'Get used to hearing the term "systems approach." Car crafters are an independent bunch, and we rarely purchase all the components for a trick engine or bitchin' suspension all at once. The time-honored piecemeal approach is much easier on the wallet. Since drag racers and quarter-mile street masters are more horsepower junkies than suspension innovators, chassis parts are often ignored in the quest for that elusive low e.t. If you consider yourself enlightened, think about planting the power your engine already makes. It might be easier than you think.
We've dismembered the front and rear suspension systems into smaller chunks of more easily digestible information. Keep in mind that the systems approach is your ultimate goal. Integrate the front and rear suspensions so the front half works as hard as the rear and you will experience a car that leaves harder than it has a right to.
Weighty IssuesWe met Jimmy Byrne at the Cordova, Illinois, NMCA race running an '88 Mustang in Xtreme Street. Byrne was runner-up based on consistent 8.30s on almost every pass that weekend with just he and his wife Lee Ann running the whole operation.
We grilled Byrne on his thoughts on improving traction. His ideas are grounded in hundreds of dragstrip passes with several stock-suspended Mustangs: "The theory says a lighter car is faster. But the fastest I ever went in my Mustang was when it was the heaviest. Most cars are nose heavy. If I take all the ballast out of my car, it will measure 55 to 58 percent of the weight on the nose. So that means you gotta put weight in the back."
That sounds counterintuitive, but the Byrne plan has merit. "For a street car, you want to make it look stock, so you're not gonna move the battery to the trunk. So you gotta add weight back there. There's a guy down the street from me who was pulling the interior out of his car to go quicker. I told him to put all the stuff back in and add 100 pounds of weight in the trunk. You also want it as far back in the rear as you can put it." This guy was running nitrous and had more power than tire, which is the focus of Byrne's suggestions. Horsepower is easy to make now, but there are fewer guys who have a handle on traction.
To give you an idea of how much difference 50 pounds can make, Byrne said on one pass he put 25 pounds of ballast directly over each rear shock and his Mustang spun the tires. He moved the weight 6 inches farther back in the trunk and the car planted the back bumper on the starting line. This ballast change represented roughly 3 percent of the car's rear weight, yet it had a drastic effect on traction. Granted, this is on an 8-second, 160-mph car, but this is no ladder-bar or four-link drag race rear suspension. Byrne's Mustang still uses stock upper- and lower-control-arm mounting points. Makes you think, doesn't it?
ShocksOne of the easiest and quickest ways to tune the suspension on a street/strip car is with shock absorbers. For normal street driving, a shock is used to dampen the oscillations of the springs. This is accomplished with hydraulic damping. That same style of damping can be used to help tune both the front and rear suspensions to perform exactly the way you and the traction gods intend for maximum bite.
For the front suspension, the old racer trick was to use "dead" stock shocks. Some racers even went so far as to drain the hydraulic fluid by drilling holes in their shocks. Then came 90/10 shocks. This generic term describes shocks with 90 percent of the control on compression (called bump) and 10 percent on extension (rebound), making it very easy to extend and difficult to compress. These shocks work to allow front-end rise to shift weight to the back for traction, but also hold the front end up through the traps, causing aerodynamic drag. A better solution is a single-adjustable shock that allows you to tune the shock to extend at a given rate. A car with lots of power tends to want a slower front-end rise than a slower car with less power.
Rear suspensions can also benefit from adjustable shocks. While single-adjustable shocks are better than nonadjustables, a double-adjustable shock that allows independent tuning of the compression and extension is the best way to go. For example, with a greasy or bald starting line, you might try a softer extension setting to allow more rear body rise to soften the hit on the tires to prevent tire spin. Conversely, a track with teeth usually will respond to a stiffer extension setting to apply more torque to the tire rather than wasting that power on body rise.
Koni, QA1, and Chris Alston's new Vari-Shocks are examples of single- and double-adjustable shocks that allow the racer to fine-tune the suspension to optimize traction based on existing conditions. Obviously, double-adjustable shocks are considerably more expensive than single adjustables, which cost more than non-adjustable shocks. How much control do you want?
Coil SpringsSprings do much more than just keep the fenders off the tires. Stored energy in the front springs can be used to help weight transfer in slower cars and can be dialed in to help calm down a quicker car. Moroso and Competition Engineering make very tall coil springs for early A- and F-body Chevys, Fox-bodied Mustangs, and several others. These springs come in various rates depending upon front-end weight of the car. The only way to know for sure is to weigh the entire car on a scale and then weigh the front and rear halves separately. Front-end weight will generally measure between 52 and 58 percent of total weight.
