Operators! Newsflash: your roller brake tests are not worth the paper they are written on.
What does your brake testing schedule look like? Maybe you have four brake tests undertaken each year, or perhaps you do a brake test every other safety inspection? If so, you are not alone; in fact, the majority of maintenance providers advise a roller brake test at MOT, plus three further tests throughout the year.
This recommendation, however, is outdated; the latest version of the Guide to Maintaining Roadworthiness states: “It is strongly advised that a calibrated roller brake tester (RBT) is used at each safety inspection to measure individual brake performance and overall braking efficiencies for the vehicle or trailer to the annual test standards.”
Roller brake testing has been used in the industry for over 50 years, yet for some reason it is still widely misunderstood by operators and maintenance providers. Having worked with hundreds of operators over the years, I can say confidently that the vast majority do not know how to read the brake test report; their attention jumps directly to the bottom of the document. The term ‘Pass‘, however, can be incredibly misleading, and relying on this may result in vehicles going on the road in a dangerous condition.
How to read a roller brake test
Before we look at performance figures, there are a few definitions to keep in mind:
- Vehicle/Trailer Details: Fairly self-explanatory, but you should make sure these correspond to the details on the plating certificate.
- DTp Number: This is the Department for Transport (DfT) number related to the braking information for the vehicle/trailer. It is very important that this is correct as it gives the brake testing machine all of the details against which the test is conducted. If this is wrong the whole test will be invalid.
- GVW and GTW: These are the Gross Vehicle Weight and Gross Train Weight respectively. It is important to check these against the plating certificate as they are used in efficiency calculations.
- TAW: Trailers will also show the Total Axle Weight
- Bind: This is a check to see if the brakes were binding at all when the brake was not applied.
- Time Lag: This is a check to ensure that each of the brakes on the same axle apply and release at the same time as each other.
- Ovality: This check is conducted on the steering axle only. The term comes from the days of drum brakes which over time could become more oval in shape, leading to an imbalance between the left and the right. These days, the term covers any imbalance or fluctuation in the braking effort as each wheel is rotated. This covers ovality of drums or warping of discs.
- Imbalance: This is a check for the percentage variation between the brakes on the same axle. The report shows the braking force of each brake on one axle and the variation percentage between the nearside and the offside.
- Max Force: This indicates the maximum braking effort or braking force for each brake. This figure was also used to calculate the imbalance previously. It is worth watching out for the (L) after the force figure which would indicate the brake locked and therefore the potential braking effort could have been higher.
- Axle Weights: Each axle will be identified and the weight of the axle during the test will be stated with the brakes present on that axle listed below. The axle weight should be between 50-65% of the Axle Design Weight (found on the plating certificate) for the brake test to be considered meaningful.
How to read a roller brake test
A roller brake test report is divided into 3 sections:
Section 1: The Vehicle or Trailer Details
This section has the details of the vehicle or trailer undergoing the test plus some details of the test centre and the date and time of the test. It is important to check these details are correct, especially the DTp Number, as this tells the computer the exact parameters of the brake test. If this is entered wrong then the test is invalid.
Section 2: The Brake Performance by Axle
This section looks at each axle and shows the axle weight at the time of testing, the report then continues to show the braking system on each side of each axle, checking the bind, time lag, ovality, imbalance and max force as well as indicating if the brakes locked . The main thing to look for here is a significant imbalance in the brakes; this is a clear indication that maintenance is required on the brakes.
Section 3: The Test Summary
The final section indicates how the vehicle or trailer has performed during the roller brake test. This section will identify the total measured vehicle weight (all axle weights added together), highlight if any axle had insufficient load, identify the values required by each braking system to pass the test, show the test value achieved during the test for each braking system, and finally the overall result of the roller brake test. We will cover how this is calculated in the next section.
Putting it all together – An example of a good brake test
- In the example above the vehicle was loaded to 87% capacity making it a meaningful brake test as it exceeds the minimum 50-65% expected by the DVSA.
- All the imbalances recorded were very reasonable with the largest being only 13% on Axle 2. However if we look closely at this imbalance we can see that the offside service brake locked which means that it did not get to its full force before it locked up. It is therefore reasonable to assume that had the brake not locked at that point it would have been able to exert more force which would reduce the imbalance further. When one side locks and the other doesn’t it is likely that the imbalance will be slightly exaggerated.
- Finally the test summary showed the test values exceeding the pass values for every braking system by a clear margin. This example shows a meaningful brake test that has been passed well within the criteria and would indicate that this vehicle can safely be put on the road with confidence between now and the next inspection date.
