Car Suspension E-Book

Julian Spender

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Veröffentlichungsjahr: 2019

Beschreibung

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.

Leseprobe

CAR SUSPENSION

CAR SUSPENSION

REPAIR, MAINTENANCE AND MODIFICATION

JULIAN SPENDER

First published in 2019 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2019

© Julian Spender 2019

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 78500 662 3

DedicationDedicated to my wife and children for putting up with my long hours in the garden shed, every Sunday, for well over a year. Without your patience and understanding this book could never have been completed.

DisclaimerSafety is of the utmost importance in every aspect of an automotive workshop. The practical procedures and the tools and equipment used in automotive workshops are potentially dangerous. Tools should be used in strict accordance with the manufacturer’s recommended procedures and current health and safety regulations. The author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

CONTENTS

INTRODUCTION

1 WHY IS SUSPENSION NECESSARY?

2 TYPES OF SUSPENSION AND HOW THEY WORK

3 REPAIRING SUSPENSION SYSTEMS

4 MODIFYING A SUSPENSION SYSTEM

INDEX

INTRODUCTION

The aim of this book is that it appeals to novice and professional alike. It doesn’t have too many complicated formulas, and it isn’t the ‘go-to’ book for a Formula One team looking for the next advantage in suspension. However, it does provide a concise overview of the different types of suspension. The reader may not be mechanically minded, but is perhaps a keen driver – or they may be actively involved in some form of motor sport, and may find that the text refreshes some basic concepts, or provides a platform for lateral thought.

Above all I have tried to keep the subject interesting, without getting too bogged down in the complicated mathematics that govern the subject. By using examples at the extremes of suspension applications, the reader will understand that the most important thing is to set out a clear objective, and to work to achieve that. Moreover, just as in many spheres, initial objectives may not be met first time, and a process of testing, analysis and modification may be required. Rinse and repeat until the objective is met – or finally realize that there is an inherent design flaw in the vehicle, which cannot easily be solved.

A ‘stanced’ VW Polo showing stretched tyres, oversized wheels, and lots of negative camber to get the wheel to fit in the arch. If you want a car that looks like this when stationary and you want to clear a speed bump, then air or hydraulic suspension is ideal. However, if the tyres are stretched too far on the wheels, they become dangerous, so if you like this look it is important to keep the speed low around corners. Tyres are usually larger in width than the rim, or are possibly the same size.

As well as well-known shocks and springs, there are other types of suspension system available, and this book will consider the virtues and foibles of these, too. Furthermore, rather than being a repair manual for various different types of suspension – this market is well served by workshop repair manuals – hopefully it will help the reader think about how the suspension on a particular vehicle could be modified to achieve the effect they want. It will also help them understand that suspension is often an area of compromise, and making an advantageous change in one area can often result in a down side in another. For example, if you decide you want a road-legal race car, it is perfectly easy to make it super on circuit, but it will be downright dangerous on a winding B road with a wet surface. And if you want a road-legal car that can be lowered 100mm (4in) when stationary, but will still clear speed bumps, clearly this cannot be achieved using simple springs and dampers.

A Flawed Design: The Magnificent Auto Union Racecar

The Auto Union Racecar was a V16-engined monster of a car, and much credit must go to the drivers who completed races around the legendary green hell of the Nürburgring in one of these frankly dangerous cars. At the time the rear suspension was considered advanced, but the limited understanding of kinematics at the time meant that the oversteer the car experienced was never fully understood. If a layman from the modern day went back in time with an inexpensive suspension design program on a laptop, he could have told them that the rear suspension design was inherently flawed. It had a badly located roll centre, terrible camber control, and jacking forces that multiplied in such a way that the car could snap into an unrecoverable spin. Throw in low grip tyres and the capability for over 320km/h (200mph), and it’s not surprising this car took the life of Rosemeyer in 1938 during an attempt on the land speed record on a German autobahn. Later in the book we’ll look briefly at chassis dynamics, and discover how steady-state understeer is designed into modern vehicles.

Auto Union Type C – the rear suspension design, roll centre location and weight distribution made this car very dangerous.

 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 to be a large amount of compression travel (movement in the upward direction).

The Ariel Nomad, possibly the only vehicle that excels both on and off road.

