What's the Diff?

January 24, 2022

The differential is one of the least understood systems in a vehicle, yet this ingenious device is one of the most critical for power delivery and driveability. A great many can describe how an internal combustion engine, transmission, or suspension works. Not the case when it comes to differentials! 

This article aims to provide a working knowledge of them including their design, history, and different types. By the end, hopefully, you will have a deeper understanding and appreciation of the critical role they play in making our cars safe and fun to drive. 

The Basics

The differential, a.k.a. the “the diff” or “rear-end,” is the final system in the drivetrain. The name comes from the fact that it differentiates the power that goes to each wheel. A diff takes the power generated by the engine, after the transmission has converted it to match the vehicle’s speed, and applies it to the axles which drive the wheels. It accomplishes this by using gears to rotate the power delivery by 90 degrees through the perpendicular rear axle, then on out to the wheels. 

A differential is a complex beast solving a complex problem of torque, rotational mass, and weight transfer, caused by the fact that the inner and outer wheels travel different distances in a four-wheel vehicle. If power were delivered equally when cornering, the inside wheel would hop in a corner under power and would resist the steering input. Net result: the car would be virtually undrivable at speed and would understeer like crazy.

3D Rendering of an Open Diff

Differentials look like an M.C. Escher drawing when you get a glimpse inside; gears and teeth are going in every direction. In a basic configuration, the power is transferred to the rear axles by the following four gears:

  • Pinion (or drive) gear - Attached to the end of the driveshaft and looks like a swirly, soft-serve ice cream cone. 
  • Ring (or driven) gear - This receives the power from the pinion and converts the power by 90 degrees to align with the rear axle.
  • Spider (a.k.a. satellite or planet) gear - The spider gear is the crux of the differential. The bevel gears allow the power to transfer to each axle individually to compensate for their different lengths of travel.
  • Side gears - These bevel gears are attached to each axle and receive the power from the spider gear(s).

The configuration is clever and tames power delivery to the wheels.

History of the Differential

Differentials were invented long before automobiles, with simple mechanical versions going back to ancient Greece and China. The precursor to the automotive application was patented by French watchmaker Onésiphore Pecqueur in 1827 for use on a steam wagon, followed closely in 1832 by an English patent by Richard Roberts for “gears of compensation” for “road locomotives.”

Automotive pioneer Karl Benz applied a simple differential designed for bicycles that worked with chain drives. An Australian named David Shearer used a gear-driven differential on a steam car in 1897. The technology wasn’t adopted at first, with many early cars delivering drive to just a single wheel, giving way to a solid axle design.

In 1932, Ferdinand Porsche commissioned German engineering firm ZF to build a limited-slip differential (LSD) for the Auto Union Grand Prix car to improve performance. The LSD (again, more on what that is, in a sec) was commercialized by General Motors, and is known in the U.S. by its brand name Positraction (“positive traction”), or "Posi" for short. It was introduced in 1957, and made famous by the movie My Cousin Vinny. Similar systems were available by other automakers under other trade names. It became a popular feature in Muscle Cars of the 60s and 70s.

The Different Diffs

Differentials, as with most assemblies, have evolved significantly over decades, from the rudimentary to the sophisticated. For the most part, it is one-way traffic, with a car of a specific era utilizing the most advanced version of its day. Occasionally engineers and automakers have taken a step backward in a bid to compromise between different design specs, but this is a rare exception indeed.

Solid Axle or Spool. As simple as it gets. One gear between the driveshaft and the axle that drives an axle and operates as a solid piece, although some use half-shafts. This makes a car understeer and lose traction, not something that is typically beneficial!

In some cases, a differential isn’t needed. Drag racers and vintage Indy Cars are good examples of how power delivery to the wheels is not compromised by both wheels getting an equal amount of power. The trade-off is that the car is lighter and more reliable. Famously, Porsche used a spool axle in the Porsche 962, allowing them to drive back to the pits, which is not possible with other configurations.

Open. This allows the power to freely transfer to the wheels through the differential. Common in older cars, the open differential has a major weakness: If one of the wheels has little traction, i.e. is slipping. In that case, the differential will send all of the power to the slipping wheel, so the car just sits and spins its one wheel and the car goes nowhere. Pretty common in the old days during a snowstorm.

Limited-Slip. Currently, this is the most common type in use. The main difference is that if one wheel starts slipping, a mechanism will limit the power going to that wheel. This means the car has a single wheel doing the job of propelling the car -- a good thing! Several technologies have come along for LSD. There’s the “clutch pack” that operates using friction plates, as well as “cone,” “hydraulic locking type,” and Torsen(r). 

Electronic. The modern variant is to not allow “stupid” mechanical systems and enable “smart” computers to do the job. In the same way that ECUs (electronic control units) can map ignition, they can also map traction. If you think of a modern sports call with different “sport” settings, these control suspension, and differential mappings accommodate high performance or comfort as the operator desires. It's a huge advancement brought forth by racing teams from Formula One on down, but they are spendy and do not offer much benefit outside of performance driving. 

Diff Vocabulary

As we conclude, just a few more items to note that we would be remiss to ignore. Here are a few common terms to remember about differentials: 

Rear-axle (or Rear-end) Gear Ratio. The ratio between the ring and the pinion gear represents how far the rear axle turns for every revolution of the driveshaft. If the pinion has 10 teeth and the ring has 40 teeth, it takes 4 turns of the driveshaft to propel the axle one complete revolution. This would be expressed as a gear ratio of 4.0:1. Adjusting the rear-axle ratio can change the characteristics of how the power is delivered, optimizing for top speed, highway cruising speed, or high torque, for example.

Transaxle. An integrated system that combines the differential, transmission, and axle into a single assembly. It is commonly used when the engine and the drive wheels are co-located, e.g. front-engine and front-wheel drive or rear engine and rear-wheel drive. However, it was used often in racing and sports cars to balance the weight distribution, and was a common way to achieve 50/50 weight distribution between the front and rear axles. The transaxle was first used on a 1934 Skoda Popular, and then in the groundbreaking Lancia Aurelia.  

Locked Differential (or ‘lockers’). In certain conditions, the driver may want the wheels to receive the same power and turn in unison, so some differentials can be locked to do just that. This is common in offroading and 4x4s when crossing rough terrain or slick conditions at low speeds. It’s also popular in the sport of drifting. 

That is a primer on differentials, but for those in the know, there is an endless amount of adjusting that can have transformative effects on how a car handles, especially in race conditions. Hopefully, this provides an appreciation of the value of differentials so that you can explore them on your own, or with your trusted mechanic’s help.

References

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