Following the recent news that Volkswagen has taken engineering creativity to new heights to enhance its emissions results, the whole way that vehicle fuel consumption and emissions figures are calculated is being questioned (see also Automotive Testing). Why are quoted consumption figures unachievable in the real world? Does it have to be that way?
The problem is that car fuel consumption varies depending on a number of factors, with the driver arguably being most important. For example, if a hybrid is driven flat-out around a racetrack, while a sports sedan simply keeps up at the same pace, the sedan can actually burn less fuel than the hybrid. This is due to the hybrid being driven as fast as it will go, whereas the more powerful sedan does not have to work as hard to maintain the same pace.
Looking at fuel consumption from a vehicle engineering perspective, both temperature and humidity greatly affect engine performance, and aerodynamic drag changes when it’s raining.
Closer to home, the rolling resistance of a tire changes if it has to move road water out of the way, and the roughness of the surfaces also has an effect. For these reasons, among others, motor manufacturers test their cars’ fuel consumption under perfect lab conditions: dry, cold air; smooth steel drums; no water; and crucially, a driver carefully following an optimized routine without worrying about outside influences, such as traffic.
Though this approach gives consistent, repeatable results, it is not an accurate representation of real-world conditions. For that, the most obvious approach is to instrument a car, drive around on public roads and measure the tires’ rolling resistance. Unfortunately on-vehicle testing rarely provides useful data, as the rolling resistance is a very small measurement, often swamped by other forces and moments acting on the tire.
The same problem occurs when testing on most large Flat-Trac rigs, as they are designed to test the tire under extreme loads and high-slip conditions where massive forces are generated. So using the same rig to measure tiny rolling resistance forces under straight-line, zero-slip conditions is like trying to measure the thickness of a sheet of paper using a tape measure. Some Flat-Tracs might be reconfigurable with more accurate (but fragile) sensing equipment in an attempt to overcome this, but ultimately the internal resistances and complexities of the rigs often rule them out.
Camber Ridge – an all-new, ground-breaking tire testing facility currently being built in Charlotte, North Carolina – will likely offer
a solution by providing repeatable rolling resistance measurements on flat asphalt. This will be achieved by testing tires on articulating trolleys mounted to an oval test track. Watch this space.
A strong contender to improve rolling resistance measurements is trailer testing, where a rig including an instrumented hub is towed behind a car or truck. The trailer separates the test tire from the rest of the vehicle, isolating it from most of the outside influences that rule out on-vehicle testing. This means a highly sensitive rolling resistance hub can be used and accurate measurements can be collected during real-world conditions. Excellent! However, the motor industry needs to be able to test tires under identical conditions so that they can be rated and fairly compared against each other. It’s here where attempting to standardize trailer testing raises some problems. For example, what temperature should the tires be? Whichever temperature is chosen, what do you do when the weather at the outdoor test facility is not at that temperature? What do you do when it’s raining?
These limitations are why the current standards for testing rolling resistance are all based on drum testing. Drum rigs can apply all the necessary load cases to the tire, including camber and toe angles. They can also be used to warm the tire if needed and many can be fitted with asphalt tiles to better represent the road surface. The curved surface of these drums will affect rolling resistance measurements and of course, 1.2m diameter drums are not entirely representative of real-world conditions. However, in the absence of an all-round better alternative, it appears that drum testing remains the best overall solution. Better the devil you know than the devil you don’t. tire
Gregory Smith is the director of Tyre CAE & Modelling Consultants, providing tire testing and modeling services to the OEM and motorsport industries. More information at www.TyreCAE.com
