Roller Brake Test Explained


Starting in April 2025, it will be mandatory for all safety inspections to either perform a laden brake test or evaluate an Electronic Brake Performance Monitoring System (EBPMS) report if the vehicle or trailer is equipped with one. This change is prompted by Traffic Commissioners (TCs) urging operators to adhere to legal obligations, ensuring that brake maintenance meets satisfactory standards.


Lets look at each element and what they mean:


Vehicle Identification

DTp Number: This unique identifier links your vehicle to its official documentation and test history. Think of it like a personal identification number for your truck.The accuracy of DTp number (Department for Transport (DfT)) is crucial for accurate brake testing. It acts as a bridge between the vehicle and the DVSA computer, fetching all the necessary information for a compliant and meaningful test. However, there are indeed potential issues that can arise:

Incorrect DTp Number: Mismatched information: Entering the wrong DTp number leads to the computer retrieving data for a different vehicle configuration, making the test parameters and pass/fail criteria inaccurate. This could result in:

  • False test passes: An HGV failing to meet certain safety standards might pass due to incorrect weight limits or test procedures.

  • Unnecessary test failures: Conversely, a perfectly safe HGV could fail due to overly stringent parameters meant for a heavier or differently configured vehicle.

  • Data integrity issues: In rare cases, a system glitch or human error might cause the DVSA computer to misinterpret the entered DTp number, leading to similar problems as described above.

Missing or Incomplete Information:

  • Outdated data: If the DVSA database hasn't been updated with recent modifications or repairs, the retrieved information might be incomplete or inaccurate, impacting the test's effectiveness.

  • Missing model-specific details: For newer or less common HGV models, the database might not have all the necessary specifications, potentially leading to incomplete tests or inaccurate results.

Consequences and Prevention:

  • Safety risks: Inaccurate testing puts both the HGV and other road users at risk by potentially allowing unsafe vehicles to operate or unnecessarily restricting safe ones.

  • Compliance issues: Operating an HGV with a failed brake test due to DTp number errors can lead to fines and legal repercussions.

Prevention: To minimise these risks:

  • Double-check the DTp number: Ensure it matches the one on the vehicle's plating certificate.

  • Report any discrepancies: If you suspect an error, inform the testing station and the DVSA immediately.

  • Regular updates: Encourage HGV operators to keep their vehicle information updated with the DVSA.

Weight & Measurement

GVW and GTW: These stand for Gross Vehicle Weight and Gross Train Weight, respectively. GVW is the maximum weight your truck can safely carry, including cargo and passengers. GTW applies if you tow a trailer, and includes the combined weight of both truck and trailer. Imagine GVW as your luggage allowance for a flight, and GTW as the combined weight allowed for you and your checked bags.

TAW: This refers to Total Axle Weight, which is the weight distributed across each individual axle of your vehicle. Each axle has a maximum weight limit, so TAW helps ensure you're not overloading any particular one. Think of it as distributing your luggage weight evenly across the wheels of your suitcase.

Axle Weights: This section details the weight on each individual axle, measured during the test. Comparing these values to the TAW limits helps identify any uneven weight distribution.

The table shows what is checked during the test and when it will fail.
Brake test values

What is Bind?

Bind: This checks if the brakes drag even when not applied, which can waste fuel and wear down components faster. Imagine your car brakes slightly rubbing against the wheels even when you're not pressing the pedal."Bind" represents a crucial measurement related to the braking system's health and safety. Here's a detailed explanation:

Bind refers to the residual braking force present on an axle even when the brakes are not applied. In simpler terms, it checks if the brakes drag on the wheels even when you're not pressing the brake pedal.

How is it Measured?During a roller brake test, each axle of your HGV is individually tested on rollers simulating real-world road conditions. While simulating no brake application, sensors measure the force exerted by each wheel on the rollers. This force, if present, indicates binding.
Why is it Important?Excessive bind has several negative consequences:

Fuel Efficiency: Constant dragging brakes increase fuel consumption due to unnecessary friction.

Premature Wear: Binding causes unnecessary wear and tear on brake components, leading to more frequent replacements and higher maintenance costs.

Uneven Braking: If bind is uneven across axles, it can lead to unbalanced braking, affecting vehicle stability and potentially pulling it to one side during braking.

Reduced Stopping Power: In rare cases, severe bind can even reduce the overall braking force achievable when you actually apply the brakes.

Acceptable Values:DVSA regulations specify maximum allowable bind values for each axle based on the HGV's weight and configuration. If the measured bind on any axle exceeds this limit, the vehicle fails the brake test. The DVSA website suggests more than 4% is a fail.

