Rolling Road Dyno

Measuring the Power

CRD Performance, Bolton, is one of a few specialist UK garages to own a rolling road (also known as a dyno’ or chassis dynamometer)

The 2WD dyno’ is frequently used for both diagnostic purposes and to measure calculated engine brake horse power (BHP) and torque.

There are two methods used to measure BHP and torque: A braked dyno’ or, the preferred modern method, an inertia dyno’. The dyno’ at CRD Performance is capable of both methods.

Braked Dyno

Braked DynoHaving ‘settled’ the vehicle and strapped it to the dyno’ the car can now be driven on the rollers in any gear and up to 125mph.

The operator chooses at which road speed or engine speed to apply a braking force to the rollers using the inbuilt eddy current brake.

Fault Diagnosis

If diagnosing a running fault at motorway speeds, for example, the rollers can be restricted to 70mph and, no matter how hard the operator accelerates, the rollers will not exceed 70mph. This fixed speed method can be used to carry out diagnostic tests and measurements to the ignition and fuel systems at various engine loads. In this mode we would normally observe exhaust gases, ignition coil output, fuel pressure and flow and even turbo boost pressure. Being able to choose any speed and engine load also allows us to to calibrate an LPG system or remap (tune) the engine’s power using, for example, the Dastek Unichip.

Estimating BHP

The more the operator presses the accelerator pedal or makes improvements to the engine power, the greater the BHP displayed on the dyno’s display panel. However, using a braked dyno’ for BHP measurements only displays the power at the wheels, not the power at the engine’s flywheel. Also, while a braked dyno’ is great for diagnostic work, mapping and before/after comparison testing, it is not suitable for accurately testing the vehicle’s BHP.

If, for example, a braked dyno’ were to be used to check the maximum BHP, the car would be rolled in the most ‘straight-through’ gear (normally 4th) at the required engine RPM. Let’s say max BHP is developed at 6500 RPM – then in 4th gear this may equate to 100mph, which the operator would find by gently driving the car up to 6500 RPM without locking the rollers.

Having now set the rollers not to exceed 100mph the vehicle is driven through the gears gently into 4th gear until 6500 RPM is reached and the braking resistance of the rollers felt. Now the operator can accelerate ‘flat out’ with the power at the wheels driving into the locked rollers, which causes a strain gauge attached to the torque bridge of the Dyno’ to move; the greater the power, the greater the strain gauge movement and higher the BHP on the display.

Now the BHP is known at the wheels we can estimate the flywheel BHP; Any losses, including transmission and tyre friction losses, have to be taken in to account. Typically, it is generally accepted, that front wheel drive cars lose around 20% through their transmission and tyres, while up to 25% loss is normal for rear wheel drive vehicles. These figures are only a guide since some well engineered vehicles, Porsche for example, are often claimed to lose only about 15%. Other factors effecting losses include non standard wheels and tyres, tyre pressures and adverse geometry (such as excessive negative camber or toe in/out).

Inertia Dyno

Rolling RoadEven though our Sun Ram X11 rolling road was originally designed to be a 300BHP braked dyno’, it has since been modified and upgraded to enable it to also work as an inertia dyno’. using the TAT software system, and is now capable of measuring up to 500BHP at up to 160mph.

The inertia method of measuring BHP does not rely, in most cases, on the eddy current brake to load the rollers. The TAT system, during installation, is configured to match the type of rolling road it is to be used with, so it therefore knows the mass of the rollers and can make its calculations accordingly.

After initial programming and calibration all the operator needs to do is drive the vehicle at 60pmh in any gear he chooses, (often top gear) and let the TAT dyno’ software know what the engine RPM is at 60 mph. The system automatically measures the workshop’s air temperature and takes a barometric pressure reading so that BHP calculation compensations can be made, be it a cold, damp day or a hot, dry day.

The operator would initially strap the car down, position the cooling fan next to the radiator and perform 2 or 3 runs to warm the tyres and transmission. During the power run it is normal to set off slowly in 1st gear to approx’ 2000 rpm, then 3rd gear to 2000 rpm, select top gear and then ‘floor’ the throttle and accelerate to the engines max’ RPM or rev-limiter. At max RPM or rev-limit the clutch is pressed (if a manual transmission) and neutral selected, then the operator waits while the vehicle coasts to a standstill, during which time the transmission losses are measured.

Rolling RoadSince the TAT system knows the engine RPM at 60mph and the mass of the rollers, it monitors the acceleration rate and deceleration rate of the rollers, from which it calculates engine torque and BHP at the flywheel. This method of calculation even allows for excessive transmission losses due to tyre temperature change during several repeat runs or even the vehicle being strapped down too tight.

We have used this inertia method hundreds of times and it never ceases to amaze us how accurate it is. We have frequently had customers come to us for power runs, claiming some wonderful exaggerated unrealistic power figures their vehicles have produced on someone else’s dyno’ – all I can say to these is that our dyno’ has done very little work from new and often measures non-modified standard cars whilst in our workshop of either an LPG conversion or to have a Superchip and the BHP measurements we see are very believable.

I did mention earlier that the inertia method does not use the eddy current brake most of the time, now let me explain the exception: When testing high-powered turbocharged vehicles it is essential that the turbo is made to work so that boost is achieved. Without any dyno’ braking the run could be over in just a few seconds and therefore maximum boost may not be achieved. The TAT system allows the operator to select a small and linear amount of roller braking throughout the whole run, to enable full boost and engine power to be realised. The amount of eddy current braking is electronically fed into the TAT software during the power run but is then switched off by the operator during deceleration to allow correct measurement of transmission losses.


You will be asked to sign a disclaimer prior to your vehicle going onto the rolling road.

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