PSV Brake Code Calculator & How It Works

A PSV brake code calculator works out the braking efficiency of a PSV (Public Service Vehicle, i.e. a bus or coach) from a roller brake test, using the measured brake forces and the vehicle’s weight. You enter the total brake force (in daN) and the vehicle’s gross weight (in kg), and the calculator returns the brake code as a percentage. That percentage is then compared against legal minimums to decide whether the vehicle passes or fails its brake test.

Formula

The brake code is the total braking force expressed as a percentage of the vehicle’s weight in newtons. In words:

Brake code (%) = (Total brake force in daN × 10) / (Gross weight in kg × 9.81) × 100

Where:

• Total brake force (bf) – the sum of all wheel brake forces measured by the roller tester, in decanewtons (daN).
• Gross vehicle weight (wt) – the actual weight of the vehicle when tested, in kilograms (kg).
• 9.81 m/s² – standard acceleration due to gravity, used to convert mass to force.

The result is a percentage. A brake code of 50% means the brakes can decelerate the vehicle at 0.5 g (about 4.9 m/s²).

Worked Example

Using the default values from the calculator:

• Total brake force = 4500 daN
• Gross weight = 12000 kg

Step 1: Convert daN to newtons: 4500 daN × 10 = 45000 N

Step 2: Weight in newtons: 12000 kg × 9.81 = 117720 N

Step 3: Brake code = (45000 / 117720) × 100 = 38.22%

So the brake code is 38.22%, which is below the 50% minimum for a PSV service brake.

Why the Brake Code Matters for PSVs

Public service vehicles (buses, coaches, minibuses used for hire or reward) are subject to strict safety checks in most countries. In the UK, the Driver and Vehicle Standards Agency (DVSA) requires a minimum brake efficiency of 50% for the service brake and 25% for the parking brake on PSVs. These values are set in the Construction and Use Regulations and are enforced during annual tests and roadside inspections.

The brake code is not just a number – it directly determines whether a vehicle is legally allowed to carry passengers. A vehicle that fails the brake test cannot be used until repairs are made and a retest is passed. The calculation is also used during routine maintenance to monitor brake performance over time.

How to Use the Calculator Correctly

To get an accurate brake code, you need the correct inputs:

• Use the total brake force from all axles, not just one wheel or one axle.
• Use the actual weight of the vehicle at the time of the test, including fuel, passengers, and load – not the kerb weight or the design GVW.
• Make sure the brake force is in daN (decanewtons). If your test machine gives newtons, divide by 10. If it gives kgf, multiply by 0.9807 to get daN.
• If the test is done on a gradient (rare for PSV roller tests), the calculation would need to include the slope – but roller tests are done on a level surface.

Common Mistakes and Misconceptions

A frequent error is mixing up units. Brake force is often displayed in daN, but some older machines show kgf. One kgf is approximately 0.9807 daN, so using kgf directly will overstate the brake code by about 2% – which could be the difference between a pass and a fail.

Another mistake is using the vehicle’s design gross weight instead of the actual test weight. If the vehicle is tested empty, the brake code will be higher than when it is fully loaded. The legal requirement applies to the vehicle in its normal operating condition, so the test should be done at a representative weight.

Some people think a brake code above 100% is impossible. In fact, it can happen if the tires are in very good condition and the brake force exceeds the weight of the vehicle – but that is extremely rare and usually indicates a measurement error.

What Affects the Brake Code Result

Several factors can change the measured brake force and therefore the brake code:

• Tire condition and pressure – low pressure reduces grip and measured force.
• Brake adjustment – worn or incorrectly adjusted brakes produce lower force.
• Roller test machine calibration – an uncalibrated machine gives unreliable readings.
• Vehicle weight – a heavier vehicle needs more brake force to achieve the same code.
• Brake temperature – hot brakes may have lower friction (fade) or higher (initially).

Limitations of the Calculation

The brake code formula assumes a level surface and that all wheels are braked. It does not account for:

• Brake balance between axles (the distribution of force).
• Anti-lock braking system (ABS) operation.
• Dynamic effects like load transfer during braking.
• The condition of the brake components (e.g., glazed linings).

Therefore, a high brake code does not guarantee that the vehicle will stop well in all conditions – but it is a good indicator of overall brake performance.

Frequently Asked Questions

What is the minimum brake code for a PSV?

In the UK, the minimum brake efficiency for a PSV service brake is 50% (0.5 g) and 25% for the parking brake. These are legal requirements set by the DVSA. Other countries may have similar or different thresholds, so always check local regulations.

Can I use this calculator for a car or truck?

Yes, the formula is the same for any vehicle. However, the minimum required brake code differs – for example, cars typically need 50% for the service brake, while heavy goods vehicles (HGVs) may have different requirements. Always refer to the relevant testing standards.

What does a brake code of 50% mean in terms of stopping distance?

A brake code of 50% means the vehicle can decelerate at 0.5 g, which is about 4.9 m/s². At 50 km/h (13.9 m/s), the theoretical stopping distance would be about 19.6 meters (ignoring reaction time and assuming constant deceleration).

Why does my brake test report show a different code than my own calculation?

Possible reasons: the test weight may be different, the brake force may be reported per axle rather than total, or the machine may use a different gravity constant (e.g., 10 instead of 9.81). Check the units and the weight used in the report.

Is the brake code the same as brake efficiency?

Yes, they are often used interchangeably. Brake efficiency is the ratio of braking force to vehicle weight, expressed as a percentage. The brake code is the same value, just a different name used in PSV testing.

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in bf | Total brake force | daN | 4500 | 0
in wt | Gross vehicle weight | kg | 12000 | 0
out code | Brake code | % | bf / (wt * 9.81) * 100 | 2
out eff | Efficiency (g) | g | bf / (wt * 9.81) | 2