1. The purpose of the differential is simple--it permits two wheels on the same end of the car to rotate at different speeds. This makes it easier for the two tires to negotiate different radius curves when the car is turning a corner.
2. Several different types of differentials have been developed.
A. Open--this type uses an ingenious set of gears that allows the two wheels to rotate at different speeds at all times as they are never locked together. Consequently, it handles corners very nicely. Its main drawback is that if one tire is slipping on a slick surface such as ice, torque is reduced so that the slipping tire spins while the other doesn't spin at all and the car won't move. A similar situation occurs on race cars in a corner because the inside tire is less loaded and has less grip than the outside tire. The open differential by itself is rarely seen on race cars although it is almost universally used on passenger cars.
B. Locked--this type is the complete opposite of the open differential. The two wheels are always locked together and rotate at the same rate. It is commonly used on trucks, dragsters, and karts. Locked differentials are seldom seen on race cars as they promote understeer. However, the 1970s Porsche 917 Turbo CanAm cars used a locked differential as other differential types of that era could not handle the car's massive torque.
What is really needed for a race car is a differential that is open for entering the corners and locked for exiting the corners and maximum acceleration on the straights. The limited slip differential (LSD) was developed to accomplish these goals. An LSD has a mechanism that locks the two wheels together under certain conditions. The two LSDs that apply to 1960s race cars and GPL are the Cam and Pawl and the Salisbury.
C. Cam and Pawl LSD--this type uses a set of plungers (pawls) that extend into slots to lock the two wheels together completely when power is applied to the differential. When power is removed, the plungers retract and the two wheels are free to rotate independently of each other. The Cam and Pawl is one of the simplest forms of LSDs and is relatively cheap and reliable; however by its very nature, the plungers tend to wear quickly and have to be replaced often. Also, the locking action is relatively quick so essentially it is either completely locked or completely open.
Cam and Pawl LSDs were used on all F1 cars of the 1960s. Hewland in England and ZF in Germany manufactured their own versions of the Cam and Pawl LSD. David Wright researched the subject and determined that the first use of a LSD other than the Cam and Pawl didn't occur until Ferrari used a Salisbury type LSD on their mid 1970s F1 cars.
This is all well and good, but strangely GPL does not model the Cam and Pawl LSD; rather, it models the Salisbury.
D. Salisbury LSD--this type uses mechanical wedges (ramps) and a series of clutch plates to lock the two wheels together. The advantage of the Salisbury is that the amount of lock is individually adjustable when the power is applied and when removed. Also, the two wheels are locked together up to a certain point, then are free to rotate somewhat independently. The locking action is more progressive than with the Cam and Pawl. By all accounts, the Salisbury is a vast improvement over the Cam and Pawl.
3. GPL Salisbury LSD Model:
A. GPL models the Salisbury LSD fairly well. The player setup menu contains settings for the ramp angles for "power" and "coast" and the number of clutches. These settings are applied to a formula to determine a "locking percent". GPL's code continually keeps both wheels/tires turning at the same rate by apportioning torque from one wheel to the other; however, when the torque difference reaches the locking percent limit, the two wheels begin to rotate independently of each other.
B. The formulae for locking percent are:
Power Locking Percent = Cosine(Power Ramp Angle) * (Number of Clutches + 1) * 5%
Coast Locking Percent = Cosine(Coast Ramp Angle) * (Number of Clutches + 1) * 5%
Note that ramp angle and number of clutches both contribute to the locking percent calculation. Therefore, it is possible to have the same locking percent by using different combinations of ramp angle and number of clutches. The attached chart shows the locking percent for various combinations.
Note that the number of clutches affects both power and coast locking percent.
GPL Setup Manager displays locking percent according to these formulae.
C. As best as I can determine from reviewing GPL's code, the car handles the same when using the same locking percent regardless of the ramp angle and number of clutches. The ramp angle and number of clutches values are not used anywhere else in the code.
D. The power locking percent is used when the player's throttle is on by any amount.
E. The coast locking percent is used only when the player's throttle is completely off! This has a major implication for left foot brakers as they will be using the coast locking percent only if they completely remove their right foot from the throttle pedal. It's all too easy to keep a slight amount of pressure on the throttle pedal which causes the power locking percent to be used inadvertently.
F. Disregard any reference to what real world cars used for ramp angles and number of clutches. They may or may not correlate to what GPL needs for a good handling car. Years of practical experience with GPL has determined that power ramp angles between 60 to 85 degrees, coast ramp angles between 30 to 45 degrees, and number of clutches between 1 to 3 can be used depending on the car characteristics and the player's skill level. These values correspond to locking percents of 1% to 10% for power and 7% to 17% for coast.
Note that some "alien" drivers may use locking percentages that are different than these. If you are an alien, you probably don't need to be reading this in the first place.
Edited by Lee200, Dec 18 2018 - 09:03 PM.