It all starts with the build, and the tune is the polish. If the build (upgraded car) is a turd, the tune polish the turd. You will still have a turd, but it will be shinier! Most vehicles cannot be built/tuned to do everything and remain competitive, and most vehicles are either predisposed to either grip (momentum) or speed (power). Very few cars can be both, and even fewer can be both at the same time! Build to the vehicle's strengths, and shy away from making shiny turds!
These are all the parts that should be completed first
Get the car to the handling number FIRST. This will usually indicate whether or not the build is going to work for the class you have chosen.
The idea here is to install all of the non-weight affecting parts first, then boost, then weigh reducing, and finally the rest until you can no longer install any combination of engine parts within max PI range.
Use these parts to fill in the last bit of PI. Rim diameter can be done here too.
If you are not experienced at tuning and/or you are intimidated by tuning in Forza, there are tuning calculators to assist in generating a good base tune (starting point) for you. Up 2 Speed Customs has one that is free. Use the U2SC Forza Tuning Calculators here! If you are willing to spend the time learning, then here are some notes on where to begin as well as fine-tuning once you have a good base tune.
Also, remember to tune for YOUR driving style. What works for you will not necessarily work for everyone else (and vice versa), but keep testing new ideas and learning. The only way to become a better builder/tuner is to try new methods and experiment.
Tire pressure is used to provide optimal contact patch and tire temperatures in Forza. Start with your tires at 30psi and do 1 lap on your test track. Once you begin lap 2, open up telemetry and watch both the "Tires Misc." and "Heat" telemetry pages. For "Stock" and "Street" compounds, the optimal temperatures in Forza is between 180-195 F. For "Sport" & "Race" compounds, the optimal temperatures in Forza is between 200-210 F.
The optimal starting point for spring rate is to take half the total weight of the car and then multiply it by the front weight percentage. That is the base front spring rate. To get the rear spring rate, simply subtract the front spring rate from half the total weight. The equations look like this
Ride Height is one of the more powerful settings. The best way to start is to leave the car at "stock" ride height and then watch the "Suspension" page in Telemetry on your test drive. What you are looking for is the lowest ride height possible without bottoming out (or getting too close to bottoming out) the front and rear suspensions. You do not have to keep the sliders symmetrical.
For Alignment, there are a few factors here. The best thing is to have starting ranges for each drive type, and then adjust the values when fine-tuning. Here are some suggested base values:
For dampers, you want to start with Bump. While test driving keep lowering bump until you feel the car start to bounce or move with the bumps in the road. You are looking for a setting the is just above the point where the vehicle transitions from absorbing bumps to moving with/reacting to them. Some vehicles are stiff enough that your are looking to minimize this as much as possible. For most vehicles, Bump will end up somewhere between 2.0 and 4.0, but there are exceptions. Once you have the bump, the rebound should be set to between 2-3 times the bump (i.e. if front bump is 2.5, then front rebound should be between 5.0 and 7.5). Rebound is also instrumental in fine-tuning the vehicle, but more on that in the Fine-Tuning sections below.
Anti-Roll Bars or ARBs, are used to increase sustained lateral grip while cornering. The values here can vary quite a lot, and are based on several factors.
Aero settings will be dependent on several factors like track, class, and personal preference. The best place to start is to set to minimum and adjust during fine-tuning.
Brakes are where the most driver preference exist. Ultimately you want the front and rear brakes to lock up together under hard braking.
There will quite a few factors like drive type, weight balance, engine power/torque, and grip at the drive wheels. The best things to have are starting ranges for each drive type, and then adjust the values when fine-tuning.
Here are some suggested base values:
Use the stock setting and adjust the final drive to get the best gearing. Play with settings here to find the best values for each vehicle you tune.
Choose the right test track! If the build is a grip/momentum build, then choose a grip/momentum track for testing. Testing a grip build on a speed/power track will not yield the best fine-tuning results. Make use you make a note of the general lap times you run in each class or division so you can compare different builds too.
Here are some of the better test tracks:
Understeer: the car does not turn enough and leaves the road
(the items below are ranked in order of effectiveness)
...at Higher Speed
...at Lower Speed
* Does the Front suspension bottom out?
** YES the front does bottom out.
Oversteer: the car turns more sharply than intended and could get into a spin
(the items below are ranked in order of effectiveness)
* Does it Oversteer while Braking?
** YES it does oversteer while brakeing
...at Higher Speed
...at Med-Low Speed
* Does the Rear suspension bottom out?
NOTE: The information below is pretty much word-for-word, with a few formatting/consolidation changes, from the Forza Motorsport 7 In-game Help Text! This information assists in understanding the impact of each tunable componemt on the vehicle, and what whappens when each component is adjusted.
All of a car’s handling, acceleration, and braking must travel through the tires to reach the road. Tire pressure is an effective way to adjust peak grip, responsiveness, and wear. Test the pressure changes in small increments, because 1 or 2 PSI can make a big difference in grip.
Excessively high or low pressures can adversely affect the contact area of the tire with the road and overall friction of the tires. Low tire pressure reduces overall responsiveness while high tire pressure can result in skittish handling.
It’s best to fine-tune tire pressures after a few laps, since tires will heat up during the race making the air inside expand, increasing pressure.
