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.
But trouble starts right away.