3.7 Brake Performance (Roller Brake Test)

Information Method of Inspection Reason for rejection
A. Roller Brake Test
The brake performance test must be carried out on a properly calibrated and maintained slow-speed roller-brake tester designated as acceptable for the statutory tests, except Vehicles for which a roller brake tester is not appropriate, or At premises without a roller brake tester where approval has been granted for the test to be carried out by other means If the vehicle is of a type which can be tested on a roller brake tester  
Vehicles not  to be tested on a roller brake tester

Certain vehicles should not be tested on a roller brake tester, eg vehicles with More than one driving axle permanently engaged Limited-slip differential Belt-driven transmission Brakes for which the servo operates only when the vehicle is moving These vehicles should be tested using a properly calibrated and maintained decelerometer or a plate brake tester designated as acceptable for the statutory tests, see Sub Section 3.7 B, page 24, and C, page 25.

A roller brake test is also not appropriate for vehicles with damaged, under-inflated or studded tyres.

Preparation    
1. Examine the tyres of the vehicle to ensure that they are not obviously under-inflated.  
2. Determine whether the vehicle has a split (dual) braking system. Note: To determine whether the vehicle has a split (dual) braking system, check the number of pipes from a hydraulic master cylinder or air foot valve.  Split (dual) systems normally have at least two pipes.  Some hydraulic systems have two master cylinders.  

3. Select the direction of rotation of the roller brake testers so that the vehicle wheels rotate forward. Positioning the vehicle Position the front wheels of the vehicle in the rollers of the brake tester and then run both sets of rollers together to align the vehicle.

Note: In some cases, it may be necessary to chock the wheels not under test.

 
With some vehicles, the required brake efficiency is just obtained or just exceeded, but the tester knows that a higher performance figure is normally obtained for the type of vehicle. Although the vehicle has passed the brake performance test, the tester should advise the vehicle presenter that the braking system appears to need adjustment or repair.

2. Start both sets of rollers and note whether a significant brake effort is recorded from any wheel without a brake being applied.  Gradually depress the service brake and watch how the braking effort for each wheel increases.

From the previous tests you will know the value at which wheelslip occurs.  Aim to stop just short of this. However, if wheelslip is caused unintentionally, start the test again. Gradually release the service brake and observe how the braking effort at each wheel reduces.  Stop the rollers.

Note the out-of-balance in braking effort between wheels on either side of the vehicle

 
  a. A significant braking effort recorded on a roadwheel, even though the brake is not applied, indicating that a brake is binding
  b. Evidence of severe brake grabbing or judder as the brake is applied
  c. The braking efforts at the roadwheels do not increase at about the same rate when the service brake is applied gradually
  d. The braking efforts at the roadwheels do not reduce at about the same rate when the service brake is released gradually
 

e. The out-of-balance of the brakes on the steered road wheels is greater than 25% at any time (see Method of Calculating Brakes out-of- Balance on page 26)

Note: Disregard any service brake imbalance when the brake effort from each front wheel is less than 40kg

Old pre-1906 vehicles
Vehicles certified by the London Science Museum as being designed before 1 January 1905 and constructed before 31 December 1905 do not require a parking brake 3. If the vehicle has a parking brake (handbrake), which operates on the front wheels, repeat the process outlined in 1 above using this brake and keeping the “hold-on” button or trigger in the disengaged position the whole time. a. A low braking effort is recorded from the parking brake on any wheel, indicating clearly that the brake is not functioning correctly
  b. see Reason for Rejection 9
  Testing the rear wheels Release the brakes and drive the vehicle forward until the rear wheels are in the rollers. Run them together as for the front wheels to align the vehicle.  
4. With one set of rollers revolving at a time, gradually depress the service brake until maximum effort is achieved or until the wheel locks and slips on the rollers.  
  a. A low braking effort is recorded from the brake on any wheel, indicating clearly that the brake is not functioning correctly
Record the maximum braking efforts and whether ‘lock-up’ occurs.  Release the service brake. b. see Reason for Rejection
 

5. Start both sets of rollers and note whether a significant brake effort is recorded from any wheel without a brake being applied.  Gradually depress the service brake and watch how the braking effort for each wheel increases. From the previous tests you will know the value at which wheelslip occurs.  Aim to stop just short of this.

