This story first appeared in the November 2017 of Drag Racing Edge, republished for you here to allow you something to read while we're not racing.
We’re not sure how or why it even got started, but the suspension system of choice, at least as far as fast cars are concerned is a four-link. Beginning with full-bodied cars, the standard four-link has been adapted to fast dragsters which has brought about some basic changes but it’s still those same four bars which have helped many to reduce elapsed times dramatically. And although it may go by a number of other names now; swing arms, rocker arms, etc.; it still comes down to those basic four bars which make up a suspension system.
What a suspension system really comes down to is a lever, the same lever most of us might have hated learning about in high school Science class. For those of us who may have forgotten; or want to forget; a lever is noted as one of six simple machines (wheel and axle, pulley, inclined plane, wedge, screw, and of course, lever) which can be used in a number of different ways to lift, move or apply a force to something; which in our case is used to apply the force of the engine to the pavement
Beginning in the early days and continuing through today in some lower horsepower vehicles, a triangulated device known as a ladder bar connected the rear end housing to the chassis and formed the basis of the lever, which is the fore runner of today’s four-link suspension system.
In order to understand a four-link or any suspension system for that matter, first we must come to the realization of what happens when a car leaves the starting line. With the tires firmly planted on the ground, rotational power is first applied to the pinion gear from the driveshaft. As the pinion attempts to turn the ring gear of the differential, that ring gear is directly connected to the tires. When that happens, the pinion attempts to “climb” up the ring gear which in turn wants to rotate the rear end housing upwards, limited by the mounting of any suspension links to the frame. In the case of the ladder bar (the lever if you will), it places an upward force on the end of the ladder bar; which is the chassis; and because theoretically it can’t “lift” the weight of the chassis, it in turn pushes the rear end and tires into the ground to aide in traction.
Because of the way the ladder bar is constructed, the position of the pivot point is limited. However, in the case of a four link, by theoretically extending the arms to their intersect point, one can easily see how the location of the pivot point can be almost infinitely adjusted to suit any number of chassis/horsepower combinations.
“The first deviation from a ladder bar that I saw was a three-link on Bill Jenkins’ first Vega Pro Stock car that was built by S&W Race Cars,” said chassis guru Ed Quay. Interestingly enough, Quay worked at S&W when that first Vega was built. He later went to work for Jenkins, working with him for two years before eventually opening his own chassis shop; Ed Quay Race Cars; which has built quite a number of championship race cars over the years. Retired today having sold the business, Quay might be one of the last few around who were truly there during development of today’s suspension systems.
“I believe Bill adapted the three-link from an early Chevrolet passenger car,” Quay added. “It utilized two lower bars; control arms if you will; and only one upper bar. While I believe it is the best suspension design ever, it was the touchiest of them all. You had to make certain that the upper bar was straight in line with the centerline of the car. Otherwise the rear housing would move sideways and that pivot would cause the car to turn, sometimes violently. A washer’s thickness one way or the other would make a big difference.
“Shortly thereafter, a fourth bar was added to the top which made it a little more forgiving and allowed the car to go straight. “The sad part,” says Quay, “is that nobody in those days used scales and the setups were ‘guesstamations’ at best. You tried something and if it didn't work, you tried something else.”
Back to the ladder bar though, the downside being its lack of adjustability. A four link consists of four bars that are independent of one another and almost infinitely adjustable to form the lever.
Over the years, that simple four-bar set up has been modified somewhat to include what is termed as a swing arm rear suspension, rocker arm style as well as other monikers, but in essence they are still basically a four-link set up. In the case of a swing arm, it comes down to the two lower four-link bars connected to one another and in some cases hooked to one or two shocks.
Regardless of the style; or the name; there needs to be a device in order to keep the rear end centered in the car. In earlier years, a simple track bar attached to the rear end housing and running parallel to the housing was connected to the frame. The problem with that is during suspension travel, the length and height placement of the bar affected lateral movement. The next improvement was the addition of a diagonal bar run from the rear housing diagonally forward to the front of the opposite side ladder bar or four link mounting point. Once again, depending on its mounting point, it affected body movement. The next step was the use of a wishbone, however it’s downfall was the constant maintenance required. Finally, the answer to it all was to attach the two lower bars of the four-link to one another and in essence the swing arm was born.
