From The Road Up – Final Drives
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.
Gearbox Through To Final Drive
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.
Drivetrain Gearing and the Diff
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.