However, if wheelslip is caused unintentionally, start the test again.

5. 

a. A significant effort recorded on a roadwheel, even though the brake is not applied indicating that a brake is binding

  b. Evidence of severe brake grabbing or or judder as the brake is applied
  c. The braking efforts at the roadwheels do not increase at about the same rate when the service brake is applied gradually
  6. Gradually release the service brake and watch how the braking effort at each wheel reduces.  Stop the rollers. 6. The braking efforts at the roadwheels do not reduce at about the same rate when the service brake is released gradually
 

7. If the vehicle has a parking brake (handbrake) which operates on the rear wheels, repeat the process as outlined in 3 above using this brake and keeping the “hold-on” button or trigger in the disengaged position the whole time.

Note: For testing transmission (prop-shaft) handbrakes, see page 23.

7.

a. A low braking effort is recorded from the parking brake on any wheel, indicating clearly that the brake is not functioning correctly  

  b. see Reason for Rejection 9
  8. Calculate the service brake efficiency (See Method of Calculating Brake Efficiency on page 26).

8. The calculated service brake efficiency is too low (see Brake Efficiency Table on page 28)

Note: The service brake percentage efficiency is considered satisfactory providing wheel lock occurs on more than half of the wheels braked by the service brake.

Note: Class VII vehicles only 

When testing service brake performance on unladen vehicles Premature wheel lock can occur, and Less than the required brake effort is achieved

The required brake effort might not be achieved due to the action of load sensing/pressure reducing equipment in the service brake system.

In either of these cases, the service brake percentage efficiency is considered satisfactory if

i. more than half the wheels lock, or

ii. both front wheels lock and at least 100kg (220lb) is achieved by each rear wheel, or 

iii. for three axle vehicles; both front wheels lock and at least 50kg (110lb) is achieved by each rear wheel

  9. Calculate the parking brake efficiency (See Method of Calculating Brake Efficiency on page 26). 9. The calculated parking brake efficiency is too low (see Brake Efficiency Table on page28)
  Note: The parking brake percentage efficiency is considered satisfactory providing wheel lock occurs on more than half of the wheels braked by the parking brake
Testing Transmission (prop shaft) Handbrakes
  10. Carry out the following procedure Place the wheels to be tested in the rollers. Run both sets of rollers together to align the vehicle. Chock the other wheels of the vehicle fore and aft. Run both sets of rollers together. Keep the handbrake ratchet disengaged for as long as the brake is applied. Apply the brake slowly and progressively without causing transmission snatch. Note the gauge readings and calculate the brake efficiency (see Method of Calculating Brake Efficiency on page 26). 10. The transmission brake 'parking' efficiency is too low (see Brake Efficiency Table on page 28)
B. Decelerometer Test

Roads used for decelerometer brake testing

The requirement for a steady road speed during a brake test by decelerometer means that the vehicle must always be driven on a road which Has a good surface Is suitable for brake tests when dry or wet Has a minimum of traffic

A particular public road should not be used for tests so much that it would cause complaints from residents.