For A-body GMs and Fox-body Mustangs, it's possible to band-aid a stock rear suspension with an Air Lift bag in the right rear to counteract the tendency for the rear axle to lift the right-rear tire under acceleration. Placing 10 to 15 psi of air pressure in the right rear bag placed inside the spring will plant both rear tires. Moroso makes rear coil springs that offer different spring rates with the right rear spring stronger than the left, eliminating the need for the air bag. Combined with adjustable shocks, these changes can create more consistent launches.
Another important tuning device is a rear-mounted aftermarket antiroll bar. Unlike the factory bars that bolt to the rear control arms, companies such as Competition Engineering and Dick Miller Racing sell a universal weld-in unit with frame mounts that extend links to the rear axle. The links use threaded rods and spherical joints that can be used to adjust preload. While preload is usually added to the right rear to compensate for rear axle lift, it's also possible to add or remove preload from both sides to fine-tune the rear suspension.
The Search For Instant CenterCars with coil-spring rear suspension are not always immune from wheelhop. For those A-body GM and Fox Mustangs that suffer from a bad case of bouncing rear tires on launch, there are some simple fixes. The easiest and quickest way is to adjust the rear ride height until the lower rear control arms are parallel to the ground. Some car crafters have been known to jack the back of their cars up with air shocks to fit a big set of tires. This not only changes the instant center but usually overly stiffens the rear suspension which can cause that evil hop. The smart move is to keep the ride height near level.
Coil-spring cars also have a tendency to squat in the rear under launch. While this may look like the car is really planning the rear tires, the reality is that when the body squats, the rear tires are momentarily unloaded. To fully plant the rear tires, a small amount of body rise is necessary, evidenced by an increase in the distance from the axle centerline to the rear fender lip under launch. For third-generation Camaro and Firebirds, Spohn Performance makes a weld-on relocation bracket for rear lower control arms to help eliminate squat, especially for lowered cars.
Another way to go is with a set of No-Hop bars from Lakewood or Bill Miller Racing. Before we get into that, we should review something called the rear suspension's instant center. The instant center is a point where imaginary lines extended forward from both the upper and lower control arms (as viewed from the side of the car) converge. By changing the rear mounting point of the upper rear control arm, this changes the position of the instant center and causes the rear suspension to rise slightly upon acceleration. Lakewood and Bill Miller Racing offer these bars for both GM A body cars and the ubiquitous Fox-body Mustangs. If there is a downside to these No-Hop bars it's that there is a limit to lowering the rear of the car since these bars reduce the clearance between the upper control arms and the floor kick-up over the rearend. In those cases, lowering the rear of the rear lower control arms is an alternative to raising the rear of the upper control arms.
Get it StraightIn order to have a fast car of any kind, it must track straight down the track. This means the alignment has to be dialed in and this goes way beyond just setting it to the factory specs. But before we get into that, you have to make sure the rest of the front suspension is in good shape. This means inspecting the ball joints and control-arm bushings. If you're working with any older musclecar, the A-arm bushings are no doubt wasted. Replacing them with a stock rubber or even polyurethane bushing is acceptable, but a better idea is a bushing that allows you to dial in a given amount of preload.
Global West makes front control-arm bushings that are near solid yet can be used on the street and are also adjustable. The aluminum-bodied Del-A-Lum bushings feature a Teflon-like material called Delrin that separates the steel shaft from the aluminum bushing body and can be preloaded. The beauty is that these bushings do not deflect thereby maintaining a very accurate alignment for a very long time.
For a drag-race-style alignment, it's important that the settings be made at race weight with the equivalent of driver weight in the seat. Then, the front suspension should be raised to the ride height achieved as the car goes down track. This is important since all three variables of toe-in, caster, and camber are ride-height dependent.
As for the specs, maximum positive caster offers excellent high-speed stability, so 4 or more degrees of positive caster is a good goal. Some strut cars like Fox-body Mustangs do not provide caster adjustments, but aftermarket plates like those from Global West can make this happen. Camber should be kept as close to zero as possible. Toe-in should be set at 11/416-inch total or 11/432-inch per side. This minimizes tire scrub that might be worth a small trap-speed improvement.
Launching With LeavesThe leaf spring is the oldest form of spring yet is still very popular in drag racing. The biggest problem with leaf springs is their tendency to cause wheelhop. This action starts with the twisting motion of the rear axle, which rotates around the rear-axle centerline. This twist imparts an S-shaped bend into the front half of the leaf spring, which eventually binds and then snaps back, forcing the tire to literally bounce off the ground, creating that evil wheelhop.
