Muscle Mustangs & Fast Fords
1995 Mustang GT Project CMC Part 4 - Road Race Tuning A Mustang GT Suspension
Our CMC Mustang Hits The Track After A Race-Prepped Alignment And Suspension Tuning.
There's great excitement with any new project, and that includes the planning, purchasing, and assembly stages. Bolting together a new race car is only half the job (and half the fun). Once it's assembled, the real fun of optimizing the performance of those parts begins. With that, our Camaro-Mustang Challenge (CMC) '95 Mustang GT went together in relatively short order. Major modifications were limited to the suspension, brakes, engine, transmission, and rearend, and we covered all the details in the past three issues. In this installment, we'll finally hit the track and start sorting out the bugs.
With the Mustang about 99 percent complete, we entered a NASA Midwest Region Time Trial event at Chicago's Autobahn Country Club for the initial track test. Shakedown/test sessions are where you find all the stupid little things that escaped your attention in the final mad-thrash to meet a deadline--like forgetting not only to install the engine oil dipstick in the block, but also leaving it at home. Note: A header bolt makes a great impromptu dipstick plug.
We also learned at the car's next outing, its debut race at Road America two weeks later, that the fuel tank venting arrangement we'd "engineered" was gushing fuel on lefthand turns like a fire hose. We learned this after being black-flagged out of the lead of the race, a victory we would most likely have earned until being pulled into the pit lane for a safety check about midway through. Fortunately, both issues, while embarrassing oversights, were easily corrected, and in subsequent testing and races, we've yet to have another mechanical issue.
As with all racing, preparation is the key to success, and successful preparation starts with attention to detail, no matter how trivial. In a class as closely matched as CMC, the best-prepared cars and drivers rise to the top. In this segment, we'll highlight some of the key procedures for aligning and tuning a road racing suspension. Believe it or not, some of these techniques apply equally to drag and street-oriented Mustangs, and getting the suspension working right is often the difference between winning and losing.
Scaling The Chassis
Corner-weighting is the process of measuring and adjusting the amount of weight applied to each of the car's four wheels to balance the chassis. For a stock SN-95 or earlier Mustang, the F/R weight distribution is heavily biased to the front, typically as much as 58/42 percent for a stock street Stang. Although the ideal weight balance for road racing is close to 50/50 with a bias to the rear, on a front-engine Mustang without radical weight reduction efforts, it's hard to achieve a weight balance much better than about 56/44, especially within the restrictions of the CMC rules. For a 3,150-pound Pony like ours, the front/rear weights are approximately 1,765 and 1,385 pounds, respectively. To make a one percent change to the rear, about 32 pounds needs to be moved from the front of the car to the rear, while getting to 50/50 requires redistributing almost 200 pounds from the front to the rear. Since CMC rules prevent the kind of radical modifications needed to achieve ideal distribution, we focused our efforts on first getting down to the class minimum weight of 3,150 pounds, then on adjusting corner weights to achieve a 50/50 crossweight, which means the sum of the left front/right rear corner weights is equal to the right front/left rear, to ensure that the car reacts equally to left and right turns. To do it, we need a set of level platform scales and a way to adjust the weight on each corner. With the car on the scales, there are two ways to make a change in corner weight--raise or lower the installed height of the spring on that corner, or physically move or add weight to the chassis. Changing ride height at one corner affects the load on the diagonally opposed corner, a process known as jacking, but as the load is changed on one corner, it affects the others too. Making corner adjustments can't alter the car's static front-to-rear or side-to-side weight distribution, it can only shift it around between opposing corners. To change the front-to-rear weight balance, ballast must be physically moved on the chassis.
Most gearheads are familiar with the basic terms of suspension alignment, including caster, camber, and toe, so we won't go into to the definitions. Suffice it to say that for a road-racing Mustang with limited suspension modifications, like our CMC racer, you will end up running as much positive caster as you can get, a lot of negative camber, and some amount of toe-out. Caster is pretty much a set-it-and-forget-it deal, especially since the Mustang's strut design is particularly challenged in this regard. Ideally, with a strut-type front suspension, positive caster should be 8 to 10 degrees or higher to negate the inherently poor camber loss characteristics of the design. Unfortunately, even the SN-95's improved geometry struggles to get much more than halfway there. To offset the loss of negative camber as the car rolls through a corner and weight is transferred to the outside tires, increased amounts of static negative camber are needed to keep the tire's tread in contact with the road surface. But there are trade-offs with adding too much negative camber. As it increases, so does inner tire edge wear, and there is also a corresponding loss of braking ability as the tire's contact patch is reduced in straight-line braking. For a street car, a range of 1.5 to 2.5 degrees of negative camber will provide a nice improvement in handling, while a dedicated road racer might need as much as 4 degrees or more. As with most tuning variables, the ideal balance of alignment setting must be sought and compromises are necessary. For example, increasing toe-out can help initiate sharper turn-in at the entry to a corner, but high amounts of toe-out increase rolling resistance and can cause darting and instability under braking.
Bumpsteer is a condition that occurs when the arc of travel of the lower control arm is not parallel to the arc of the steering tie rods. It commonly occurs when a Mustang is lowered too much, and the result is unintended changes in toe in or out as the suspension cycles through compression and rebound. Excessive bumpsteer may cause the car to dart side to side and feel skittish over rough road surfaces. Remedying this condition is relatively simple with a bumpsteer gauge and adjustable outer tie-rod ends, commonly called a bumpsteer kit. Bumpsteer is also affected by changes to caster, so bumpsteer corrections should only be made after the car has been aligned.
Tires and TuningThere are dozens of ways to affect the dynamic handling or balance of a car. Adjustments to spring rates, ride height, track width, sway bars, tire pressure, and more can change the way the car reacts in a turn. It takes a lot of time, money, and other resources to comprehensively test and sort out all the possible setup combinations on a race car, a process that's beyond the reach of most budget racers, so we'll focus on just a couple that are easy to adjust: sway bars and tires.
The goal of tuning is to achieve neutral handling where the front and rear of the car have equal levels of grip. Understeer occurs when the front looses grip before the rear and additional steering input is needed to keep the car steering along the intended path. Oversteer occurs when the rear end breaks traction before the front. The natural tendency of a front-heavy Mustang is towards understeer, but the design of the stock four-link rear suspension also lends itself to sudden and unexpected oversteer at the limits of traction. Finding an ideal balance can be tricky, and what works in one corner or section of the track may not be the ideal balance for other sections, so again, the trick is finding the best overall compromise that results in the fastest lap times for the track conditions. This is all part of the fun and challenge of racing.