When you start adding more power to your vehicle, the next question is how to put it down to the pavement. While getting a good set of tires is a good start, the differential is an equally important component.
Like tires, all vehicles have a differential, with AWD and 4WD having two, although digging into this further, there are multiple types. However, we can break it down to open and limited-slip differentials (LSD) for simplicity’s sake.
As this is a rather technical topic, we’ll just scratch the surface of the three main LSD types, some common applications, along with their advantages and disadvantages.
In basic terms, an LSD transfers torque between the wheels that are slipping (or spinning) to those that have traction. While burnouts may be fun, it also means you are not going anywhere fast, which is less than ideal in a drag race.
Likewise, for off-roading, it may be the difference between spinning your tires and getting stuck or being able to pull yourself out. Suffice to say, traction can make all the difference, which is what LSD’s are all about.
While that is the easy explanation, it comes down to choosing the right type of LSD for your application. Obviously, this is a technical topic, so we’ll stick to the basics and briefly cover them.
What Is A Limited Slip Differential?
First, let’s take a look at the two extremes, open and locked differentials. An open differential is little more than two axle shafts with a gear on each end. Inside the differential housing, they are connected to each other by one or two additional gears.
As you accelerate, power is transferred to both wheels up until the point where one tire begins to spin. However, when that occurs, the torque will keep going to the wheel with less traction.
An open differential is used for most cars since it is simple, cheaper, and allows both wheels to spin freely, which is beneficial when turning.
In comparison, a closed or locked axle has the same components, although the axle shafts are locked (or welded together). This provides maximum traction when accelerating, which is ideal for drag racing and some off-road applications.
Likewise, it is commonly used for drifting since it provides more predictable steering by forcing both wheels to spin at the same speed. On the other hand, (and for that reason), a locked axle is not recommended for street use as it will negatively affect handling.
Types Of Limited Slip Differentials
Mechanical (clutch-based) LSDs
In contrast to an open differential, a mechanical LSD will transfer power between the wheels when one starts slipping. This is done by using clutch plates, and the slippage will lock them up with power being transferred to the wheel with more traction.
However, it should be noted the percentage will vary to some extent. Under normal conditions, mechanical LSD functions like an open differential, and the tires will spin freely when turning.
The main disadvantage of a mechanical LSD is maintenance, and the clutches can wear out over time and need to be replaced.
A viscous LSD uses fluid to create the resistance needed to lock and unlock the differential instead of clutches. They are simpler and have fewer moving parts, although there is more of a delay in transferring power from the wheels that slip to the wheels that grip.
However, the transition is smoother compared to other types. In addition, a viscous LSD can transfer more torque to the wheels with traction as opposed to a clutch or torque-sensing unit where it is usually based on a predetermined percentage.
In addition, they are relatively maintenance-free as there are no clutch packs that need to be replaced.
However, viscous LSDs have some disadvantages. Most notably, they tend to have a short life span, and they typically have to be replaced as an entire unit. In addition, when the fluid heats up under hard use, they become less effective.
Helical/Torsen (torque sensing) LSDs
The torque-sensing or Torsen differential is the new kid on the block, and they started appearing in vehicles during the 1980s. A noticeable difference is that it does not depend on clutches or fluids to create resistance.
Instead, it uses worm gears to transfer power to the wheels with more traction. Likewise, since they meshed together on each side of the differential, power is transferred almost instantly.
While this is one advantage, they can be designed with different ratios like 5:1, which means 80% of the power will go to the wheels with more traction. Lastly, there are no clutches that have a tendency to wear out and become less effective over time.
On the downside, a Torsen LSD is considerably more expensive than the other two.
Now that we know the differences between them, albeit in basic terms, it should be noted each type has distinct advantages and disadvantages when used in certain applications.
Mechanical (clutch-based) LSDs
A mechanical or clutch-based LSD is a huge improvement over an open diff for many street and track applications. Likewise, if your vehicle has an LSD from the factory, it is probably of this type.
Most aftermarket units are usually constructed with better clutches, hence an upgrade over the stock one. As there are multiple types of mechanical LSD’s, we’ll lightly touch upon them.
Likewise, we’ll skip over lockers and spools, which are intended for drag racing, drifting, and off-road use instead of street-driven vehicles.
The main advantage of clutch-based LSD is to provide better traction without negatively affecting handling. For hard track use, like drifting and road racing, a two-way differential is preferred as it gives the driver more braking predictability, although again, it is less than ideal for street use.
On the other hand, a 1.5-way will have the same characteristics when accelerating. However, it permits the axles to spin more freely when you let off the gas. As such, a 1.5 is preferred for events that involve a lot of turns, such as autocross.
Likewise, it is good for the street as well.
Another advantage of clutch-based LSDs is the availability of parts. The clutches can be replaced if worn out, and you can use different types to fine-tune the slippage and power transfer between the wheels.
Lastly, mechanical differentials are cheaper and more readily available than a Torsen unit.
Viscous LSDs are mainly used in stock applications due to their lower cost and simplicity. For the most part, they are used in AWD vehicles as they can effectively transfer power between the front and rear wheels.
However, in recent years, many manufacturers have switched to a Torsen differential. It offers the same benefits as a viscous LSD, not to mention it is stronger and more durable.
Helical/Torsen (torque sensing) LSDs
Torsen or torque-sensing differentials didn’t come about until the 1980s, and as noted above, they have replaced viscous LSD in some applications. Likewise, they are fairly low-maintenance and do not use clutch plates.
For street and mild track use, the Torsen is an excellent choice, especially if your vehicle had an open differential, to begin with. The other advantage is the ease of use and installation as they are essentially an install and forget item.
For competitive track use, there is a debate as to whether or not a Torsen diff is superior to a mechanical clutch-type LSD. However, for certain applications like road racing, where a wheel has a tendency to lift off the ground during hard cornering, a clutch-type LSD is preferred.
Likewise, the same can be said for rock crawling, where the same issue can occur. In both situations, when that occurs, it will act like an open diff.
As for the torque bias argument, again, this is debatable. Sure, Torsen differentials can be had in different ratios, although, unlike the mechanical type, you cannot swap out components later.
Likewise, there is a higher upfront cost of torque-sensing differential.
Should I Upgrade To A Limited Slip Differential?
With that said, any LSD is a huge advantage over an open differential. However, for daily driving, it makes little sense to install one just for the sake of doing it.
Likewise, if your vehicle came equipped with an LSD from the factory, that unit is good enough for most conditions and can handle some off-road and track use as well.
Where an LSD comes into play for racing and off-road driving, although it depends on the events you participate in. A 1.5-way or 2-way for more extreme events is probably a better choice for competitive events with many turns, but you probably already know that.
For off-road use, again, it depends on the usage. However, aside from rock crawling, Torsens are popular and are better able to transfer power between all four wheels.
Keeping that in mind, a Torsen or a clutch pack LSD is a noticeable upgrade over an open-diff or even a stock unit in many applications. If you are serious about putting down more power, they are both worth a look.
Understanding Limited Slip Differential – YouTube