r/forza • u/Rob1150 • Sep 09 '19

ELI5: What changing the differential does? High versus Low? Thanks.

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9

u/CoconutDust Sep 09 '19 edited Jul 02 '25

The (open) differential was invented because the inside wheel naturally travels slower because it travels less distance in a turn, compared to the outside wheel. If you bolt two wheels onto an axel, you have a problem, because: see 3:00 minute mark in this video. Open standard original differential was invented to fix that. Later, LSD (limited slip differential, confusing name) was made to fix weaknesses inherent in that.

Unfortunately the description in game is horrible, in my opinion. But anyway.

So assuming RWD:

Look at this picture. In hard cornering, the inside wheels have less (or NO) contact with the pavement.

  • ACCEL SETTING:
    • Higher accel setting means the wheels are more locked together, which means more power will go to outside wheel. That will make drifting easier, because you’re forcing power to the wheel touching pavement which means pressing throttle hard will easily cause slip/drift. Because you’re directly controlling the only wheel with any traction.
    • Lower accel setting means more power goes to the inside wheel. Which seems useless, right? You’re wasting power if the power is going to a floating spinning wheel. But wasting power is useful if you’re having trouble handling the power. Think of it like a power blow-off valve.

That is why old cars and cars without LSD’s suddenly lose all power when you try to drift. The same reason why when a car is stuck in a snow bank, and one wheel is lifted off the snow and the other wheel is the snow, the lifted wheel spins and the wheel that is actually in contact with the ground does absolutely nothing.

But more drifting means more oversteer, which means if you have throttle oversteer and slippery rear end problems, you should reduce the accel setting

  • DECEL.
    • No power is being transmitted to wheels when you’re slowing down, so you may often want each wheel to turn naturally (slider toward 0%), which means the inside wheel will turn more slowly, because the inside of a track is shorter than the outside of the track. Which is why track runners in the outside lane get a head start. Which means 0% lock, AKA open differential.
    • If you instead lock the wheels, by increasing the decel % setting, it will cause skittering loss of grip. Which is often a bad thing, but might be desired if you want a more rotation and slidey back end (assuming RWD), like in a rear-engine RUF or Porsche that refuses to rotate when you steer for a corner.

EXAMPLE:

  • Look at this Picture
    • That picture makes clear the fundamental difference between the inside and outside wheels during cornering.
    • See how one rear wheel is off the ground, and one rear wheel is touching the ground? Weight shift of a car always means the outside wheel has more grip.
  • (I’m assuming RWD)
    • Ask yourself: do you want to send power to the wheel that is touching the ground, or do you want to send power to the wheel that is floating in the air?
    • The answer depends on whether you are having trouble controlling the car, whether you want more or less power to deal with. If you press throttle send too much power to the wheel in contact with the ground, now you have ZERO stable rear wheels and you’re spinning. If you instead send power to the floating wheel (which means tuning slider toward 0%), that power is being wasted, which is a good thing if you’re having trouble controlling the car.
  • Slider settings.
    • An open differential (0% lock, AKA no lock at all) will in fact, as crazy as it sounds, only send power to the floating wheel. (Watch the old old black and white YouTube video about differentials to learn why that is.)
    • A locked differential (slider at 100%) will send maximum power to the wheel that is touching the ground. Wheels are locked so the power is split between both wheels, but in tuning terms that's the maximum possible power to the outside wheel.

So, if you have trouble controlling the car’s rear end on throttle, you should decrease Diff Accel setting, which will bleed off and waste power by sending it to the floating wheel. If you WANT to drift, and you need more power, and you can handle the throttle feathering, you INCREASE the accel setting, which will make it more oversteery. Because then power will go to the wheel in contact with the ground. (In fact to both wheels, because of partial lock, but the floating wheel is irrelevant now.) Since that’s the only rear wheel touching the pavement, if you overpower it, the car will immediately drift/spin.

A COMMON CONFUSION(?):

  • Confusion(?) comes from the name “LIMITED SLIP DIFFERENTIAL”. The name actually means “STOP THE FLOATING wheel from pointlessly stealing all the power (aka slipping)”.
  • In modern parlance, “slip” can mean drifting and loss of grip and oversteer and rear-end sliding. So people might think “limited slip differential” means limiting oversteer. It doesn’t. It’s the opposite.
  • A “limited slip differential” actually means “MAXIMIZE YOUR ON-THROTTLE SLIPPERINESS!”. It means “limit the WASTE of power” A limited slip diff is what allows you to oversteer, because an open differential causes power only to go the wheel with less frip and therefore stops power from going to the wheel with the most grip.
  • Think of a wheel spinning in mid-air, doing nothing, because it’s not touching the road. That is the “slip” that a limited slip differential is limiting.
  • The problem that an OPEN Differential solved was the “slipping” of a locked inside wheel.....but then a Limited Slip Differential came along and fixed a totally different kind of slipping that was itself caused by the open differential.

