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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
DIFFERENTIALS AND YAW CONTROL
I've been reading some statements on various Internet message boards (yeah, I know) regarding how a Quaife or Torsen type of differential adds some level of yaw control to a RWD vehicle above and beyond what a CLSD provides. Given that many CLSDs are adjustable for preload and lockup percentage, what additional (if any) yaw control will an automatic torque biasing differential provide?
Maybe a more direct question is: Can a passive differential of any type provide any yaw control beyond the inputs from a driver's right foot?
The quick answer is: no, a passive differential cannot provide yaw control, in the sense that we usually use the term nowadays. However, more loosely, the action of passive diffs does affect the car's yaw behavior, and the driver's ability to control the car's yaw behavior, so semantically maybe there's some sort of case for calling that yaw control. That's a stretch, though.
For readers less familiar with these devices, CLSD stands for clutch limited slip differential. CLSD designs commonly used in road racing have split differential carriers that can spread apart slightly when the carrier applies torque to the pinion shafts. This spreading is used to load packs of clutch plates on either side of the diff which create a locking effect. The amount of spreading force per unit of drive torque depends on the angle of flats on the pinion shaft, which bear against corresponding ramps on the carrier halves. There is one set of flats and ramps for forward torque, and another set for reverse torque. By selecting components with different ramp angles, the unit's properties can be varied for propulsion torque and deceleration torque independently. Additionally, the clutch packs can be preloaded to provide an initial locking torque which is always present, even when no torque is being applied by the engine.
A Torsen or Gleason-type differential uses worm gears rather than clutch discs to produce locking torque. Normally only one set of worm gears is available, so Torsens are less tunable than CLSD's. We can normally preload the gears to some extent, although the preload tends to be highly wear-
sensitive. One interesting difference is that reverse torque in a Gleason will actually relieve the preload and reduce locking torque, at least up to some value, whereas with a CLSD, preload adds locking torque in all conditions. "Automatic torque-biasing differential" is another term for a worm-gear LSD.
Actually, all limited-slip diffs bias torque automatically. The torque to the output shafts is allowed to differ by an amount less than or equal to whatever locking torque the mechanism creates. Whenever the torque bias, or difference, is less than the locking torque, both shafts are forced to turn at the same speed. Any time the shafts are turning at different speeds, their torque differs by the locking torque.
By contrast, with an open differential, the torque to both shafts is always identical, and their speeds can differ freely. It is also possible to design open diffs that split torque unequally, in a fixed ratio. These designs are commonly used for center diffs in all-wheel-drive systems, sometimes in combination with an active or passive locking mechanism. Open diffs in a front or rear axle are always 50/50.
Yaw is rotation of the car about a vertical axis: change in the direction the car is pointing. Strictly, then, yaw control would be anything that controls such motion – the primary mechanism for this being the car's steering. However, in current usage, yaw control means active, computer-controlled creation of yaw moments, intended to augment the driver's control of the vehicle over and above that provided by the steering. Usually, this is done by selectively applying one or more of the brakes.
The propulsion or retardation thrusts from the tires create yaw moments. If we have two equal propulsion thrusts from the rear wheels, and the car's c.g. is exactly centered right to left, the two yaw moments exactly cancel each other, and there is no net yaw moment from propulsion. If the c.g. is not exactly centered, the rear tire thrust forces have unequal moment arms about the rearward inertia force acting at the c.g., and there is a yaw moment toward the heavy side. However, when the c.g. offset is small, this yaw moment is likewise small.
When there are unequal thrusts from the rear tires, and the c.g. is centered laterally or nearly so, we get a yaw moment away from the greater thrust. For example, if there is more thrust on the right, the car tends to turn left.
We noted earlier that a limited-slip diff allows the rear wheel torques to differ by the amount of the locking torque. So when the rear wheels have unequal grip, there is a yaw moment away from the side with more grip. The greater the locking torque, the greater this yaw moment can be.
If the road surface has dramatically and erratically varying grip levels – for example, randomly distributed patches of snow, ice, and bare pavement – and the diff has a lot of locking torque, under power the car will tend to erratically snake right and left. Absent any computerized yaw control, the driver will have to make constant corrections with the steering to keep the car pointed in the desired direction. With an open diff, the car will have better directional stability. Unfortunately, it will also
have less available propulsive thrust.
When the rear tires have unequal amounts of grip, we face an inescapable tradeoff: thrust versus stability. No device or control strategy can give us both at once. Either we let the thrust of the drive wheels be unequal, to let the wheel with greater grip take advantage of its grip, and accept the resulting yaw moment, or we restrict that thrust to make it more equal to the lesser thrust, and gain yaw stability at a price in propulsion.
Even if we have a computer controlling the brakes and/or diff, the requirements for traction control and yaw control inescapably conflict. To achieve traction control, we need to brake the wheel with less grip. To achieve yaw control, we need to brake the wheel with greater grip. If we do both at once, we are merely turning fuel into heat and brake pad dust. The one thing we can do with computer control that we cannot do otherwise is to set some limits on the car's yaw behavior, below which the system functions to optimize thrust, and above which it changes character and optimizes stability instead.
The differential with the most driver-friendly properties, the one that makes it easiest for the driver to control the car's yaw behavior, is a completely open diff. Comparing limited-slips, the one that comes closest to this will be the one that has the smallest locking torque, under the conditions being examined. In general, worm gear diffs have less locking torque than clutch pack diffs, although this depends on the specifics of the units being compared. If the worm gear diff locks less, a car equipped with it will be more stable in yaw under power, and will therefore feel more like a car with computerized yaw control. However, the same car with more locking torque will be able to put more power to the road and will therefore be faster, provided the driver can keep it pointed straight.
So it is defensible to say that, in general, worm gear diffs provide a measure of yaw control, over what we have with CLSD's. However, this comes at a price in speed, and it is due to the worm gear diff acting more like an open diff than a CLSD does. If the worm gear unit does not provide less locking torque, it also does not provide greater yaw stability. And it is really not correct to suggest that either alternative is fully equivalent to computerized yaw control.