Transmission System The mechanism which converts the engine

Transmission System The mechanism which converts the engine output into different speed and power combinations at the road wheels There are many different types of transmission. For driving licence purposes they are only classed as Manual or Automatic Pictures produced with kind permission from Volvo Bus Sweden – copyright applies

In the transmission diagram different size gear ratios join together to enable the road wheels to turn at a wide variety of speeds within the engine limits.

Gear Ratios In the picture to the right, suppose the smaller gear has 13 teeth The second, larger gear has 26 teeth. The gear ratio is therefore 13/26 or 1/2 (written as a ratio of 1: 2) For every revolution of the smallest gear the largest gear has turned much slower – only ½ a revolution

Typical Example Gearbox Ratio Road speed at 1000 revolutions of the engine First – lots of engine power but not much speed 2. 97: 1 7. 7 mph Second 2. 07: 1 11. 0 mph Third 1. 43: 1 15. 9 mph Fourth – here the ratios are the same 1. 00: 1 22. 7 mph Fifth – more speed but not as 0. 84: 1 much engine power 27. 1 mph The gear selector is used to change gear ratios Warning! - Because low gears have more power but not much speed it is easy to lose traction

In easy terms – you can accelerate quickly in a low gear but you can’t attain a high speed You can reach a high speed in a high gear but you can’t accelerate as quickly

Transmission Systems - All The gear selector does three things • It provides a permanent means of disconnecting the engine from the driving wheels – NEUTRAL • It provide different ratios to allow the optimum and most economical engine speeds to be maintained at all speeds and loads • It provides a means of driving the vehicle backwards

Transmission Systems - Auto There are many different types of gearbox and gear selectors – In general most buses and many coaches have some form of automatic gears – Often, this modern type of transmission senses and selects the appropriate gear according to road speed and the load on the engine – The driver is usually able to override the system by use of the gear selector or ‘kickdown’

Transmission Systems - Auto ‘Kickdown’ – By depressing the accelerator fully down this provides for quicker acceleration and: – It may also result in the vehicle automatically changing down to a lower gear – The engine may rev higher than normal before changing gear

Transmission Systems - Auto Selecting drive – Application of the footbrake before selecting drive is a very important safety feature for any automatic transmission – It ensures foot placement is correct before releasing the handbrake – Some transmissions will not allow drive to be selected unless the footbrake is applied and the handbrake is on

Transmission Systems - Auto Using the handbrake – Fully applying the handbrake whenever you are stationary is important on an automatic vehicle – Automatic vehicles may move forwards or backwards in gear with the handbrake off – this is called ‘creeping’ – Secure the vehicle and select NEUTRAL when stationary for a long period to avoid danger and damage. Never leave the vehicle in gear with the engine running.

Transmission Systems - All Manual selection – This gives the same flexibility as a manual gearbox – It is especially useful for uphill or downhill gradients – It also allows you to hold the vehicle in the same gear – On downhill gradients selecting a low gear will assist braking, especially if the vehicle has a retarder fitted.

Transmission Systems - All Driving safely – You should know how to change gear – When to select gears – Which gear to select – Some coaches have high and low ratio transmissions which can total up to 16 different gears!

Transmission Systems - All How to change gear – Consult the vehicle manufacturers handbook – Seek advice from a qualified trainer – There are many different types of gearbox that will require further training

Transmission Systems - Manual How to change gear • Engage the clutch smoothly when moving away and changing gear • Select the appropriate gear • Avoiding taking your eyes off the road when you change gear • Ensure your foot does not remain on the clutch pedal when you are not using it

Transmission Systems - All When to change gear – Steep downhill gradients may require the selection of a lower gear. This will increase engine speed but reduce the vehicle speed – Modern buses and coaches have a braking retarder. Efficiency may be improved when the engine speed is increased – The engine braking effect and retarder will avoid excessive use and overheating of the service brake

Transmission Systems - All Which gear to select – Select the correct gear(s) to combine safety, performance and economy – Avoid maximum acceleration through the gears where possible. This may compromise safety, comfort and cause excessive wear of mechanical components – Reduce speed safely before selecting lower gears

Transmission Systems - Manual Which gear to select • Change gear in good time before a hazard or junction • Choose the most appropriate gear for your speed and the road conditions • Avoid holding on to the gear lever unnecessarily • Avoid ‘coasting’ with the clutch pedal depressed or the gear lever in neutral. It is illegal

Using the green band on a rev counter as a guide will allow the optimum and most economical engine speeds to be maintained.

Ensure the appropriate speed is attained before selecting the right gear for the circumstances or manoeuvre

However, not all automatic gearboxes allow for a manual selection.

Transmission Systems - All Understanding how the gears work will help you to get the most out of them. You should be aware of the manufacturers advice for the particular vehicle that you drive Don’t forget to check your driving licence! Many drivers are restricted to driving automatic passenger carrying vehicles only

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