What is a Limited Slip Diff

Lets Start With The OE Open Diff

When you turn a left hand corner, your right hand wheel rotates faster than your left hand wheel as it has more distance to travel. Your original equipment (OE) open diff allows the driven wheels to do exactly that by having sets of gears inside that are able to rotate at different speeds whilst still exerting driving force in a single direction, in this case forwards. When you turn a right hand corner, similar stuff happens.

Car manufactures like these open diff units because they are cheap, do not cause any additional stress on driveshatfs, they won’t wear the tyres out and the overall action of the diff in operation is so smooth, you generally can’t feel anything clever going every time you turn the steering wheel.

The Problem With an Open Diff

Generally, all open diffs are geared diffs. It is the way in which the gears operate that allow the difference in wheel speed whilst maintaining forward motion. The problem is geared diffs are lazy. As soon as you open the throttle and the torque of the engine goes through the diff (torque is the engines turning or twisting motion that drives the car), it send the torque down the path of least resistance.

Let us consider the following, as your accelerate around your left hand bend, the left hand side wheel has become lighter on the road that the right hand wheel (due to the body of the car leaning to the outside of the corner due to the shift in weight). If you apply too much throttle, the left hand wheel will start to spin.

This is the diff sending engine torque to the light left hand wheel. Because the open diffs gears allow it to be lazy, the diff just send more and more torque to the spinning left hand wheel and sends very little torque to the right hand wheel. This results in lots of wheelspin, very little grip, very little control and sod all forward motion.

The only thing you can do is stop accelerating and maybe even slow down so that traction is restored. Slowing down (especially on a race track) is a crap idea for petrol heads but if you allow things to continue, your front wheel drive car will plough straight off the road or your rear wheel drive car will probably spin round in the middle of the road.

And So to the Cure – Fit A Limited Slip Diff!

So, you are accelerating around this left hand bend but this time you have a Gripper Limited Slip Differential fitted. As you accelerate, the left wheel again goes light and starts thinking about spinning. The magic inside the limited slip differential sees this. Instead of allowing the left hand wheel to spin, it locks the left hand wheel to the right hand wheel. So now we have all the engine torque being sent to both driven wheels. The result now is that both driven wheels are gripping the road, wheelspin is all but eliminated and forward traction is maintained. You can actually keep applying the power whilst exiting the corner and the car gets round the corner onto the straight at a much higher speed than it did before with an open diff!

So, what is a limited slip differential? Its magic is what it is! Simply put, fitting a limited slip diff into your car will give you the best increase in traction money can buy! And less simply put…….

This is Why - The Magic Inside a Limited Slip Diff!

Drive is transmitted from the engine to the crownwheel which is bolted to the diff casing. It is then transmitted to the pinion gears which turn the side gears which turn the driveshafts and hence forward motion of the car is achieved.

As the car accelerates around a corner, there is a difference in wheel speed. This difference applies a force to the crosspins which are now forced to move sideways. As the crosspins sit in an angle cut into the ramp black, the sideways movement of the crosspins moves the ramp block towards the clutch pack which then compresses. This compression of the clutch pack makes it rotate at a single speed.

One clutch pack is fitted either side of the diff between the casing and the side gears and acts on the corresponding driveshaft. Each clutch pack is made up of external and internal plates. The external plates have tabs that run in grooves in the diff casing so they connected to the diff casing and rotate at the diff casing speed.

As the external plates are rotating at the diff casing speed and are now locked to the internal plates (due to compression of the clutch pack), they too are now rotating at the diff casing speed. The internal plates are connected to the side gears so now both side gears are rotating at the diff casing speed. As the side gears are connected to the drive shafts they are now rotating at casing speed limiting wheel speed difference and transmitting torque to both driven wheels. This reduces wheelspin and increases traction maintaining forward drive.

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