Dialing in a race car is a long and complicated process. We do it because of the satisfaction and frustration it brings, and, more importantly, for the friends we make along the way. If we claimed in this article that the parts we installed on Ron Burgundy, our Fox Mustang project, took us from wheelspin to wheelies, we'd be lying. Although these parts did solve some of the basic problems that prevented us from getting an accurate baseline, they represent the first steps in making this car fast and reliable.
In the Jan. '13 issue, we took the Fox Mustang to the Auto Club Dragway in Fontana, California, and ran some respectable 7.80s at about 90 mph or about 12.25 in the quarter. The car has a single 61mm turbo from a Hellion Street Heat kit on what is essentially a stock engine, a 3.27:1 gear ratio, and a good 4R70W transmission from Performance Automatic. Not too bad, but on paper the car should be able to run 11.70s at its current weight and horsepower.
Looking at the photos from the Fontana event, we noticed several problems with the attitude of the launching car. It would pull hard to the right, as evidenced by the right front tire rolling under as the driver counter-steered to keep the car straight, and it would bounce and unload the rear tires once the front end came down, indicating that the dampers (shocks) weren't controlling the springs. The result was a 2-second, 60-foot time with a lot of uncontrollable wheelspin and a Fox Mustang.
The solutions to these problems are relatively simple. Since the Fox Mustang has to be the most drag raced body in recent history, there are many companies with a lot of knowledge about how to dial in one of these cars. One such company is Maximum Motorsports (MMS), which employs guys like Jack Hidley, who know just about everything about these types of door-slammers. We gave him a call to get some solutions. We'll get to that, but first, a little theory.
Center of Gravity
Everything revolves around the Fox Mustang center of gravity (CG). If you placed a jackstand in the center of the car under the CG, it would be balanced in the air with neither the front nor rear tires touching the ground. The CG is a fixed position. Looking at the car from the side, the four-link's two upper and two lower control arms form a triangle that intersects at an invisible point called the instant center (IC). The term “instant” is used because as the car and suspension begin to move, the IC moves as well. As you raise and lower the car's ride height, you change the position of the IC. The same is true when you raise or lower the upper or lower control arms. Controlling the IC means controlling how the suspension rotates around the CG and how fast and hard the tires are loaded when the car begins to move.
On cars with a solid axle, there is an imaginary line extending horizontally from the contact patch of the rear tire to where the front-axle vertical centerline reaches the height of the CG. This imaginary line is called the neutral or antisquat line. Using suspension and ride-height adjustments, you can move the IC above this line to hit the tire with more torque, or lower the IC to soften the hit on the tire. The IC can also be moved forward (closer to the front axle) or rearward (closer to the rear axle), which affects the rate at which the load is applied. Generally, the closer the IC is to the rear axle, the quicker the load is applied.
This is the adjustable, 11⁄4 -inch- od x 0.250-inch-wall-thickness, heavy-duty, drag-race
The job of the sway bar is to keep the axlehousing from rotating up into the chassis on a
The MMS sway bar is also adjustable. As the attachment location moves toward the rear of t
The springs and dampers control the speed of the car’s pitch rotation, slowing down or spe
The Tokico dampers adjust both jounce and rebound at the same time with 7.5 turns of adjus
The front struts, now converted to coilovers, do it all. Using the adjustment collar, we c
Ron's Rear Suspension
While we were trying to determine the correct tire pressure and launch rpm for the track, wheelhop became worse as we delivered more torque to the tire. Wheelhop is a result of spring wrap in leaf-spring–equipped or worn bushings and components in four-link cars. Also during the launch, it was clear the car was spinning the tires from a huge hit of torque when launching off the transbrake, hooking, then unloading again when the right rear tire rotated up into the body as the axle was loaded with engine torque. This occurred several times, making the car bounce or porpoise before finally gaining traction. The solution to both of these problems is a set of adjustable rear control arms. The new bushings solve the wheelhop problem, and the adjustability allows control of the initial location of the IC.
We also ordered a sway-bar kit. On the occasions the car did hook, it darted to the right and again unloaded the tires, causing a bit of scary fishtailing. The sway bar acts as a spring that keeps the axle housing from rotating up into the body and unloading the right rear that steers you into the guardrail. All these factors are either working with you or against you when launching a lightweight and powerful car like the Mustang at the dragstrip.
