VMM Software

The Testers Manual

3.7 Brake Performance (Roller Brake Test)



Information
Method of Inspection
Reason for rejection
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

Certain vehicles may not be suitable for testing using a roller brake tester. Testers should take account of the vehicle’s drive configuration, transmission type and any Vehicle Specific Information before deciding whether a full or partial roller brake test is appropriate.
If additional information is available from the vehicle manufacturer, then this should also be taken into account.

Unsuitable 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 3.7B and C.

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

When testing a veteran car or a vehicle with special controls the driver should be allowed to drive during the test if he/she wishes.
Vehicles with automatic transmission must never be roller brake tested with the gear selector in the “P” park position.

For vehicles with servo assisted or power braking systems, the engine must be idling while the service brake is being tested.

Where it is not possible to read the gauges of the roller brake tester while sitting in the driver’s seat, an assistant may be used to apply the brakes during the test.

ATL approved test stations use a Computer Controlled RBT. Testers should follow the sequence of instructions displayed on screen and on completion of the test print off the results.

For Class 4 vehicles the ATL system will automatically weigh the vehicle and this presented weight will be used for the brake efficiency calculations.

For Classes 5 & 7, brake efficiency must be calculated using the vehicle’s DGW (see Section 3.8).

Vehicles of unknown test weight should be tested as normal. However, if the vehicle does not meet the locked wheel criteria explained in the notes following RfRs 9 and 10, carry out a further brake test using a decelerometer to determine the brake efficiencies.
A. Roller Brake Test


For vehicles of a type which can be tested on a roller brake tester.

Preparation
Examine the tyres of the vehicle to ensure that they are not obviously under-inflated.

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.

Position the front wheels of the vehicle in the rollers of the brake tester and then run both sets of rollers together in a forward direction to align the vehicle.

Note: In some cases, it may be necessary to chock the wheels not under test.
Testing the front wheels
1.
With one set of rollers revolving at a time, (see information column if ATL approved) gradually depress the service brake until maximum effort is achieved, or until the wheel locks and slips on the rollers.

Record the reading at which the maximum braking effort is achieved and whether “lock-up” occurs. Release the service brake.

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 wheel slip occurs. Aim to stop just short of this.
However, if wheel slip is caused unintentionally, start the test again.
Note the out-of-balance in braking effort between wheels across each steered axle, disregarding any imbalance when the brake effort from each wheel is less than 40kg.

Gradually release the service brake and observe how the braking effort at each wheel reduces. Stop the rollers.
1.
a.
Little or no braking effort is recorded from the brake on any wheel, indicating clearly that the brake is not functioning correctly

b.
see Reason for Rejection 9.

2.
a.
significant braking effort recorded on a road wheel, 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 road wheels do not increase at about the same rate when the service brake is applied gradually

d.
the out-of-balance of the brakes on the steered road wheels is greater than 25% (30% for Class 5 vehicles) at any time whilst brake effort is increased gradually (see Method of Calculating Brakes Out-of-Balance in Section 3.8)

e.
the braking efforts at the road wheels do not reduce at about the same rate when the service brake is released gradually.
Vehicles designed before 1 January 1905 and constructed before 31 December 1905 do not require a parking brake.

For the Method of Calculating Brake Efficiency see Section 3.8.

If the required brake efficiency is only just met, but the tester knows that a higher performance figure is normally obtained for the type of vehicle, the vehicle presenter should be advised that the braking system appears to be in need of adjustment or repair.

If the vehicle is fitted with an electronic parking brake, refer to VSI or the manufacturer’s operating instructions for the test procedure.

Large vehicles fitted with spring brakes, lock actuators or air assisted parking brakes may require an applied brake test in respect of parking brake efficiency as follows:

. apply the parking brake fully (the service brake may be used to assist in setting the parking brake) and then release any power assistance

. start each brake roller in turn and note the maximum effort recorded.
This test must only be carried out using an approved Class 5 roller brake tester with the appropriate ‘Applied Brake test’ programme.
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.

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, (see information column if ATL approved) gradually depress the service brake until maximum effort is achieved or until the wheel locks and slips on the rollers.

Record the maximum braking efforts and whether ‘lock-up’ occurs. Release the service brake.

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 wheel slip occurs. Aim to stop just short of this. However, if wheel slip is caused unintentionally, start the test again.

6.
Gradually release the service brake and watch how the braking effort at each wheel reduces. Stop the rollers.
3.
a.
Little or no braking effort is recorded from the brake on any wheel, indicating clearly that the brake is not functioning correctly

b.
see Reason for Rejection 10.

4.
a.
Little or no braking effort is recorded from the brake on any wheel, indicating clearly that the brake is not functioning correctly

b.
see Reason for Rejection 9.

5.
a.
A significant effort recorded on a road wheel, 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 road wheels do not increase at about the same rate when the service brake is applied gradually.

6.
The braking efforts at the road wheels do not reduce at about the same rate when the service brake is released gradually.
To avoid possible damage when testing transmission parking brakes, the parking brake efficiency requirement must be calculated using the appropriate vehicle weight before the brake is tested.

Testing of the brake must cease as soon as the minimum efficiency requirement is reached when progressively applying the brake.

When testing service brake performance on unladen Class 7 vehicles premature wheel lock can occur resulting in less than the required brake effort being achieved.
This may be due to the action of load sensing or pressure reducing equipment in the service brake system.
In such 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.

If after applying the above criteria the minimum brake efficiency is still not met, then a decelerometer test should be carried out.