As you can see from the picture above, the requirements of suspension for this surface are completely different to that needed for the smooth tarmac of a racetrack. Off-road vehicles not only employ suspension with much more travel, but four-wheel-drive versions make use of differential locking: this locks the drive between the two wheels – much like a race kart, which has no differential and a solid rear axle. With differential locks engaged, the vehicle will not want to turn, but it will have considerably more drive on a slippery and uneven surface.

The latest four-wheel drives use a range of electronic technology to enable them to traverse as wide a range of surfaces as possible, without the driver interaction that would normally be required – although a recent episode of Amazon’s Grand Tour showed that this kind of technology is not infallible. In fact it demonstrated that the popularity of SUVs on the road has made them severely compromised off-road.

This is the single biggest challenge of suspension. It is extremely hard to take one system and one style of vehicle and make it work in all environments: there has to be an element of compromise. Any vehicle with significant off-road ability will tend to be quite hampered on road. In the next chapter we will also see that, just as the suspension system needs to be fit for purpose, crucially so do the tyres. There are plenty of modern SUVs that look as if they could go off road, but as they are supplied with low-profile summer tyres, they have no real benefit over a car fitted with all-season tyres.

There is arguably one vehicle that crosses the boundary between high performance on road, and high performance off road, and that is the Ariel Nomad. An exquisitely brazed chassis (Gordon Murray’s Rocket was also brazed – it gives a stronger joint than welding, and prevents warping) is fitted out with top name components, from the Honda VTEC engine to Bilstein suspension and Alcon brakes. This machine has phenomenal pace both on and off road, and because it is light, the long travel suspension can work in both spheres of operation. Granted, compared to the Atom there is more body roll on road, but the flexibility of the vehicle is a good demonstration of the virtuous circle that comes from light weight.

 CHAPTER TWO 

TYPES OF SUSPENSION AND HOW THEY WORK

In this chapter we will be looking at the different types of suspension, how they work, and what their benefits are. However, first we must look at tyres.

THE IMPORTANCE OF TYRES

Tyres are often said to be the most important part of the vehicle, and this really is true, assuming that everything else is fit for purpose, and designed from the tyre upwards. For example, there would be no point in building an incredible racing car, optimised for a racing slick tyre, and then entering the first race on road tyres. Likewise, a four-wheel-drive vehicle can never properly operate off road without a specialist tyre – at the very least it would need an all-season tyre, although this would compromise its performance in summer.

When we look at tyres and how they interact with suspension, the subject is nearly as broad as that of suspension itself. Where they are similar is in the quest for a solution that is fit for purpose.

THE COMPLEXITIES OF GRIP

For a racing tyre we are looking for maximum lateral grip, and a favourable set of characteristics to enable the driver to maximize performance over the life of the tyres. Lateral G-Force readings may be as much as 5G with aerodynamic downforce. For an off-road tyre the ability to grip on slippery surfaces while still offering good performance on tarmac is crucial. The off-road tyre is not focused on high lateral G, and will be lucky to achieve 0.6G. The tyre generates lateral grip by interacting with the surface it is on.

There are two fundamental types of grip: adhesion and hysteresis. Adhesion results from the interaction between the tyre and the surface of the road. When the road is wet, adhesive grip is reduced. This would also occur with oil on the road surface, or any substance that reduces the coefficient of friction.

The other type of grip is hysteresis driven. As the tyre doesn’t give back all the load put into it, it suffers from hysteresis, which means there is a delay in its reaction to the surface. Although at first this situation doesn’t sound as if it would generate grip, what the tyre is actually doing is creeping over the surface and interlocking with it over its vertical imperfections, and the delay in the tyre’s reaction to the surface actually increases its grip and also temperature. The level of hysteresis – it is generally increased with the softer compounds – dictates the performance and behaviour of the tyre.

The shape of a tyre and its tread are specifically designed to work on a chosen surface. Thus a slick racing tyre for smooth tarmac will have no tread at all. A wet racing tyre will be similar, but is made from a softer compound, with large grooves to drain out water. A winter tyre is often characterized by being siped. Siping is the process of cutting thin slits across a rubber surface, or in this case small grooves across larger tread elements to improve traction in wet or icy conditions. More sipes give more traction in snow or mud, and also on cold, wet surfaces. Typically, wide, straight grooves have a low noise level and good water removal. Siping was invented and patented in 1923 under the name of John F. Sipe, but it was his son Harry Sipe who applied the technology to pneumatic tyres.

