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

The gear selector is used to change gear ratiosWarning! - Because low gears have more power but not much speed it is easy to lose traction

Gear 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 much engine power

0.84:1 27.1 mph

Typical Example

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

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’

‘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

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

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.

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.

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!

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

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

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

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

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

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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