The Focus RS is a weird animal. It features an advanced rear drive unit (RDU) - really an electronically controlled system that routes torque to either or both of the rear wheels under the direction of a dedicated computer.
The system uses hydraulically activated clutches that act independently on each side and, though controlled friction (or slip) in the clutches, sends more or less torque to the wheels depending on a number of factors, like throttle position, steering angle, vehicle speed, and vehicle dynamics such as yaw angle and lateral acceleration.
The system can leave both wheel entirely undriven (the car is then operating like a regular front-wheel drive) or, on the contrary, the rear wheels can get a share of the torque, either an even one or an uneven one, depending on what the computer tries to promote.
For example, during hard launches, maximum pressure is applied to both the left and right clutches in the RDU and the car briefly behaves like a true 4x4 equipped with a strong limited-slip differential keeping both rear wheels rotating at the same speed.
In other circumstances, like for example in drift mode, when the computer detects a large steering input together with a large throttle input, it sends pressure to the outer wheel’s clutch much more than to the clutch driving the inner wheel, and promotes power-oversteer, making it easy to break traction on the outer wheel, causing it to lose its lateral grip abilities, and induces a strong oversteer.
The system is infinitely variable: pressure can be finely applied to either or both clutches and the computer monitors the slip by looking at the rotation speed of the output flanges, so it can adapt to aging, and get the right amount of torque to the right wheel at the right moment to promote the desired effect.
In contrast, the front is equipped with a simple, open differential of the most common type. The problem with open differentials is they don’t do anything to prevent slippage and if one of the front wheels looses traction, it will start spinning uncontrolled and all the torque sent to the front wheels will escape through that one spinning wheel, leaving next to zero torque for the other wheel, and hurting the car’s front traction.
Front traction losses occur randomly. The road surface of course plays a big role, a patch of gravel, mud or ice, but also a bump unloading the wheel, a track kerb causing the inner wheel to loose contact with the ground and all and any other reason for the tire to lose traction will instantly cause the affected wheel to spin uncontrolled, and just as instantly remove all torque from the other wheel.
Depending on the terrain and situation, the loss of traction can be brief but also randomly affect any or both front wheels in random succession. On the font axle, you can find yourself with two, one, or zero tracting wheel(s), in any order and any number of time over the course of the same second of time, hurting efficiency and feedback to the driver.
In other situations one of the front wheels can spin “indefinitely” until either the driver lifts off the throttle, traction improves drastically, or some electronic nanny like the ESC comes into play and applies a dab of brake pressure to that one wheel to slow it down, and/or limits the engine torque until the spinning stops.
Reducing engine torque because of one spinning wheel goes against the goal of achieving maximum acceleration. Remember this can happen on a bright, dry sunny day with slick tires on a warm track: just climb on a kerb on hard cornering and your inner front wheel no longer touches the ground. If you happened to be hard on the throttle at that same time, the inner wheel will instantly start spinning in the air and all the torque directed at the front wheels will escape through this spinning, instead of pulling the car forward.
If ESC is on, braking the spinning wheel will help send some torque to the other wheel (this is how differentials work) and counter-intuitively will promote traction, but the braking itself will consume energy in the form of heat in the brake, and that lost energy will this not help pull the car forward. Finally, only a limited amount of braking can be applied as to not accelerate the other wheel too much and cause it to lose traction.
The real solution comes in the form of a limited slip differential, aka LSD.
Limited slip differentials comes in many forms. Some of them are not limiting ship in any way but are still being called that way. Let’s see what’s commonly available, in increasing order of efficiency, from worse to okay to excellent.
TOrque SENsing or Automatic Torque Biasing - same thing.
Viscous couplers
Plated differentials (ZF) <- the ones that work, TBC...
The system uses hydraulically activated clutches that act independently on each side and, though controlled friction (or slip) in the clutches, sends more or less torque to the wheels depending on a number of factors, like throttle position, steering angle, vehicle speed, and vehicle dynamics such as yaw angle and lateral acceleration.
The system can leave both wheel entirely undriven (the car is then operating like a regular front-wheel drive) or, on the contrary, the rear wheels can get a share of the torque, either an even one or an uneven one, depending on what the computer tries to promote.
For example, during hard launches, maximum pressure is applied to both the left and right clutches in the RDU and the car briefly behaves like a true 4x4 equipped with a strong limited-slip differential keeping both rear wheels rotating at the same speed.
In other circumstances, like for example in drift mode, when the computer detects a large steering input together with a large throttle input, it sends pressure to the outer wheel’s clutch much more than to the clutch driving the inner wheel, and promotes power-oversteer, making it easy to break traction on the outer wheel, causing it to lose its lateral grip abilities, and induces a strong oversteer.
The system is infinitely variable: pressure can be finely applied to either or both clutches and the computer monitors the slip by looking at the rotation speed of the output flanges, so it can adapt to aging, and get the right amount of torque to the right wheel at the right moment to promote the desired effect.
In contrast, the front is equipped with a simple, open differential of the most common type. The problem with open differentials is they don’t do anything to prevent slippage and if one of the front wheels looses traction, it will start spinning uncontrolled and all the torque sent to the front wheels will escape through that one spinning wheel, leaving next to zero torque for the other wheel, and hurting the car’s front traction.
Front traction losses occur randomly. The road surface of course plays a big role, a patch of gravel, mud or ice, but also a bump unloading the wheel, a track kerb causing the inner wheel to loose contact with the ground and all and any other reason for the tire to lose traction will instantly cause the affected wheel to spin uncontrolled, and just as instantly remove all torque from the other wheel.
Depending on the terrain and situation, the loss of traction can be brief but also randomly affect any or both front wheels in random succession. On the font axle, you can find yourself with two, one, or zero tracting wheel(s), in any order and any number of time over the course of the same second of time, hurting efficiency and feedback to the driver.
In other situations one of the front wheels can spin “indefinitely” until either the driver lifts off the throttle, traction improves drastically, or some electronic nanny like the ESC comes into play and applies a dab of brake pressure to that one wheel to slow it down, and/or limits the engine torque until the spinning stops.
Reducing engine torque because of one spinning wheel goes against the goal of achieving maximum acceleration. Remember this can happen on a bright, dry sunny day with slick tires on a warm track: just climb on a kerb on hard cornering and your inner front wheel no longer touches the ground. If you happened to be hard on the throttle at that same time, the inner wheel will instantly start spinning in the air and all the torque directed at the front wheels will escape through this spinning, instead of pulling the car forward.
If ESC is on, braking the spinning wheel will help send some torque to the other wheel (this is how differentials work) and counter-intuitively will promote traction, but the braking itself will consume energy in the form of heat in the brake, and that lost energy will this not help pull the car forward. Finally, only a limited amount of braking can be applied as to not accelerate the other wheel too much and cause it to lose traction.
The real solution comes in the form of a limited slip differential, aka LSD.
Limited slip differentials comes in many forms. Some of them are not limiting ship in any way but are still being called that way. Let’s see what’s commonly available, in increasing order of efficiency, from worse to okay to excellent.
TOrque SENsing or Automatic Torque Biasing - same thing.
Viscous couplers
Plated differentials (ZF) <- the ones that work, TBC...
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