Archive for the 'My Car From The Road Up' Category

From the previous articles there now exists a small collection of tyre/wheels isolated from a bodyshell/chassis and that in turn can be steered and has a dual circuit service brake system with a separate mechanical park brake.

Depending on the vehicle type, either two or four wheel drive, engine power has to be fed through the gearbox and reach the driven wheels; this is the job of the final drive. The component list will vary from model to model but will usually be drawn from the following; live axle(s), driveshafts, propeller shafts and differential. Common to the vast majority of exposed transmission shafts is the requirement for the shaft to be able to accommodate the movement of the wheel/axle assembly; this is achieved by the incorporation of universal joints of one kind or another at either end.

Taking probably the most common vehicle type, the front engined front wheel drive car, rotation is taken from the engine, through the gearbox, transmitting directly to the integral differential unit which will turn gearbox output through right angles also allowing each driven wheel to turn independently. Drive then exits the differential via a driveshaft to each side. With the front engine rear wheel drive arrangement gearbox output is passed directly to a propeller shaft, which carries rotation to the rear mounted differential unit. Should the vehicle have a solid live axle the differential passes the drive to either wheel via half shafts encased within the axle, with an independent rear suspension system the output is carried to the drive wheels by exposed driveshafts which, as already mentioned, have universal joints allowing suspension movement. Moving on to four-wheel drive vehicles, power needs to be distributed to all the wheels and to achieve this a transfer unit is mounted at the end of the gearbox with two shafts exiting, one to the front and one to the rear. The shaft arrangement is virtually the same as either of the two systems above depending on whether the vehicle has all round independent suspension or not. As with the differential at either end of the vehicle allowing either side wheel to turn independently, four wheel drive vehicles have a centre differential to enable the front and rear axles to turn at different speeds, an addition to this is the diff lock which ensures that power distribution is evenly split front to rear when traversing slippery terrain thus retaining grip and allowing progress to be maintained. This feature is normally available to the driver as a lever mounted next to the normal gear lever or as a switch mounted in the dash area.

As a brief aside, many four wheel drive vehicles have a selector arrangement which allows the drive to the front axle to be disconnected at the whim of the driver, this disconnection will have the effect of reducing fuel consumption and tyre wear.

The principles and construction behind transmission shafts is simple enough, a bar or tube with a splined or bolt fixing at either end. The decision to specify bar or tube largely depends on the length and the power to be handled, a long propeller shaft is better made from tube so as to keep vehicle weight under control, a short driveshaft maybe measuring around half a metre is probably easier produced from a bar. By and large a correctly designed and manufactured tubular shaft is capable of handling all vehicle requirements and has the added advantage of less weight.

Lastly, what is a differential? As well as providing the vehicle manufacturer with another avenue to specify drivetrain gearing, the differential is absolutely vital in allowing the vehicle to turn left and right. Imagine a car travelling clockwise in a fixed diameter circle, if you were to measure the circumference scribed by the left hand (outer) wheels you would find that distance greater than the distance travelled by the right hand (inner) wheels. As all the wheels are fixed to one body (the car) the outer wheels must be allowed to turn faster or the car would be forced to travel straight ahead. Forcing any deviation left or right from the straight on via steering input would result in massive tyre wear, transmission damage and a huge increase in fuel consumption not to mention making the vehicle virtually undriveable.

It must be fairly obvious what the braking system is for; it allows the vehicle to be slowed in varying degrees and also provides some method of holding a stationary vehicle without having to continually apply the service (foot) brake.

The braking system on any motor vehicle, whether it be the smallest glass fibre two seater city car or the fully laden heavy haulage tractor unit and trailer relies on the basic principle of friction. It must be mentioned at this point that the majority of heavy commercial vehicles and buses/coaches have supplementary braking systems which do not use friction, instead they have various devices/systems which have a direct effect on the engine or exhaust, alternatively a system utilizing the principles of electro-magnetism mounted into the drivetrain.

Returning to the average family car, two entirely separate systems are fitted; 1) the service or foot brake system and 2) the park or hand brake system. With the service brake, the brake pedal is connected to the brake master cylinder and when the pedal is pressed hydraulic pressure is applied through the rigid metal pipes and the flexible brake hoses to the brake cylinders or brake calipers mounted at each wheel.