Spotting a bad one – what a bad brake test looks like
It says “Overall Result: Passed“…and that’s good enough for me!
Unfortunately that is the attitude of many people, especially the fitters, who have been taught to get a pass on locks so that they can sign it off and begin the next job. This is not helped when a DVSA MOT Tester will accept this brake test as a pass during the MOT inspection.
But what is acceptable for the MOT tester, may be unacceptable for their colleague; namely, the DVSA inspector who is investigating the operation. DVSA agents are known to dismiss ‘Pass’ certificates as meaningless. So why is this a bad test?
- The vehicle has been presented for the test weighing 51% of it’s total capacity however most of the weight is over the first axle which means that axles 2 and 3 have insufficient load.
- There are some significant imbalances in the braking systems, specifically on the third axle which are potentially dangerous and should not be left without further investigation. Even if you do think that this is okay now, do you really think it will be okay by the time the vehicle next gets an inspection?
- The test summary showed the test value of the service brake being less than the required pass values however the summary still shows this as a pass. Why? – Well the service brake locked up on all wheels on all axles so in this example the report shows this as a pass on locks. So despite the service brake performance being 14% less than the required pass mark it is still allowed to show as a pass.
How to calculate brake performance
As mentioned above the brake performance is calculated and shown in the test value column within the test summary section. The formula for this calculation is :
- The Brake Effort is the Max Force exerted by the brakes on each side of each axle all added together.
- The Weight depends on which brake is being measured and is identified next to the pass value in the test summary section.
Based on the brake test above for the Iveco tractor unit with the registration N20 EDS we would take the following steps to get the brake performance figure for the service brake:
Step 1 – Add all of the braking force figures for the service brake on each axle together to give you the total braking force.
Axle 1: N/S 1,776 kgF + O/S 1,884 kgF = 3,660 kgF
Axle 2: N/S 1,912 kgF + O/S 1,651 kgF = 3,563 kgF
Axle 3: N/S 3,023 kgF + O/S 3,131 kgF = 6,154 kgF
Total Braking force for the Service Brake across all 3 axles = 13,377 kgF
Step 2 – Divide the total braking force by the weight as specified in the pass value column. In this case the Gross Vehicle Weight (GVW) of 24,000 kg
13,377 ÷ 24,000 = 0.557375
Step 3 – Multiply by 100 to get a percentage, rounding up or down to the nearest whole number
0.557375 x 100 = 55.7375
Rounded up to the nearest whole number = 56%
Therefore the service brake in this example has achieved a performance score of 56% which is higher than the required pass value of 50% meaning that the service brake has achieved a passing score.
Main things to watch out for on a roller brake test
Insufficient Load on axles – For a brake test to be meaningful each axle should be loaded to a minimum of 50-65% of the designed axle weights. While there is an exemption for tri-axle semi trailers it is worth noting that any brake test conducted on an unloaded vehicle or trailer can not be considered to be meaningful. All modern vehicles and trailers have load sensors which increase the pressure in the braking system when a load is present. Testing a vehicle or trailer unloaded will lead to less braking force being exerted, the brakes locking earlier than they should and a false positive test result which gives no indication of whether the vehicle will be able to stop safely when fully loaded.
Passing on Locks only – As in the bad example above it is possible for a test result to show a pass even though the brake performance of a braking system was less than the required pass value. This is very common when a brake test is carried out with insufficient load over the axles.
Dangerous Imbalance – Any imbalance in the brakes on either side of an axle could lead to the vehicle pulling to one side under heavy braking and potential failure in emergency braking situations. An imbalance of over 30% will lead to a fail on the test unless it passes on locks. It is recommended that any imbalances over 10% are investigated and brakes adjusted to address the imbalance.
What can you do going forward
Ensure your maintenance provider knows how to read the roller brake test correctly. – Email them the link to this article or share it with them on social media if it helps.
Set up or adjust your service level agreement with the maintenance provider to ensure all roller brake tests are carried out with sufficient load. – It may cost more if you need the maintenance provider to load the vehicles and trailers for you but every time you get a brake test done with insufficient load, you are just throwing your money away.
Have roller brake tests carried out every time the braking system has work carried out, not just four times per year. – Most operators get brake tests done in accordance to their maintenance schedule, however if you’ve just had new discs and pads put on, wouldn’t you rather find out if they have been fitted correctly and are working efficiently on a roller brake machine rather than when you plow into a family saloon when leaving the workshop.
Read and critique every line on the brake test report before allowing the vehicle on the road. – Please don’t just look at the bottom of the test sheet for the overall result. As you now know this can be very misleading, you must check every line on the test to ensure the vehicle you are putting on the road is safe.