Introduction

There used to be a boring 10 minute read here, either skip to the next bit or tap your fingers for 10 minutes..

Differential

You probably owned one of those ‘pedal karts’ where you could pedal your way around the schoolyard under the impression that you had the coolest wheels of the class.. Think back.. do you remember the understeer those things had? The rear wheels just wanted to push the car forward when you where about to impress that chick with some flashy cornering.

When a car is taking a corner, its outside wheels travel a longer way than the inside wheels. If you put a solid axle between the left and right rear wheels the inside and outside wheel start working against each other. They can’t have different speeds as they are connected to one sturdy steel axle. The result is that the rotation speed of the most heavily loaded tire (with the same underlying surface) will become the rotation speed of the entire rear axis. The other rear wheel tries to push the car forward in this case. . You also see this with karting, the lack of independently rotating rear wheels can give chronic understeer in some situations. (i think a kart 'steering geometry' makes it nearly lift the inside rear when cornering to counter this)

Because taking corners fast is all what racing is about, they came up with a clever solution for this problem. Instead of a solid rear axle, there is one short axle coming from each rear wheel. Where they meet are 2 or more clutch plates. Depending on the force at which these plates are pressed together, the rear wheels will either

1) rotate independently easily

2) behave almost like a solid rear axle

With very little force on the clutch plates, they are easily ‘rubbed’ over each other. One guy can hold the left rear, and with hand force another guy would be able to rotate the right rear. With a lot of force on the plates, they barely rotate relatively to another. Even with two large blokes you would barely be able to rotate the wheels independently. The differential in GPL allows you to set the force at which the clutch plates are pressed together. This is also known as the differential’s lock. (more force = more lock)

You can set the amount of lock independently for when you are on the gas and when you are off on the gas. Independent of being on or off the gas (i.e. as much influence on both) have the amount of clutches. 1 set of clutch plates can withstand a certain amount of torsion before the maximum amount of friction is reached and they start to rotate at different speeds. Logically, the more clutch plates you put in, the harder this gets, regardless if you are on the gas or off the gas.

In the car setup menu you can select a differential from 30/30 to 85/85 with small steps between these extremes. The first number is the ON gas value, and the second number is the OFF gas value. A smaller number means a bigger force (more lock) and a bigger number means a smaller force (less lock) You can set the amount of clutches anywhere from 1 to 6. As said, the clutches amplify the lock, set with the xx/xx numbers.

Those numbers (30...85) are the angles at which cogwheels in the differential are placed. Depending on the angle, a certain amount of force will go in radial direction on the axle and clutch plates (i.e. not pressing them together) and a certain amount will be in axial direction, i.e. pressing the clutch plates against each other. Lets go back to physics class again for a second. A force not directly in the x or y axis (i.e. not exactly at 0 or 90 degrees) has a x component and an y component. As shown below.

The size of the components depends on the angle at which the force is applied. At 85 degrees (almost ‘vertically up’) there is only a very small horizontal component. Now back to the GPL differential. When you set GPL to a 85/85 diff this means that both ON and OFF the gas only a very little amount of force is applied at the clutches. When you set it to 60/30, more force is applied on the clutches when you go on the gas, and even more when you are off the gas.

GPL drivers talking about differentials is a sight to behold. ‘I like 85/45’ one says. Another guy enthuses the crowd about 60/30... The differential is perhaps the best way to prove that the guys who setup a car by ‘feel’ can’t really interpret the ‘feel’ that GPL gives them. First of all, they usually forget to mention the amount of clutches, and we’ve just seen that the clutches are directly related to the amount of lock. You can’t really blame the people who don’t understand though. Presenting the differential in a xx / xx / x format (power angle / off power angle / clutches) isn’t a direcly usable format.

We’ve seen with the wheelrates that they are instantly usable. This doesn’t go for the differential. In order to make the differential ‘workable’ you need to express the numbers in ‘amount of lock’ and not some weird looking 45/30/2 or something. You want to know how much lock you get when on the gas and how much you get off the gas. 45/30/2 means that you have a bit more differential lock when you are off the gas than on the gas, but how does it compare to 60/45/4? The angles are bigger, meaning less lock.. but there are more clutches, meaning more lock. If you knew 45/30/2 means, say 1.5 lock on the gas and 1.8 off the gas, and 60/45/4 is 1.4 on the gas and 1.7 off the gas, only then are you able to work with the differential in a proper way.

Well, and this is perhaps the most ‘groundbreaking’ bit of this setup guide, Marc Mercer (with some aid from above) found a way to convert all differentials in workable lock numbers! So from now on you can see the exact amount off lock for the on and off gas angles with a certain amount of clutches all in the handy table below.

On the gas

You are on the gas coming out of a corner. Your goal is to exit the corner as fast as possible. Now what ‘on gas’ angle and clutch combo do we need for that? Since we’re looking only at the ‘gas’ side lets take 85/xx/1 first. Look at the table, this gives you 0.17 lock. Now we try to accelerate out of a right hand corner. Most of the pressure is on the left rear tire. The clutch plates are barely pressed together. This means that the engine power soon overcomes the clutch clench force, and it will start to power the least loaded wheel: your inside rear tire will start to spin, leaving the outside with not much drive force. You will leave a lot of smoke at the inside yet you will crawl out of the corner.

Ok so 0.17 isn’t enough lock, we need more power on the heavy loaded left rear tire in order to properly accelerate out of the corner. Lets do 30/xx/6, giving us 38 times more lock than 85/xx/1 Now when we accelerate out of a corner, a lot of power goes to both wheels. Should you get wheelspin then you will have a hard time trying to correct it as both rear tyres will loose grip at the same time as opposed to just the inside wheel with 85/xx/1.

