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Living With Limited Slip Differentials

Tail Lights
Jim Allen | Writer
Posted July 1, 2010

LSD Use Made Simple

In the hardcore wheeling world, maximum traction at any cost is the order of the day and lighter-duty options are often forgotten. The cost-effective answer for many on the low-adrenaline side of fourwheeling and work-truck environments is still the plain and simple limited-slip differential (LSD), and it's past time we shed light on the topic.

Open differential performance is only as good as the traction from the tire with the least grip. It all comes down to one basic rule: the average speed of both axles will always equal the speed of the ring gear. Going straight down the road, if the ring gear is turning 100 rpm, so is each axle ([100 + 100]/2 = 100). If you put one tire on ice and the other on dry, sticky pavement, torque takes the path of least resistance to the low-traction tire, which spins, and the high traction tire is stationary. If ring gear speed is 100 in that circumstance, then the spinning tire is turning 200 rpm if the stationary tire is 0 ([200 + 0]/2 = 100).

The same rule applies to a limited-slip, but it can shuttle some torque to the high-traction tire. The amount of torque that can be transferred from the low- to the high-traction side is commonly expressed as the torque bias ratio (see mini-glossary on page 68). The bias ratio is achieved by applying a "brake" or artificial torque/traction to the low-traction side. If you slow the spinning wheel down by any amount, the stationary wheel has to speed up by just as much.

In general, there are two LSD types: clutch and gear. The clutch-type LSD comes with plates or cone clutches on each output (axle) side of the differential, behind the side gears. With a torque load on the axle, gear-separation forces between the side and spider gears force the side gears outboard, squeezing the clutches between them and the diff case. Increased resistance ("braking") is the result. Preloading the clutchpacks via springs adds to that resistance. If there is very little grip at the tire, the preloaded clutches are the torque ("artificial traction") that the differential needs to sense on the low-traction side before it can shuttle some of the torque. A gear-type LSD achieves the same braking via built-in rotational resistance in the gears.

For reference, many OEM LSDs are under a 2.0:1 bias ratio, and you will barely know they are there (on the street or the trail). The aftermarket units pick up from there and the more streetable ones will run to about 3.0:1. Around 2.5:1 is very common because it offers a noticeable improvement on the trail without bad manners on the street. In most rigs, bias ratios between 2:1 to 3:1 will be fairly transparent. A unit above 3:1 may be ill-mannered, depending on the weight and size of the vehicle. Gear-type limited slips can edge higher and stay more mannerly because they are typically not preloaded. If an aftermarket limited-slip is built for front axle use, it's usually no more than around 2:1.

The reason we have a differential in the axle is to allow turns without tire scuffing. In a turn, the outer tires travel a longer distance than the inners. Without differentiation, one tire scuffs. The LSD operates like an open diff, allowing one wheel to speed up and the other to slow down (always proportionally), but the clutches resist. Most LSDs have a certain amount of preload in the clutches, which means that the clutches are partially applied all the time. The question becomes: Will the clutches break loose before the tire does? Before the clutches can slip, a certain amount of "breakaway torque" is needed. In a high bias LSD, that torque may be more than the tire can support, so the tire scuffs (and barks). In a turn, weight transfer unloads the inner rear tire, so that's usually the barker. For this reason, a high bias LSD may be squirrelly on slippery roads due to tires skidding. If you live in a winter environment, you don't want a high bias LSD!

Sources

AUBURN GEAR, INC.
Auburn, IN 46706
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