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Sep 12, 2023
Tinton Falls, NJ
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You may have heard the terms Trak-lock, traction assist, posi, limited slip, open rear, etc. This will explain the difference between these types of differentials.

A differential is a device that allows power to be delivered to both driven wheels, while also allowing those wheels to spin at different speeds relative to one another. This is required on a street car to allow tight turns. The most basic differential is called an "open" differential, which is simple and effective in most situations, however in cases of extreme differences in available traction between the wheels, the result is the power goes to the wheel with the least amount of traction. When you do a burnout and only one wheel spins, or when you get stuck with one wheel on ice or in mud, this is a demonstration of the shortcomings of an open differential. Ford has dealt with this shortcoming in a couple ways over the years, and specifically in our cars.

Trak-lock was the first way that Ford dealt with the limitations of the open differential. Trak-lock is Ford's term for a clutch-type limited slip differential. GM calls this same setup "Positraction", or posi for short. Chrysler called it "Sure-grip" Limited slip means exactly that; limited amounts of slip. A trak-lock diff will allow small differences between wheel speeds, however as the slip, or difference in speed between the wheels increases, the trak-lock will resist that slip and start applying more power to the other wheel. The way it does this is through clutch packs between the side gears and the carrier. As one wheel spins faster than the other, it turns the spider gears. If the spider gears start turning faster, they will exert an outward force on the side gears, pushing them into the clutches with more force. The drag from this force transfers the power through the carrier to the other side gear, and out to the other axle, thereby making both wheels spin. The advantage of this is more traction is available on slippery surfaces due to power going to both wheels, and most importantly, both tires spinning during your big smokey burnout! The downside to this setup is that it can sometimes be noisy, and the clutches will wear out over time making it less effective. Also if you are driving in slippery conditions, while an open differential is more likely to leave you stuck on the side of the road, a limited slip differential is more likely to cause you to lose control, since when going around a corner both wheels will spin, leaving no traction to keep the back end from sliding around. While we as performance enthusiasts definitely want a limited slip, the average driver did not like the noise and control issues, and so manufacturers looked for another soluition. What they came up with is Traction Assist!

Traction Assist is not located in the differential. A car with Traction Assist will have a regular open differential. Traction assist works through the anti-lock braking system. The wheel speed sensors measure how fast each wheel is spinning, and if one of the rear wheels starts spinning too much faster than the other, the system will apply the brakes to that wheel only. By applying the brake to the slipping wheel, the natural function of the open differential will be to send power to the wheel with less resistance, which now becomes the wheel that does have traction. The advantages of this system are no noise under normal operation, and no wearable components inside the differential. The downsides are that in extremely low traction situations, the system cannot keep up and will be constantly fighting itself by applying the brakes back and forth on both wheels. The result is the same as the open differential, in very slippery conditions, you go nowhere! Under normal driving, you can hear and feel when the traction assist kicks in, in which case the best thing to do is ease off the gas anyway, both to regain traction and prevent excessive wear on brakes, etc., as your engine and brakes are working against each other. If you are going to be driving in very slippery conditions such as snow and ice, it is important to turn traction assist OFF before driving. Turn it back on if you get stuck and need to get out of a particular situation, however once moving, it is better to have it off as the traction assist randomly applying the brakes to one wheel can definitely cause a loss of control, and an open differential will be much easier to control.
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