A Gripping Tail: LSDs, The Key to Superior Car Control

August 1, 2013
Story By
Evan Griffey

A limited slip differential, or LSD, puts the ‘action’ in ‘traction’ by maximizing the grip of a car’s drive wheels. All is well in a car with an open diff as long as things are calm - but its downfall is realized when driving intensity is ratcheted up a notch or two (as I'm sure you Driving Line readers are prone to do from time to time). An open diff is designed to apply power to the drive tire experiencing the least resistance. Get busy and push the suspension through the corners to the point where the inside tire is unloaded, and you end up with wheelspin because the open diff directs more torque to the unloaded tire. The same thing happens when sudden acceleration causes the unloading of a drive tire. LSDs funnel power to both tires in these situations, making the car more stable and better connected to the track through the turn. There are two distinct LSD designs: gear drive and clutch pack.

Gear drive LSDs fall into two categories; helical gear and Torsen - with the big difference being how the gears are situated within the diff. Helical diffs feature horizontally situated gears, while Torsen designs run gears up and down in the case. A gear-drive style of LSD may also be referred to as torque-biasing or torque-sensing.

Instead of a gear set, a clutch-style LSD uses clutch packs located on each side of the pinion shaft to apply power to both axles. There are some noteworthy differences in how these designs operate. If the contact patch on a gear-drive LSD car is lost, the unit will unlock and act like an open diff - but clutch-pack LSDs will remain active or locked as long as torque is being applied to the clutch packs. Gear drive units require less maintenance, are quieter and tend to last longer than clutch set-ups - but clutch types are fully rebuildable where gear types are not. 1.5-Way or 2-Way…What’s the diff? The big difference surfaces upon deceleration.

A 2-Way provides lock-up during deceleration and is generally ideal for high-speed road racing and drifting use, as it provides better braking stability for competition cars in heated situations. Since a 2-Way is fully locked during both accel and decel, it might however, become disadvantageous in a very technical track with tight turns, depending on the driver’s experience and skill. One by-product of the 2-Way is the ease in which oversteer can be induced in certain applications, which makes this style a good call for beginning drifters.

The 1.5-Way diff will lock the drive wheels during acceleration, which can be looked at as the ‘1’ in 1.5 but is 50-percent less aggressive in decel mode than a 2-Way, which can be seen as the ‘.5’ part of the equation. The 1.5-Way LSD’s less aggressive decel personality makes it better suited for tight tracks, time attack, drag racing and autocross. It is also considered the best design for daily driving because its less aggressive lock-up during deceleration and/or braking translates into less push from the rear, much more neutral steering and improved control under duress-braking and the like. Reputable LSD manufacturers include, under the clutch-style banner: Kaaz, OS Giken, Cusco and Tomei. Gear type makers include: Quaife, Torsen, Wavetrac, Auburn Gear and Detroit Truetrac.

LSDs are made for front- and rear-drive cars and can be especially effective when big power is added to the equation. For instance, a turbocharged Honda or other FWD platform will exhibit dramatic bump steer and be a handful to keep in a straight line under max boost. An LSD balances out the distribution in the driveline and puts the power down in a more orderly fashion…sure, you maygranulate two tires instead of one - but think of the fun you’ll have making those smoke signals. No matter your set-up, upgrading to a limited slip differential will ensure you get the most from your tires - the key is picking the right LSD for your set-up and driving needs. -Evan Griffey

A Gripping Tail: LSDs, The Key to Superior Car Control

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