B. Decelerometer Test  
1. If the vehicle is of a type which cannot be tested on a roller brake tester, set up the decelerometer in the vehicle in accordance with the equipment manufacturer’s instructions drive the vehicle on a level road at a steady speed on approximately 20 mph (32 kph) and note the brake efficiency recorded when applying only    
a. the service brake a. the service brake efficiency recorded on the decelerometer does not meet the requirements specified in the Brake Efficiency Table on page 28
b. the parking brake b. the parking brake efficiency recorded on the decelerometer does not meet the requirements specified in the Brake Efficiency Table on page 28
c. while the vehicle is decelerating under the action of the service brake, note if the steering wheel tends to pull or the vehicle tends to swerve c. when the service brake is applied there is a severe grab or judder, or there is a severe pull one way on the steering wheel, and/or the vehicle swerves appreciably  
Testing transmission handbrakes
When using a decelerometer to test a transmission handbrake, keep the ratchet disengaged for as long as the brake is applied.  Take the efficiency reading without the occurrence of transmission snatch or judder.    
Vehicles which just pass
With some vehicles, the required brake efficiency is just obtained or just exceeded, but the tester knows that a higher performance figure is normally obtained for the type of vehicle. Although the vehicle has passed the brake performance test, the tester should advise the vehicle presenter that the braking system appears to need adjustment or repair.    
Plate Brake Testing
The brake performance test must be carried out on a properly calibrated and maintained plate brake tester designated as acceptable for the statutory test.  (Alternatively a slow-speed roller-brake tester can be used-see section 3.7 ‘A’).

C. Plate Brake Test Preparation

1. Determine whether the vehicle has a single or dual (split) braking system.

Note: To determine whether the vehicle has a dual (split) braking system, check the number of pipes from the hydraulic master cylinder or air foot valve.  Dual (split) systems normally have at least two pipes.

Some hydraulic systems have two master cylinders.

 
Vehicles which only just pass

With some vehicles, the required brake efficiency is just obtained or just exceeded, but the tester knows that a higher performance figure is normally obtained for the type of vehicle.

Although the vehicle has passed the brake performance test, the tester should advise the vehicle presenter that the braking system appears to need adjustment or repair.

2. Class IV vehicles: Obtain the vehicle test weight from the data chart.

Class VII vehicles: Establish the vehicle actual presented weight.

The brake efficiency on class VII vehicles will be calculated using either:

the actual DGW where the presented weight is 2000Kg or over (the DGW is obtained from the Department or Transport plate or the manufacturers plate fitted to the vehicle – see notes 1 and 2 on page 27), or

a nominal DGW figure of 2600Kg if the presented weight is less than 2000Kg.

 
Testing transmission handbrakes
When using a plate tester to test a transmission handbrake, keep the ratchet disengaged for as long as the brake is applied.  Take the efficiency reading without the occurrence of transmission snatch or judder. 3. Enter the appropriate data to conduct the test.  
Testing the Vehicle
  1. Drive the vehicle forwards at a steady speed of about 4mph up to the plate tester.  Just before the wheels are on the plate high friction surfaces, apply a light constant pressure to the brake pedal.  Do not stop on the tester.  Note the way in which the brake efforts fluctuate. 1. Excessive fluctuation of brake effort with a constant brake pedal effort indicating brake judder.
  2. At the same steady speed of 4mph, again drive the vehicle forwards onto the plate brake tester.  As soon as the wheels are on the plate high friction braking surfaces, apply the service brake progressively until maximum effort is achieved.

2.

a. a significant brake effort recorded on a roadwheel, when the brake is not applied, indicating that a brake is binding

Note. The way in which the brake efforts increase . The maximum values achieved.

b. the braking efforts at both roadwheels on an axle do not increase at about the same rate when the service brake is applied
 

c. the out-of-balance of the brakes on the steered roadwheels is greater than 25% at any time

Note: Disregard any imbalance when the brake effort from each front wheel is less than 40Kg force

  d. the service brake efficiency is too low (see Brake Efficiency Table requirements on pages 26 & 28)
  3. Repeat 2 above using the parking brake. 3.
  a. a low brake effort is recorded from the parking brake on any wheel, indicating clearly that the brake is not functioning correctly
  b. the parking brake efficiency is too low (see Brake Efficiency Table requirements on pages 26 & 28).
  4. Repeat 1, 2 and 3 above a second time to confirm any reason for rejection  
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