Non-independent suspension

Live / Dead axle



Until the late 70s, most cars still used this simple non-independent suspensions, especially at the rear axle. Basically, it is a rigid axle fixed between left and right wheels. The car body is suspended by leaf springs or coil springs on the axle / wheels unit.

As you can see, the wheels are not independent. When one wheel rides on a hump, the shock will be transferred to another wheel. Besides, both wheels will be cambered, thus non-neutral steering is inevitable.

If the axle is also the driving axle, it is called Live Axle. Live axle is very heavy. It consists of the final drive / differential, drive shafts and a strong tube enclosing all these things. Since the whole axle is rigidly fixed to the wheels instead of suspended by springs, the so-called Unsprung Weight is very high.

 

Solid-axle leaf springs 

The first rear suspension built by M.E. Hertel in 1897 had liitle difficulty providing drive to the rear wheels. It was a solid-axle, independent suspension, of the same type than the front suspensions we could find at the same period :
 

                                     
                                                               Solid-axle leaf springs rear suspension
 
 

It had a lot of success especially in America thanks to its cheap price and to its simple design. It was the basis of a large number of successful rear suspensions like right up to cars as later as the mid eighties 2.8 Capri.
 
 

Solid-axle coil springs

Basically the  Same as above but with coil springs replacing the Agricultural leaf springs :
 

                                                             
                                                                Solid-axle coil spring rear suspension system
 

Compared to the previous system, leaf springs have been replaced by coil-spring combos and two arms have been added to provide lateral support.

Driven with coil springs.



 

What is the result of high unsprung weight ? Assuming a live axle meets a hump and "jumps" quickly upward, the more weight it has, the more momentum it gains (because momentum = the product of mass and velocity). That means the more momentum the springs have to deal with. Of course, springs cannot absorb all the momentum, so eventually part of the latter will be transferred to the car body in the form of shock. Therefore live axle is never good at ride quality.

If the axle is not the driving axle, it is called Dead Axle. Without the driving mechanism incorporated, dead axle has much less unsprung weight, so its ride quality is better than a Live Axle. 

Anyway, comparing them is useless - it does not make sense for a car to use dead axle in the non-drive wheels while using a suspension advanced than live axles in the driving wheels. 

(ED But it does still find its way into a lot of cars. Many Mini/superminis like the Fiat Uno use Mac Struts up front, at the rear it used a simple coil sprung beam axle. )

Live / Dead Axles have another disadvantage - body roll is not sufficiently suppressed. Springs are the only element which control the body roll, however, stiffening the springs will inevitably deteriorate ride quality. Moreover, if coil springs are used, lateral force due to cornering will lead to transverse movement of the car body, thus result in weight transfer and affect steering response.

The popular solution was to add some control arms between the car body and the axle, such as Panhard rod and Watt link.
 

Note:

A live axle is possibly the simplest and crudest way possible to connect two wheels to a body.  You still see it on most trucks and 4*4 in the rear and sometimes in the front.

The live axle is almost extinct in cars today, even four wheel drive ones. So why is the live axle evil? The most obvious reason is that shock that is encountered on one side is transmitted to the other. Live axled vehicles tend to be more easily upset if they hit a bump while turning.

The less well known reason, however, is that a live axle carries with it a large amount of unsprung weight. Unsprung weight is the portion of the vehicle that is not sitting on top of the suspension. Examples of unsprung components are the wheels, tyres and brakes. Unsprung weight is a highly undesirable attribute in suspension design, because the more inertia the unsprung components have, the less inertia the sprung masses have to remain unperturbed over a bump. This means that with less unsprung weight the suspension is able to react faster to bumps rather than simply transmitting the shock into the cabin.

The ratio of sprung to unsprung weight is a key determining factor in how smoothly a car rides. A heavier car rides better than a lighter car? It is because the heavier car most likely has a higher sprung to unsprung weight ratio than the lighter car. This also means that lighter cars can be made to ride better than heavier cars if they manage to keep their unsprung weight down. And it also explains why these heavy offroaders still ride so roughly while handling so poorly compared to cars much lighter than them. They have too much unsprung weight, much of it comes from using antiquated live axles.

Compared to the modern independent suspension, a live axle has the entire differential assembly, protective casing and all final driveshafts as unsprung masses whilst the main driveshaft is partially sprung. All the variations of modern independent suspension only have the final driveshafts partially unsprung. The main driveshaft(s) and differential assemblies are all part of the sprung masses attached to the body. Thus one can see how much of an unsprung weight penalty there is by using a live axle.

A high performance car with measures taken to reduce unsprung weight can trade the gains in ride quality for stiffer suspension. Designers of high performance cars go to extreme lengths to reduce unsprung weight. In the Porsche 911 turbo for example, not only are the wheels made of lightweight alloy, the spokes are also hollowed out. All this in a fanatical attempt to reduce unsprung weight. Some higher priced cars have also introduced suspension components that are made of aluminum instead of steel. The aforementioned Porsche, the Audi A4 and the BMW 5 series are some examples.

With low unsprung weight one has a car capable of sizzling handling while riding reasonably. Conversely, a 4*4 despite its sprung weight, with its high unsprung weight has to sacrifice what little handling it has for a tolerable ride. The use of live axles and leaf springs can be traced back to at least 50 years ago,  Yet strangely the use of coil springs can be found in railroad cars dating back to the 19th century.

 

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