“We had originally termed it as a swing-link,” says Michael Weney of S&W Race Cars, “finally changing the name to a swing arm. In reality, all it did was eliminate a wishbone or any other rear end centering device.”
Travis Colangelo of American Race Cars says, “The swing arm on our dragsters creates a bridge of sorts of which a single shock is connected to, although we have made some with a dual shock, but it’s actually a true swing arm style.”
In a typical four-link set up, there are two shocks mounted behind or in some cases in front of the rear end housing. In addition, there are also equal and unequal length arms, of which Quay has always been a fan of unequal length bars. “I’ve always felt an equal length four link had an inherent bind when the pivot points did not coincide on the vertical. Anyone can see a bind if they do not coincide horizontally.” But in all reality, it comes down to that single term: a lever.
Back to the typical four-link though, in that fashion, the car can have a tendency to “roll” from side to side. The addition of an anti-roll bar which used a set of links attached to a bar mounted laterally to the chassis. This eliminated to an extent body roll but added another device which requires adjusting.
“In addition to eliminating the anti-roll bar, our swing arm makes the car very rigid especially when you’re down at the top end ripping the throttle,” said Colangelo.
In an earlier story on four links, the late Scott Weney of S&W Race Cars said, “We’re more concerned with bar angles than anything else. While those angles form the basis for the instant center it’s those angles and spreads on the rear and front mounting which do the job to make the suspension work.”
Mark Horton of American Race Cars said, “We also subscribe to angles and spreads when it comes to four links, but the I.C. [Instant Center] also comes into play.
“Typically, a shorter I.C. equates to a short lever and vice-versa,” Horton added. “A longer lever will use more of the car’s weight to plant the tires but the rear end will move slower initially. Conversely, a shorter I.C. point allows the rear to move faster initially. And changing that top and bottom bar angle can have an effect on the car squatting.”
While intersect points (I.C.) might be an easy way to explain how the system works, Quay said, “I’ve always had a problem with explaining it in that fashion as there is much more to it than that. It really is the angles of the bars which allow the car to work properly.”
There are probably thousands of engine/power/transmission combinations that dictate four-link placement. In addition, the type track you’re racing on can add another variable. Adjustments can be made to adjust a car to allow it to accomplish just what you’re trying to do. In our last issue we spoke about the misconception of what a shock absorber does when the car leaves the starting line (DRE, September 2017, Shocking Discoveries). In that story we explained how a shock separates when the car launches. The four-link can have a dramatic effect on that separation, which ultimately affects bite.
In the beginning, the upper bars of the four-link used typical rubber bushings for whatever reason. Quay might have been one of the first to use rod ends in all the links. “I have no idea why,” he said, “but no one thought using all rod ends could be made to have the car go straight. I also used an upper rear mounting position that was forward of the center line of the housing as a further way of reducing bind in the travel. At the time, no one did that. That of course was on my full-bodied cars. As far as my dragsters were concerned, because I use a shorter top link the angle changes quicker during suspension movement. I actually lengthened the top bar by moving the pivot point back to the centerline and therefore slowed the suspension reaction to a height change.”
As we mentioned previously, by theoretically extending the four link arms out, you easily find the intersect point or I.C along with the height of it in the chassis. However, also as mentioned earlier, the location of the point is not the only thing to concern one’s self with. The actual angle of the bars and their spread on both ends of their mounts can also be critical when it comes to suspension tuning. It may look confusing, but it really all comes down to basic common sense and simplicity.
Drag racing itself is very simple. Cut a perfect reaction time and run dead-on your dial or be the faster car and chances are you’re going to win. A four-link suspension is much the same when it comes to simplicity. It’s all about a lever. However, in both cases, the actual implementation of the theory is where the hard part comes in.