Features - Technical
OCTOBER 17, 1998
Differential braking
BY PETER WRIGHT
It is not every day that technology flows from Super Touring Cars to Formula1 but, with both Benetton and Williams testing front axle differentials, this is exactly what has happened.
Having raced and won with their 4-WD A4, until prevented from doing so in 1998 by the Technical Regulations, Audi understand just about everything there is to know about individual wheel torque control. Masters of the centre and axle differential, they have explored almost every mechanical and electronically controlled device in racing (STC and DTM) and rallying, as well as on their Quattro road cars. They know from experience that when tyres are near their limit under braking or accelerating, control of torque across an axle can be used for stability and directional control. Forced to go FWD in 1998, Audi were confronted by the tendency of FWD racing cars to lift and lock their inner rear wheel while braking into a corner. A locked rear wheel is a product of a forward weight distribution and the need for a high rear roll stiffness to counteract understeer. However lightly loaded, a locked wheel will disturb the car as it regains contact with the road and attempts to re-establish traction, ultimately limiting the useable braking effort at the rear.
Audi's solution was to apply 4-WD technology to solve the problem - they connected the two rear wheels together with half-shafts and a viscous differential. They also drove the alternator from one half-shaft, moving its mass to the rear of the car and perhaps hoping to disguise the true reason for the system. The viscous differential transfers torque whenever there is a speed difference between the wheels, enabling the brake torque from the unloaded (slower) wheel to be transferred to the outer (faster) wheel, rather than locking it up. Shortly after Audi's competitors discovered exactly what they were doing, the system was banned in the BTCC.
Around the same time McLaren had sharpened everyone's minds in Formula1, to the potential for steering the car using differential braking on the rear wheels. In fact they had done this to such an extent that Ferrari protested the system and it was banned. However, it was inevitable that thoughts subsequently turned to other ways of controlling braking torque across the car, and Audi's solution caught the attention of those Formula1 teams that were connected to STC teams. The gains to be had from the type of system that Benetton and Williams are rumoured to be developing for the front axle, are slightly different from the system used in STC.
Much of the difference between a qualifying lap and a race lap is the speed held by the driver as he brakes and turns into a corner. This is also the time when most overtaking occurs and where most loss of control takes place. Anything that can provide more braking, without an associated loss of stability, will be a large potential advantage as it contributes directly to grid position and to the gaining of places in a race. The rear brake proportion is limited by locking up the rear wheels with the inevitable consequence of a spin. Much effort went into variable fore-and-aft balance systems, to provide optimum rear braking as the speed and downforce reduced, but they got out of hand and were banned. More braking on the front axle stabilises the car up to the point where the inner wheel locks. A high front roll stiffness exaggerates the problem by increasing the load transfer away from the inner wheel. How hard the driver can brake as he turns in, is limited by how much of a disturbance he can tolerate as the inner wheel locks and then grips again.
Connecting the front wheels together, as per the Audi rear wheel system, potentially improves this situation. Just what kinds of differential the two Formula1 teams are testing is well covered up down in the depths of the footwells. However, a fairly stiffly set-up viscous differential would not be a bad place to start. No doubt, if the system proves to be a worthwhile advantage, electro-hydraulically controlled front axle differentials will become the norm. Anything that researches and develops technologies that improve stability and control is fruitful work and has applications to road cars. The loss to the TV-viewer will be the disappearance of the smoking front tyre to indicate just how hard a driver is trying.
Shortly after this was written, the FIA Touring Car Commission has stated that "ÉÉthe homologated drivetrain must be retained. As a 2-WD car does not have the undriven wheels coupled they cannot be coupled under Super Touring regulations with or without a differential." That provides no problems for Formula1. However, the same statement continues "É..Coupling the undriven wheels would also contravene Art.262.4.6.9 which prohibits anti-lock brakes." This view is not shared by the Formula1 technical department, as the same could therefore be applied to the rear axle and its differential.
The difficulty with writing regulations is that they can only ever cover that which has been imagined up to the point in time when they are written.