4 wheels speeds tell you 90% of the story
All is about getting the right balance
Understeer and Oversteer Balance
Car balance is described by the driver as oversteer (rear sliding first, leads to spinning if uncontrolled) or understeer (front sliding first, rear ‘pushing’ the front of the car, slows the car down a lot and stops the driver from going back on power early on exit). Understeer will make the car go slower around a lap but helps the driver keep control of the car.
In 2015, Nissan's ground-breaking GTR LM Nismo had a WD of 65%!
First order parameter in car behaviour is how much static weight is reacted by the front tires relative to total weight. A ‘balanced’ racing car has a weight distribution of anything between 44% and 48%. For example, Formula E have got a low weight distribution of 39.5% due to the weight of the battery. This rearwards weight distribution leads to an oversteer balance on entry and exit of corners, a tendency to overheat rear tires and sliding of front tires in mid corner (understeer). This is far from ideal as the balance shifts from oversteer to understeer to oversteer from entry to exit of the corner. P424 is a 4 wheel drive car with a WD close to 48% which is a much more balanced platform to work from.
Other very important parameter in car behaviour is how much downforce is reacted by the front tires relative to the total downforce. ABal is generally kept 3-4% lower than WD to ensure that the car remains stable in high speed corners. So for example with P424 we would target ABal of around 45%. Generally on track we are trying to push the ABal as far forward as the driver can cope with as this is done by adding total downforce on the car (more front wing angle). When grip is low (rain) the ABal needs to go rearwards by 2-3% to help the driver control the car with a bit more understeer into it.
Formula 1 oversteering
Roll Stiffness Distribution
Third essential parameter in car behaviour is how much lateral load transfer is reacted by the front tires relative to total load transfer. KRollBal is always >50% (front stiffer than rear) mainly to keep the rear soft and help with traction performance at the exit of corners (keeping both rear tires evenly loaded is required to get the power to the ground). The fine tuning of anti-roll bars is generally a fast setup tool during sessions to adjust the balance of the car. P424 KRollBal baseline is 62.5%.
Vehicle Dynamics Optimisation
Generally to have a fast car, it is best to keep all those parameters away from extreme numbers. A well balanced car will always be better in various tracks and conditions. To achieve balance, the first challenge during the design process is to make sure the weight distribution is correct. For electric cars it means using a 4 wheel drive system, a long wheelbase car (P424) to increase the distance between the battery + MGU and the rear axle.
Studying wheels speeds enables to judge if the car behaves correctly: