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Setup Dev 10 - Diff

rFactor setups, GTR2 setups, GTL setups, RACE setups, Sim Racing setups
Again, some terminology; As I refer to a 'tighter' differential, I will be referring to an increase in the diff value. 'Looser' will refer to a decrease in value.

I have seen many people explaining the diff as something you can simply set one way or another based on how your car is handling; "If you are experiencing under-steer, then increase/decrease this setting to fix it". This simply is not true and is misleading anyone who reads that into a poor setup development process. Either, tightening and loosening a diff, can cure or cause both over- and under-steer. What matters is WHY you are over- or under-steering and how the diff can help cure it. There is no "do this to fix it" quick-fix with the diff.

You must first understand what the diff does before you can start adjusting it;
The looser your diff, the more independently your rear tires will spin when on or off the throttle. The tighter the diff, the more they spin at the same rate. While not completely accurate, you can exaggerate that a 100% diff is equal to a solid, locked rear axle where both tires are going to spin at the exact same rate. Having a locked axle would not be very beneficial when try to go around a corner since both rear tires need to travel different distances around two different corner radius.

Power and Coast settings are as they appear; The diff settings are adjustable for either when you are on the throttle (power) or off the throttle (coast).

A very loose diff (0 - 25%) will allow the inside and outside tires complete freedom to spin at different speeds. This is helpful in the corners, but can induce both over-steer and under-steer for different reasons. When you begin to lose traction with a loose power-diff settings, all drive power is redirected to the wheel that has lost grip, usually the inside wheel). This prevents the outside wheel from losing any more grip and hopefully prevents you from spinning. Our standard passenger cars usually come with a 15-25% diff setting for this very reason.

A loose diff may be preferred in some corners where you need to be on the throttle while still deep in the corner (Parabolica at Monza is a great example). Having too loose of a diff can result in under-steer because of too much grip. A certain amount of slippage is ideal. It may also lead to over-steer if you begin to break traction with that inside wheel and it begins to 'spin-up'. Once that wheel begins to spin-up, it is increasingly more difficult to regain the grip to it, causing more and more over-steer until you are finally facing the wrong direction.

A tight diff (75 - 100%) increasingly prevents the inside and outside tires from independent rotation. This will force the rear wheels to share the workload more and is more suitable for straight line acceleration, such as exiting a tight hairpin or chicane. The downside is that when you begin to lose traction on one of the tires, it forces the other tire to begin to lose traction too. A dragster will have a very tight diff (completely locked axle) as a dragster only needs to focus on straight line acceleration and not any cornering at all.

Again, a tight diff may be preferred for those hairpins and chicanes, but having too tight a diff can result in under-steer as both rear wheels are spinning at or near the same rate. This causes the inside rear wheel to 'push' the car into a straighter line, causing under-steer. It can also induce over-steer as once you begin to lose traction on either of the wheels the other is certain to follow and a spin is not too far away.

As a rule of thumb; A loose diff is easier to drive as long as you can manage that inside tire from spinning up. It will be more forgiving to accidental loss of grip, but you can easily lose time if that inside tire is constantly spinning up. A tighter diff can be less forgiving as you will feel more "snap over-steer" with little time to recover from it. Still, having both wheels working together can be beneficial if you can manage those 'snap' tendencies.

You need to know why the car is under- or over-steering before you can make any diff changes. Is it spinning up that inside tire? Is the loss of grip to the outside tire causing the inside tire to also lose grip? Is there too much grip to that inside tire? These are the questions you need to be asking.

If the track you will be racing on has numerous long, fast, sweeping corners (Barcelona), your setup may benefit from a looser diff setup. If the track is chocked full of chicanes and hairpins (Monza, Montreal) you may want to consider a tighter diff setup to focus on the straight line acceleration and braking. There is no perfect setting, as usual, and you will just have to test different settings to find what is comfortable to you.

The preload settings are fairly straight forward. The higher the value, the greater the diff is preloaded (or "prepared") for the transitions between power and coast. You may be like me, and never feel a difference with this adjustment. However, it is never wise to overlook adjustments. In theory, if the car is very twitchy or over-reacts as you transition on/off the throttle mid-corner, you may wish to reduce your preload. If you can handle a little twitchy-ness, added preload will speed up this transitional phase, and speed is what it is all about right?