The Appeal of Pushrod Suspension: Why and Why Not

Chris Alston Chassisworks gStreet rocker-arm IRS rear suspension.

 

Pushrod suspension is still something of an oddity in the world of production and passenger cars—and this goes for road cars as well as their racing variants. Save for several Lamborghinis and a smattering of other supercars, few road-going machines employ this style of suspension, which always grabs the eye and screams exotic. Even plenty of successful, high-tech, frighteningly fast racing cars have their shocks mounted in the conventional upright position—so other than the cachet, what’s the appeal of pushrod suspension and how exactly does it work?

What’s the Appeal?

The pushrod setup has its own specific appeal in regards to damping—but its greater appeal is that it maximizes performance in a very broad sense. Because of its flexible nature of design and the way it can be implemented, a pushrod setup can be shaped to benefit overall vehicular performance. For this reason, this style of suspension is popular on thoroughbred racing cars which are tailor-made with the fewest compromises made in the name of comfort or practicality. Here, we’re talking IndyCars, formula cars, prototypes, or anything else that has no road-going variant, unlike production-based racers like a Porsche GT3 Cup, a Spec Miata, or a Spec E30.

Pushrod suspension is something that can be easily shaped to suit the greatest concerns in vehicle design; most specifically in the areas of weight distribution and maximization of the aerodynamic shape, as these are critical aspects of design in high-end racing cars. After all, aerodynamic grip is what sets apart racing cars by the largest margins nowadays. However, pushrod suspension can contribute to more than just these areas.

The Lovefab Enviate must remain compliant over pockmarked public roads used for hillclimbing. Therefore, it helps to keep unsprung weight to a minimum to ensure minimal wheel deflection.

Since some of the suspension components are moved inboard with a pushrod setup, their weights are no longer carried by the lower control arm and unsprung mass is reduced. This is a massive issue with racing cars, though it still influences the ride quality of road cars. The less weight associated with wheel assembly, the more effectively the suspension can keep the wheel in full contact with the road.

A pushrod setup allows designers to stick the dampers, springs, and any ancillary components in the center of the car. The more the suspension is moved towards the center of the car, and the lower the center of gravity, the less body roll and more efficiently the suspension moves. Finally, the geometry of levers and pivot points permits maximum movements to be transmitted into minimally sized equipment, which further reduces overall weight.

How it Works

There are innumerable variations to this design, but a typical pushrod suspension utilizes a double wishbone-and-coilover setup, with the coilover mounted inboard. “When the suspension compresses, the pushrod causes the rocker to rotate, which in turn compresses the shock and the spring,” notes Lino Chestang, marketing manager at Chris Alston’s Chassisworks.

The end of the pushrod furthest from the wheel is connected to a bell crank lever, also known as a rocker. The distance between the rocker and coilover mounting locations varies from one design to another, because it determines the motion ratio and stiffness required for the job. For street cars, the 1:1 ratio is typically retained. One practical area in which this design holds an advantage over conventional uprights is it can corner balance the car via adjustment of the pushrod.

The beauty of the bell crank implies an air of exclusivity. Photo credit: fiero.nl

Appropriately for something quite so specific in its design, the level of adjustability offered by a double-wishbone setup with pushrods is impressive if the user is looking for a multiplier or looking to change the motion ratio. “You can also use the rocker to change progression/digression of the wheel rate,” adds Chestang. Additionally, it produces variable rate geometries quite easily, and the motion-spring ratio can be modified via the rocker. However, that makes more sense with racing cars, not road cars.

Aerodynamic Advantages

As mentioned above, one of the most valuable aspects pushrod suspension provides is the way it can be incorporated to the rest of the vehicle’s overall shape. As aerodynamics are at the top of the list in design criteria for single-seater or prototype cars, the bodyshaping takes precedence over most other areas. Therefore, the more compact the suspension can be made, the tidier the packaging, and the greater the overall aerodynamic benefit.

50 years ago, designers knew no better than to leave the dampers out in the airstream, but unfortunately, this enlarges the frontal area considerably, and disrupts the airflow alongside the car. As aerodynamics have taken precedence in racing-car design over the last 30 years, pushrod suspension “allows designers to relocate the bulky dampers and put them into an area that is out of the wind,” states Lex Carson, veteran suspension expert/consultant from Carrera, JRZ, Moton, and Motion Control Suspension.

Observe the streamlined arms on a Formula 1 car; they’re not cylindrical — they have a sharpened leading edge to minimize drag. Photo credit: formula1-dictionary.net

This stratum of racing cars are reliant on the flow of air over the front wheels (or wheel arches), past the middle of the car, and over the rest of the body. In a sense, these recessed dampers are largely responsible for the overall level of downforce, since they dictate the amount of air passing over the rest of the body.

These benefits improve other areas of performance as well. It’s not simply about downforce and grip; an unbroken airstream helps with engine performance, too. By minimizing the drag via the suspension components, the radiators—which are typically mounted mid-length in the car’s sidepods—are fed with a greater quantity of air. They’re also supplied with cooler air, since a damper in the airstream would warm the air fed into the inlets. Though it might seem like a stretch, damper location does have a dramatic effect on engine performance.

In addition to increasing downforce, pushrod suspension reduces drag, and can help with maintaining an extremely stiff platform. The ability to multiply the damping forces offers a lot of stiffness with little wheel movement, and that helps keep the body stable, which helps provide a constant aerodynamic platform to take advantage of the air flowing around the car. Since precision racing cars depend on aerodynamic stability for their otherworldly levels of grip and agility, the appeal of pushrod suspension grows more and more understandable—but only for an elite bunch of racing machines.

