Rolling Road
A Rolling Road is also known as a chassis dynamometer
F1 Automotive is fortunate to be one of only a few specialist garages, even throughout the UK to have one of these in their workshop.
We frequently use the 2WD dyno’ 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, these are either to use a braked dyno’ or the preferred modern way by using and inertia dyno’ F1’s dyno’ is capable of both methods.
Braked Dyno
Having settled and strapped the vehicle 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. If diagnosing a running fault at motorway speeds, the rollers may be restricted to 70mph ad 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 ignition and fuel systems at different 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 tune or re-map the engines power using for instance the Dastek Unichip or to calibrate an LPG system.
The more the operator presses the accelerator pedal or makes improvements to the engine power then the more BHP is displayed on the dyno’s display panel. Using a braked dyno’ for BHP measurements however only displays the power at the wheels and not the engines flywheel. A braked dyno’ is great for diagnostic work, mapping and before and after comparison testing to see if alterations are heading in the right direction but not to accurately test the vehicles BHP..
If for instance 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, lets 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” the power at the wheels driving in to the locked rollers 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, transmission losses including tyre friction losses have to be taken in to account. Typically it is generally accepted that front wheel drive cars tent to lose around 20% through their transmission and tyres whilst up to 25% loss is normal for a rear wheel drive vehicle. These figures are only a guide since some well engineered vehicles, Porsche for example are often said to lose only about 15%. Other factors effecting losses include non standard wheels and tyres, tyre pressure and adverse geometry, excessive negative camber or toe in / out for example.
Inertia Dyno
Even though our Sun Ram X11 rolling road was originally designed as a 300BHP braked dyno’ it has now been modified and upgraded to an inertia dyno’ using the TAT system and is 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 being used to brake the rollers. The TAT system when originally installed is programmed with which type of rolling road it is being used with so it therefore knows the mass of the rollers.
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 tell the TAT software what the engine RPM is @ 60 mph. The system automatically measure the workshops air temperature and takes a reading of the BAROMETRIC pressure so that compensations can be made in BHP measurements between cold damp days and hot warm and dry days.
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 RMP, select top gear and then “floor” the throttle and accelerate to the engines max’ RMP or rev limiter. At max RPM or rev limit the clutch is pressed (if a manual) and neutral selected and the operator waits motionless whilst the vehicle coasts to a standstill, during this coast down period transmission losses are measured.
As the TAT system knows the engine RPM @ 60mph and the mass of the rollers, it looks at the acceleration rate of the rollers followed by the deceleration rate and from this 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 how accurate it is. We have often 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 on 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 not 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.
Disclaimer
You will be asked to sign the following discalimer prior to your vehicle going onto the rolling road.
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Whilst every precaution is taken by representatives of F1 to ensure incident free vehicle testing whilst using the Rolling Road, the following points have to be brought to your attention:-
It is the customer’s responsibility to ensure the oil and water levels have been checked prior to testing. F1 will keep the engine at an acceptable operating temperature using an additional cooling fan if necessary.
Engine failures during Rolling Road Power Testing are extremely rare as long as the engine is in good mechanical condition. The load imposed on the engine and transmission is no different to that of normal road use.
An engine failure could occur for various reasons during Rolling Road Testing eg. Cam belt failure, due to wear or incorrect fitting, Cylinder Head Gasket failure, engine overheating, due to faulty cooling system i.e. blocked radiator etc., contaminated engine oil or restricted oil filter, i.e. lack of maintenance.
F1 therefore insist that the customer be fully aware of the above points and accept full liability should any mechanical failures occur during testing.
VEHICLE…………………………………………………………………………………………….
REG’ NO’……………………………………………………………………………………………….
MILEAGE……………………………………………………………………........................
I have read and understood all the above and should any mechanical failures occur during Rolling Road Power Testing I accept full liability.
SIGNED………………………………………………………………………………………………
PRINT NAME……………………………………………………………….………………………
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