Note: It must be remembered that a decelerometer test should not be carried out if the vehicle has already failed for ‘little or no braking effort is recorded from the brake on any wheel, indicating clearly that the brake is not functioning correctly’.
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.

Testing transmission (prop shaft) parking brakes

8.
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 parking brake ratchet disengaged for as long as the brake is applied

. apply the brake slowly and progressively without causing transmission snatch.

9.
Record the appropriate results of the service brake test via the VTS Device, which will calculate the results. Where the VTS Device is unserviceable see paragraph 3 of the Introduction.

10.
Record the appropriate results of the parking brake test via the VTS Device, which will calculate the results. Where the VTS Device is unserviceable see paragraph 3 of the Introduction.





7.
a.
Little or no braking effort is recorded from the brake on any wheel, indicating clearly that the brake is not functioning correctly

b.
See Reason for Rejection 10.

8.
See Reason for Rejection 10.

9.
The calculated service brake efficiency is too low (see Brake Efficiency Table in Section 3.10).
Note: If more than half the wheels braked by the service brake lock, the service brake percentage efficiency is considered to be satisfactory.

10.
The calculated parking brake efficiency is too low (see Brake Efficiency Table in Section 3.10).

Note: If more than half the wheels braked by the parking brake lock, the parking brake percentage efficiency is considered to be satisfactory.
Decelerometer brake testing must always be carried out on roads which:

. have a good surface

. are suitable for brake tests when dry or wet

. have a minimum of traffic

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

Vehicles with damaged or significantly under inflated tyres should not be decelerometer brake tested.

When using a decelerometer to test a transmission parking brake, keep the ratchet disengaged for as long as the brake is applied. Take the efficiency reading without the occurrence of transmission snatch or judder.

If the required brake efficiency is only just met, but the tester knows that a higher performance figure is normally obtained for the type of vehicle, the vehicle presenter should be advised that the braking system appears to need adjustment or repair.

Additional braking devices, e.g. retarders, should not be operative during the brake test.
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 of approximately 20mph (32kph) and note the brake efficiency recorded when progressively applying only:
a.
the service brake.

b.
the parking brake.

2.
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.

3.
While the vehicle is decelerating under the action of the parking brake note if the vehicle tends to deviate excessively from a straight line.

Record the appropriate results of the brake test via the VTS Device. Where the VTS Device is unserviceable see paragraph 3 of the Introduction.
1.
a.
The service brake efficiency recorded on the decelerometer does not meet the requirements specified in the Brake Efficiency Table in Section 3.10

b.
the parking brake efficiency recorded on the decelerometer does not meet the requirements specified in the Brake Efficiency Table in Section 3.10.

2.
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.

3.
When the parking brake is applied the vehicle deviates excessively from a straight line.
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’).

If the required brake efficiency is only just met, but the tester knows that a higher performance figure is normally obtained for the type of vehicle, the vehicle presenter should be advised that the braking system appears to need adjustment or repair.

When using a plate tester to test a transmission parking brake, keep the ratchet disengaged for as long as the brake is applied. Take the efficiency reading without the occurrence of transmission snatch or judder.

If a vehicle fails any aspect of the plate brake test, the check should be repeated to confirm the Reason for Rejection.
C. Plate Brake Test

Preparation

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.

Class 4 vehicles: Obtain the vehicle test weight.

Class 7 vehicles: Establish the vehicle actual presented weight.

Brake efficiency on Class 7 vehicles will be calculated using either:

. the actual DGW where the presented weight is 2000kg or over (the DGW is obtained from the manufacturer’s plate fitted to the vehicle, or the ‘Ministry’ plate if one is fitted - see notes 1 and 2 in Section 3.9), or

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

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.
Note the way in which the brake efforts increase and the maximum values achieved.
Disregard any imbalance when the brake effort from each front wheel is less than 40kg force.

3.
Repeat 2 above using the parking brake and note the maximum values achieved.

4.
Record the appropriate results of the brake test via the VTS Device, which will calculate the results. Where the VTS Device is unserviceable see paragraph 3 of the Introduction

 
 
 
 
 
 
 
 
 
 
 
 
 
1.
Excessive fluctuation of brake effort with a constant brake pedal effort indicating brake judder.
2.
a.
A significant brake effort recorded on a road wheel, when the brake is not applied, indicating that a brake is binding
b.
the braking efforts at both road wheels 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 road wheels is greater than 25% at any time (30% for Class 5)
3.
Little or no effort is recorded from the parking brake on any wheel, indicating clearly that the brake is not functioning correctly.
4.
a.
The service brake efficiency is too low (see Brake Efficiency Table requirements in Sections 3.10)
b.
the parking brake efficiency is too low (see Brake Efficiency Table requirements in Sections 3.10).

This test may be conducted as an alternative to a decelerometer test on certain large vehicles with a dual circuit braking system (min. 16% efficiency required) such as Motor Caravans fitted with a prop shaft brake not suitable to be tested on a roller brake tester.   

Care should be taken to avoid any risk of ‘grounding’ the vehicle when reversing onto and driving off the test slope.
D. Parking Brake Gradient Test

1. Reverse the vehicle onto a 16% gradient, apply the parking brake and release power assistance

Note:  When the parking brake is being tested by the gradient test, the service brake (foot-brake) may be used to assist in setting the parking brake

Record the appropriate results of the brake test via the VTS Device.  Where the VTS Device is unserviceable see the Introduction section paragraph 3.
1. When fully applied, the parking brake is incapable of holding the vehicle on a 16% gradient
Issue Date: 01 January 2012