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Many people assume that at least two wheels receive power from the drivetrain. Their logic assumes both front wheels pull a front-wheel-drive vehicle or both rear wheels push a rear-wheel-drive vehicle. In about 95 percent of all vehicles, this is an incorrect assumption. In actuality, almost all vehicles on the road use an “open” differential; a mechanism that dates back to the third century in China. As a result, there are situations where the open differential only directs power to a single wheel. Having all of the power directed to a single wheel can obviously limit the full potential of a vehicle. By understanding the purpose that a differential serves, we will explore the function of open differentials, limited-slip differentials and the spool.
When a vehicle is going in a straight line, all wheels are traveling at the same rotational speed. This, of course, assumes that you have the same size tires on the left and right sides of the vehicle and that both the front and rear tires are of the same height. If different size tires are being run from the front to the rear, the rotational speed of the front wheels will not match the rotational speed of the rear tires. However, as long as the tires match from the left to the right side of the vehicle, both front tires will share the same rotational speed while both rear tires will share the same rotational speed. While traveling in a straight-line doesn’t require a differential, the need for a differential becomes apparent when you begin to understand the dynamics that occur when a vehicle makes a turn.
During a turn, the outside wheel must travel at a higher rotational speed than the inside wheel. If that sounds weird, consider this situation for clarification. If a vehicle makes a 90-degree turn on a radius of 25-feet to the inside tire, the outside tire travels on a radius of 25-feet plus the track width of the vehicle (distance between the center points of the two tires). For this example, let’s say that the track width is 60 inches or 5 feet. This would mean that the outside tire must travel on a radius of 25 plus 5 feet or 30 feet. In this example the total distance traveled during this turn by the tires would be:
Inside tire: (25 feet x 2 x π) / 4 = 39.25 feet
Outside tire: (30 feet x 2 x π) / 4 = 47.10 feet
For some platforms, you may find that the OEM offers an LSD as an option or even as standard equipment. While OEM LSDs have the same function as aftermarket LSDs, they are designed with stock power outputs in mind. As a result, the construction of OEM differentials is in many cases significantly weaker than aftermarket models. These differences can often be seen in the number of spider gears or the number and size of the clutch plates used within the OEM LSD. Hence, if you are considering the swap of your open-differential to a limited-slip differential, it’s worth the additional cost to use an aftermarket unit if you are planning to make more than stock-level horsepower.
Many different limited-slip designs are currently employed in both production and aftermarket LSDs. Cone and clutch type limited-slip differentials have been popular from the dawn of the performance era. Gear type and viscous-type LSDs are newer to the performance arena, but have proven to have merit. To say that one particular design is superior to another is not just. In reality, each design has its own merits. The focus of a performance enthusiast should be to select the best LSD to accomplish their specific performance goals. The following four questions should help to select the best LSD for your application.
Aug 26, 2015, 12:24 PM
