Terra Firma - Rear Suspension Guide - Popular Hot Rodding

The Three-Link Set-Up
Another suspension type common on the racetrack but not seen very much on street cars, is the three-link. Historically, three-link suspensions have not been common with OEMs, but they have recently started showing up, most notably on the new '05 Mustang. Many racecars have won with this design, but there are not many choices for this in the aftermarket short of fabricating your own setup. One company that is working on a solution to this is Lateral Dynamics in Carlsbad, California, where mechanical engineer Katz Tsubai and owner Mark Magers have been hard at work making this design work in our classic cars. We asked Mark to give the quick explanation of what a three-link suspension is. Mark told PHR: "Three-link rigid axle suspension systems derive their name from the fact that the axle is located under the car with three trailing arms, in conjunction with either a Panhard bar or Watt's link to locate the axle assembly laterally. In virtually every case, there are two lower arms that connect under the axle assembly, and one upper link that is attached to the axle assembly in the middle, right above the pumpkin."

One big advantage to using a three-link: there's virtually no bind associated with the design. Each link has one job and one job only. The two lower links working together prevent the axle from skewing relative to the vehicle axis and the upper link is responsible for controlling pinion angle changes in bump and droop. This allows the system to be set up for whatever type of driving you plan on doing, whether it's drag racing, road racing, or a combination of the two. When we asked Mark and Katz why they chose the three-link approach, they said, "Whether we want to accept it or not, all suspension systems are faced with compromise; you simply cannot have everything in one system that you'd like to. You need to make choices as to what is most important. In our opinion, the three-link approach arguably offers the fewest high-performance compromises of any rigid axle suspension system. In addition to being very simple and highly tunable, the three-link also provides the lightest unsprung weight--in a rigid-axle suspension--relative to all other setups. These facts are the reasons that the fastest road race rigid-axle cars predominantly use this setup, such as today's Trans Am and SCCA GT1 cars."

Mark was quick to point out that, like all rear suspensions, the three-link also has a couple of pitfalls. The biggest downfall with a well-executed three-link application on early musclecars is packaging. A good three-link setup will usually require some intrusion between the rear seats in the interior of the car for the upper link, though this can be minimized, and a custom rear seat can be installed that will easily accommodate clearance requirements. A quality three-link system will also set you back a good stack of cash. Lateral Dynamics Mechanical Engineer Katz Tsubai also added this in regards to three-links: "Not all three-links, or any other suspension configuration for that matter, are created equal. These advantages and disadvantages not only show you inherent characteristics of each configuration, but also design possibilities and flexibilities. The suspension is only as good as how you can implement those possibilities into the design. For example, let's say someone decides to build a three-link. His design consists of 10-inch long upper and lower links, all three mounted level to the ground, with a 20-inch Panhard bar mounted above the third member. Technically, it's a three-link and there is no roll bind, but it is hardly a good suspension system. Engineering, design, and execution are just as important as the suspension configuration itself."

Independent Rear Suspension
Independent rear suspension, or IRS, has been around on production cars for decades. It works in very much the same way as your front suspension does but without the steering provision. There are upper and lower control arms, along with a coil spring and shock. The main advantage an IRS has is its ability to react over uneven surfaces. Unlike a solid axle, the movement of the left wheel is not affected by the right wheel hitting a dip or bump, and visa versa. Several companies, including Heidt's and 21st Century Street Machines, offer rear IRS kits. We contacted Ed Bednar of 21st Century for his take on independent rear suspensions. When we asked Ed why he would steer a customer toward an IRS he said, "The real performance of a car and its suspension begins with the performance of the tire. In order to maximize the performance of the tire, as much of the tire contact patch as possible must be maintained with the road. The contact patch is maximized when the tire is vertical to the pavement. IRS geometry is designed to try and maximize the contact patch for overall performance." An IRS allows for greater control of camber settings in an effort to keep the tires as vertical as possible to the road surface. This ability to maximize the contact patch and to react 'independently' to pavement variations is why almost all modern sports cars run an IRS. Overall, the upside to this system is that you can get new-car ride and handling characteristics in your classic musclecar.

Down sides to IRS would be that they are expensive to buy, and in most cases, not very easy to install. They are also at a disadvantage when it comes to drag racing compared to their stronger solid-axle counterparts. According to Bruce Griggs of Griggs Racing, "In many cases people talk of the lower unsprung weight of an IRS system, however, they forget that the overall weight of the system usually negates the unsprung savings." More than a few owners of new Cobra Mustangs have found themselves swapping in solid-axle rear ends in place of the factory IRS since the IRS was hurting them so much at the drag strip. Again, picking the right system for how you will use the car is critical.

Truck Arms--The NASCAR Favorite
Chances are, the only time you have seen a truck-arm style (aka center drive) rear suspension is if you looked under a NASCAR racer. Winston Cup, Busch Grand National, Craftsman Truck, and IROC drivers have been tweaking this technology for the last 30 years. Hot Rods To Hell, in Burbank, California, has been retrofitting classic cars to center drive technology for over 10 years, so we hit them up for their insight on this system. HRTH had this to say, "To start with, a truck-arm suspension cannot bind under any type of condition. The suspension stays supple, the arms converge on an actual instant center and, because of their length, they remain neutral throughout bump, droop, and roll travel. The axle travels straight up and down so the wheelbase also remains constant. Your driveshaft slip yoke barley moves, and torque-induced chassis twist is eliminated. Also, the chassis is no longer being pushed around from the rear outer extremities by short little links, or the front half of leaf springs." HRTH also mentioned that the system eliminates wheel hop and is fully adjustable. These adjustments allow you to set the ride height and still maintain a level Panhard rod.

To find out a bit more about how a truck-arm suspension works, we hit up suspension specialist Kurt Binkley for his take. Kurt works for PPC Racing, which handles the cars for Kenny Wallace, John Andretti, and Craftsman Truck Series driver, Terry Cook. In Kurt's words, "Well, the standing joke is that all NASCAR cars are basically '67 Chevy trucks with headers and an aero-package!" Kurt went on to explain how the truck-arm suspension works, "The truck arm is an I-beam arrangement that pivots on a mounting point just aft of the trans tailshaft and is hard-mounted to the axle. There are various ends used at the forward mounting point depending on racetrack distance. The adjustable Panhard rod allows the opposite sides to be set for proper handling at various tracks. Coil springs rest in pockets on top of the truck arm, just forward of the axle. The upper pockets have a vertically adjustable plate that can compress or decompress the spring to change the ride height for specific tracks and conditions. When a driver comes into the pits and the pit crew adds or removes height, they are adjusting this plate up or down." This design allows for quick fine-tuning of the suspension, including the pinion angle, by the NASCAR mechanics. When we asked Kurt why NASCAR continues to use this technology he said, "It's required that we run this type. NASCAR is still old school." It also can be argued that this setup gets the job done on some of the fastest racecars around.

According to mechanical engineer Katz Tsubai, "The pros of a truck arm suspension are stable IC location, good roll steer characteristics, and good yaw response. This results in consistent behavior under both acceleration and braking. As for downsides, he points out the overall weight of the system, both sprung and unsprung, is quite high. Also, since the trailing links are rigidly mounted on the axle, components must elastically deflect in order for the suspension to roll. Try visualizing the whole suspension system as a giant anti-roll bar, with the axle assembly being the bar and trailing links being arms. You can see why the system restrains the roll." It should also be noted that running a full exhaust system with this type of rear suspension is quite a challenge due to how much real estate the components eat up under the car.