[Physics Update] GTO - 18/12/2021

Note: the #1 rule here is that old setups will not work with any updated cars. Trying to use these setups will result in a very poor handling car. They must be deleted!

Introduced by IMSA in 1971 the GTO class catered for grand touring type cars of 2.5L displacement or more, the letter O referring to "over 2.5L. By the time the early 80s had passed the 2.5L limit had been increased to 3.0L with a maximum capacity of 6.0L.
Performance equalisation was achieved through a sliding scale of engine configuration and allowed weight. For example, 3.0L cars were required to weigh 860 kg whereas 6.0L cars had to weigh no less than 1200kg. Turbocharged engines with two valve heads were required to run 15% heavier, while cars with four valve heads were required to run 20% more weight.

Chassis design was free, typically tube frame and were strikingly similar to Group 5 cars under their skin. Aerodynamically they were relatively simple and ground effects were not allowed. The car's general shape was required to closely resemble the production car, although track could be widened and fenders reprofiled. Bodywork was often fibreglass for ease of repair and low cost with only the production car's steel roof retained.


Tyres

By the mid 80s the transition in motorsport from cross-ply to radial tyres was in full swing. Top-line American motorsport was no exception, fuelled by fierce competition between several manufacturers. Amongst others, BF Goodrich, Goodyear, Hoosier, Firestone, Yokohama and Avon were all battling in a development war. Tyre technology was often shared with the faster GTP and Group C cars, and ours in game are also closely related. Closer to a modern GT tyre, these are very stiff tyres which are a leap forward from the older Group 5 crossplys, however they don't quite have the 'stick' of a current tyre.


Suspension

The biggest development here compared to the earlier Group 5 cars is in the damper department. In the early 80s various inventions enabled dampers to engage a different lower damping rate when suspension velocities increase above a certain threshold. Inside the dampers ball bearings held in place by springs are pushed away from the holes they cover, allowing more oil to flow inside the dampers under certain conditions, thus changing the damping rate. What this means to the driver is that the car can handle curbs, bumps and jumps much better. The damping rates don't have to be such a compromise any more.

Now, a quick introduction to the cars one by one;


Audi 90 Quattro

Quattro is the name of Audi's four wheel drive system which had already conquered all in the World Rally Championship. Now the company were looking out for other ways to prove their concept to the world. Powered by a 2.2L 20-valve 5-cylinder engine which was mounted well forwards of the front wheels, it also made use of a huge turbocharger to kick out silly power numbers without breaking a sweat. It was developed with a 6-speed gearbox which was actually not permitted by the rules so had to be blocked off in order to comply. Tyres were an incredible 355mm wide all round, which actually makes them the widest front tyres ever fitted to a race car. The cars only raced for one year, skipping the early season endurance races of Daytona & Sebring. They won many races but did not win either championship. At the end of the season Audi's board of directors had decided to concentrate on the domestic DTM series, so the GTO project was shut down.




Ford Mustang

Produced in plentiful numbers the Mustang can be considered a very typical GTO car: small-block V8 up front with a live axle suspension at the rear. These cars were a fixture in the series for almost a decade.
As a none-turbo car it was permitted to run at a lighter weight, which allows it to shine on twister tracks. It's lack of turbo lag also makes it an easier car to race others with. Aero is simple but works, with a huge fixed spoiler on the rear deck pushing the rear tyres into the ground.
In later years a new Mustang was developed which made use of a smaller 4-cylinder turbocharged engine, in order to take advantage of weight breaks.
NB: A live axle links both the back wheels together, so bumps on one wheel will affect the other. There is also a matter of torque reaction across the axle, which when accelerating pushes one wheel into the ground while lifting the other. This gives you a slight yaw to the right when hard on the power.



Nissan 300ZX

A personal favourite of mine designed by one of my heroes, Trevor Harris. He's the guy responsible for the Shadow Can-AM cars as well as the dominant Nissan GTP and Nissan NPT series cars of the GTP category.
These cars were commissioned by Nissan as a way of promoting the newly released 300ZX road car, and visually they were probably the most similar to their roadgoing counterparts.
At the heart of the beast is the Electramotive Engineering produced aluminium-block 3.0L V6 with two turbochargers. Proven to be bulletproof in the GTP cars, they were an absolute powerhouse in the GTO category too.
Trevor Harris's design concept called for a stiffer anti-roll bar setup than other cars, which we've represented in RaceRoom. That philosophy allowed for softer road-springs than usual, which gives for better compliance over uneven surfaces.
Aerodynamically there's not much to differ it from the other cars in class. The rear wing also being fixed and not adjustable.




Some more stuff

It's worth noting that the fuel tanks of all these cars are all mounted between the rear wheels and not close to the centre of the car as is normal these days. This means that the centre of gravity is subject to more change than you might be used to as fuel is burned off.

Now some cool videos:

 

Simply to accommodate the gearbox, front differential and drive shafts. Their options were either to put the engine above the gearbox, which would be a bad move, or they could put the gearbox behind the engine.
Nowadays it's not so much of an issue as 4wd systems use twin prop shafts and geared drivelines which allow the drive shafts to sit either side of the engine. Back then they either hadn't thought of that, were trying to keep weight down, or were trying to keep the system (relatively) simple.




Wheel covers were bang on trend in the 80s on sports cars. They were a way of smoothing the air flow over the wheels and providing increased brake cooling as they doubled up as extraction fans for the heat produced by the brakes. They were used on everything from Porsche 962s to garagiste specials:

 

In this situation the cars with engine maps are balanced with cars which do not on the default map mode, which is usually #3.
Conversely cars with adjustable engine maps actually have a huge advantage in qualifying.