Interestingly there is a lizard called a gecko, whose ability to grip on a vertical surface works in a similar way. A gecko foot under magnification can be seen to have microscopic hairs that effectively increase the grip between its feet and the surface, and enable it literally to walk up walls and even a vertical glass pane. However, as is often the case, there are compromises, and although a gecko is an excellent all-surface specialist, it isn’t the fastest lizard.

Gecko foot showing the microscopic hairs that improve its grip.

Similarly, winter tyres, and ‘mud and snow’ tyres, may have thousands of sipes and give good traction in difficult conditions, but on a hot summer’s day when both the tyre and the road surface are warm they may feel ‘squirmy’.

Incidentally in snowy conditions it is interesting to note how much grip you get from a space-saver wheel and tyre, as supplied for a spare tyre on most modern cars. If you have one fitted and you are driving in snow, you will soon appreciate that the space-saver is the only tyre keying in to the surface.

Diagram showing the ‘circle of grip’.

THE CIRCLE OF GRIP

The ‘circle of grip’ is a circular representation of the capability of a tyre (see diagram). Most of the time when driving, a tyre is combining more than one force. For example, as you enter a corner you are decelerating, and generally would use a technique called ‘trail braking’, where you will be turning the car into the corner whilst gently releasing the brake. At this point the tyre is having to cope with both deceleration and cornering forces. If a tyre is capable of generating 700kg of lateral force for 450kg of vertical loading, the maximum G-force generated will be 700/450=1.55G. At maximum speed around a left or right turn with no deceleration or acceleration the adhesive limit will be on the edge of the circle at the left or right turn location.

However, by adding acceleration or deceleration forces, the maximum lateral grip is then compromised. If you are already at maximum lateral acceleration you cannot add any acceleration or deceleration without straying beyond the circle: any more loading will result in a slide. Only a combination of acceleration or deceleration, and lateral grip that falls within the circle, is achievable. In the diagram the blue line shows a combination of lateral force and acceleration that is right on the edge of the circle. In this instance no more lateral force is available without a reduction in acceleration.

If ever you have driven ‘on the limit’ on a circuit you will know the feeling of breaching the circle of grip, which will be manifested as either understeer, oversteer, or even a four-wheel drift, depending on the set-up of the car.

Typical G-Force Figures from Tyres

The tyre is the governing factor here, however chassis stiffness and set-up are crucial to ensuring that both the front and rear tyres work to the best of their ability. In addition, aerodynamics that generate downforce provide what is known as ‘free grip’. Generally speaking, most typical road tyres in a modern car will generate approximately 0.85G. Softer and ‘grippier’ tyres will be able to push this to 1G, which is about the limit on a typical road tyre. Then there are ‘track day’-style tyres, which try to combine the benefits of a slick racing tyre with a road-legal tread. These tyres are typically three to five times softer than a normal road tyre, and with a well-designed chassis will generate something like 1.2G to 1.4G. A kart with no suspension and slick tyres can generate over 2G, and F1 can reach 5G. Nevertheless, however much grip the tyre can generate, the overall lateral and longitudinal mix is still governed by the circle of grip.

Drifting Wastes Grip

With the circle, it is easy to understand how a car that is drifting cannot generate as much lateral grip as one that isn’t (assuming all other things are equal). When a car is drifting sideways, accelerative forces have overcome the lateral grip capability of the tyre, to the point where the tyre is often spinning as well as gripping. The reason for this is the contact patch given by extremely wide tyres. A typical drift car will run much wider wheels and tyres than standard, as these offer maximum lateral grip at the expense of tractive grip, which is exactly what the drifter needs.

The Right Contact Patch

If you have a flat tyre and replace it temporarily with the space-saver from the boot, you might think that its grip would be easily overcome both acceleratively and laterally. In fact when fitted, the space-saver has the same size contact patch as the wheel you removed, it is just that the shape of the patch changes – the space-saver contact patch is narrow (laterally) but fatter. The (flat) tyre you remove gives a contact patch that is wide laterally but thin. So although in a picture the contact patches might look very different, the area is the same.

