First Rule: Fix the end of the car that has the problem. If it’s the rear sliding around don’t go adjusting the front. Start with adjusting the back end to stabilize it first. You can then go up or down in unison to stiffen or soften the car.
Tires and Temperature
Getting the correct temperature/pressure combination is like fitting together a big puzzle. Stiffer spring = more temperature and stiffer roll bars = higher temperature and on and on. It all fits together and the tires are the only thing we have direct control of. It’s often the most overlooked part of tuning yet it is the most important.
I’ll use the recommendations that I have gathered from Pirelli for temperature as some manufacturers differ +/- 10° F. The optimal grip for a tire will differ by what compound you put onto the vehicle. The softer the tire the lower the temperature needed for optimal grip.
PSI needs to be 30-35 when hot. I generally settle with 33.
Temperature range can be from 180° to 210° F. A softer tire will lose grip more rapidly once it gets past 210° than a harder compound.
How does tire temperature relate to pressures?
Higher pressure = Lower temperature
Lower pressure = Higher temperature
This works in small amounts. On FWD cars in the game you can have a really high PSI in the rear and it will slide all over the place.
Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher
Some of the below quotes do not apply to FM5 with the high camber ratings.
Alignment:
Camber
You always want negative camber
Negative = More inside
Positive = More outside
There is a sweet spot in this setting that usually doesn’t change unless you move the Caster around too much once everything is set. You also always want more camber in the front than the rear. Rear camber isn’t as necessary as those tires don’t turn and I want them with a touch of negative camber when throttle is applied on exit.
Toe
Front toe favors a positive number (more responsive and better turn in) between .1° and .5°. Anything above .3° will cause excessive drag in the straights and gets really bad above 160mph.
Rear toe favors a negative number and becomes more responsive through turns at 1°. However anything over 2° and the back end loses a lot of stability in acceleration and braking. On bumpy tracks any amount of rear toe will cause the car to lose stability and you will have to make corrections even in a straight line costing you time.
EDIT: To clarify you shouldn’t use more than .3 degrees in either direction unless the circumstance is extreme.
Caster
Allows the tires to roll with the chassis giving a larger contact patch through a turn. It also helps to balance the temperature of the tires on tracks with predominant left/right turns.
A larger number here will also help absorb bumps on the track while decreasing stability as too much caster causes the tires to wander a bit.
I personally like a really high Caster (5.5-7.0) in the lower classes but on tracks with longer sweeping turns less caster is better… Most cars aren’t sensitive to caster changes.
Adjustments Decrease Understeer Decrease Oversteer
Front Wheel Camber More Negative More Positive
Rear Wheel Camber More Positive More Negative
Front Wheel Caster More Positive More Negative
Anti Roll Bars
Anti roll bars control how much side to side roll a car has. In theory you would want to minimize body roll as much as possible. This is especially true in the higher classes but on occasion in FM3 some body roll helped things. I guess we will see in FM4. The end of this one is long winded but helps to understand it all.
Rear ARB soften = decrease oversteer or increase understeer
Rear ARB stiffen = decrease understeer or increase oversteer
Understanding what the ARB’s do
“Limiting the lean of the body is good because it means that when you take a quick set into a turn, that the body isn’t still moving sideways after the tires are at their limits. Otherwise you turn in quickly, the tires grip, then the body finally finishes leaning, when it stops, the tires lose grip.”
“It limits camber changes. The camber directly impacts the angle at which the tire cross section meets the road and thus controls lateral grip. As the suspension compresses the camber angle generally changes relative to the chassis. With a normal McPherson strut that hasn’t been lowered, the camber goes from positive to more negative as the lower A arm swings out straight, and then back to positive as it swings up. That swing up into positive camber is BAD. At that point the chassis is already leaned over so the tire may be starting to roll onto its sidewall. Changing the camber even more positive is just nasty. A big sway bar will prevent the body roll in the first place, and prevent the suspension compression on the outside which causes the positive camber change relative to the chassis.”
“Here’s where it gets really tricky: If decreasing the size of the rear bar doesn’t help enough, the next thing you do is increase the size of the front bar. When the outside front compresses in a corner, it causes the inside front to compress and may actually lift that tire completely off the ground. The car is now sitting on 3 tires and guess where the weight that was on the inside front goes? Outside front? Some of it. The rest goes to the inside rear where we need more grip. The total weight of the car hasn’t changed. It’s just been redistributed, and a sway bar at one end, actually transferred weight to the other end of the car. Increasing the front bar then translates into more motive grip at the rear, and thus more acceleration”
Springs & Ride Height
From speaking with Dan the spring tuning and adjustments have changed drastically from FM3 to FM4. Every car is now different based upon what it is in reality as opposed to just one generic suspension being stuck under every vehicle like Forza titles in the past. (his direct words were not all suspensions are the same and the same springs weights don’t do the same thing on every single car)
Any change in the spring rate will change the ride height due to the rate of spring deflection. They also change the compression(bump) and deflection(rebound) ratings of the shocks.
I like using my springs to control the suspension travel and balance the weight of the vehicle. Aero also affects the weight on the car and needs to be figured in. Some tracks require a softer spring while others require stiffer. I have went away from using the below formula but will leave it here as a baseline since some like it. I just reduce the stock spring setting by about a 1/4 on front and rear for my base tune.
Ride Height
Keep this as low as possible without the car bottoming out
Damping (shocks)
The extension when the tire is unloaded (force back out) = REBOUND
The compression of a shock (making it smaller) = BUMP
Braking
Under braking the front axle will compress while the rear will rebound. As the weight shifts the front axle will have a higher velocity than the rear which is good because the front should settle before the rear. If the front does settle too quickly the tires in the front will lock a little premature and the same for the rear. Brake bias only masks the problem and moves it to a different portion of the track.
Acceleration
Under acceleration the rear axle will compress while the front will rebound (think of a see-saw). On a RWD for example if the rear squats before the torque reaches peak it reduces the time the driver has to feather the throttle as the tires begin to break loose. Try to balance the compression against the required grip of the torque. FRONT REBOUND SHOULD BE FASTER THAN REAR COMPRESSION.
FWD is different in a sense. For acceleration increasing the front rebound will give more grip initially while increasing rear rebound increases a sustained level of grip.
Downforce and Braking
Downforce
More at rear = more understeer and a stable back end
More at front = more oversteer and increased turn in.
More downforce = less speed in straights, more speed through cornering.
Don’t forget that any downforce adjustment = a spring adjustment to compensate for the added weight. Especially at higher speeds.
Braking
Braking pressure is a preference of the driver in my experience.
Your brake bias controls understeer and oversteer while braking.
More front bias = more understeer and can bind the front tires up to where you can’t trail brake if you prefer to.
More rear bias = more oversteer and can cause the rear tires to lock up faster than the front which is really bad. Will cause the car to spin. This can also be a preference as some like to go early in fast out and some find it easier to brake later and trail brake through the corner. Those two can create a full scale argument.
Differential Settings
Differential settings increase the amount of lock your inside wheel has while turning. At 100% both the tires turn at the same speed which on a rear differential creates oversteer (on FWD it eliminates torque steer). A higher differential will allow you to use the throttle to help turn around a corner but can cause mid-exit oversteer.
On deceleration when you lift off of the throttle one tire locks faster than the other. This can cause the back end to want to kick out over the locked tire in extreme settings (under 15%). A lower setting does help things and the further apart the accel and decel are on the same differential the more unstable the car seems to get in my experience.