What to Do if Bind is Detected:Several factors can cause bind, including worn or seized brake components, misaligned drums or discs, or even issues with the brake actuation system. If you find excessive bind in your HGV's brake test report:

Seek professional help: Consult a qualified HGV mechanic to diagnose the exact cause of the bind and perform necessary repairs or adjustments.

Address the issue promptly: Don't ignore bind, as it can lead to increased operating costs, safety hazards, and potential legal repercussions if your vehicle fails a subsequent brake test.Additionally, while bind is primarily measured for service brakes, some reports might also include information on parking brake bind, which is equally important for ensuring your HGV remains securely parked.

What is Time Lag?

Time Lag: This measures the difference in time it takes for each brake on an axle to apply and release. Ideally, all brakes should react simultaneously. Think of it as the brakes on each wheel of your bicycle reacting with the same speed when you pull the lever.Time Lag, as measured in an HGV brake test, refers to the difference in time between when the driver applies the brakes and when each wheel on an axle actually starts braking. Ideally, all wheels should react simultaneously for optimal stopping power and vehicle stability.

How is it Measured?During a roller brake test, each wheel on your HGV is equipped with sensors that measure the braking force applied and the time it takes for that force to increase. The time lag is calculated for each wheel, compared across the same axle, and reported in the test results.

Importance of Minimal Time Lag:

Prompt & Synchronized Braking: Minimising time lag ensures your HGV responds quickly and consistently to brake application, crucial for reducing stopping distances and avoiding collisions in emergency situations.

Vehicle Stability: Consistent and simultaneous braking across all wheels on an axle helps maintain vehicle stability and prevents it from pulling to one side during braking.

Smooth Braking Experience: Reduced time lag contributes to a smoother and more predictable braking experience for the driver, enhancing overall control.

Acceptable Values:While DVSA regulations don't directly specify pass/fail limits for time lag, excessive delays can raise concerns about potential issues in the braking system. Some testing stations might have their own internal guidelines or best practices for acceptable time lag values.

Potential Causes of High Time Lag:

Air brake system issues: Leaks, worn components, or inadequate air pressure can delay brake actuation in air brake systems.

Hydraulic brake system problems: Faulty valves, air in the lines, or worn components can cause delays in hydraulic systems.

Mechanical wear and tear: Loose linkages, worn brake pads, or misaligned drums/discs can increase the time it takes for the brakes to engage fully.

Consequences of High Time Lag:

Increased stopping distances: Even a small delay in applying the brakes can significantly increase stopping distance, especially at higher speeds.

Uneven braking: If delays vary across wheels on an axle, it can lead to unbalanced braking and loss of vehicle control.

Safety risks: High time lag poses a significant safety risk by compromising your HGV's ability to stop quickly and safely in critical situations.

What to Do if Time Lag is High:

Consult a qualified HGV mechanic: They can diagnose the root cause of the delay and recommend necessary repairs or adjustments.

Address the issue promptly: Don't delay addressing high time lag, as it directly impacts your HGV's braking performance and road safety.

Regular maintenance: Regular brake inspections and servicing help prevent excessive wear and tear, minimizing the risk of time lag issues.

What is Ovality?

Ovality (steering axle only): This checks if the brake drums/discs are perfectly round. Irregular shapes can cause uneven braking and vibrations. Imagine the brake drum being slightly egg-shaped instead of perfectly circular.Ovality, in the context of brake drums, refers to a condition where the drum is no longer perfectly round, but instead has become slightly egg-shaped. This can occur due to uneven wear, heat distortion, or improper handling.

How does ovality affect braking performance?When a brake drum is oval, the brake shoes or pads won't make consistent contact with the entire surface of the drum. This can lead to several problems:

Reduced braking force: With less contact area, the brakes won't be able to generate as much stopping power, potentially increasing stopping distances.

Uneven wear: As the brake shoes or pads rub against the high and low points of the oval, they'll wear unevenly, further exacerbating the problem.

Vibrations: The uneven contact can cause the drum to vibrate, which can be felt through the steering wheel and brake pedal, making the ride uncomfortable and potentially affecting handling.

How to detect ovality:There are a few ways to detect ovality:

Visual inspection: If the drum isn't perfectly round, you might be able to see the distortion with the naked eye.

Micrometer: A micrometer can be used to measure the drum's diameter at different points to identify any deviations from roundness.

Dial gauge: A dial gauge can be mounted on the hub and used to measure the runout of the drum as it rotates. This is the most accurate way to measure ovality.

What is Imbalance?

