What are Plate Type Differentials, and How Do They Work?

what are Plate Type diffs and how do they work MAZDA MX-5

What are Plate Type Differentials, and How Do They Work?

The Basics of a Differential.  

Nearly every car on the road today will be using some sort of differential.  

In short, on a two-wheel-drive car, a differential allows one wheel to move at a different speed to the other while being fixed to the same axle. 

This is needed as the outer wheel will have to travel a further distance, and spins at a different speed than the inner wheel when you go through a corner.  

If you have 10 minutes to spare, take a look at the video below. This is one of the clearest explanations of the basics of a diff I could find.  

It’s also much easier to understand the principles behind a diff when you see one in action. Note the type of diff they explain in the video is called an Open Differential.  

The problem with Open Diffs 

Most factory-fitted diffs are Open Differentials. This is a simple and very reliable type of differential and is ideal for 99% of road cards.  

There are two key principles to remember about open diffs: 

  • It can only ever transfer the same amount of torque to both wheels 
  • The torque will always follow the path of least resistance. 

Now imagine this, you’re sitting at a red light in an open diff MX5. It’s a warm, dry day and you have a clear road ahead of you. 

You haven’t realised, but one of your rear wheels is in a large patch water while the other is on dry tarmac.  

Since it’s a nice day, you decide to drop the clutch and head towards your favourite B road. 

However, as soon as you set off the wet wheel begins to spin while the other doesn’t move at all. 

This happens because the water has removed the friction between your tire and the road surface, which makes the amount of torque needed to spin the wheel tiny.  

Because an open diff can only transfer the same amount of torque to both wheels, the wheel on dry tarmac is now receiving the exact same (tiny) amount of torque as the wet wheel, this usually results in one wheel spinning pointlessly while the other doesn’t move at all, often referred to as “one tyre fire”. 

This problem, however, is not limited to standing starts.  

If you’re taking a fast, tight corner with an open diff, you can often feel the inside wheel sort of giving up and not pushing you through the corner. When this happens, you’ll notice you’ll be forced onto a much wider line and may not be able to carry as much speed through the corner as you’d like. 

 

So, how can we stop this?  

By using a Limited Slip Differential (LSD). This does exactly what it says on the tin, it’s a differential that limits the amount of slip between each wheel. An LSD will still allow one wheel to turn at a different speed to the other, but limits the difference each wheel can spin. 

 

Helical/Torsen Type LSD 

A Helical Diff is a similar idea to a Torsen diff found in some Eunos Roadsters. This uses clever gearing to achieve something called Torque Biasing. In essence, when one wheel starts to spin faster than the other, a Helical diff will transfer more torque to the slower wheel.  

This has a dramatic effect on how the car handles. If you remember the example from earlier, let’s reimagine that scenario but with a Helical Diff.

You’re taking a fast, tight corner this time with a Helical Diff. When your car starts to lean into the corner, instead of the inner, slower spinning wheel giving up, it will start to receive more torque as the outer wheel is spinning faster.  

This not only means you’ll feel more confident chucking your car into corners, but also that you’ll be able to get a much tighter and more predictable line coming out the corner.  

Helical limited-slip differentials are great for fast road and light track use, they generate no noise, require no maintenance and are ideal as a fit and forget LSD. 

The downsides of this type of LSD is that you cannot alter any of the ratios or characteristics of the unit. 

If, further down the road, you decide you need a different ratio, or you want more lock under load, you’ll need to get another diff entirely. 

Another potential downside to this type of diff is that it can never fully lock, so you won’t be able to get the torque down as aggressively as a plate type diff. 

 

Mazda MX-5 Limited slip differential explained how do they work

 

Plate/clutch type LSD. 

Before reading on, take a look at the image above. In particular, note where the cross pin and ramps are on the diff. I found it makes it a lot easier to understand how this works if you can visualise how they fit together. 

A plate type LSD is a very different configuration to a Helical. Unlike a helical diff, which can never fully lock, this diff uses plate packs (sometimes called clutch packs) to effectively lock the entire rear axle under load. 

It does this by using a cross pin and a ramp ring. The cross pin is a long cylindrical pin that sits in the middle of the diff between either one or two ramp rings. In the case of the 3J diff above, only one ramp ring Is used. 

These ramp rings have literal ramps machined into the side, when under load the whole ramp ring rotates causing the ramps to ride up against the cross pin, forcing the ring apart and “upwards” in the case of the image above.

As these ramp rings are forced apart, they push several friction plates together on either side of the diff.  

This essentially acts like a clutch, but instead of applying the clutch to a moving shaft, it applies it to a stationary plate until there is enough friction between the plates to momentarily lock the diff. 

Since this diff relies on friction to lock the axle, it does require some level of maintenance to check and, if necessary, replace the friction plates.

Under heavy motorsport use, it’s best to get the diff serviced at the start of every season. For road and light to moderate track use, the diffs can go for years and not need a service. 3J say, as a rule of thumb, get the diff serviced as soon as you notice any drop in performance. 

With a plate type diff, you also get a lot more control over how it handles by altering things called preload and ramp angles. 

 

Preload 

Pre-load is the amount of pressure that is already applied to the friction plates before the ramp rings push them together. The more pre-load you have, the more pressure will be exerted onto the friction plates. This means you’ll need less torque from the engine to lock the diff. 

If you want your diff to frequently lock to get the best possible grip on track, you’ll want a high amount of preload. If you want something that is at least somewhat useable on the road, you’ll want a low amount of preload so the diff doesn’t lock every time you turn the car. 

It is possible to set the preload to always have the diff under some level of lock. When this happens, the preload will apply enough pressure to the plates for them to generate some friction but not enough to completely lock the diff. 

 

Ramp angles 

Ramp angles are literally the angle of the ramp that is machined into the ramp rings. Depending on the angle of the ramp, it will be easier or harder for the ramps rings to ride up the cross pin and lock the diff. 

If you have a shallow angle, say 25 degrees, the ramps will easily be able to slide up the cross pin under low – medium acceleration and deceleration; making locking the diff easier and more frequent.

If you have a harsher ramp angle, say 50 degrees, it’s going to need a lot more torque to be able to get the pin to slide and lock the diff.  

 

Mazda MX-5 LSD Limited Slip Differential close up

  

1-way, 1.5-way and 2-way diffs, what’s the difference? 

This has to do with how the diff locks under both acceleration or deceleration.  

A 2-way diff is capable of locking under both acceleration and deceleration. It does this by having exactly the same ramp angles for both acceleration and deceleration. 

A basic 2 way set up would have a 45o ramp angle on each side (45/45) of the cross pin. According to 3J, this roughly equates to 25% lockup while driving under normal conditions. This is the best option for heavy track use or drifting.  

1-way diffs are only capable of locking under acceleration. This is the “kindest” type of LSD and is normally only used for fast road driving. This will always have a deceleration ramp angle of 90o as the cross pin cannot slider over a 90o ramp, ensuring it cannot lock. 

A 1.5-way diff aims to find a middle ground between the two by making it easier to lock the diff under acceleration and harder under deceleration. This is done by having a lower ramp angle for the acceleration side than the deceleration side. For example, a 35/55 configuration (Accel ramp angle / decel ramp angle) would lock easier and more frequently under acceleration due to the shallower ramp angle on the acceleration side.  

 

Final Thoughts 

Whether you’re on track, going for a weekend B-road blast or daily driving, some form of LSD will always be helpful. 

They help to keep traction, inspire confidence in corners and let’s face it, are much more fun.  

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