Braking systems questions LGV theory test

This topic has 63 practice questions. Here are 10 of them with the right answer and why.

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  1. When is trailer swing most likely to occur on a lorry and drawbar combination?

    • While you're braking on a bendRight answer
    • While you're using an endurance brake
    • While you're steering at slow speed and fully loaded
    • While you're moving off at an angle

    The forces that act on your vehicle if you brake while cornering can lead to instability and may cause trailer swing. You should try to plan ahead so that you brake smoothly and in good time as you approach a bend.

  2. Your vehicle is fitted with a 'diff-lock'. When would you normally use it?

    • While driving on straight roads
    • While towing an empty trailer
    • While driving on muddy construction sitesRight answer
    • While uncoupling a trailer

    The differential allows the drive wheels to rotate at different speeds; this is necessary for the vehicle to negotiate corners and bends. On a slippery surface, however, it can cause problems: if one wheel has less traction, it can spin, resulting in a complete loss of drive. The 'diff-lock' prevents the wheels from turning independently and so helps to stop this happening.

  3. What colour is the auxiliary line on a three-line braking system?

    • Red
    • BlueRight answer
    • Green
    • Yellow

    If you're driving an articulated vehicle or a trailer combination, it's vital that you understand the rules that apply to coupling and uncoupling the brake lines. If you take a practical driving test with a trailer, you'll be expected to demonstrate this during the test. The lines are colour-coded: red is the emergency line, blue is the auxiliary line and yellow is the service line. They must be connected strictly following the correct procedure.

  4. The emergency line is common to both two-line and three-line brake systems. What colour is it?

    • RedRight answer
    • Blue
    • Black
    • Yellow

    The red emergency line is common to both two-line and three-line braking systems. Always set the parking brake before disconnecting any brake line.

  5. Air-brake systems usually have 2 lines. Which additional line is fitted on a three-line system?

    • Emergency
    • Service
    • Electrical
    • AuxiliaryRight answer

    The blue (auxiliary) line is not used when connecting to a two-line system. Follow the manufacturer's instructions about what to do with this third line.

  6. In frosty weather, what precaution could a lorry driver take to prevent moisture from freezing in the air-brake system?

    • Drain the air tanks dailyRight answer
    • Cover the air tanks with a blanket
    • Keep the engine at high revs when starting
    • Pump the brakes

    You should make sure that you drain the air tanks daily to avoid moisture in the system. Most modern vehicles have an automatic draining system, which should be checked regularly.

  7. How can a lorry driver avoid brake fade?

    • Make sure that the air tanks are drained before journeys
    • Check that the air pressure is correct
    • Make sure that the parking brake is applied before stopping
    • Select an appropriate gear before downhill gradientsRight answer

    Using the brakes to control a lorry's speed as it's driven down a hill will cause them to get hot. If they overheat, they can become less effective - a condition known as brake fade. By selecting a low gear, the engine's resistance will slow the lorry. This reduces the demands on the brakes, making them less likely to overheat.

  8. When are exhaust brakes most efficient?

    • At high engine speed, while in low gearsRight answer
    • At low engine speed, while in high gears
    • During stop/start town work
    • While travelling at a constant speed on a motorway

    An exhaust brake alters the engine's exhaust flow, using it to help control the vehicle's speed. It's most efficient when the engine is running at a high speed and the vehicle is in a low gear, such as when descending a long hill. Using the exhaust brake can relieve the service brakes, preventing them from overheating and fading.

  9. What's the principal braking system on a lorry called?

    • The endurance brake
    • The service brakeRight answer
    • The parking brake
    • The jake brake

    The service brake is usually operated by the brake pedal. It's used to control the speed of the vehicle and to bring it to a halt safely. It may also incorporate an anti-lock braking system.

  10. You're driving an articulated lorry. What could happen if you change to a lower gear while you're going too fast?

    • The vehicle could jack-knifeRight answer
    • The engine could stall
    • The brakes could fail
    • The trailer could uncouple

    Severe braking or selecting a gear too low for your road speed can cause the tractor unit to slow more quickly than the trailer. The trailer can then pivot around the coupling (fifth wheel), causing the vehicle to jack-knife. Jack-knifing is more likely to occur with an unladen vehicle, particularly on a bend or curve.

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