Differential Equations - Differential Tech

It's often said that it's never too late to learn, and that's great-but then youthful curiosity is a wonderful thing too. As an example, how many budding street rodders discovered early on that it was possible to make just one rear tire of just about any car go up in smoke every time you went around a corner? If that sounds like a vaguely familiar activity from your formative years, congratulations are in order; you learned early on the basic weakness of a differential.

While standard differentials (or open, as they're often referred to) have some shortcomings, the fact is they are a necessary component, and in most cases their performance is perfectly acceptable. In fact, driving a car on the street without a differential would be difficult at best. The reason for this is simple enough: Imagine watching from directly above as your street rod goes around in a circle. The outside wheels travel in a larger circle than those on the inside, and as a result, they have to turn faster. The front wheels aren't connected, so they turn independently, but in the rear it's the differential axle that allows this to happen.

Although the differential performs a necessary function, there is one major drawback. In normal operation, an open differential will equalize the power delivered through both wheels, but if one tire loses traction, the differential will transfer all the power to that wheel. Remember that one smoking tire from your past?

While a differential is obviously a necessity in a street-driven car, in a performance environment it can be a nuisance if all the power makes one tire go up in smoke. As you can guess, it didn't take hot rodders long to learn that eliminating the differential allowed power to be equally applied to both wheels, and the result was better traction. Some early enthusiasts took the direct approach and simply welded the internals of the differential into a solid mass; the result was a locked rearend that tied both rear wheels together.

Granted, welding the differential gears together was crude, but it was effective. There is a more sophisticated means of eliminating the differential, however; it's with something called a spool. Spools are used in many drag race cars; they replace the differential assembly and mount the ring gear, connecting both axles solidly together. And while they work great in a straight line, going around corners is another matter. Imagine what would happen in our overhead view scenario of your car turning in a circle if both drive wheels were locked together. The inside wheel would turn as fast as the outside wheel and that would mean something would have to give-the inside tire would have to spin on the pavement, the car would try to go in a straight line rather than turn, or more than likely, there would be a combination of both taking place. A Sprint Car that slides around on a dirt track, or a drag racer that goes mostly in a straight line won't have problems with a spool, but street cars will.

Open Differentials
The rearend of a car serves a number of functions in a front-engine, rear-wheel-drive configuration. It provides the final gear reduction with the ring-and-pinion, turns the power 90 degrees from the driveshaft to the axles, and it contains the differential.

In a standard differential the pinion gear turns the ring gear, which is mounted to the differential case. The ends of both axles slide into the case and engage what are called side gears. Now here's where things get interesting. The axles and side gears are not attached to the case. Another set of gears, commonly called spiders, is hooked to the case by a shaft; it's these gears that engage the side gears on the axles.

When the car is going down the road in a straight line, power from the ring gear is delivered through the case and spider gears to the side gears on the axles. When the side gears are both turning the same speed, the spiders remain stationary on their shaft. But when the car goes around a corner, or one wheel loses traction, the axles, and as a result the side gears, begin turning at different speeds. At that point the spiders compensate by spinning their shaft. That's how one wheel can turn faster than the other and also why all the power goes to the wheel without traction.