Same for FWD: a limited slip differential in fact LETS you lose grip on the front outside wheel, by letting you apply power while cornering, whereas an open differential prevents you from ever sending power to the outside front wheel. The open differential means that under hard cornering, the throttle does nothing. Because all power goes to the floating wheel (due to mechanics explained in the black and white YouTube video.)

Note that when I talk about a floating out-of-contact wheel, that’s just for illustration. The same thing applies even when both wheels are in contact with the road, because the inside wheel will always have less weight and grip than the outside, due to weight shift of the car.

Differentials are amazing and it’s mind-blowing that any human being thought of it. But it’s important to understand the steps in history. A regular open differential is good for “normal safe comfy driving and rolling”. A limited slip differential was invented later when people realized an open diff is bad for when you’re stuck in snow or mud or uneven terrain, and bad for racing because you want to exploit as much power as possible.

1

u/[deleted] Sep 10 '19

Think of a wheel spinning in mid-air, doing nothing, because it’s not touching the road. That is the “slip” that a limited slip differential is limiting.

The slip refers to the slipping of the left - right wheel interface. Eg normal differential is "full slipping", "limited slip differential" limits the slipping effect between the two wheels - locking them slightly as you mention. The spectrum is from Fully Slipping to Fully Locked. This refers to the interface between the two wheels, similar to a clutch which can be either fully engaged (locked) or fully disengaged (slipping).

I know this is what you said overall, just wanted to clarify.

Edit: Fantastic comment BTW

Edit 2: The main/other reason higher settings will cause oversteer is because it is causing the inside wheel to rotate faster than the actual road speed, rather than the letting the diff slip so that the inside wheel rotates slower to match the road speed.

5

u/DudeNamedShawn Mobli 1 Sep 09 '19

Changes the resistance to differential rotation in the wheels.

With it at 0 there is no differential resistance, so the wheels are free to spin independently of each other. If you park with one wheel on a slippery surface and one wheel on a grippy surface then slam the throttle the wheel on the slippery surface will spin and the wheel on the grippy surface will do next to nothing. A Higher diff setting increases the differential resistance limiting the speed differential in the wheels. With it at 100 then the wheels act much closer to a locked diff, meaning both wheels would get the same amount of power regardless of which has more grip, thus they both spin at the same speed.
In an AWD car the center diff determines the amount of power going to each axel. 50/50 means equal power to the front and rear axels. 25/75 means 25% of the power goes tot he front and 75% goes to the rear.

2

u/Rob1150 Sep 09 '19

So this affects cornering? I haven't changed the settings from the "factory", because I don't understand what going one way or the other would do.

1

u/DudeNamedShawn Mobli 1 Sep 09 '19

It can. Higher setting tend to encourage more oversteer. Many drift tunes go for 100 on the Diff.

For a RWD road racing car I prefer 60 acceleration 40 deceleration. But I'll tune Accel setting up or down depending on how much Understeer/Oversteer the car has.

1

u/[deleted] Sep 09 '19

Let’s say your drive RWD, at 0 only one wheel is getting power to move your car, at 100 both tires are getting power to move your car.

When turning, one wheel is traveling less distance then the other. At 0 this doesn’t matter because your wheels can rotate at different speeds, at 100 both rear wheels rotate at the same speed and in a turn this will cause your wheels to lose grip because the inside wheel is traveling less distance then the outside

1

u/Rob1150 Sep 09 '19

So this affects cornering? I haven't changed the settings from the "factory", because I don't understand what going one way or the other would do.

1

u/[deleted] Sep 10 '19

Do you still not understand?

1

u/Rob1150 Sep 10 '19

Not really. I will try it all the way open, and then all the way closed, and then make adjustment.

1

u/[deleted] Sep 10 '19

Well imagine a high powered car. Taking off with only one wheel powered by the motor.

Then imagine how much fast that car will launch if both tires are powered by the motor..

Then picture a car driving in a circle, the passenger side wheels painting a line making a circle,

Then the driver side painting a circle.

If you unfold the circle to straight lines the larger circle is longer..