Three of the main components of a vehicle's underbody are the suspension, shock absorbers and last but not least, the springs.
These three components have to work in symmetry, and even if one were faulty, driving would be a much less pleasant experience, with every bump on the road transmitted directly to the vehicle's body.
A vehicle's suspension ensures a driver and passengers can travel in comfort and be kept safe by ensuring that the wheels are in constant contact with the road surface, dramatically enhancing its road-holding capabilities.
The role that springs play in the equation is defined by their "spring rate" – in simple terms, the stiffness of the springs.
The spring rate defines the relationship of how far a spring will expand and contract in response to the load imposed on them, governed by the material used to produce the springs, its specific qualities and its exact design.
Nowadays, the springs found on the underbody of almost every vehicle are produced used with an alloy comprising steel, silicon and manganese.
There are the three elementary spring designs:
- Leaf springs
- Coil springs,
- Torsion-bar springs.
Leaf springs are the oldest known and most common springs design, with a history that goes back to the horse-drawn carriage days.
Consisting of several metal layers bound together under considerable tension, gradually moved out of favour in the private car sector because of their substantial weight and bulk, are still widely used in heavy commercial vehicles.
Coil springs have also been around for a long time, gradually replacing leaf springs with the arrival of the first machines that could wind springs in volume in the early Nineteen Twenties.Since then, coil springs have been fitted as standard in most private cars, estate cars and parcel vans throughout the World.
The principal advantage of coil springs is their ability to absorb sizeable loads while remaining lightweight and compact.
A coil springs' actual load-bearing ability is defined by how tight its coils have been wound, harnessing its tensional strength of a long spring steel cable within a compressed footprint.
While the coil spring's arrival generally meant the end of the leaf spring, during the Fifties and Sixties, it was still possible to find vehicles fitted with a combination of independent front suspension with coil springs in the front and leaf springs in the rear.
Torsion-bar springs, a favourite of European car manufacturers, bear a strong resemblance to a coil spring that has been partially unwound. The major difference is that torsion bars run along the vehicle's width – one on each of its four sides.
With one end attached to the fame and the other to a wishbone, when the vehicle is travelling on an uneven surface, any resultant vertical movement is dissipated due to the action of the torsion bar soring and its torsion bar.
Although the tension bar concept has been around for a long time, they were only sparingly fitted in cars due to the cost involved. Most of the UK manufacturers prefer the less expensive, more comfortable to work with coil spring.
Any restorer appraising a classic car with a view to restoration will find it difficult to define any problems with springs. However, a simple eye check can provide a general impression. Things to look out for are as follows:
Other than that, submitting the vehicle to a pre-purchase vibration test will provide a picture of general problems with the suspension or steering and a rough appraisal of whether the springs are faulty and will need replacing.
Replacing a vehicle's springs is a job that can be carried out in-house, although most restorers do not have either the patience or the courage to tackle the job.
In the case of leaf springs, a lot of patience and knowledge will be needed to painstakingly add one leaf to another, while clamping the pieces together at the end can be pretty dangerous.
Removing and installing coils springs involves using a coil spring compressor, a moderately dangerous and quite expensive piece of equipment.
Unless the restorer plans to replace the springs on their vehicle regularly and has all the neccessary tools and experience to do so , this is a job best left to the specialists.