The calipers/cylinders are absolutely no different in their principle or operation to the huge hydraulic rams seen on earthmoving machines or in the tipping gear of trucks it is only their appearance that is so different. Simply, hydraulic pressure exerted within a sealed space with a moveable section at one end (a piston) can be used to move or operate another mechanism, in the case of a brake system, a friction device. All that is needed is the friction device (a pad or shoe) to be pressed against a rotating body (a disc or drum) in turn attached to the wheel assembly, as the exerted pressure is increased so is the clamping effort on the disc or drum thus slowing its rotation.

Obviously the metal construction of the pad or shoe will need to be faced with a material which not only provides an effective degree of friction and a high resistance to heat, which is an unavoidable by-product of friction, but have excellent wear characteristics without producing premature wear in the disc or drum. The maximum amount of hydraulic pressure generated will depend not only on how hard the driver presses the brake pedal but also the relationship between the diameters of the master cylinder and the cylinders/calipers at each wheel.

One of the most important features of the modern service brake system is the dual circuit. With the old single circuit designs all the hydraulic lines were shared so that if a leak occurred all brake effort was lost. The dual system offers split circuits so that in the event of failure two brakes will work albeit with much greater pedal travel. The split will normally be arranged so that the two remaining brakes will be one at the front and one at the back usually diagonally opposed.

With the advent of the dual circuit system less emphasis is placed on the second brake system fitted to the motor vehicle, the park or hand brake. Now the car has effectively three brake systems, two almost separate hydraulic service and one completely separate mechanical only park. Regulations dictate that the park brake must be purely mechanical in operation, the most common being a centrally mounted lever in the passenger compartment coupled to the rear brake assemblies via a cable arrangement. Pulling on the handbrake tensions the cable(s) pulling levers inside the brake assembly applying the shoe or pad against the disc or drum. As the park brake must be able to be applied and locked in the on position, a ratchet mechanism is incorporated in the handbrake lever so the brake can be set only releasing when required by the driver.

Every fitment in or on a motor vehicle has to be mounted to the bodywork or structure. By and large the bodywork performs various functions aside from the aesthetics; it provides a weather resistant environment for those inside and also facilitates the fitment of equipment to make that environment very comfortable indeed. Everything from the latest in multi-adjusting seats, through climate control, road noise suppression, a whole glut of airbags and the most up to date in satellite driven route finding not to mention mind blowing in car entertainment systems. From a purely engineering point of view all this equipment has to be securely fixed if it is to function as the manufacturer intended.

The forces generated by not only the weight of the vehicle itself plus occupants, their luggage, fuel and hundreds of items of ancillary equipment but also the side effects of the vehicle simply being driven along must be correctly calculated and distributed throughout the structure. This can only be achieved by producing that structure from a wide range of material specifications. This means close attention to not only the type of material used but also its dimensions and shape; what may be the ideal profile for one body panel will not necessarily mean that it would be correct for another.

The two main structure types are separate chassis and monocoque (sometimes referred to as integral construction). The monocoque, although probably the most common in car/light commercial production, has been in existence long before the internal combustion engine, take a look at the construction of the type of trailer pulled by a horse, the drawbar is fastened to the body, the body is either flat or adapted to carry a load requiring side panels and the rear may have a moveable tailboard for unloading, it has no separate chassis. The main reason for the success of this type is that it suits the modern production methods including the ubiquitous computer driven robot. The separate chassis construction is not as old as the integral type but does date back many years, its strength lies in the adaptability of a simple frame, make a powered frame with the essentials fitted, transmission, steering, brakes, etc and then bolt on the body which suits your particular needs, be it multi seating, flat back, tipper, tanker and so on. In the world of heavy commercial vehicles it is normal practice, when required, to cut a chassis and add in extra length for the fitting of a particular and/or unusual piece of equipment, this can include extra axles to spread the vehicle load.

Common to both structure types in the design and production of modern motor vehicles is the requirement to protect the occupants from crash injury. This is not done by simply making the whole vehicle a rolling bank vault, that would be prohibitively expensive and the result would be something akin to a main battle tank, complete with fuel consumption to match. The answer lies in the very careful engineering of certain areas e.g. the uprating of the mounting points for the seats and seatbelts, the fitting of side impact protection bars and the strengthening of the A, B and C pillars, it can also include the incorporation of “weak� areas, the much vaunted “crumple zones�. These zones are specifically designed to deform under impact folding and compressing thereby absorbing as much crash energy as possible before it reaches the driver and passengers. Very often, a car involved in a head on collision will display a completely destroyed frontal area but will have a relatively untouched passenger compartment.

