Muscle Mustangs & Fast FordsHow To Tech Qa
The Science of Straight
Understanding the Mustang chassis and suspension and how they relate todrag racing.
Drag racing is one of most basic yet most exciting forms of motorsport.The concept is simple to understand and virtually anyone can getinvolved.
It doesn't matter if you're driving a stock street car, aslightly modified boulevard bruiser, or a purpose-built drag racemachine--there's a class for everyone. Fortunately, many Fordproducts--especially the late-model Mustang--lend themselves to the dragracing arena. And if you modify your machine to run quicker elapsedtimes with a greater trap speed, you'll quickly realize you must alsomodify the chassis and suspension to get the power to the ground.
There's a science to applying horsepower. To do it you'll needtraction--and lots of it. But you can't sacrifice control or stabilityin the process. You'll learn early on that the quickest way to thefinish line is a straight line. You'll also learn those who make wildruns are not the good drivers; they're the ones who end up with slowe.t.'s. Even worse, they sometimes end up in the wall. Maximizingperformance takes a basic understanding of chassis and suspension, andwith that knowledge you'll be on the fast track to low e.t.'s.
When it comes to building a quick, reliable, and safe Mustang,you should begin with the chassis. As it's the foundation, it must besturdy.
Late-model Mustangs utilize a unibody chassis (also called theframe). A unibody chassis is built from a thick, sheetmetal floorpanwith most of the body structure welded to the floor. This creates arigid platform to which you can mount the suspension components. Itdiffers from a full-frame chassis (which resembles a ladder) that usesframerails supported with cross bracing to which a body and suspensionare bolted.
There are many benefits to unibody construction, includinglighter weight and reduced construction costs. It also allows for thefloor of the vehicle to be lower. These factors make unibody cars greatfor racing, but there are some important modifications that should bedone to prepare a unibody car.
First and foremost, you must strengthenthe chassis. When it comes to Mustangs, the unibody structure was notoriginally designed to handle the torque loads of a racing engine orslick tires. Simply stated, when the power is increased or slick tiresare added, you increase the load on the suspension components, thepick-up (or attachment) points, and the actual unibody structure.
It istherefore critical to install bracing that will increase the overallrigidity of the unibody and the pick-up points. If this isn't done--anddone properly--the unibody will flex, the suspension pick-up points maymove, and the suspension can't work as designed. The car will beunpredictable and it may handle poorly.
Because of this, we also nowknow to add a set of frame connectors. These are neces-sary because theytie together the front and rear subframes of the vehicle, and theyrein-force the unibody chassis structure.
Next on the list for anyserious racer should be a rollbar or rollcage. This type of structurewill add crash protection for the driver and strength to the unibody. Itis important to have the structure tie in to critical points in the carthat will enhance the strength. Simply welding it to the floorpan is notas good as having it welded to the stronger bracing points, namely themain floorboard crossmember, the rocker panels, and the rear shocktowers. With the engine applying torque to the K-member and thesuspension prying on the floor in the back, it becomes increasinglyimportant to also add kicker bars connecting to the torque boxes wherethe suspension parts mount.
During launch, maximum power is applied to accelerate the vehicle in the most efficient manner. Thismeans you must be able to use all the power to accelerate forward, whilenot giving up traction and while maximizing all the variables, of whichthere are many, including track conditions, weather, horsepower andtorque output, launch rpm/converter stall speed, trans-mission and rearaxle gearing, suspen- sion type and adjustment, and tire size and type.And let's not forget about the weight of the vehicle, availabletraction, and driver skill.
In that first moment of launch, power istransferred from the engine to the transmission, either through a clutchor a torque converter. Torque is multiplied by the transmission gearing,and the flow of power is sent to the driveshaft and to the rearend. Thering-and-pinion gears further multiply torque. Then the differential orspool directs the torque to the axles in order to drive the wheels.Finally, the tires rotate and the car begins forward motion.
If the rearhousing was bolted or welded solidly to the chassis, as in a dragster,you would rely solely on the tires for traction. But in the case oflate-model Mustangs, the suspension allows the four wheels and tires tomove up and down independently of the chassis structure. And because thechassis is suspended, force applied to the sprung weight causes weighttransfer. Generally, when accelerating, the weight moves rear-ward; whenbraking, it transfers forward.
But what many don't realize is the act ofweight transfer is actually caused by engine torque and gearing. Becauseevery action has an opposite and equal reaction, the rear housing, whichis attached to the axles, will rotate in the opposite direction of thewheels. Because of this reaction, force is applied to the upper andlower control arms, which are attached to the housing.
Underacceleration, the upper arms are pulled rearward, while the lower armsare forced forward. This force is applied directly to the vehicle'schassis at the control-arm attachment points.
