The Rolling Road Myth buster
04 Jul 2018

The Rolling Road Myth buster

Rolling roads are a way to measure the power of a vehicle. However, they can also lead to many an unhappy and confused car owner as results can often be misread due to a general lack of understanding of the limitations, drawbacks and also benefits of a rolling road.

There are a few fundamental things you should know about rolling roads what I refer to as myths.

Myth #1 It is impossible for a rolling road to simulate real world driving. Rolling roads provide linear load on the engine and do so with a slow pull from low RPM’s to redline. It can take up to 45 seconds to reach redline. This amount of time and constant load on an engine and turbo charger will inevitably raise intake temperatures well beyond what the real world driving will see.

Myth #2 Rolling Roads are very accurate. It is very common for a car to have less rolling road performance and yet maintain strong power on the tarmac. This can be attributed to several factors. The fan we usually see in front of the vehicle when a vehicle is on the rollers is providing a linear flow of air (constant flow/volume/temp), usually to a small portion of the frontal area of the vehicle. If we were to compare this to real world conditions, we can prove airflow is proportionately increased as speed is increased. This isn’t replicated with the airflow on a dyno.

Myth #3 The Desk fan at the front of a rolling road somehow mimics the massive airflow over a car in the real world. This then leads to heat soak. A term used when an engine doesn’t get adequate cooling for the given running conditions and the ECU will be forced to compensate for the additional heat.

When a car suffers from heat-soak the ECU will typically go into an Exhaust Gas Temperature protection mode; lowering the requested load and increasing the amount of fuel in attempt to cool things down, this results in the curve tailing off much quicker than expected and a lower than realistic power output.

Myth #4 A rolling road magically calibrates for your car amongst dozens of others on a rolling road day. Really ? Unless you have booked a bay for a whole day to yourself  no account is made of any any potential issues that could be inherent with a car, the amount of heat already in the car before it goes on the rollers, or any inconsistencies between operators and rolling road calibration.

For Example: vehicle weight, number of cylinders, transmission type, air temp, air pressure. All factors not entered into the dyno software correctly that can lead to BS figures.

Myth #5 Are you really good at complex maths ? Wheel and flywheel figures can be a source of confusion, and there’s a danger of back-calculating flywheel figures from a chassis dyno. Power at the wheels is more meaningful and fairly accurate so long as the ambient and intake temps are reasonable. Certain rolling roads calculate force applied at the rollers, everything from there on is a mathematical equation and as such don’t necessarily give you accurate figures.

Myth #6  One of the biggest mistakes people make is to take a figure from a Rolling Road as gospel. There are so many varying factors between different dynos that can affect the output, (as mentioned above), that can differentiate vastly from one rolling road to the next. Realistically, a Rolling road can be a great tool to show differences from the fitment of additional hardware, but in the guise of a rolling road ‘shoot-out’ for a one off reading they are a waste of time.

Myth #7 The gospel myth – the number will be replicated all day every day.

“A figure or power curve only shows what the car is doing on that dyno, on that day, in those conditions.”

Myth #8  Using a Rolling Road to show the difference between cars or show the increase from software is a realistic check without correct preparation. If you have two identical cars running the same quality fuel, tyre pressures, etc. you can still have a variable within the ECU due to differing driving styles and conditions the cars see. One car might have been used much more aggressively than the other and have a much larger correctional factor due to adaptation from excessive heat. This can vastly affect the power output of a vehicle. Something else to be aware of is after programming an ECU the car will take a certain period of driving time to adapt (short/long term fuel trim), this period of time is dependent on driving style and conditions.

Myth #9 The Peak figures myth.  The peak numbers you get on a rolling road are typically referred to as ‘Pub Talk’ numbers; who has the most power and torque! In reality the Peak numbers are largely irrelevant; it’s the power/torque throughout the rev range and the power delivery that’s important, the peak number gives very little indication to how capable a vehicle is or how well it drives. Unfortunately there’s a lack of understanding in the industry and still a requirement to quote peak figures?

The general consensus is the higher the number the better, numbers sell. A 300hp car sounds much better than a 280hp car! But in reality what can you tell by these numbers?

First of all, quoted power and torque numbers are peak figures, they are the highest seen figures on a power or torque curve. They tell you what power and torque a vehicle is making on that dyno but those figures are really only useful for bragging rights. For a true indication of how a vehicle will feel and where the benefits of tuning can be seen you need to assess the power and torque curves

In summary if you come to us with a full technicians report on your vehicle carried by a recognised rolling road centre. Where you have hired exclusively the rolling road and calibrated it to your vehicle and carried out multiple tests over different days and conditions then let’s talk we are all ears.

If not frankly it’s just pub talk and we have never advocated drinking and driving.