Apart from starting and stopping probably the most obvious function required of a motor vehicle is that it must be able to be directed in varying degrees to the left and to the right.

So what, other than changing direction, is required of the steering system? One of the most important is the transmission or “feel� from the steered wheels to the driver. This feedback will give a good indication of the condition of the road surface being driven on and also an indication of the state of the front tyres. Rolling over a frozen surface will produce far less noise and vibration than a good road surface; the steering will almost certainly feel much lighter and will become rather vague. A tyre operating at a reduced pressure or one that is almost flat will induce drag, make the steering feel unresponsive and cause the vehicle to wander. Correct interpretation of the above via the steering may well prevent an accident caused by loss of control.

Starting at the steering wheel, a component usually made up of a metal alloy structure covered with a trim which provides bulk and grip for the driver. Driver input is transmitted, via a shaft, through to the steering mechanism. The shaft itself not only has the task of rotating but also it must either collapse or move away from the driver in crash situation. This is normally achieved by a combination of joints and crushable sections. The added advantage of the inclusion of these joints is that they will allow the steering shaft to exit the passenger compartment at angles more easily connectable to the steering mechanism; the positioning of this mechanism may well be dictated to some extent by the design of the body shell and/or subframes.

There are six main types of steering mechanism; worm and sector, screw and nut, recirculating ball, cam and peg, worm and roller, rack and pinion. By far the most common system used on modern cars and light commercial vehicles is the rack and pinion. This system is simple in operation, lends itself easily to the addition of power assistance, is relatively lightweight and has little or no maintenance requirements other than routine inspection. All in all, the steering mechanism itself is low maintenance; however, it is important that regular inspections are made in the area of basic wheel alignment, commonly referred to as tracking. Faults in this area normally manifest themselves as irregular tyre wear and/or poor handling. On a less frequent front the owner may wish to have a more comprehensive steering/suspension check done of the castor and camber angles. All of the above factors will have an effect on one other desirable feature, that of self centreing. It will become very tiresome if the driver is forced to manually return the steering wheel to the straight ahead position after every turn, correct steering geometry is vital for this to happen.

In the previous article “From The Road Up - Tyres� it was explained that the first suspension components in line were the tyres, now the suspension components proper fall under the spotlight.

The basic principle of any form of suspension is the isolation of one component or medium from an adjacent component or medium. In the case of a motor vehicle it is desirable, if not essential, that the car body and its occupants are isolated from the harsh road surface. There are of course hundreds of components which would not take too kindly to the relatively high levels of vibration induced by road travel, but in this article comfort for the driver and passengers is all that will be considered.

In order to achieve the isolation required it is necessary to use a device or mechanism which will locate securely to the vehicle and locate just as well onto to the components which will eventually support the wheel/tyre assembly. The best thing for this task is some form of spring usually positioned one at each corner of the vehicle, but there are variations on this theme. The spring used will, to a certain extent, deform under the weight of the static vehicle, then deforming further as a result of extra vehicle loading and/or road impact whilst travelling.

There are three main types of suspension found on today’s vehicles; steel, rubber and fluid/pneumatic. Each of these have certain factors in common; they will deform as a result of impact, they require a robust mounting system and they all have a tendency to “rebound�, sometimes at an alarming rate.

Steel springing will normally take one of three forms; leaf, coil or torsion bar. The leaf spring has been fitted to many vehicle types over many, many years and has the advantage of being able to locate, an axle for example, without much in the way of extra linkages. The coil spring, which is probably the most common form of car/light commercial springing, has no such advantage requiring turrets, linkages and platforms for accurate location but it does have the very useful attribute of being compact and therefore easier to incorporate into the modern vehicle design. The less common torsion bar is quite simply a square or round section bar fixed at one end to the suspension arm (moveable) and the other to the structure/bodywork (fixed).  The operation of each of the above as a result of impact is for the leaf spring to deflect whilst increasing in length, the coil will simply compress and the torsion bar will absorb the movement by twisting about its length.

