With the new updates to Automation it has become much more accessible for building race and rally cars. So I thought i'd revise my guide on rallycars. fot this guide I will walk trough the making of an 70s car and explain the decision making and engineering that goes into making a car like this. You can use this guide for different classes aswell, I will touch on components and rules of different era's.

A rallycar chassis decides the style of driving your car will have, a short wheelbase has less inertia so the car will rotate quicker in corners and be more lively overall. a long car is more stable and easier to control but will struggle in technical stages. In FWD cars having the chassis as short as possible will ensure you can get the car pointing in the right direction as fast as possible and thus get on the throttle earlier. for RWD cars the engine position is very important. engine in the front and your car will be easy to control and catch over-steer. engine in the back and you will be able to quickly get your car rotating and have more traction but they spin very quickly.
For the suspension almost all options are viable except solid axles in the front. for mid and rear engine i'd advise using dubble wishbones in the rear. these increase camber at speed and can make the car recover from over-steer.
A little summary of the differences:
Solid Axel: a single beam with the wheels attached on each end. cant have camber and aways keeps wheels level with ground.
Mc pherson strut: basic independent suspention. compression results in wheels moving upward and inward while keeping camber.
semi trailing arm: mc pherson for the rear, only wheels now rotate around mounting in front of wheel
torsion beam: very basic independent suspention, instead of coil springs its just a beam with rods on the end that twists under compression. handels similar to semi trailing but cant carry power
Double wishbone: more advanced suspention that adds camber with compression.
multi link: highly advanced supention that adds camber and toe angle with compression.
Push rod: dubble wishbone with the weight moved inboard of the car

Engine tuning is very important for drive ability. A poor power band can make a car stall in hairpins.

NA engine's are pretty easy. Just keep the max torque at 1/2 to 2/3 of max rpm and tune the exhaust for max torque instead of power (usually one tick down). Also throttle response is good, so use a light bottom end and quite rich fuel.

Turbo's are more difficult. Lag can make the car stall easily. to get a good powerband use a low cam profile and high boost. make the compressor big enough so the turbo doesn't overheat and the turbine as small as possible without surging. AR should be so it just encompasses the power band. the low cam profile is crucial to stop the turbo from overheating.



The gearbox is just as important as the engine. for rally cars wheelspin is good, it enables you to powerslide and it gives more traction on gravel. so make the gears short. on most tracks you will never exceed 180 kph so going above that is pretty useless. if you have low power go even lower id say set max speed at 220 kph at 400 hp to 120 kph at 100 hp but it kind of depends on your engine. the worse your powerband is the shorter your gears need to be, The graph should have a flat line at the top, big gaps will make the car transition from wheelspin to traction and back, this will at best stall the car and at worst send you flying of the track. if you cant get this to work tune the engine less aggressively

In the early years the automatic locker is your friend, but because it can only be on or off you need to have quite a lot of rear grip.

I tested all promising tire compounds in beam on several surfaces, From hard long life, Chunky offroad and All terrain was All terrain the obvious winner, it pulled the most G on all surfaces in all directions. hard long life was equal on tarmac but a lot worse on light and heavy gravel, sand, grass or mud. chunky offroad did a bit better in mud but worse everywhere else. on Italy gravel hillclimb 1 chunky was 30 sec slower than all terrain (driver error was 3 sec over 3 runs each) it also pulled 0.8 g compared to 0.9 in all directions except braking, here it was 0.8 compared to 1.2

the tires work best with narrow profile with a high tire wall. 15 inch 630mm by 160 mm all round on my AWD Group B car

I also would not give the wheels a big offset. it doesn't really help performance and tucking them in protects the suspension from collisions, you also see this on real cars.

Rally suspension is very important and a bit weird, good tuning can make your car from an unstable slow mess to a nicely balanced, very fast rocket on any surface.
On tarmac you want your car low and stiff with lots of camber, just press race setup and fiddle with camber and roll-bars to get as much cornering G as possible.
On gravel its a bit different. a good setup keeps the tires on the ground as much time as possible, and makes the springs strong enough to absorb bumps. Because cornering speeds are much lower you don't need as much camber, between -0.5 to 1.5 is best.
Springs can be quite stiff, usually a bit above sport setup, to make the rear-end more stable you can lower rear spring stiffness to halfway comfort/drivability, this is useful on all chassis layouts but mostly for RWD and mid engine cars.
Dampers are much softer than track cars, depending on the stage they are between comfort and drivability
You can adjust the balance of the car by adjusting sway bars or camber. for sway bars making the rear stiffer adds oversteer, making the front stiffer adds understeer. extremes are bad so keep the values between 500 and 2000. also making it very asymmetric can give weird handling. the driven wheels are often softer than the non driven, this keeps them on the ground more in hard corners and allows you to get on the throttle earlier.