Brake Efficiency Calculator turns roller brake tester readings and test weight into service brake efficiency, parking brake efficiency and axle imbalance, checked to UK MOT limits.
How the Brake Efficiency Calculator Checks a Roller Test
The Brake Efficiency Calculator turns roller brake tester readings into the efficiency figures used in the UK MOT. Garage technicians, MOT testers and owners use it to see whether a car, van or minibus passes on service brake efficiency, parking brake efficiency and side-to-side balance.
Pick the vehicle type, then enter the test weight and the brake effort at each wheel from the roller tester printout. Add the left and right parking brake readings as well.
Weight and effort can be entered in kilograms and kilograms-force, or pounds and pounds-force. The efficiency math works the same either way, since it is a ratio.
The Brake Efficiency Calculator Formula
Brake efficiency is the total braking effort as a percentage of the vehicle’s weight.
$$\text{Efficiency} = \frac{\text{Total brake effort}}{\text{Vehicle weight}} \times 100$$
With the defaults, the four service brake readings of 320, 300, 170 and 150 kg add up to 940 kg. On a 1,400 kg car, that is 67.1% efficiency.
The effort goes on top and the weight on the bottom. Some online versions divide the other way, which gives impossible results well over 100%.
A common mistake is entering effort readings in daN or newtons while the unit is set to kg. Roller testers vary, so check the unit on the printout before you type.
MOT Minimums by Vehicle Type
The Vehicle Type menu sets the minimum service brake efficiency. Under the MOT inspection manual, cars first used on or after 1 September 2010 need at least 58%, a limit that took effect in June 2013.
Older cars and N1 vans use 50% in the tool. Minibuses and buses first used from 1968 use 50%, and those used before 1968 use 45%.
The first card of the Brake Efficiency Calculator shows how far you sit above or below that minimum. At the defaults, the car clears 58% by 9.1 points, and it would need only 812 kg of total effort to pass.
The same card shows the level the tool treats as dangerous, which is half the minimum. For a 58% car, that is 29%.
Side-to-Side Brake Imbalance
Imbalance compares the left and right wheels on the same axle. It uses the difference as a share of the stronger side.
$$\text{Imbalance} = \frac{\text{Higher effort} – \text{Lower effort}}{\text{Higher effort}} \times 100$$
The MOT limit is 30% on every axle, measured at maximum effort whether or not the wheels lock. A steered axle with more than 50% imbalance counts as a dangerous defect.
At the defaults, the front axle shows 6.3% imbalance and the rear shows 11.8%. In the Brake Efficiency Calculator, the second card shows the worse of the two, plus the share of braking done by the front axle, 66.0% here.
The tool skips the imbalance check on an axle when both wheels read 40 kg or less. The alert then asks you to check those low readings, since a wheel with very weak braking can still fail.
Parking Brake Efficiency
The third card of the Brake Efficiency Calculator works out the parking brake the same way, using the two parking brake readings. The MOT minimum for most vehicles is 16%.
At the defaults, 150 kg and 130 kg give 280 kg, or 20.0% efficiency. The imbalance between the two sides is 13.3%.
The card also turns efficiency into the steepest slope the parking brake could hold. A 20% brake holds about a 1 in 5 slope, and the 16% minimum works out to about 1 in 6.25.
The tool treats a parking brake under 8%, half the minimum, as dangerous. The pre-1968 minibus option has no parking minimum in the tool.
Efficiency as Deceleration
Efficiency and deceleration are closely linked. A brake efficiency of 67.1% equals a deceleration of about 0.67 g.
The fourth card uses that to estimate braking distance. At 0.67 g, the car stops from 30 mph in about 13.7 m and from 60 mph in about 54.6 m.
Those figures assume steady braking and exclude driver reaction. Real stops vary with tire condition, road surface and reaction time.
To work the other way, from weight, speed and stopping distance to the force at the tires, the braking force calculator covers that and checks it against tire grip.
What the Numbers Cannot Show
The roller test also checks for binding, grabbing, lag and effort that fluctuates as the wheel turns. The Brake Efficiency Calculator cannot see any of those from a single reading.
A second rule can also help a car that falls short. If more than half the wheels lock on the rollers, the efficiency requirement counts as met, and the alert notes this when the result is under the minimum.
Test weight must be greater than zero, and each effort must be zero or more. Anything else gets flagged in red.
Reading Mistakes to Avoid
Using the gross vehicle weight from the plate instead of the test weight lowers the Brake Efficiency Calculator result. Use the weight the tester recorded for the test.
Typing a rear reading into a front box changes both axle results. Match each reading to its wheel on the printout.
Adding the parking brake readings into the service brake total inflates service efficiency. Keep the two sets separate, as the tool does.
Brake Efficiency Questions
How do you calculate brake efficiency?
Add up the brake effort from all four wheels, divide by the vehicle weight, and multiply by 100. For example, 940 kg of effort on a 1,400 kg car is 67.1%. The Brake Efficiency Calculator also checks the result against the MOT minimum for your vehicle type.
What is the minimum brake efficiency for an MOT?
Cars first used on or after 1 September 2010 need at least 58% service brake efficiency. Older cars need 50%. The parking brake needs at least 16%, and no axle can show more than 30% imbalance between left and right.
What causes brake imbalance?
Uneven pad or shoe wear, a sticking caliper or wheel cylinder, a seized parking brake cable, or a hydraulic problem on one side are common causes. An imbalance makes the car pull to one side under braking. Over 30% fails the MOT, and over 50% on the steered axle is a dangerous defect.
What does 16% parking brake efficiency mean?
It means the parking brake produces a braking effort equal to 16% of the vehicle’s weight. That is enough to hold the vehicle on a slope of about 1 in 6.25. A reading of 20% holds about 1 in 5, which is a steeper slope.