The path of a clean airstream around a formula car. Photo credit: garcfd.weebly.com

A Hierarchy of Needs

Unfortunately, the aforementioned design considerations are only made when the budgets aren’t all that strict. The reality is only prototypes and racing cars at the upper echelons of the sport are subject to all these considerations. What’s more—these considerations are given priority over other parts depending on the specific platform and the overall design ethos.

Essentially, most thoroughbred racing cars are built to a very strict set of objectives, which are all performance-related. “First, the center of gravity is lowered, then the weight is typically reduced as much as possible, and then the track is widened,” instructs Dan Gardner, owner of DG-Spec and three-time national champion road racer. Every successive requirement on the checklist must bow to the one which precedes it—even the single-purpose racing cars that could never drive to the supermarket are compromised, albeit in a very narrow sense.

Making Sense with Production-Based Cars

For the lion’s share of the sporty machinery out there, the concessions are more obvious. Production cars and their racing versions are designed not just to go quickly, but to provide comfort, visibility, and usability in a variety of situations. Crucially, they’re not dependent on aerodynamics quite like the thoroughbreds are. For the few that do use wings, canards, and splitters, the aero grip is more of a secondary worry. Even GT3 cars are fundamentally compromised where the aerodynamics are concerned, and rarely do they use pushrod suspension.

With these cars, the limitations are much more obvious. To suit the demands of the street, road cars and their production-based racing brethren, have higher centers of gravity and narrower tracks than thoroughbred racing machines. With the packaging constraints, it’s quite difficult to retrofit a sedan for pushrods; it usually entails cutting and widening fenders. Additionally, rules in production-based racing series rarely allow for this sort of modification. It’s quite easy to see why this style of suspension is such a rare sight with production cars.

“In the case of the Mustang suspension, the shocks were moved inboard primarily for packaging purposes with the added benefits of additional tire clearance and a linear spring rate,” adds Chestang.

Yet, there is a demand among those looking for a hint of the exotic with their muscle car. “Pushrod or rocker-arm suspensions are positioned at the high-end of what’s currently available in the Pro Touring market,” notes Chestang. “Chassisworks manufactures a retro-fit system designed to fit under the stock bodywork of 1964 to 1970 Mustangs and Cougars, offered through our Total Control Products brand, as well as a clean-slate designed IRS back-half intended for the top-tier Pro Touring market and track day enthusiasts from Chassisworks Pro Touring.”

“The IRS system from Chassisworks is actually available in two versions, a pushrod or rocker-arm style and a traditionally mounted coilover. Both the pushrod and the traditionally-mounted shock attach to the lower control arm in the same position. Chassisworks again, chose to keep the rocker ratio at 1:1, so performance of the two styles is nearly identical. There is a minor difference in unsprung weight, but accessibility for shock damping adjustments is greatly improved with the shocks now residing inside the trunk. Either style offers great performance, but the pushrod layout gets the nod for making your buddies jealous,” exclaims Chestang.

 

Chassisworks’ gStreet Rocker-Arm Independent rear suspension.

Some believe that having the shock as close to the wheel as possible is ideal for control. “Nothing works better than a double A-arm with the shock as near as possible to the wheel,” proclaims Carrera’s Carson assuredly. Furthermore, there are plenty of challenges when moving the shock away from the wheel; for instance, it requires greater spring rates. “Typically when custom fitting coilovers to a rear suspension, the bottom of the shock is positioned close to the wheel or inline with the lower suspension link, and the top of the shock is leaned inboard of the frame rail for the additional floor height,” elucidates Chestang. “Severe shock angles result in digressive wheel rates, making for a less-than-ideal tuning scenario. Chassisworks’ use of the rocker arm allows the shock to operate as if mounted in a vertical position with a predictable linear rate.”

When looking to widen wheels within a production-based car’s confined wheelwell, the challenge becomes all the more noticeable. The only way to move forward is by freeing up space within the wheelwell itself — that is, once the track and wheel has been moved as far outward as possible. Even for the factory teams, this really isn’t a cost-effective means of finding speed.

Even quarter-million-dollar racers like the Audi R8 LMS use conventional uprights. Photo credit: Supercars.net

In Summary

“From a practical standpoint for club racers, relocating the suspension for minor advantages far outweigh bread-and-butter principles which will make their car faster in a cost-effective fashion,” says John McNulty, motorsports engineer at DG-Spec. Simply put: “money is better spent on quality dampers and lots of testing, since the juice isn’t worth the squeeze,” proclaims DG-Spec’s Gardner.

There are numerous advantages to the pushrod design, but it’s an expensive and elaborate way to go racing. However, if the builder has an innovative vision for the car, if the car needs to have a certain degree of shock value, if cost is no object, or if improving the aerodynamic performance tops the list of concerns, pushrod suspension holds plenty of promise.

 

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About the author

Tommy Parry

Tommy Parry has been racing and writing about racing cars for the past seven years. As an automotive enthusiast from a young age, he worked jobs revolving around cars throughout high school, and tried his hand on the race track on his 20th birthday. After winning his first outdoor kart race, Tommy began working as an apprentice mechanic to amateur racers in the Bay Area to sharpen his mechanical understanding. He has worked as a track day instructor and automotive writer since 2012, and continues to race karts, formula cars, sedans, and rally cars in the San Francisco region.
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