Front-to-rear balance is important, too. Heavier braking puts more heat and pressure in the front tires. Try to balance for each.
The Final Drive is the last multiplier or torque before it reaches the wheels. As such, it has an effect in every gear.
Adjusting the Final Drive ratio impacts acceleration and top speed. A higher ratio provides better acceleration while a lower ratio provides better top speed.
It’s best to choose the ratio where the car reaches its top speed in it’s highest gear, near redline. This provides maximum acceleration while still allowing the car to reach top speed.
Adjusting individual gear ratios impacts acceleration and top speed. A higher ratio provides better acceleration while a lower ratio provides better speed.
The difference between each ratio changes how far the engine’s RPM will drop when shifting. Adjust the ratios to keep the engine at peak power as much as possible. This usually means keeping the ratios close together, but not so close together, but not so close that the car can’t accelerate off the line or achieve top speed.
Camber refers to the angle of the tire relative to the ground. Adjusting the camber impacts grip in a balance between cornering and a straight line.
As a car corners, the area of the tire contacting the ground is distorted. By adjusting camber, drivers can correct this distortion and improve cornering. However, peak braking and acceleration occur at zero camber, with the tire straight up.
Start with about one degree of negative camber (-1.0) front and rear, then tune from there for peak values.
Remember, camber changes dynamically during suspension travel and every car is different.
Adjust toe (the inward or outward angle of the wheels) to sharpen turn-in response (the transition between driving and straight ahead and turning).
Avoid extremes, because excessive toe-in or toe-out can wear tires quickly.
Caster refers to the angle of the steering axis from the front to the back.
Increasing positive caster pushes the top of steering axis rearward, increasing camber as the wheels are turned. This allows for less front camber, keeping the tires perpendicular to the road when traveling in a straight line, which is optimal for braking.
Too much positive caster will create an undesirable amount of camber while turning, resulting in unpredictable handling.
Anti-Roll Bars (also called anti-sway bars or ARB) provide extra roll stiffness when cornering by tying the left and right suspension together. This allows control of body roll without affecting load transfer under braking and acceleration.
Front ARB tuning is most effective at turn entry when the front of the car is loaded, as it is in braking.
Spring stiffness controls how the car’s weight is transferred under acceleration, braking, and cornering.
Ride height determines the car’s ground clearance and center of gravity.
Lowering ride height lowers the center of gravity, which improves cornering. However, lowering it too far can cause bottoming out and sudden loss of control. Generally, drivers should lower their ride height as much as possible without bottoming out.
Rebound damping controls how quickly the car’s suspension expands when the wheel moves away from the car, as the inside wheels would during a turn or the rear wheels would during braking. It determines how quickly load is transferred in transitions during turning, braking, and acceleration.
The best practice is to set the bump-damping stiffness first, then move to rebound damping. Lower the car’s rebound damping setting until the car transitions harshly through tight chicanes. Then slowly raise it until these transient movements smooth out.
Drivers can also use rebound damping to aid in straight line grip.
Experiment with different damping stiffness for the front and rear to find peak grip.
Bump damping controls how quickly a car’s suspension compresses when the wheel moves towards the car, as it would when going over a bump. It controls the movement of the unsprung mass and keeps the tire in contact with the ground.
Start with a low setting for both bump and rebound damping then slowly increase bump damping to smooth tire contact with the ground across bumps, curbs, and harsh transitions. Increasing front/rear bump damping too high will cause the tire to skitter across bumps, resulting in a loss of grip.
Once bump damping is set, move to rebound damping.
Air flowing over a car creates lift, which reduces grip and impairs handling.
Increasing downforce will press the tires onto the road, improving handling while heating the tires more quickly. However, increasing downforce increases drag, which reduces speed.
Carefully adjust downforce to balance the handling of the car at high speeds.
A car’s level of grip and the way it handles weight transfer under acceleration or deceleration impacts its brake balance. As drivers upgrade and tune their cars for better performance, they will probably need to adjust brake balance to maximize brake performance.
By controlling the relative distribution of hydraulic pressure between the front and rear brakes, brake balance impacts which tires lock up first under heavy braking. This impacts braking distance and understeer/oversteer balance during trail braking (braking while turning in).
Avoid extremes, which can increase braking distance and cause instability.
Adjust overall brake pressure so the tires will lock under hard braking, but won’t lock prematurely with just a small amount of brake application.
The differential allows the tires on each side of the car to turn at different rates, since the inside tire travels a shorter distance around a turn than the outside tire. A limited-slip differential limits this difference in speed by equalizing the torque going to each wheel, providing maximum traction under acceleration and/or deceleration.
Adjusts the amount of torque that will transfer to a wheel first.
The Acceleration setting adjusts how much torque can be applied from one side of the car to the other when accelerating.
Adjusts the amount of torque transferred from side to side when the wheels slips.
The Deceleration setting adjusts how much torque can be applied from one side of the car to the other when the engine isn’t under load.
Increasing the Deceleration setting makes the differential tighten harder and more quickly under deceleration, which in small amounts can stabilize the car.
A center differential controls the relative distribution of drive torque between the front and rear axles in all-wheel-drive cars.