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.
Without adjustable coilovers...
Without adjustable coilovers or spring perches, both of which are illegal in CMC, the installed height of the springs must be adjusted manually with shims between the spring and seat to increase the ride height on that corner and jack weight onto the diagonally opposite corner.
For a road racer, constant...
For a road racer, constant trips to a local alignment shop are too costly and inconvenient to be practical, so it's worth investing in the tools and skills to align your car at home or the track. It doesn't take a huge investment in time or equipment to achieve excellent results equal to even the best computerized alignment racks. We use a simple Longacre bubble-type caster/camber gauge with a hands-free wheel adapter. Digital gauges are also available that can be calibrated to account for uneven shop floors or paddock spaces, making the job even easier.
There are several methods...
There are several methods for measuring toe. We use a set of aluminum toe plates at the track to make quick adjustments. For a drag or street car, the wheels should be close to parallel (zero toe), or even slightly toed-in for the best straight-line stability and tire wear. On a road racer, the wheels will generally be toed-out anywhere from 1?8- to -inch.
We used a Maximum Motorsports...
We used a Maximum Motorsports bumpsteer gauge and adjustable outer tie-rod kit to measure and correct our car's bumpsteer. With the springs removed and the car and suspension set at static ride height, toe changes are measured with a dial indicator as the control arms are cycled through droop and compression. Each side of the car must be measured and adjusted independently to account for variations in suspension components.
Shims are used to adjust the...
Shims are used to adjust the height of the outer spherical bearing tie-rod, and the bumpsteer is remeasured until the curve is optimized. Ideally, bumpsteer should be as close to zero as possible through the working range of the front suspension. Slight amounts of toe-out under compression are preferable to any amount of toe-in. Toe changes should be kept to a maximum of 0.010-0.015-inch through the first couple of inches of suspension travel in compression or rebound, preferably only in the direction of toe-out.
Tire pressure is the most...
Tire pressure is the most basic handling adjustment, so an accurate tire pressure gauge is one of a racer's most important tools. Through experience, we've found that the CMC-spec Toyo Proxes RA-1 competition tire works best at a hot pressure of around 37 psi in the front and about 32-34 psi in the rear. Small adjustments can produce big changes in handling, so you'll need to work up to your own preferred settings. An old racer's rule of thumb is "add air, add grip." One of the single biggest handling improvements we made to our Mustang over the course of the season's testing was a 2-psi bump in rear tire pressure. We found that our SN-95 chassis's improved front geometry put less heat into the tires compared to our old Fox chassis, which required a 2-psi pump in static front-tire pressure to reach our target hot pressure of 37-38 psi.