Engine power in the form of rotation is delivered to the final drive from the gearbox. The gearbox is normally in the form of a cast metal casing which not only contains the individual gears, the selector mechanisms and the means of securing all the bearings necessary to allow the gears and accompanying shafts to rotate freely but also a reservoir for the lubricating oil vital to ensure a long service life and smooth, quiet operation.
High To Low Gear
Why do we need a gearbox? Without going into the depths of gear ratios and the like, which may be the subject of a later article, imagine jumping on a bicycle and trying to pedal away in the highest gear, huge amounts of effort are required just to keep moving without falling off and acceleration is extremely difficult. With the huge effort comes excessive strain on the entire drivetrain; pedals, chainrings, chain, rear block and the rear wheel assembly, all in all very inefficient. Now repeat the exercise starting off in the lowest gear, moving away and accelerating becomes easy no longer requiring the huge amounts of lung and leg effort. The whole outfit also becomes much more stable and further acceleration through the gears increases the speed far more comfortably without loading the components beyond their normal limits. Now instead of accelerating through the gears see the rider staying in the lowest gear, acceleration and top speed will be limited by the speed the rider can spin his legs and there is a definite limit to this. Using a selection of gears permits the progressive acceleration up to the required speed and the facility to choose a gear to suit prevailing road conditions.
The Effects Of Acceleration And Speed
The effects felt by the rider are very much like the effects experienced by a motor vehicle when used without a selection of gear ratios being available. The high gear scenario will require the extensive slipping of the clutch, greatly shortening its service life, and the general fuel efficiency will suffer. A vehicle fixed in a low gear will obviously move away from rest with no problem but further acceleration and the maximum speed will be greatly limited by the noise from the engine just prior to it failing in a most catastrophic and expensive manner!
Gear Ratios and Intended Uses
The specification of gear ratios and the number eventually offered by the gearbox will largely depend on the vehicles intended use. This is the reason for large commercial vehicles having, in most cases, twelve to sixteen forward gears. The requirement to overcome initial resistance to movement caused by a much larger vehicle coupled with a sizeable load means that the laden truck must have a greater number of low ratio gears. This also applies throughout the acceleration process with a larger number of intermediate gears being required before reaching the higher ratios and the desired final speed. As this process is a much more drawn out affair in a commercial vehicle the driver will endeavour to make the most efficient use of engine power, this is usually achieved by taking much greater notice of engine speed and therefore making greater use of the power range indicated on the dashboard engine rev counter, the so-called green band.
Trailers and Caravans
A much smaller vehicle will have no such requirement even if it is asked to tow a trailer or caravan of an approved weight, so the provision of four, five or even six forward gears will suffice.
To summarize, the gearbox is fitted to provide a range of ratios allowing easier acceleration and the maintenance of a required road speed without either under or over revving the engine thus promoting greater fuel efficiency.
One last, but very important feature, the gearbox offers the facility to go backwards!