The pros and cons of the torque sensing differential

Background

Traction management is a concern for both automotive engineers and drivers. Think for a moment about your car going around a sharp curve at a reasonably high speed. If you know a bit of basic physics, it's obvious that the tires on the outer side of the car will have to move faster than ones on the inner side (similar to the "crack the whip" ice skating game: the outermost person on the whip ends up careening around way faster than the one in the middle). This results in a difference in applied torque between the tires on an axle. If this is not done, the car is more likely to slide off the road or spin out while cornering. The wear on the axles and drivetrain will also be unacceptable. Cornering is not the only time that tires can lose traction, as anyone who's driven in a snowstorm or even in the rain knows. It is surprising how often one or more of a car's tires will momentarily lose traction, which can result in slipping, sliding, and accidents.

The Standard Option: Open Differentials

Thus, it is neccessary to differentiate, or separate, the tires on each side of the vehicle to allow for faster or slower rotation. Most cars use what is called an "open" differential, which is basically just a bevel gear gear set in the middle of the axle. Unfortunately, the open differential doesn't really support large differences in torque. Basically, when an open differential kicks in, the wheel spinning slower (the "inside" wheel, if you consider the cornering example) continues to have a fairly high torque applied to it. This leads to an undesirable rate of spin on the lower torque wheel, which causes wear and tear on the drivetrain and is not the most effective solution to traction loss. If you've ever had a tire or two stuck in a mud puddle and had your wheels "spin" with little or no result, you've experienced this phenomenon personally.

The Torsen Differensial

You may have heard on a commercial for Subaru something along the lines of "it automatically sends power from the wheels that slip to the wheels that grip". This is precisely what the Torsen (TORque SENsing) differential does (The Torsen is an improved form of Limited Slip Differential, but not every LSD is a Torsen).

I'm not going to get into the mechanical details of a Torsen. They're very complex and the way in which it works can be confusing unless you've got a strong background in automotive or mechanical engineering. I'll just say that the Torsen uses invex gearing: a modified crossed axis helical gear mesh. This gearset allows a proportional amount of torque to be applied or removed from individual wheels as the traction conditions and torque requirements change. To compare with an open differential, you get about 2.5 times the available torque if you use a Torsen. Take my word for it...this is a good thing. =)

Pros

  • Unlike an LSD, the Torsen does not use friction clutches to control torque application. This means that a Torsen has far less wear and lower failure rates than a standard limited slip differential.
  • The Torsen does not interfere with anti-lock braking systems (ABS).
  • The Torsen is a fully mechanical system, which eliminates the risk of a computer or sensor breaking. The Torsen works instantly (or at least a lot faster than a LSD), and doesn't need external sensors to operate.
  • The Torsen is not sensitive to changes in air temperature or environmental changes. Again, it's completely mechanical, so unless it breaks, it'll work.
  • The Torsen's gear ratios can be modified to improve performance under specific use conditions. This is why the Torsen is so incredibly popular for military vehicles (the Hummer, for example) and on the racing circuit.
  • When you don't need it to work (dry, straight roads), the Torsen doesn't limit performance...but when you need it, it's the best option for traction management.

Cons

  • The big problem with the Torsen is price. Only one company (Zexel Torsen, Inc.) manufactures it. If you want a car with a Torsen, you're probably going to have to buy an Audi, BMW, or a Lexus (although some Toyotas and VWs have them). Don't even think about an aftermarket install unless you're Bill Gates. One of these things runs anywhere from $1200-1600 US...and then you're got to find (and pay) someone to install it...which can require serious modifications to the rest of the drivetrain.
  • The only other con that I can think of is possibly maintenance/repair issues, especially if you want modifications for certain driving conditions. Honestly, I wouldn't really know, since the Torsen I work with is not being modified, and will probably never need repair.

The Bottom Line

Torsen is good technology, and is a definite improvement in both design and performance over an open or limited-slip differential. However, the cost seems to keep it from being more widely adopted.

As of 2004, the Torsen differential has become much more accessible. In addition to Zexel Torsen, a company named Quaife now produces automatic torque-biasing differentials. ATBs are manufactured for virtually any car, from the original Mini to the Viper SRT-10. A Torsen or Quaife differential for a Ford Focus, for example, can be had for under $700, and requires little modification to install.

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