Imbalance measures the difference in braking force between the left and right wheels on the same axle of your HGV. Ideally, both wheels should contribute equally to slowing down the vehicle. However, various factors can cause one wheel to exert more or less force compared to the other.
How is it Determined?During a roller brake test, each wheel on your HGV is individually tested on rollers that simulate real-world braking conditions. The maximum braking force generated by each wheel is measured, and the imbalance is calculated as the percentage difference between these forces.

Acceptable Variance:According to DVSA guidelines, GTMWR, an HGV fails its brake test if the imbalance on any axle exceeds 30%. However, even imbalances below this threshold can be a sign of potential issues and deserve attention. Some sources recommend investigating and addressing imbalances above 10% as a precautionary measure.

Importance for Safety:Brake imbalance directly impacts your HGV's stopping ability and stability during braking. Here's why:

Uneven Braking: If one wheel applies significantly less force than the other, it pulls less weight, reducing the overall braking effectiveness. This can increase stopping distance and make it harder to control the vehicle during an emergency.

Instability: If the imbalance is significant, it can cause your HGV to pull to one side during braking, potentially leading to loss of control or skidding. This is especially dangerous on wet or slippery roads.

Excessive Wear: Uneven braking forces can cause uneven wear on brake components, leading to premature failure and potential safety hazards.

Overall, understanding and addressing brake imbalance is crucial for ensuring the safety of yourself, your passengers, and other road users.

Additional Tips:

  • Regular Maintenance: Regular brake inspections and adjustments are essential to prevent and minimise imbalances.

  • Professional Expertise: If you detect an imbalance, consult a qualified mechanic or HGV specialist for diagnosis and repair.

  • Driver Awareness: Be aware of any pulling sensation during braking, which could indicate an imbalance. By staying informed and taking preventive measures, you can ensure your HGV's brakes operate safely and efficiently, contributing to a safer driving experience for everyone on the road.

What is Max Force?

This measures the maximum braking force each axle can generate. It's crucial for ensuring your truck can stop safely in emergencies. Imagine this as the "power" of your brakes, similar to how hard you can squeeze the brakes on your bike.Max force, in the context of an HGV brake test report, refers to the maximum braking force generated by each individual axle during the testing process. It's a crucial metric for understanding the overall effectiveness and safety of your vehicle's braking system.

What are we measuring and why?The max force essentially measures the maximum stopping power your HGV's brakes can achieve on each axle. This directly translates to the vehicle's ability to decelerate effectively and safely under various braking conditions, especially during emergencies.

Here's a breakdown of what we're measuring and why:

Braking force: This is the force exerted by the brakes on the wheels, ultimately slowing down the vehicle. Max force captures the peak value of this force for each axle.

Individual axle values: Measuring each axle separately helps identify imbalances in braking performance. Ideally, all axles should contribute equally to stopping power.

Safety implications: Adequate max force across all axles ensures your HGV can comply with stopping distance regulations and come to a safe halt even when fully loaded.

Technical aspects:The measurement of max force typically involves:

Roller brake tester: The HGV is driven onto rollers that simulate real-world road conditions.

Force sensors: Each wheel is equipped with sensors that measure the braking force applied.

Data acquisition: The maximum force generated by each axle during the test is recorded and displayed in the report.

Interpreting max force values:

Pass/fail criteria: Each axle has a specific minimum max force requirement based on DVSA regulations and the HGV's weight and configuration. Failure to meet these criteria constitutes a failed brake test.

Safety margins: Even if an axle passes the minimum requirement, a higher max force provides a safety margin for unforeseen circumstances, like wet roads or emergency braking.

Technical implications for road safety:

Insufficient max force: If an axle falls below the minimum requirement, it significantly increases the stopping distance required for your HGV, potentially leading to collisions or loss of control, especially at higher speeds.

Uneven force distribution: Imbalances in max force between axles can cause the vehicle to pull to one side during braking, compromising stability and increasing the risk of accidents.

Predictable braking: Knowing the max force of each axle allows drivers to adjust their braking technique for optimal control and safety in different situations.

Beyond the basics:

Vehicle factors: Max force can be influenced by various factors like tire types, brake pad/shoe material, and suspension condition. Regular maintenance and appropriate component selection are crucial.

Advanced testing: Some testing facilities might measure peak force and sustained force separately. Peak force is the initial maximum, while sustained force represents the braking force maintained over a longer period. Both are important for understanding overall braking performance.

By understanding the importance of max force and interpreting its values correctly, you can ensure your HGV's braking system operates within safe limits and contributes to responsible driving on the road. Remember, regular brake inspections and maintenance are essential for maintaining optimal max force and overall braking performance.