Downforce and Braking
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 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 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.
There isn't a ton I can find on differential settings so sorry for being so brief here and it is pretty simple. A higher number creates oversteer and a lower number prevents it.
VEHICLE – POSSIBLE CAUSES
Straight Line Instability
* Too much rear wheel toe-out.
* Not enough rear downforce.
* Too much front downforce.
* Not enough rear toe-in (under hard acceleration) –presumably for RWD.
* Too much front toe (either in or out) – car darts over bumps.
* Front ARB is way too stiff – car darts over bumps.
Instability Under Brakes
* Front end darts or wanders – too much front brake bias.
* Car wants to spin – too much rear brake bias.
* Car feels heavy and unresponsive.
o Too much downforce.
* Car feels sloppy and is slow to take a set in corners.
o Too little shock.
o Too much body roll (not enough spring and/or ARB).
* Car responds too quickly, is twitchy, and slides easily.
o Too little downforce.
o Too much shock, too much spring, and/or too much ARB.
o Too much tire pressure.
* Corner entry understeer – car won’t turn in at all.
tires not wide enough.
o Too much front roll stiffness – ARBs and/or springs.
o Not enough front bump – shocks.
o Not enough front downforce.
o Too much dynamic camber on front wheels (not enough static negative camber).
* Corner entry understeer – car turns in initially then starts to push.
o Too much front toe-in.
o Rebound too stiff – shocks (not enough droop travel).
o Not enough front downforce.
o Bump not stiff enough – shocks.
* Corner entry understeer – car turns in and then darts.
o Insufficient front suspension travel in either or both directions – shocks.
* Corner exit oversteer – gets progressively worse from the time throttle is applied.
o Too much rear roll stiffness.
o Too much rear camber.
o Too little rear downforce.
o Too little rear toe-in.
o Not enough rear spring, shock, or ARB – allows car to roll over on outside rear wheel.
* Corner exit oversteer (sudden) - car takes its set then breaks loose.
o Not enough rear suspension travel (too much shock in either bump, rebound, or both).
CAUSES – EFFECT ON VEHICLE
* Too much spring – overall
o Harsh and choppy ride, lack of tire compliance.
o Can’t put power down on corner exit - excessive wheelspin.
o Car slides too much.
* Too much spring – front
o Initial understeer.
o Front end breaks loose in corners.
o Front end breaks loose over bumps.
* Too much spring – rear
o Oversteer when power is applied on corner exit.
o Excessive wheelspin.
* Too little spring – overall
o Car bottoms out.
o Car feels like it’s floating.
o Sloppy response.
o Car is slow to take its set.
* Too little spring – front
o Front end hits ground under brakes.
o Too much body roll on corner entry.
o Initial understeer – car won’t point in.
* Too little spring – rear
o Too much squat under acceleration, and the resultant increase in negative camber.
o Car falls over on outside rear wheel causing power-on oversteer.
* Too much ARB – overall
o Very sudden turning response and little feel.
o Car slides or skates instead of taking its set.
o May dart over one wheel bumps or diagonal bumps.
* Too much ARB – front
o Corner entry understeer that gets progressively worse.
o Steady state understeer in the middle of sweeping turns.
* Too much ARB – rear
o Corner exit oversteer when throttle is applied.
o Excessive sliding coming out of corners.
* Too little ARB – overall
o Car is sloppy and lacks response.
o Car is slow to transition, especially in chicanes and esses.
o Car rolls too much resulting in too much dynamic, positive camber and the resultant loss of cornering power due to decrease in tire traction.
* Too little ARB – front
o Car rolls over onto outside tire on corner entry then ‘washes out’ (understeers).
o Car lacks steering response and is slow to change direction.
* Too little ARB – rear
o Back end doesn’t want to rotate on corner exit under power (difficult to throttle steer).
* Too much shock – overall
o Very sudden car with harsh ride, sliding, and wheel patter.
o Car crashes over road surface irregularities.
* Too much rebound adjustment
o Wheels do not return to road surface quickly after displacement.
o Inside wheel pulls off the road surface in a corner.
o Lack of tire compliance over bumps and surface undulations.
o Car may be jacked down in long corners.
* Too much bump adjustment
o Initial reaction to bumps and curbs is harsh.
o Initial chassis roll slow to develop.
o Car slides rather than sticks.
o Driven wheels hop when the power is put down.
* Too little shock – overall
o Car floats a lot in ride and oscillates after bumps (underdamped).
o Slow and sloppy response.
o Chassis rolls too quickly.
* Too little rebound adjustment
o Oscillates after bumps.
o Doesn’t put the power down well.
* Too little bump adjustment
o Initial bump reaction soft.
o Car dives and squats a lot under brakes and under power.
o Car rolls quickly and falls over outside tires – front tire on corner entry and rear tire on exit.
* Front toe-in – too much
o Car darts over bumps, under the brakes, and during corner entry.
o Car won’t point into corners.
* Front toe-out – too much
o Car wanders under the brakes.
o Straight line instability especially over one wheel and diagonal bumps.
o Car may point into corners then refuse to take a set
o Understeer as a result of tire scrub in long corners.
* Rear toe-in – too much
o Rear feels light and unstable on corner entry.
o Car slides a bit in corners rather than rolling freely.
* Rear toe-in – too little
o Power on oversteer during corner exit.
* Rear toe-out – any
o Power oversteer during corner exit.
o Straight line instability.
* Front wheel caster – too much
o Excessive physical steering effort (probably non-applicable with the XBOX controller)
* Front wheel caster – too little
o Too little steering feel and feedback (also probably non-applicable with the XBOX controller)
* Camber – too much negative
o Inside of tire will be hotter than the rest and wear faster.
o Front tires – reduced braking capacity (dive)
o Rear tires – reduced acceleration capacity (squat)
* Camber – too much positive
o Outside of tire will be hotter than the rest and wear faster.
o Rear tires – corner exit oversteer and reduced tire traction.
o Rear tires - If extreme may cause corner entrance instability.
o Front tires – too much body roll and understeer after car turns in.
* Too much tire pressure
o Harsh ride.
o Excessive wheel patter, sliding and wheelspin.
o High temperature at center of tire.
* Too little tire pressure
o Soft and mushy response.
o High tire temperatures at inner and outer edges.
o Reduced contact patch