Decoding De Dion Suspensions

February 28, 2022

In my reading about race cars from the 40s and 50s, the term de Dion suspension would come up time and time again. The technology was ubiquitous across race cars from Germany, Italy, England, and the U.S. from the 1920s through the early 1960s.

De Dion suspension is an ingenious solution to the tricky problem of powering the rear wheels while keeping maximum traction. The idea had been around since the very beginning of the automobile but was largely forgotten. That was, until being dusted off by the great race car builder Harry Miller, who utilized the suspension to dominate oval racing in the mid-1920s.

The de Dion suspension single-handedly advanced automobile performance. Let’s dive into this invention to understand what it is and why it was the key difference between winning and losing.

A Brief History of De Dion

The de Dion axle was invented in 1894 by a French engineer named Charles Trépardoux who was the co-founder of De Dion-Bouton, a French automobile manufacturer from which the device took its name. He developed his invention for use on a steam-powered tricycle, but it didn’t catch on until it re-emerged in an unexpected place: the Los Angeles workshop of American racing visionary Harry Miller. 

De Dion clearly visible on 1925 Miller 122 FWD

In 1924, Miller built an Indianapolis race car unlike any before it. The revolutionary Miller 122 FWD utilized front-wheel drive and was the brainchild of Jimmy Murphy and Riley Brett. Revered as one of the all-time greats, the 122 took victory after victory in the U.S. and Europe. The de Dion suspension made the radical configuration of the 122 FWD possible. The next generation Miller 91 won the Indianapolis 500 in 1926, 1928, and 1929.

The de Dion reappeared on the fearsome Mercedes W125 in 1937 which dominated Grand Prix racing that year, and also tasted victory at the 1937 Vanderbilt Cup held on Long Island, New York. Auto-Unions quickly copied the W125 in the Type D in 1938, although with a simpler design. 

After the war, de Dion suspension was considered state-of-the-art for race and sports cars, until the 1960s when independent suspension superseded it. Almost every Grand Prix car of the era used the system, including Ferrari, Maserati, Vanwall, BRM, and so on, including iconic cars like the Aston Martin DBR1 and Maserati 250F. 

This type of suspension was used in road cars too, debuting in the Lancia Aurelia Series 4 in 1954, as well as by its successor, the Lancia Flaminia. Other manufacturers used it in production cars including Iso, Bizzarini, Alfa Romeo, Ferrari, Aston Martin, Maserati, Mazda, Chrysler, and Alfa Romeo, to name a few.  

Division of Labor

For simplicity’s sake, we are going to use as our example a car configured with a front-engine, that drives the rear wheels. (The front-wheel-drive Miller 122 was the exception, not the rule!) Once drivetrains had moved beyond belt and chain drives to prop shafts and differentials, the standard way to drive the rear wheels was through a “beam axle,” a.k.a. “solid axle,” “live axle,” or Hotchkiss device. It was ubiquitous because it was functional, but had many drawbacks that were not well understood until more powerful engines came along.

Live axle

A beam-style axle combines the differential, prop shafts, and axle into a rigid casing. In essence, it’s an all-in-one solution that connects the wheels and then drives them simultaneously. The beam axle has three drawbacks: 1) it’s a heavy component that is 100% unsprung weight; 2) the wheels are rigidly linked, so if one is disturbed, the other is as well, resulting in poor traction; and 3) the torque reaction, due to the prop shaft and axle rotating in opposite directions, loads the springs and tends to lift one rear wheel so power isn't applied evenly in corners or hard accelerations. (If you watch NASCAR and recall references to “wheel hop,” that is the torque reaction from the use of live axles.)

In particular, the issue of unsprung weight has a huge and detrimental impact on the vehicle’s dynamics, most notably having a higher ratio of unsprung weight reduces the traction of the tires, especially on uneven road surfaces. 

De Dion axle

Enter the de Dion suspension, also known as “dead axles” and often referred to as “de Dion tubes” after their characteristic tubular component. A de Dion axle separates the two functions of the rear axle of suspension and power delivery. The wheels are connected to each other by a large cross-member that is often a large-diameter tube to better balance the strength-to-weight ratio. 

Here’s How It Works

  • The De Dion tube sits behind the wheels and connects to each side’s wheel bearing mounts. It typically curves rearward so it keeps clear of the differential that is situated directly on the axis between the rear wheels.
  • The differential is now attached to the chassis, not the axle, which makes it and the half shafts sprung weight. 
  • The half shafts are then connected to the wheels via a pair of universal joints on each side so the power is transferred evenly as the wheels move up and down in a vertical plane. 
  • A device keeps the axle centered with the chassis so the wheels only travel vertically and maintain 0-degree camber. These include a Watts linkage (shown in diagram below, Panhard rod, or A-frame central link. (Watch this demo video of a Watts linkage to visualize how it works.) 

To sum it up: the tube connects the wheels and keeps them centered and vertical while the drive shafts propel them. 

Diagram of De Dion rear suspension from 1958 Vanwall GP car using a Watts linkage.

The De Dion Difference

Compared to the primitive live axle, the de Dion dead axle dramatically advanced the performance of automobiles in the following ways:

Reduces unsprung weight. The lower the unsprung weight, the better the performance. When weight is sprung, the vehicle’s suspension can control it which improves traction, cornering and road isolation. De Dions significantly lower that amount by moving the drive components from the axle to the chassis.

De Dion: Wheels stay vertical

Rear tires are always vertical. The wheels are always at 90 degrees to the road surface no matter how much the body is rolling, so the tire contact patch is constant for improved traction.

Eliminates wheel hop. The torque reaction is eliminated, because the half shafts are isolated from the axle, so the power goes down smoothly through both wheels simultaneously. 

Durable components. De Dion axles are beefy so are not prone to failure, unlike independent suspension systems that have more complex and fragile components that are prone to failure.

In the end, a superior technology came along. Independent suspensions were first used on a Grand Prix car with the dominant Mercedes W196 in 1954. However, the technology didn’t take off at the moment, mainly due to Mercedes’ withdrawal from racing following the 1955 Le Mans tragedy.

The semi-independent de Dion suspension was the bridge from the primitive beam axle to the modern independent suspension that we use to this day. However, “dead axles” are still used, and continue to be popular in Muscle Cars, where power delivery is prioritized over cornering ability. In its day, de Dion was responsible for a huge step in performance and driveability that showed a suspension that optimized traction and power delivery could be as important as brute horsepower.

References

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