These are the 2 extremes. Now what is the 'right amount of lock'. A good question. With too little lock you will get understeer and lack of acceleration when the engine's energy is wasted on the inside (not heavily loaded) tyre. Too much lock and you'll be having the 'solid axle pushing the car forward' understeer behaviour, and this is more likely to cause spins as wheelspin is harder to correct. (more sudden less forgiving) Somewhere in the middle there is an area providing lock that makes the inside tyre neither spin and wasting power, nor being too locked and pushing the car forward or oversteer suddenly with wheelspin.. Depending on your 'style'  (most have none and are just hanging on for dear lifet) and other setup parameters. you need to look for lock values of about 1 to 1.5.

But how do you set the exact on power lock? The aim in GPL is to drive smoothly. When done properly smooth driving is faster, easy on the tyres, and it is the best base for consistent driving.  If you use a 60/xx/2 diff you got a lock of 1.5. If you're very sensitive with your right foot this will provide a good 'power oversteer' based differential setting. Most likely however you won't be very smooth with this as all too soon the rear wants to step out. 60/xx/1 will be a lot easier but you may notice that you can't 'steer the car with the power' as nicely anymore.  you can try settings from 'harsh' lock to 'easy' :

60/xx/2 = 1.500 'snappy' tricky on power behaviour
70/xx/3 = 1.368
75/xx/4 = 1.295
75/xx/3 = 1.036 more controllable less oversteer

What will you end up with? just try them :) get GPL race engineer here which enables you to set those 'weird' non standard 70 and 75 ramp angles. Don't worry, if they wanted a 70 or 75 power ramp angle they could've manufactured one back then, its perfectly sane and definitely not cheating. If you're wondering why I'm always using at least 2 clutches, read on!

Off the gas

You are off the gas braking for, and entering a corner. Your goal is to enter the corner not really as fast as possible, but to do it nice and controlled, so you can line up for a quick exit. You can win 0.1 with a good corner entrance, but you will loose 3 tenths with a bad exit. Lets try the same extremes as we did with the ‘on gas’ testing. So xx/85/1 is up first, giving us the same 0.17 lock but this time when we release the gas pedal. We try to enter a corner and we find that there isn’t talk of the ‘nice and controlled’ entrance as talked about earlier. The reason is simple. The lack of lock makes the car almost rotate into the corner by itself because the rear wheels are almost ‘eager’ to rotate at a different speed .Xx/30/6 on the other hand reintroduces the ‘solid rear axle’ argument with which I started the differential topic. In this case the car doesn’t rotate at all, it just wants to go straight on. You need a certain amount of lock to avoid automatic or over rotating into the corners. But you don’t need too much: that pushes the car forward instead of nicely into the corner.

Yes that's right, if you use 30/85/1 (as often seen in 'hotlap' setups) and you keep some gas on while braking (as done by a LOT of drivers) you're using basically 30/30/1 as the lock while braking and turning into a corner remains the 'on power' lock. 

Anyway, for us proper drivers who take their foot off the gas pedal while braking, we have an additional differential setting to use namely the 'coast' or 'off power' lock. To avoid the car 'auto rotating' into the corner as mentioned earlier, you need a decent amount of lock.Even with a ramp angle of 30 you still need at least 2 clutches or it will be too oversteery! This depends on the entire setup (rollbars / wheelrates / toe) but to avoid too much oversteer I usually end up between 2.5 and 3.5 'coast' lock. I like oversteer and with little toe and wheelrates based on the car's weight distribution I most definitely get an oversteery car on entry using xx/45/3  With more toe and a different setup you might not experience this as oversteery. Again, smoothness is the key. You will end up being more consistent and 'safe' using a slightly understeery setting, like xx/30/3 or even xx/30/4. Some coast settings from 'oversteery' to 'understeery' would be:

xx/30/2  = 2.598 oversteery ish when releasing gas fully
xx/45/3  = 2.828
xx/40/3  = 3.064
xx/30/3  = 3.464
xx/30/4  = 4.330 understeery ish when releasing gas fully

So what diff to use

From 'hard' to 'easy' with an optimum probably being in the middle somewhere for most drivers, these are some differentials you could try:

60/30/2 Harsh and oversteery both on and off power
70/45/3 Considerably nicer but still oversteer based depending on the rest of your setup
75/45/4 Not too 'tricky' but you can't 'steer with the pedals' as accurately 

It would be utterly foolish to say 'which is faster' because:
1) all GPL cars are overpowered, undergripped mean machines that theoretically are nearly as fast regardless of exact car setup
2) this means its all in the driver and what inputs he gives to the car.
3) a good setup IS important to make the driver able to race with as smooth and few inputs as possible, but the achievable laptime won't differ much between setups.

Simply put, someone could do world record laps using any of the above mentioned differentials. The driver is the alien, not the setup. I really like 70/45/3 as this gives me 'just about controllable' oversteer both on and off the gas. What is the right differential for you? try them out and find out for yourself! You now know what the 'power' and 'coast' lock does so you should be able to try a few diffs and feel the effect they have.

It does take a half decent driver to 'feel' a racing sim. Decent doesn't mean how fast he or she is but how well he or she receives what GPL is saying to them. I didn't notice small changes in a GPL setup until one day ... I saw the light so to speak :) Chances are you're as ham footed as an elephant just like I was one year ago and you can't really tell 75/45/5 from 60/30/2. Don't fool yourself then. I'm sure half of the GPL racers are too inexperienced to feel the subtle effects. You have to drive very consciously and record the car behaviour in your black box (brain).. At first I really wasn't capable of telling one setup from another. It came with the years (literally) GPL is meant to be fun and if you can't really 'feel it' well enough yet, don't worry just go back to whatever differential you seem to like and enjoy the ride!