. Rover wasn't the only one still using the live axle though. The new Opel Ascona of 1975 also used a live axle lay-out almost in the same manner as the SD1, incorporating a torque tube. The Volvo 240, Opel Rekord and Ford Granada's of those time all had live axles. Well, you can see, Rover was far from alone in the good old live axle arena.

The advantages of the live axle are the low production costs and the fact that engineers have considerable knowledge of its problems and solutions. The Rover's live axle is a hefty affair connected to two coil springs which carry the body. The shock absorbers are also attached to the axle and the body. Lateral support of the axle comes from a pair of radius arms. The front ends of these are attached to the chassis, the rear ends to the axle. The name live axle comes from the fact that the gears and shafts are moving with the suspension. Their rotations gives torque reactions to the axle casing which in turn transfers it to the springs. The axle wants to rotate in two directions.

Rotation like a propeller around its center in the direction of the pinion (see figure). If not properly cured this rotation effect will cause axle tramp and loss of traction because one side of the axle is lifted and the other side pressed down.
Rotation around the drive shaft axis.

To cure the rotation around the center of the axle Rover has fitted a torque tube. The tube encloses the propeller shaft with the rear end fixed to the differential and the front is connected to the body by means of a locating arm. This way the torque tube effectively eliminates the torque reactions that try to rotate the axle. The disadvantage is that the torque tube increases the unsprung weight of the already heavy axle......nothing comes for free.

The rotation around the drive shaft axis is taken by the two trailing arms which don't permit the axle to rotate. A Watt linkage takes the longitudinal forces. It is called a Watt linkage because the principle was used to operate the valves of steam engines. The linkage permits free travel up and down but prevents any lateral movement of the axle.

To sum it all up:

Driving and braking forces are taken by the radius arms.
These arms are quite long to make the fore/aft movements of the axle as little as possible
The lateral forces are taken by the Watts linkage
The vertical forces are taken by the coil springs and the shock absorbers
Torque along the pinion center is taken by the torque tube
Torque reactions along the driveshaft center are taken by the radius arms

As you can see....The axle of the SD1 is very well located!

The roll center height is the same as the rotation point of the Watt linkage. So around the center of the differential casing. Compared with many IRS (Independend Rear Suspension) systems this is reasonably high.And more important it does not shift off the center wile the car is hitting a bump or while cornering. This gives predictable handling. By having a reasonably high roll center the SD1 also doesn't squat too much when accelerating (No nose up attitude..).

Now what are the advantages/disadvantages?

+ Cheap, The main reason Rover chose this setup!
+ On cornering, no camber variations (a good thing for handling in curves, racing!)
+ Track is constant (not so with IRS, a plus for the live axle)
+ Constant roll center, set reasonably high
- Very high unsprung weight....made even worse by the torque tube
- Camber variations when one wheel hits a bump, a bad thing!
- There is some fore/aft movement of the axle
- Harsh ride because wheels influence eachother

Hotchkiss drive

The P2/P3/P4 and P5 all had a Hotchkiss drive. With Hotchkiss drive the leaf springs provide the wheel travel and location of the live axle and also carry the weight of the car.

B.B.Hotchkiss had a gun factory in Paris and became involved in building cars. Georges Terasse designed a very simple rear drive system for the company without any radius rods in 1905. In those days leaf springs with radius rods were almost standard. Hotchkiss drive was considerably cheaper because the leaf springs also took care of the driving thrust and no radius arms were necessary. It became very popular among American cars in the twenties and thirties and was reintroduced to Europe by Citroen, Morris, Fiat and Opel amongst others.

However during the years with cars becoming more and more powerfull Hotchkiss systems became less popular because they gave rear steering reactions when full power was applied. Definately not something you want. By using long leaf springs which flattened out under load the flaws of the system could be hidden somewhat but it's days were numbered and gradually Hotchkiss drive made place for live axle systems with coils and proper location by radius arms and Panhard or Watt's linkage.

L et's compare the SD1 rear suspension with it's predecessor the P6. As you can see it's quite a complicated affair. The forces and torques are kept under good control as the next list shows:

Driving and braking forces are taken by trailing arms and the two upper leading arms.
Especially the trailing arms are long to minimise fore/aft axle movements

The lateral forces are taken by the lateral locating arms

The vertical forces are taken by the coil springs and the shock absorbers

Torque along the pinion center and the driveshaft center are taken by the car body because the whole differential is mounted against the body of the car. The differential is unsprung weight.

Compared to the live axle we see a large reduction in sprung weight because the differential and axles are now unsprung weight. The disk brakes are also unsprung weight! This means the De Dion suspension can have relatively softer springs than a live axle giving a smoother ride. The De Dion tube links the two wheels, so independent movement still isn't possible. This also means that there are no camber changes during cornering. There still are camber changes when one wheel hits a bump, like on the live axle. Now the P6 setup looks like a standard De Dion system. Well Rover wouldn't be Rover if they had not built in something strange....

With a live axle the driveshafts are of a fixed lenght and move with the axle casing. With independent rear suspension systems the driveshafts move with the suspension in an arc. This causes camber changes of the wheel and track variations. The track is the greatest on normal level and decreases when the wheel moves up or down. Many IRS designers have eliminated the problem of track variation by making it possible for the driveshafts to slide a bit in and out of the differential by using splined driveshafts. Thus eliminating track variations. However on early IRS systems the splined shafts often locked up under power, and as such gave a bad ride especially in corners when accelerating. The Rover's De Dion would have needed splined driveshafts to correct for track variation. This is not the case though. The P6 has fixed length driveshafts. This means that the track of the P6 varies a bit. This is not much though because of the length of the shafts. The whole setup meant that the DeDion tube linking the wheels also had to compensate for track variations. This was done by giving the tube a sliding joint. As the tube is not influenced by driving reactions lock up wasn't existing here, problem solved!

Now what are the advantages/disadvantages of the P6 system?

+ Very low unsprung weight....(The De Dion tube itself is not a weighty affair)
+ Cheaper than IRS but more expensive than the live axle
+ On cornering there are no camber variations
+ Constant roll center, set high at spring anchorage point
- Track is not constant
- Camber variations when one wheel hits a bump
- There is some fore/aft movement of the axle

So for the rear suspension the comparison between SD1 and P6 is again in favour of the latter, although the margin isn't that big as with the front .

Is there room to improve the SD1's rear suspension?
Sure, watch the next points:

Go for Vitesse wheels to reduce the unsprung weight.

Use polyurethane suspension bushes. They provide better horizontal location of the suspension , especially the rubbers in the trailing arms wear fast and give a very vague ride by introducing steering effects into the rear axle!

Thanks to: Adriaan Briene

SD1 suspension......the basics

SD1 front suspension......The MacPherson

SD1 Spring codes