A few weeks ago, I went to the amazing Auburn Cord Duesenberg Automobile Museum in Indiana, where I learned about an axle that has not just one gear ratio like in pretty much all modern cars, but two. Here’s how this fascinating contraption works.

Nowadays, we’re seeing transmissions with eight, nine and 10 gears as automakers strive to make advancements in acceleration performance and fuel economy. But back in the 1930s, before these wild transmission ever came onto the scene, and even before the highway-oriented “overdrive” caught on, Indiana-based car company Auburn used a different method to get the most out of its engines: a dual-ratio differential.

The contraption was designed originally by Ohio-based Columbia Axle Company—which, like Auburn, was a subsidiary of Cord—and introduced by Auburn way back in 1932. I saw it underneath this gorgeous 1936 Auburn 852 Phaeton, though essentially the same design was later also an option on Fords, Lincolns and Mercuries, according to Hemmings:

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Here’s a photo of how the setup looks from the outside. There’s a wide differential cover in the front, with a big vacuum canister and two vacuum hoses going to it:

The dual-ratio differential is exactly what its name suggests: the rear differential provides two different gear ratios: a shorter one for maximum tractive force for faster acceleration or steep grade-climbing capability, and a taller one for quiet, fast highway cruising (the Columbia two-speed axle was said to be able to drop the Ford V8's 55 mph cruising RPM from 2,713 to 1,940).

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Image taken at Auburn Cord Duesenberg Automobile Museum

The Auburn Cord Duesenberg Automobile Museum describes how this works in its display, saying:

“Drivers opted for either high or low ratio from a lever mounted on the instrument panel or steering wheel. This lever was linked to a vacuum cylinder. By moving the lever, the vacuum cylinder moved a mechanism in the rear axle, either bridging into operation a planetary gear set for the high ratio, or locking it out for the low ratio.”

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While there were different axle ratio options depending upon the engine, on average, the museum exhibit says, the low ratio was about 4.80:1, and the taller ratio was 3.20:1. For comparison, that’s a bigger spread than is available on a Jeep Wrangler JK’s differential gears, which go from the fuel economy-oriented 3.21:1 rear axle ratio on lower trims to the torquey rock crawling-oriented 4.10:1 ratio on the Rubicon.

Image from Auburn Cord Duesenberg Automobile Museum

A normal car’s axle ratio is set by simply the quotient between the number of teeth in the ring gear (which ultimately spins at the same rate as the wheels when a car is traveling on a dry surface in a straight line) and the number of teeth found on the pinion gear (which is spun by the driveshaft coming from the transmission or transfer case).

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But in the Auburn’s Columbia dual-ratio differential, there’s actually a planetary gear set built into a housing around the differential (you can see the housing in the instagram video above); that planetary gear set’s sun gear or “sliding clutch gear” can be locked or unlocked, ultimately changing the rear axle ratio.

I found a good description of how this works in what appears to be a repair manual posted by Restorecarsclassified. It reads:

[The Auburn Dual-Ratio rear axle] consists of a planetary reduction gear built in a housing around the differential assembly ([the] ring gear [is] mounted on this housing so that it transmits drive from the engine to planetary pinions which are likewise mounted on it). Internal gear is integral with differential housing and sun gear is mounted on a sleeve which is free to revolve on the axle shaft within the dual-ratio housing.

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It goes on:

The control mechanism consists of a sliding clutch sleeve splined on the sun gear sleeve which is shifted by a vacuum cylinder so as to engage clutch teeth on a stationary gear bolted to the left hand differential bearing pedestal (high or dual-ratio drive through planetary gears), or similar clutch teeth on an extension of the dual-ratio housing (low or direct drive with planetary gears locked sot hat the entire mechanism revolves as a unit). 

Here’s the full-page explainer showing all the components in an Auburn two-speed axle. Notice how the pinion gear (labeled “A”) spins the ring gear (labeled “B”), which rotates the case (labeled “C” and “D”), which then turns the planet gears via their pins (labeled “L”). The differential case itself (labeled “M,” which includes the spider gears that turn the axles) gets spun by the planetary gear set’s ring gear (labeled “K”).

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Image from Auburn Cord Duesenberg Automobile Museum

The aforementioned manual I found online describes how the switch uses vacuum to yield two ratios, beginning the discussion with direct drive (in which the pinion gear drives the ring gear just like on a normal axle):

When the vacuum line from the intake manifold is connected to the rear end of the operating cylinder, the piston moves toward the axle, rotating the yoke and shifting the clutch sleeve in so that the dual-ratio housing (planetary pinion drive plate) and the sun gear are locked together and rotate as a unit. The pinions are thus prevented from rotating on their shafts and act merely as clutch teeth to revolve the large internal gear at the same rate as the housing (ring gear speed), affording direct drive.

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It goes on, telling how the dual-ratio “overdrive” works, saying:

When vacuum is connected to the forward end of the cylinder, the clutch sleeve is shifted out so as to engage the stationary gear bolted to the axle housing (differential bearing pedestal) so that the sun gear is held stationary. The rotation of the dual-ratio housing causes the plantary pinions to revolve as they are carried around the sun gear causing the internal gear on the differential housing to revolve at a different rate, affording a high axle ratio.

So in other words, the driver’s switch—mounted to the steering wheel or dashboard—simply sends vacuum to one side of that cylinder mounted on the front of the axle, which ultimately slides a sleeve so as to either lock the planetary gear set’s sun gear with the axle’s “bearing pedestal” (which is part of the axle housing itself—in other words, this stops the sun gear completely) or to lock the sun gear with the rotation of the “dual ratio housing” in which it sits (“C” and “D” in the image above).

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Image take at Auburn Cord Duesenberg Automobile Museum

The actual shift takes place once the clutch is engaged, so that its’ not as clunky, as Hemmings describes:

Normal operation of the overdrive unit involves getting the car up to speed in 3rd gear, then depressing the clutch pedal and pulling on the overdrive cable. After hearing a solid clunk (telling you the overdrive’s been activated), you release the clutch and you’re in overdrive. The second speed reduced the axles’ original ratio, resulting in a 30% reduction in engine rpm. Although intended for highway speeds, you could engage the overdrive in 1st or 2nd as well; it actually worked in any forward gear.

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And of course, there’s a special two-speed gearbox adapter for the speedometer that takes adjusts the speedo cable’s output to the gauges to make sure the reading is accurate whether in the high or low rear axle gear.

Dual-ratio differentials like the Auburn’s have since died off in the auto industry, presumably because of their clunkiness and because of transmissions that can accomplish the same job (it’s worth noting that the current Jeep Cherokee KL technically does have a “two-speed” rear drive-module, though it works markedly differently and is used for low-speed crawling). That said, it’s not uncommon to find such vacuum-controlled two-speed axles on older work-trucks. The video above depicts a gentleman showing off how he operates the two-speed differential on his old Chevy.

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And here’s a great video explainer at how Eaton’s design, which is found mostly in heavy duty trucks:

Sure, the two-speed axle has fallen out of favor in passenger cars, though that doesn’t mean it’s not fascinating. I’ll be keeping my eye out at the junkyard to find one, if only just to tear it apart.