Our technical analysis tracks down which compromises are due to the architecture and which come down to set-up
Subtle they were not – bewinged saloon cars with wheel arches on steroids, big fat slicks, 450bhp and a galaxy of F1 stars at the helm. A no-holds-barred BMW versus Ford contest set the European Touring Car Championship alight in the early 1970s.
Dickie Meaden has set the context and given us a vivid insight into the challenges involved in driving these cars on the limit.
When historic racer Grant Tromans asked GSD RaceDyn to look at his GAA Capri in late 2015, we were delighted – we had long wanted to get our hands on a Cologne Capri or a BMW ‘Batmobile’.
So let’s take a detailed look at the Cologne Capri and assess how its theoretical behaviour compares with Dickie’s driving impressions.
The pressed-steel chassis is seam-welded and reinforced (where regulations permit) at the key load-bearing points. A full roll cage adds significantly to torsional and beam stiffness, in addition to primary safety. However, the chassis must deal with loads generated by the wide slick tyres and powerful engine – fatigue cracks are going to be inevitable over the course of a tough season.
The 460bhp 24-valve V6 drives through a five-speed ZF syncromesh gearbox to a live rear axle located by four trailing radius rods and a Watts linkage. A ramp-and-plate LSD is used. Light, largely ineffective glassfibre leaf springs meet the regulatory requirement to retain the existing springing medium – but the real work is done by concentric coil spring/damper units. Front suspension is by McPherson strut, using lightweight factory=developed track control arms, compression struts and hub carriers.
Current tyres are 11.0/23.5/16 front, 12.5/25.0/16 rear.
The car is very much a homologation special and this shows clearly in the wind tunnel-developed bodywork and aerodynamic package, which includes a large front air dam/splitter, a large rear spoiler and sill skirts. Huge wheel arches cover the wide slicks. Water radiators and engine, gearbox and axle oil coolers are rear-mounted (for aerodynamic and weight distribution reasons), fed by large forward-facing ducts in the rear wheel arches. We don’t have wind tunnel data and we have not been able to undertake measurements, but we estimate that the car generates 360-370kg of drag and 80-90kg of downforce at 150mph.
Front suspension geometry and kinematics are generally good. Roll camber correction is less than ideal at 34 per cent – for every degree of chassis roll, 0.66deg positive camber is applied to the outside front wheel. Castor was high at eight degrees – this causes higher steering loads, but applies beneficial negative camber with steering lock. At this point we were not concerned with the high steering loads, because the car had electrically operated power steering. This would come back to haunt us…
Analysis showed that the key inherent problems with the car were its 55 per cent forward weight distribution, high centre of gravity, a very high rear roll centre at 304mm plus the torque reaction and high unsprung mass caused by the live rear axle.
As delivered, key set-up issues were: front springs 23 per cent stiffer than we considered ideal, rear springs 18 per cent stiffer than ideal, a front anti-roll bar that was far too soft and an unnecessary rear anti-roll bar.
Stability analysis suggested that the car, as delivered, would be unstable over bumps and would oversteer excessively under power in the corner-exit phase. When cornering on the limit, the Capri’s high centre of gravity causes weight transfer such that over 83 per cent of the car’s weight is on the outside wheels. Even a slight touch on an inside kerb will push the car onto two wheels.
The car was briefly tested as delivered – and both drivers confirmed the instability over bumps and excessive power oversteer.
The car was then equipped with softer springs, a much stiffer front anti-roll bar, revised differential ramps and re-valved dampers. The rear anti-roll bar was removed.
Stability analysis showed that this should give a precise, stable turn-in and good balance under power, the inevitable penalty being moderate mid-corner understeer. A test at Silverstone in early May 2016 was curtailed after only eight laps due to gearbox failure, but Dickie reported that the handling was much improved. Steering was good in fast corners, but a little heavy in slow corners – this with power steering.
With limited testing, the first race event exposed a number of problems. Scrutineers determined that power-assisted steering was not permitted – and with assistance removed, the steering was impossibly heavy. In addition, the brakes overheated and failed after only a few laps and throttle response was unacceptably sharp.
Mike Purse, Pete Johnson and the RaceWorks team improved the throttle response and solved the braking problem with improved cooling and ceramic barriers behind the pads to reduce heat transfer to the caliper pistons and fluid.
The steering issue was more difficult. Several factors contribute to steering ‘weight’ – tyre self-aligning torque, castor, steering axis inclination, scrub radius, steering ratio and steering wheel diameter. Analysing these showed that force at the steering wheel rim when cornering on the limit was 33kg. Research shows that the maximum force a fit person can exert at the steering wheel rim is 35kg. So the steering really was much too heavy.
Theory suggested that by reducing castor and scrub radius and chipping away at the other factors, we could reduce force at the steering wheel rim to 21kg – still heavy, but viable over a one-hour HTC race. The longer ETCC races would have been tough… With these changes implemented, the car has been consistently competitive and is a regular winner.
It is interesting to compare the Capri with the 24-valve BMW 3.5 CSL. In many ways, the cars are remarkably similar, using a front-mounted 24-valve six-cylinder delivering similar power. Both used McPherson strut front suspension, but the BMW’s semi-trailing arm rear is superior, giving a lower roll centre, less unsprung weight and avoiding torque reaction at the tyres. Published data shows the Capri to be 30-40kg lighter than the BMW at 1040kg. However, we have measured and analysed several Capris and BMWs – and on average, the BMW is 105kg heavier. In theory and in practice, the cars deliver similar performance.
Finally, both initially suffered from heavy steering – neither could run power steering. BMW’s solution was to move the stub axle centre line ahead of the steering axis – termed negative trail. This means that high levels of castor can be used, giving camber gain without making the steering heavy. The downside is that it is harder for the driver to sense the limit of adhesion. If insufficient castor is used, self-centring may be entirely eliminated.
It would seem that BMW drivers were given a – perhaps imperfect – technical solution to the heavy steering problem, but Ford drivers just needed to head for the gym…