What's the diff: Part 2

Images by Dave Green

What's the diff: Part 2

SCCA / SportsCar Magazine

What's the diff: Part 2

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Without the right limited slip differential in your RallyCross car, you may find yourself spinning your wheels. But which diff is right for you?

In our last installment we covered what a differential does and how the various types of limited slip differentials function. This time, we’ll cover some pros and cons of each type of differential, which vehicles come with LSDs from the factory, and how to choose the right one for your car, surface conditions, and driving style.

An important part of choosing the correct differential setup has to do with your vehicle. An MR2 or Boxster has a mechanical advantage in traction thanks to engine placement. A Mustang or Camaro, however, do not. Consequently, consider this a warning that your mileage may vary. Your driving style and the surface you intend to run on will also influence your decision – if the surface is mostly hard pack, you may want a unit with less lock on acceleration or deceleration. Conversely, if you’re dealing with ice, snow, or mud, a differential with a higher lock percentage may be the best selection.

The cheapest differential is always the one that’s already in the car. As such, there are several ways you can snag a car for RallyCross that’s already equipped with an LSD. Some examples of popular RallyCross vehicles that were available with factory LSDs are some models of the venerable Mazda Miata, the B13 and B15 generations of the Nissan Sentra SE-R (Spec V version only for the B15 chassis cars), and, of course, the STI versions of the Subaru Impreza. Some hidden gems with factory LSDs include the Mazdaspeed Protegé, the Mini Cooper S (optional on 2005-’09 model years), the Chevrolet Cobalt SS, the Saturn Ion Red Line twins (optional on both), the 2004 and later Toyota MR2 Spyder, and certain E36 BMWs. Some vehicles even came with different types of LSDs within the same generation. For example, the early 1.6L Miatas equipped with LSDs came with a viscous unit, while the later 1.8L cars received a Torsen. Many will tell you that the Torsen is the superior unit, but some prefer the earlier viscous unit and it’s buttery-smooth operation.

 

If your car didn’t come with an LSD and you’re on a tight budget, a welded differential may be for you. If you have the knowledge and equipment, a welded differential only costs time and gas. If you don’t, finding a competent welder is usually less expensive than purchasing an LSD.

RallyCrossing a vehicle with a welded differential is like fishing with dynamite. It’s not precise but can be very effective if used properly. The key with a welded differential is not to be shy with the “go” pedal. Remember that welding the spider gears together means the drive wheels will always spin at the same speed. The net effect is that the car will not want to turn off power (especially in front-wheel-drive applications). The upside of a welded differential is absolute traction no matter the conditions. When driven properly, cars with a welded differential can be sickeningly fast on mud, snow, and ice. They work on firmer surfaces, too, (many front-drive road racers still swear by them), but their advantage starts to erode as surface grip increases.

If a welded differential doesn’t sound like your cup of tea, there are plenty of other options. For two-wheel-drive cars, most folks will recommend either clutch type or helical gear LSDs. The biggest advantage of a clutch-type LSD vs. a gear-driven unit is that if one wheel becomes completely unloaded, a clutch type will still split torque, whereas a gear type may become an “open” unit – a helical gear also biases power based on available traction, so in very slick conditions, its effectiveness can be limited.

Meanwhile, the nice part about a gear-driven differential is that they are essentially maintenance free and are often more durable than LSDs relying on clutches. On my personal front-drive car, I use a helical gear unit and have been very happy with it under most circumstances. The setup is undoubtedly giving up some performance to a clutch-type setup, especially against any front-driver with a one-way unit, but I love that my helical LSD is nearly bomb proof.

For the all-wheel-drive vehicles, RallyCross National Champion Z.B. Lorenc has many thoughts on the topic. “The best setup for all-wheel drive is, of course, torque vectoring front and rear differentials with an automatic center differential, but that’s super expensive and unobtainable to most of us,” he explains. “The next best thing is a clutch pack type rear differential with helical front and an automatic or driver-controlled center differential – sort of what you see in a WRX STI. Next down the line would be viscous type rear and center differentials, usually paired with an open front diff – like in the WRX.

“Bottom line,” he says, “in the rear you want a clutch pack to provide maximum drive out of the corner. Center controllable diffs are expensive, so a good clutch pack type is a more viable option. For the front, helical or even open [is best]. A front LSD can induce understeer, which is why I always prefer open units.”

Hopefully, these last two columns have helped shed some light on how differentials work, as well as what might be the right selection for your car. Diffs are a very important component in any serious RallyCross build, and there’s a lot more information to be learned about them than we’ve covered here. So, keep researching the topic for your specific vehicle and vehicle type, and ask plenty of questions both online and at RallyCross events before taking the plunge.

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