The quick fix is a set of traction bars. These bolt-on devices work on the track but tend to bind the rear suspension anytime the snubber hits the spring. Many bolt-on bars are universal and somewhat short, placing the snubber short of the spring eye, which will eventually bend the spring. Ideally, the snubber should be placed directly under the spring eye. Eliminating deflection with a solid bushing is also a good idea, but they don't live long on the street. Global West makes a Del-A-Lum bushing for these applications; they work like a solid bushing but without the wear problems.
Calvert Racing and Competition Engineering both offer an excellent traction-bar alternative that places an adjustable link below the leaf spring, placing the leaf spring in tension rather than in compression. The link pivots from underneath the front spring eye and is adjustable for preload to compensate for the rear axle's tendency to lift the right (passenger-side) rear under acceleration. Either of these bars is superior to standard traction bars but are more expensive. Calvert and Landrum Spring also offer drag-tuned monoleaf springs that eliminate the leaf-to-leaf friction of multileaf springs.
One significant variable with leaf springs is that as load is applied and the rear suspension either squats or rises, this forces the rear spring shackle to move. According to AFCO Performance, this shackle movement also changes the effective spring rate, which varies the load applied to the rear tires. Ideally, if the shackle never moved, this would maintain the same amount of load on the tire. The original Mopar Performance books used to preach keeping the rear shackle pivot point on the body located ahead of the rear spring eye, which would increase load with body rise. There is some great tech information about leaf springs on Afco's Web site (afcoracing.com) that explains this more fully. The solution for this shackle movement is a slider assembly. A slider assembly is a steel roller that travels in a slot designed to fit between the rear axle and the spring to allow the rear to move fore and aft under load. AFCO sells these sliders, designed mainly for Mopar-width springs in drag racing-only applications.
Tach Tuning If you're really serious about chassis tuning on the dragstrip, it makes sense to invest in a data logger to record each run and compare them to see exactly how the car used the power on each pass. Several NMCA racers we spoke with use the Auto Meter Ultimate II Playback Tach and matching software to log engine and driveshaft rpm to evaluate torque-converter efficiency as well as traction. The data is recorded by the tach and then downloaded to a computer that uses the Auto Meter Tach Facts software to plot the rpm curve of each pass. The Ultimate II tach logs up to four runs with a rate of 100 samples per second.
Most racers use the data to study driveshaft rpm in search of tire spin. This information can then be used to make chassis changes to improve 60- or 330-foot times. For example, we had a Chevelle that would initially launch and then unload the rear tires and spin roughly 30 feet off the line. Slowing the rate of front suspension rise changed the point at which the front suspension topped out and the car went quicker. The key to using any data-logging device is to accurately evaluate what the car is doing and how to use that information to improve e.t. Of course, there are dozens of other data-logging options. We did a story called "The Racepak" on an affordable data recording in Nov. '04.
|CHASSIS PARTS PRICES |
|DESCRIPTION ||PN ||SOURCE ||PRICE |
|Auto Meter Payback II ||6871 ||Summit racing ||$399.88 |
|Auto Meter Tach Facts v3.1 ||6890 ||Summit racing ||142.95 |
|Shackle kit, '67- '69 Camaro ||103-SH ||Global West ||192.30 |
|Del-A-Lum upper bushings ||1012 ||Global West ||107.06 |
|Del-A-Lum lower bushings ||1020 ||Global West ||129.08 |
|Upper rear control arms ||TBC-88 ||Global West ||289.00 |
|QA1 shocks, rear star, pair ||TC1574P ||Summit racing ||139.95 |
|QA1 single adj. rear shock, pair ||MU7855P ||Summit racing ||139.95 |
|QA1 dbl. adj rear shock, pair ||DMU7855P ||Summit racing ||277.69 |
|Edelbrock rear-brace kit, pair ||5210 ||Summit racing ||99.95 |
|Slide-A-Link, '69 Camaro ||2100 ||Summit racing ||325.95 |
|Leaf-spring slider assembly ||20236S ||Afco ||51.95 |
|Comp Eng. antiroll bar ||2020 ||Summit racing ||261.88 |
Auto Meter Products
413 W. Elm St.
12035 Burke St., Ste. 13
Santa Fe Springs
AFCO Racing Products
P.O. Box 548
42156 10th Street West, Unit V
Landrum Spring Service
P.O. Box 6796
Chris Alston's Chassisworks
8861 Younger Creek Dr.
Moroso Performance Products
Dick Miller Racing
Smart Racing Products
1455 N. Linden Ave.
494 E Lincoln Ave.