Since the control arms areattached directly to the chassis of the car, it is this force, ortorque, that causes the nose to rise and the weight to transferrearward. The upper control arms pull the chassis or unibody up and thelower arms drive it forward and up.
Meanwhile, the front springs, whichhave been compressed, possess stored energy. When the nose begins torise, the springs release this energy and force the nose up quicker thanit could be lifted on its own.
The sprung weight will be supported bythe springs. Springs separate the wheels and tires from the chassis andallow the tires to remain in compliance with the road or racing surface.Springs also allow the wheels and tires to absorb bumps and vibrationwithout transferring them fully to the vehicle's occupants.
If the carhas enough power or enough gearing, it will lift the nose off theground. This lifting can be further enhanced with front struts that havelittle resistance on extension. Naturally, a tighter strut will slow therise of the nose. Many racers use an adjustable strut to controlfront-end rise. A properly designed strut should also be engineered tobe stiffer on the compression side so the car doesn't slam down andbottom out the suspension.
Racers can also alter the rate or dampingeffect of the rear shocks to change the rate at which the rear housingreacts when acted upon. Ultimately, it is the tuning of these two areasthat make up a good suspension. Of course, you can alter the way yourcar launches by moving weight, or by changing the launch rpm, thegearing, and sometimes tire pressure, too.
As power is increased orstiffer gearing is installed, the load or force applied to the controlarms, the bushings, and the chassis is also increased. Therefore, itshould be noted that the first thing any Mustang owner does beforehitting the track is beef up the chassis, the suspension attachmentpoints, and the control arms and bushings.
As you can see, alteringanything relating to the engine, chassis, suspension, gearing, and so onwill affect how the car hooks and drives down the track. That's why it'simportant to understand what each part does and to buy only parts thatwill complement the entire package.
In some applications, racers will install a specially designedbag inside the rear spring(s). This bag can be inflated to increase thespring rate of that particular spring. In most cases, increasing therate of the right-side rear spring can help limit roll rotation onlaunch, which helps the vehicle drive straight. A drag race-style, rearantiroll bar, however, is the preferred method of limiting andcontrolling roll rotation.
AntiRoll Bar (rear)
Many chassis builders and aftermarket suspensionsuppliers offer rear antiroll bar kits to enhance dragstrip performance.Antiroll bars (also called sway bars) are actually torsion springs, andwhen applied to the rear suspension they can limit roll rota-tion (seenas twisting) and enhance vehicle stability as well. The drag-style rearantiroll bar features two bushings or bearings that are mounted betweenthe chassis rails at a location above the rear housing. A torsion bar isthen located between the bushings or bearings, and the bar is connectedwith links that drop down and attach to the rearend housing. When launchis viewed from the back of the car, you'd see that the body rolls to theright and the rear housing actually rolls to the left. The antiroll bar,which is connected between the body and the rear, works to resist thisopposite loading and keep the rear and body from twisting apart. Theresult is equal loading to both sides of the rear housing, which alsomeans more equal traction to each tire.
AntiRoll Bar (front)
Most vehicles will be equipped from the factorywith a front antiroll bar. This bar works under the same principle asthe rear bar, but it is connected to the chassis and to the front A-armsvia end links. Since the front antiroll bar ties the lower A-arms to thechassis, it has a tendency to limit front-end lift on launch. That'sbecause the antiroll bar prevents full extension of the lower A-arms.For the purpose of drag racing, therefore, it is beneficial to removethe front antiroll bar as this will result in more rapid and increasedpitch rotation of the front end.
At points where moving parts meet or touch there willbe a certain amount of resistance or friction. It is necessary toinstall a material softer than those touching each other to reduce thisfriction. To accomplish this, bearings or bushings are installed. Thesecan be found between suspension links, inside the wheels, and inside theengine. Shown above are polyurethane bushings used on various suspensioncomponents. These generally replace softer rubber bushings, which tendto flex or distort more than polyurethane.
Anyshock or strut that is fitted directly with a coil spring is consideredto be a coil- over shock. Most coilovers are more compact and lighter inweight than stand-alone spring and shock, or strut combinations. Thisallows them to fit neatly between wide rear tires commonly found in dragracing. It also allows racers to change springs more easily.
Late-model Mustangs from 1979 to 2004 (excluding'99-present Cobras and Cobra Rs) utilize a four-link rear suspension,with two springs, four shocks, and a small antiroll bar that linksbetween the two lower arms. This system incorporates two upper and twolower control arms (also called trailing arms). The arms connect to therear housing and to the vehicle on the opposite end. Stock arms areformed from stamped steel and use rubber bushings at each end. The softrubber limits noise and vibration, but it is not optimum for racingapplications because rubber tends to deflect excessively. Theaftermarket produces quite a few variations of the upper and lowercontrol arms. Some are made from billet aluminum, some from round-tubealuminum, and others from boxed aluminum or steel.