Diagram to illustratethe different contact patch shape between a narrow and a wide tyre.

Pneumatic Trail

The trail-like effect to the contact patch when subject to vertical load is known as pneumatic trail. It is generated as the tyre finds its grip over the surface. Specifically, it is the distance generated by the forces of side-slip that occurs behind the geometric centre of the contact patch.

Theoretically, then, wider tyres will have no benefit. But we all know that this is not true, as a wider tyre can resist cornering forces better. However, it is not as good at straight-line traction and deceleration. This is easy to visualize in your mind as you will no doubt be aware how a wide wheel and tyre offers less grip on slippery surfaces.

Another good example would be a dragster tyre, which is comparatively narrow, as cornering is irrelevant and maximum tractive grip is the aim. The tyres are generally very soft and also inflated to a low pressure – it is possible to see the huge forces at work as a dragster launches off the line.

A dragster tyre is tall and narrow as compared to a racing-car tyre, which is shorter and wider to give maximum accelerative grip.RICHARD SHUTE

In the picture above you can see the effect of a high G launch on a specialist dragracing tyre, which is sometimes known as a ‘wrinkle wall’, the idea being that rather than wheel spin, the tyre torques up like a clockwork motor, and then releases its energy, resulting in a faster getaway from the line. In addition the tyre expands radially, which affects the gearing on the vehicle.

Tyre Grip Versus Vertical Load

Some tyre manufacturers are able to provide load data versus lateral force. This is very useful when designing or modifying suspension and chassis. Essentially, if you know what the tyre is capable of, then the suspension can be built and designed around it. So if you aren’t already sure which tyre you are going to run, then see if any of the suppliers on the shortlist of tyres you have selected can provide vertical versus lateral load data for their product.

The graph shows the varying lateral force generated by a tyre at different vertical loadings. For example, as you can see, the 450kg vertical loading can generate a much greater lateral force for a given slip angle than the 300kg vertical loading – but the lateral force diminishes more quickly as the limits of the tyre are approached. This load data would be for a racing tyre: as you can see, at a 450kg vertical load the tyre generates 700kg lateral load. Divide 700/450 to get 1.55, and this tyre could generate 1.55G of grip.

Armed with this data it is much easier to work out what kind of spring and anti-roll bar rates will be required to maximize cornering force.

Tyre load data, showing the lateral grip a tyre can generate versus its vertical load.

Managing the Circle of Grip

Recently a leading magazine ran a feature on a certain Rob Wilson, a New Zealander who has had a long and varied career in motor sport. What was interesting is that allegedly half of the F1 grid have called on his services – which might seem surprising, as you would expect them to know what they were doing already. However, it demonstrates how crucial it is to a fast lap time to manage the overall grip available: the old adage ‘smooth is fast’ is more appropriate than ever. As a driver you have to manage the weight transfer around the car and around the tyres constantly, and in different ways, to ensure fast progress. This explains why the traditional racing line is not always the fastest.

TYRE TESTING – THE MAGIC FORMULA

The most popular formula that is used to test tyres is the so-called ‘magic formula’. These formulae were models originally developed by Hans B. Pacejka, and so named because there was no solid scientific basis for the structure of the equations. However, as an empirical model the magic formula works very well. Each tyre is characterized by a number of coefficients, and these are then used within the formula to calculate the force generated by the tyre in response to vertical load. The parameters used in this formula are designed to offer a good representation of what is actually happening when the tyre interacts with the ground. Whilst the model is significantly simpler than the physics actually involved, it is one that is nonetheless intimidating for most.

The general form for the formula is:R(k)=d-sin{c-arctan[b (1-e)k+e-arctan(bk)]}

The Overturning Moment

Tyre models are particularly useful at modelling the possibility of rollover. With the seemingly unending appetite for SUV-style vehicles, the greater risks of rollover must be managed. Most normal cars (and particularly high performance cars) will slide before they roll (in most circumstances). SUV-style vehicles, with their higher centre of gravity, are much more prone to roll over. This phenomenon can actually be measured on a tyre-testing machine, where the model used will be honed to ensure the vehicle can be developed to ensure it won’t roll over during real world testing.

Diagram showing the overturning moment present under heavy cornering loads.