Car Suspension

Repair, Maintenance and Modification

Julian Spender

• Éditeur: Crowood
• Année: 2019
• ISBN:
  9781785006623
• Sujet: Technology & Engineering
• Sous-sujet: Aviation

This book provides an easy-to-follow practical guide to the maintenance, repair and modification of the different types of suspension used in cars. With over 170 illustrations, including colour photographs and diagrams, this practical book explains what suspension is and why it is needed; it reviews the different types of suspension of available; it covers the key maintenance and repairs that an owner can undertake, and finally, describes modifications in detail with step-by-step photographs.

CHAPTER ONE

WHY IS SUSPENSION NECESSARY?

The definition for suspension in the Oxford English Dictionary is as follows:

The system of springs and shock absorbers by which a vehicle is supported on its wheels.

The fundamental reason for suspension is to partially protect the driver of the vehicle from the disrupting effects of the inconsistencies of the road surface beneath the vehicle. The suspended wheel, tyre and hub assembly is able to move over the irregularities in the road surface whilst maintaining grip between the tyre and the road, and causing minimal disruption to the driver within the vehicle. So arguably the first job of suspension is to provide ride comfort. However, as vehicles have advanced in design, the modern suspension system tries to find the ultimate compromise between comfort, handling grip and engagement.

Horse-drawn carriage.

A BRIEF HISTORY OF SUSPENSION

The simplest form of suspension was first used on horse-drawn carriages. Straps made of iron chain or leather would be used to allow the carriage to move independently of the wheels below. If you look at the carriage in the picture you can see the leather strap facilitating, and limiting, transverse movement of the carriage.

The first patent logged for the use of springs was by Obadiah Elliot in 1804 for mounting carriages on elliptical springs attached to the axle – though this, of course, was not the first use of a spring. In ancient times the principle of the spring would have been used for siege weapons, such as a catapult. However, the use of springs to provide suspension eluded these earliest of engineers.

Elliptical leaf spring.

Brush Runabout.

Once the elliptical spring came into being, it rapidly became the most popular suspension solution for carriages, and latterly vehicles. Although leaf springs are rarely found in cars today, they are still popular in larger commercial vehicles.

Next was the coil spring, first seen in a vehicle application in 1906 on the Brush Runabout.

Coil springs are now the most widely used spring component in modern suspension systems – although notably the Chevrolet Corvette has been using a transverse leaf spring in its rear suspension right up to the present day, citing packaging benefits, and the durability of the now composite material leaf.

THE FUNCTION OF DAMPING

Although, as we have seen, the Oxford English Dictionary begins its definition of suspension as ‘The system of springs and shock absorbers…’, the correct term for a shock absorber is actually a damper. This is because its function is to damp the oscillation of the spring. However, with early suspension systems it was quickly realized that although they made uneven surfaces more comfortable, they still had an Achilles heel, which was discovered when velocity was increased. The undamped spring would start to work like a pendulum with each movement in one direction, being compensated for by movement in the other. By travelling faster and faster, movements would eventually go beyond the spring’s maximum travel (to go ‘coilbound’), and possibly result in the occupants being flung from the carriage, or the carriage turning over.

The first to use a form of damper on an automobile was Mors, in 1902. Although this is a book on suspension, it is probably a safe bet that you like engines as well, and it is useful to refer back to the specification of the Mors engine, to put the era in context. It was powered by a 10-litre, V4 side-valve engine, with magneto ignition and dry sump lubrication. It would reach a heady 950rpm and produce just 6bhp per litre, making in total 60bhp. A Honda S2000 engine makes 240bhp from 2 litres and hits 9,500rpm! The reason for the comparison is the fact that many cars still use a conventional spring and hydraulic damper. Although there have been significant advances in suspension technology, in its most basic form the conventional spring and damper looks and performs remarkably like the products of yesteryear.

We will see later in the book how a number of other suspension designs came into being, all of which offer significant advantages over the simple spring and damper. We’ll also see how, strangely, they never gained the mass-market appeal that the designers would originally have thought possible.

ONE SUSPENSION SYSTEM FOR ALL SURFACES?

‘A picture speaks a thousand words’ is a truism in the field of suspension, just like many others. The next three images show three different vehicles, each with varying levels of cornering grip; however, it will be necessary to read the captions in order to appreciate which vehicle corners the fastest.

It then becomes apparent that the kart (which can be bought for as little as £1,000 second-hand) generates significantly more grip than a cutting-edge supercar. So that’s it then, we don’t really need suspension, and the book can stop here! … If only it were that simple.

The G-force to which the vehicle subjects its occupants is not, as some might think, centrifugal force. A vehicle with tyres is generating something known as ‘centripetal force’ (from the Latin ‘centre’ and ‘to seek’), which is a force that makes a body follow a curved path. The centripetal force is directed at right angles to the motion, and also along the radius towards the centre of the circular path. We feel this force in the vehicle as it attempts to make us slide across the seat.

Although the kart has no suspension, its chassis is designed to work in harmony with the tyre, and is complex – in fact kart set-up, although it might appear simple, can be just as involved as a car. Sometimes it can seem counter-intuitive to that of a car, but equally, learning how it works can often enable the user to solve a problem in the car’s handling, which previously had them stumped.

This superkart is capable of generating cornering forces of over 2G, and it has no suspension whatsoever.

McLaren 600LT: this modern McLaren sports car is generating cornering forces of around 1.7G – it has a sophisticated, hydraulic, computer-controlled suspension.

A standard production car on road tyres cornering at 0.95G. XTREME SPORTS PHOTOGRAPHY

Discovering that a kart with no suspension can corner faster than the modern supercar isn’t perhaps entirely fair, because due consideration to modern formula cars has not been given. A modern Formula One car does have a complicated suspension system, and can corner at even higher G-forces than a kart. However, most of this advantage comes from the use of aerodynamics. Thus a modern Formula One car manages the airflow that passes over it, to increase downforce, and the interplay between the lateral grip of the tyre, versus the vertical load on it, is key to making the aerodynamics work correctly. Too much aerodynamic load on the tyre may cause it to fail prematurely, or will decrease outright grip, as the maximum loading of the tyre would have been reached. Too little aerodynamic load will mean that the tyre doesn’t reach its optimum grip-versus-load point.

Unimog 404 chassis showing the extreme wheel articulation available.

TRAVELLING FAST ON BUMPY SURFACES

So for high speed, smooth circuit use, very little travel seems to work well. However you wouldn’t get very far up the side of the mountain in a racing car or a kart.

Maximum speed around a corner is a function of track width and the height of the vehicle’s centre of gravity. Thus it is easy to see that an SUV will have a lower theoretical cornering speed than the equivalent car, because the vehicle is higher up and has a higher centre of gravity (although it is usually wider to help prevent it rolling over). Conversely is also easy to visualize how this kind of vehicle should be better off road, with its long wheel travel and off-road tyres – though that is not, of course, to say that such an appearance is a guarantee of performance. Some cars, however much they look the part, are good neither on road nor off.

The main reason that the vehicle geared for off-road use is superior in such an environment, is the travel and independent nature of the suspension. On off-road surfaces there may be large undulations over a short distance, requiring the suspension to have significant amounts of droop (movement in the downward direction) to meet with the downward direction of the surface, while equally on the opposite wheel there needs...