Rubber suspension is much the same as steel in that it is the principle of deflection and temporary deformation which absorbs the impact. Unlike steel, rubber has the ability to absorb a larger amount of energy per unit of its mass and it also produces much less rebound. Mounting the rubber suspension unit is a relatively simple case of mounting a strut assembly between the structure (fixed) and the suspension linkage (moveable); upon impact the force from the tyre/wheel is transferred through the strut to the rubber spring.

Fluid/pneumatic suspension systems which rely on the transfer of fluid, sometimes under gas pressure, from one area of the car to another can be, by their very nature quite complex. Some systems require only a pressure sphere mounted at each wheel to provide the absorption required. The deflection within the unit is provided by the compression of a volume of gas, usually nitrogen. Interlinked systems can be as simple as the Hydrolastic/Hydragas suspension found on British Leyland/Rover vehicles of some years ago, or as complex as the type fitted to some Citroen models. These systems provide bump energy absorption via pressure spheres and/or the controlled flow of a special fluid from one part of the vehicle to another.

As previously mentioned, all spring mediums are able to absorb varying degrees of impact thus shielding, to a reasonable extent, the vehicle occupants from the harshness of the road surface. However, springs have the nasty habit of “bouncing� and this aspect of their character must be brought under some form of control if the vehicle concerned is not to leap around like a scared rabbit! This is the task of the shock absorber, or more correctly, the damper. To drive a vehicle on the road without some form of effective damping is akin to a suicide mission and a frightening experience to say the least. The suspension damper not only controls the magnitude of spring deflection but also shortens the time taken for the spring to return to its normally laden state. This also has the desirable effect of keeping the tyres in contact with the road. Fluid/pneumatic suspensions are to a large extent self damping thus removing the requirement for external damping.

The vast majority of people would regard the tyre as a subject in its own right and there is absolutely nothing wrong in that, however, very few people would consider tyres to be a component part of the suspension system. Consider running over a small object in the road such as the ubiquitous cats eye, the first thing to react to this impact is the tyre, the treat area will deflect inward towards the wheel and the sidewalls will expand outward on either side. Despite this two plane deformation the remainder of the tread will have the capability to maintain contact and therefore grip with the road surface. All this without the normal suspension componentry, springs, dampers etc having to do much work at all.

So that’s the suspension aspect of the tyres job, what else does it have to do? Unlike most other vehicle components the humble tyre is faced with dealing with a multitude of tasks including maintaining grip on the road surface, coping with extremes of temperature (both road and atmospheric), resisting mild but often repeated impact, virtually instantaneous clearance of water from the tread/road contact point, clearing itself of mud, ice and snow all this and still must retain the ability to provide directional stability when manoeuvring and, depending on vehicle type, transmit engine power to the road surface.

Tyre manufacturers expend vast resources in tyre development but despite this the demand from both vehicle manufacturers and customers in the aftermarket is for a product range to address differing vehicle needs, the rugged and aggressive construction of a type suitable for off road applications will be vastly different to that required for the average family car. It would be virtually impossible to produce a tyre of one construction to suit all needs simply by changing the tread pattern. To achieve these requirements tyre manufacturers will consider the vehicle end use, the off road tyre will be able to self clean larger amounts of tread blocking mud and it will have a far greater resistance to puncture in both the tread and sidewall areas than its purely on road cousin. The flip side to these attributes is a tyre far noisier on normal road surfaces, less precise handling characteristics and its extra weight will reduce fuel efficiency. For Mr/Mrs Average, tyres need to be of a reasonable price, have a good service life, be as quiet as possible in operation and provide grip over a wide range of road surfaces.

So what is a tyre actually made from? Most people would say rubber, a few less would say rubber with some sort of reinforcement and fewer still would say anything more complex. The average car tyre is made up from variations on the following basic mix; rubber 38%, fillers (carbon black, silica) 30%, reinforcements (steel, rayon, nylon) 16%, various oil and resin based plasticizers 10% and a 6% jot of various chemicals to help resist sunlight and other atmospheric conditions.

Hopefully the above will show that the tyre is not just a ring of black rubber, it is one of the most underrated components of the modern motor vehicle, further, and almost to the point of being incredible, the average family car sits on four contact patches each little bigger than the sole of a size 9 shoe, at 60mph on a good road surface covered in rainwater, the tyre at each corner will be able to clear 1 gallon (5 litres) of water from that contact patch every second!