To get maximum droop, the MMS kit uses spacers to lower the strut in the car. We started w
On the left is the factory and on the right is the MMS camber plate. Aside from the obviou
This is the new, heavy-duty lower-rear-control-arm kit with ride height adjusters (arrows)
Ron's Front Suspension
Another place to look for problems is the front suspension. Under acceleration, weight transfer occurs, creating front suspension rise. The dampers control the speed in which the front end rises and droop allows the suspension to move without topping out and unloading the rear tires. The size of the front wheel and tire also have a direct effect on the 60-foot times, but we will get into that in a later story. Droop is simply the distance at which the tire and wheel move away from the body when the car is on jackstands, allowing the car to pitch-rotate and load the rear tires. The dampers we're using from Maximum Motorsports will allow us to control the amount of droop with spacers that fit under or over the upper strut bearing plate.
The springs (or a small front sway bar) help control roll stiffness, or the resistance to roll rotation, as engine torque twists the car clockwise (as viewed from the rear) toward the passenger side. Increasing front roll stiffness creates an equal and opposite reaction on the rear axlehousing, as torque lifts the driver front suspension and compresses the passenger front suspension. When torque is applied, the rear axlehousing tries to twist around the pinion gear, rotating the axlehousing in a counter-clockwise direction. Tuning and the 60-foot
The theory is that for every 0.10 second you can shave off the 60-foot time, you get 0.20 second off the e.t. in the quarter-mile. Fast race cars have a quick 60-foot time, so that is where you should start.
We spoke about deflection in the bushings causing wheelhop. Here you can see the differenc
On a solid-axle vehicle, the ratio of the front roll stiffness to the rear roll stiffness
This is the key to moving the IC. As the car is lowered, the front mount for the lower con
With the new suspension in place, we needed a baseline. The recommendation was to set the front damper at full soft to allow the front end to rise and add the thick bearing spacer to achieve as much droop as possible. In the rear, we set the shocks in the middle of the adjustment range.
Using the recommendation from Mickey Thompsons, we set the tire pressure to 12 psi and set the ride height to 273⁄4 inches using the adjustable rear suspension. With the stock suspension, Ron had a 60-foot time of 1.80, and with the new suspension and settings, the car immediately ran a 1.73. As the track got hotter, we moved the launch rpm from 3,000 to 4,000, and the car began to rotate and use the suspension. After multiple runs, and incrementally lowering the rear ride height, Ron ran a best 60-foot of 1.59 with a 7.53 e.t. at 92 mph, or 11.72 at 116.29. That is right on the power to weight ratio calculation for the e.t. and mph.
Ride height is the way to keep accurate notes about the adjustment of the rear suspension.
Grizzled racing veterans have told us many times that you aren't really working the suspension until you have 600 hp. We read that as, “you aren't doing wheelies until you have 600 hp.” And wheelies are what we want. Since the stock 302 is telling us through oil-pressure readings and subtle coolant use that it wants out, we are going to add a Ford Racing 363-inch crate engine to the car and see if we can “work” the front end off the ground. Stay tuned.
Rules of the antisquat
If your car...
Then you should:
- is heavy
- has a numerically lower gear ratio
- uses a big tire
- makes no torque
Hit the tire harder using an IC above the antisquat line
If your car…
Then you should:
- is light
- has a numerically higher gear ratio
- uses a small tire
- makes a lot of torque
Soften the hit using an IC below the antisquat line
In this sequence, you can see the car hitting the tire hard and immediately moving to the right (from the driver's perspective), as the rear dampers fail to control the speed of the car's rotation and the sway bar and front suspension allow the passenger rear tire to rotate up into the body and unload. As the car moves through the frame, you can see the nose dive as the front suspension fails to slow down the weight transfer to the front of the car, bottoming out the suspension and starting the second bounce, again unloading the tires. The MMS parts solved all these problems at once and gave us the ability to fine-tune the car.
|MMS caster camber plates
|MMS 13⁄4-inch rear sway bar weld-in
|MMS front coilover conversion kit
|MMS R lower control arms
|MMS R spring conversion
|MMS rear coilover springs
||Varies by spring
|Tokico D-Spec struts
|Tokico D-Spec shock
|Flaming River manual rack kit
|Mickey Thompson ET Streets
Tokico Performance Shocks
550 W. Artesia Blvd.
Ford Racing Performance Parts
15021 Commerce Drive S
Micky Thompson Tires
3430 Sacramento Dr., Unit D
San Luis Obispo