Millimetres and degrees
Once the Impreza WRC2007s have been rebuilt before an event, the Subaru World Rally Team mechanics move to the next stage of on-event preparation. The corner weights, camber, tracking and ride height of each car are all carefully measured.
the suspension components have been replaced on the cars, it is essential to ensure the geometry is exactly as intended. If any aspect of the above is incorrect or mismatched, it can result in uneven and accelerated tyre wear and unbalanced handling. In an environment in which the team are chasing tenths of a second over stages of up to 30km, this cannot be afforded.
The first step is to drive the Impreza onto mobile car scales, which consist of a frame of flat aluminium raised a few inches from the floor, the same size as the wheelbase of the car, so that each wheel sits in a corner on an electronic scale. The scales are height-adjustable at each corner, so that the team, with the aid of a spirit level, can achieve a flat floor on just about any surface. Again, this is essential as if the car is not sitting absolutely flat, the measurements will not be reliable. The scales are so important that they have a permanent bay in the SWRT service area for the duration of every rally.
Once the car is flat on the scales, the measuring can begin.
By interpreting the figures from the four scales, the mechanics can see exactly what the weight distribution of the car is. The team tends to set weight distribution by adding ballast to the floortray where necessary. Each front wheel needs to match, as does each rear, so that weight balance is equal between driver's side/passenger's side. Think of it as ensuring your passenger sitting up front is exactly the same weight as you in order to ensure optimum lateral distribution.
Having completed weight distribution, camber comes next. Using specially designed aluminium brackets that clamp to each wheel hub, the mechanics can reliably measure the camber angle of each individual wheel. Camber is the degree to which a wheel leans from vertical when measured along the wheel's face. If you were to measure the distance between the top and bottom edges of the front wheels, across the engine bay, negative camber would mean the top edges would be closer and positive vice versa. In reality however, digital camber gauges are used to measure the line of the aluminium brackets for complete accuracy.
If you would never have thought of the above two for your roadcar, then the next measurement will be far more common, although perhaps not conducted in the same way. Every time you have new tyres fitted the garage will track your car; that is to ensure all four wheels are pointing parallel to each other to avoid scrubbing and excessive wear (save a degree of intended toe-in or out). The same procedure is conducted on each Impreza WRC2007, but measured carefully by hand to ensure pin-point accuracy.
With horizontal tracking bars fitted to the bonnet and boot lid, bowline (like fishing line) is run along the length of the car between the two tracking bars. This is used as a reference point to measure the leading and trailing edge of each wheel from, immediately determining the track angle of each wheel. Think of camber as the difference from vertical, and tracking the difference from horizontal.
Finally, the ride height of each Impreza WRC2007 is measured, and this of course is again where having a flat floor is absolutely essential. Using fixed reference points in the suspension and on the chassis, the difference between the two is calculated as the ride height.
It is important when it comes to the difference between gravel and asphalt setup, where ride-heights can differ by as much as 70-75mm, but it also provides an important parameter to the engineers. They will use the measurement to ensure suspension geometry and angles are set correctly to get the most efficient use of the dampers, making the car more balanced and tractable over bumps.
So next time you visit your local tyre garage, specify all the above and see their reaction!
Subaru World Rally Team