by Jean-Louis Gassée
This week, no iPad disquisition, no large companies engaged in contorted Kama Sutra embraces, no Google-Apple-Microsoft love triangle. We’ll revisit these topics in due course but, for the time being, let’s go back to a geeky topic unadulterated by geopolitics or markitecture: software and brakes. Last month, we looked at the software invasion of automotive braking systems. More specifically, we looked at the interplay between braking and kinetic energy recovery in the Toyota Prius.
Today, we’re going back to “soft brakes” for another set of applications: differentials and stability control.
Differentials. Fifty years ago, they wore out, they made noises, they had to get careful periodic checks and special lubrication. Now, with progress in metal allows, high-precision machining and modern lubricants, they’re rarely seen or heard of. Yet, they perform and important role and their basic design suffers from one no less critical flaw.
(Here, we’ll assume a rear-wheel drive car. The concept applies to all drive configurations.)
The important role goes like this: when the car turns, the two wheels on the same axle draw circles of different radii, smaller radius for the inner wheel, larger for the outer. As a result, the outer wheel must turn faster that the inner one. This is no problem for the front axle whose wheels are “free”, not driven. But, for the rear axle, we’re in trouble: the drive shaft attached to the gear box connects to the axle through a 90 degrees angle gear. This causes each wheel to be driven at the same speed. This is fine in a straight line but causes wheel slippage when we turn as the wheels must rotate at different speeds.
This was the arrangement when the very early automobiles mimicked horse carts. On carts, wheels on the same solid axle did slip in a turn, but said wheels didn’t have to provide any traction, the horse did. In a car, the axle provides traction and wheel slippage works against stability and comfort, to say nothing of tyre wear.
So, the differential was invented. It’s a little counterintuitive at first but it works beautifully.
See this touchingly kitsch Chevrolet video. Or this learned Wikipedia article. Relatively simple, once you get the hang of the planet gear’s role. And universal.
But trouble starts right away.
The differential provides “on-demand” difference in rotating speed. Great, but what happens if one wheel is in the mud, or snow and the other on solid ground? The differential is tricked in “believing” the wheel on slippery terrain needs more speed, all the speed. We end up with the wheel on solid ground not rotating at all and the wheel in the mud spinning madly. Just the opposite of the required solution: stop the madly spinning wheel and direct all the power to the “good” wheel, the one on stable terrain.
As you can imagine, the problem was discovered right away.
One crude but effective solution: temporarily locking the differential, that is preventing the “differentiating” planet gear (see the illustration in the Wikipedia article) from rotating. Many real 4 by 4 SUVs offer locking differentials. The crudest arrangements force you to get out of the vehicle to lock a wheel hub, but it gets you out of the hole.
More refined, automatic or semi-automatic arrangements are called limited-slip differentials. A mechanical device, a kind of centrifugal speed governor “notices” the excessive difference of speed between the two wheels. By construction, we know the minimum turn radius for the car, this gives us the maximum difference in wheel rotation. Anything more, we must intervene. The Limited Slip Differential was born. Helpful but complicated, fragile, expensive and, a more recent concern, adding a bit of weight to the car.
Enter software modulated brakes.
As discussed in the February 7th Monday Note, modern cars feature individual wheel speed sensors and fast electrohydraulic actuators reducing or applying brake pressure.
The differential’s birth defect is now a “mere matter of software”. The central computer detects wheel spin and, instead of letting the differential be fooled, the software applies brake pressure to the spinning wheel. The “good” wheel keeps receiving torque and we get out of the hole. As you can imagine, programmers delight in applying delicate variations to the braking action, thus providing a barely visible intervention.
Software being “free” we have a neat, inexpensive solution, not as good as straight mechanical locking in extreme off-road situations but good enough for many everyday situations.
Stability control. By this we mean helping drivers stay on the road. Oversteer is when the “tail” comes out, when the rear-end of the car gets outside the intended trajectory. This can end up in a spin, in the ditch or into the inner guard rail. It depends more on the car than on the driver whose instinctive reaction can make things worse. (“Lifting”, that is reducing throttle, can result in weight transfer from rear to front that exaggerates the spin.)
The same applies to understeer, when the car “pushes”, that is when it turn less than what the driver wants. Here, instinctive driver reactions are a little less problematic as they usually increase traction for the drive wheels; this is the origin of the “safer but duller” reputation of front-wheel drive cars.
Enter more sensors.
Modern cars have accelerometers sensing yaw (horizontal let-right movement) and steering wheel angle sensors. Using these, the central computer can compare intention, where the driver points the car, and result, what actually happens to the cars direction, oversteer or understeer.
For example, if the nose doesn’t want to come in, understeer, we apply braking to the rear wheel “inside” the intended turn. To complicate things, the computer also plays (gingerly, think weight transfer trouble) with the electronic throttle.
Over time, “resolution” has increased everywhere: sensors capture more delicate nuances, actuators offer finer control steps, software models of car motion become more detailed and Moore’s Law makes electronics less expensive. That’s why all modern cars feature some permutation of the devices we just discussed. This annoys some drivers who think it deprives them of fun, of the opportunity to sharpen and demonstrate their driving skills. Manufacturers react by providing ways to turn these “electronic nannies” off. Or, so they say, because attorneys are watching. In fact, the Off switch ought to be labelled Partially Off, even in German sports cars…
There is more, much more, as in amazing feats of hardware and software used to improve engine efficiency and cleanliness. Fodder for a future Monday Note.
by Jean-Louis Gassée