Driveshaft Safety Loop
A driveshaft safety loop is a device made ofsteel that surrounds the driveshaft 360 degrees and is designed tocapture the driveshaft in the case of driveshaft or U-joint failure. TheNHRA and IHRA have specific requirements for size, type, and placementof the driveshaft safety loop.
Most automobiles utilize one of two types offrames--ladder or unitized body. The late-model Mustang incorporates aunibody design, which is lighter than a full-frame car and morecost-effective to build. The unibody consists of the floorpan, the bodypanels, the front radiator support, and the front and rear subframes,all of which are welded together during original vehicle construction.
Because the Mustang uses a unibody frame, it isnecessary to install aftermarket frame connectors to link the frontsubframe with the rear subframe, thus strengthening the entirestructure. Installing frame connectors increases the stiffness of thefloorboard and chassis and greatly reduces flexing of the structure,resulting in better performance from the suspension and increasedlongevity from the chassis. Frame connectors come in many shapes andforms and they can be either bolted in or welded. For the ultimate instrength, chassis builders will section the floorboard of the vehicleand drop in custom connectors, which are then welded directly to thefloor of the vehicle.
These double-adjustable upper controlarms use Heim joints at the front mounting point. A Heim joint is acaptured bearing that provides movement for the suspension link withoutany bind or deflection.
The K-member in a late-model Mustang is used to support theengine, along with the lower control arms. It gets its name from theK-shaped design. The stock unit is a steel member that attaches to thefront subframe of the body and provides attachment points for the lowerA-arms and the engine mounts. Aftermarket K-members are usually builtfrom tubular or boxed steel and therefore can be built lighter than thestock stamped-steel K-member.
Simply stated, pitch rotation is the technical term forweight transfer. It is beneficial to have sufficient pitch rotation tohelp hook, but excessive weight transfer can cause giantwheelstands--which don't necessarily make you go fast, but they do getyou on magazine covers.
This is noticed as a twisting movement seen on some dragcars during launch. A car that roll rotates badly will lift the leftfront tire on launch much further than the right front. You will alsonotice on Mustangs that roll rotate badly there is a lot of chassisseparation between the left rear tire and the wheelwell, and littleseparation on the opposite side. Excessive roll rotation is wastedenergy that could otherwise be used to drive the car forward. This alsocan upset the geometry of the suspension and lead to poor handlingcharacteristics. Excessive roll rotation can be caused by a number ofthings, including a lack of frame connectors, lack of a rear antirollbar, or mismatched springs. It can even occur from an abundance oftorque or gearing. It can also be caused by worn bushings, a bentchassis, or broken/tweaked torque boxes. Generally, you can solve thisproblem by installing a rear antiroll bar, a stiffer right rear spring(or an airbag), and a rollbar or rollcage to stiffen the chassis.
Most drag cars are equipped with a rollbar or rollcage.The rollbar/cage is a steel structure made from mild steel or lighterchrome-moly tubing built into the car's interior to provide rolloverprotection and to increase the stiffness and/or rigidity of thechassis/frame. A rollbar generally incorporates a main hoop that sitsdirectly behind the driver and is supported with forward and rearwardbars. A rollcage is similar to a rollbar, but it incorporates additionalbars that are placed above and forward of the driver. In addition todriver protection, the rollcage adds strength to the chassis--and insome cases is used as the chassis--to which all the suspension membersare attached. This type of vehicle is termed as a "tube chassis" car.Modern doorslammers (shown) incorporate a Funny Car-style cage to addextra protection for the driver.
Shocks or struts are damping devices that are mounted ateach corner of the vehicle and control the rate at which the wheelsoscillate. For the purpose of drag racing, it is beneficial to havefront struts that are loose on extension because this allows for a rapidrise rate of the front end, which assists weight transfer.
Springs act to provide a cushion between the wheels and tires,and the body of the vehicle. Drag racers will vary the size, shape, andrate of the springs to alter the effect of weight transfer duringlaunch. A tall, thin-wire front spring can be compressed quite a bit andwill possess a great amount of stored energy, whereas a short, thickspring will be much harder to compress. A short spring is thereforebetter suited for handling because it can resist compression. Commonspring types are coil springs and leaf springs.
Torque boxes are specially designed components on a stockMustang that are welded to the unibody. They provide an attachment pointfor the rear trailing arms (also called control arms). In factory trim,the torque boxes (upper and lower) are tack welded to the body of thecar, and under abuse they tend to tear away from the body. For racing orheavy street use, it is necessary to reinforce the torque boxes. Thiscan be done by welding the seams of the boxes directly to the unibody,or by installing an aftermarket reinforcing kit such as the Battle Boxessold by Wild Rides Race Cars.
Photography by the author and courtesy of the manufacturers