Front suspension tuning can be one of the most critical aspects of getting your race car to handle properly. That is why it is unfortunate that many Street Stock racers simply throw in a heavier right-front spring and never experiment beyond that. We wanted to start with a Street Stock that was set up for racing and take it in another direction to see how much improvement was possible. We started with the #37 Street Stock that Bob Carpenter races at Ventura Raceway on the 1/4-mile dirt track.
This car has run for several years with a 1200-pound spring on the right front, a 1000-pound spring on the left front, a 200-pound leaf-spring on the right rear, and a 176-pound leaf-spring on the left rear. The car has been racing with no anti-roll bar on the front or rear. The A-arms (control arms) utilized the stock rubber bushings.
We hooked up with Herb and Matt Adams of VSE to tinker with this '78 Camaro in a progressive fashion so that we could track the effects of each change.
Because dirt tracks have constantly changing track conditions with corresponding erratic lap times, we decided to use our dirt track "before and after" testing time more for anecdotal and seat-of-the-pants comparisons. We arranged to spend two days testing on the 1/4-mile asphalt track at Kern County Raceway in Rosamond, California, for precise lap-time comparisons. Track Promoter Ron Hall makes it easy for anyone to test on this slightly banked oval track, and we thank him for his help in arranging this test.
To run on the asphalt we needed to take off our Towel City Retread dirt track spec tires that we run at Ventura and replace them with Towel City Retread racing slicks. Rather than do a wholesale tire swap we simply borrowed a set of wheels from Circle Wheel and used them for the test. The tires and wheels performed without incident throughout the testing.
Our baseline laps on the track revealed a push (understeer). This was a direct result of the very high spring rates, according to Adams, and the deflection caused by the stock rubber A-arm bushings.
Evidence of the front control arm bushing deflection can be seen in the lead photograph of the car during hard cornering. The Camaro had about five degrees of negative camber on the right-front while sitting in the pits. That is all that could be had with the existing upper control arm bolts. The photo shows several degrees of positive camber on the right-front during cornering. Rubber control arm bushings can deflect about 1/4 inch each, so this is the obvious cause of the camber change.
The reason why Adams believed the front spring rates were too high is because the right-front tire was working too hard in relation to the right rear. Because the driver reported understeer we know the right-front tire was working too hard.
Under hard cornering most of the weight is on the outside tires, so we need to analyze them carefully. Taking some weight off the tire during cornering will allow it to work less. This can be done with less static weight or by reducing the weight gain during cornering.
We can't really reduce the total increase in right-side weight during cornering. However, by using different spring rates we can adjust how much of the total right-side weight gain is absorbed by the right-front tire versus the right-rear tire.
With a softer right-front spring, less of the right-side weight increase would be absorbed by the right-front tire. This means the right-rear tire would need to absorb more of the total weight increase, so it would need to work harder in relation to the right-front tire.
Further evidence of our baseline understeer condition can be seen by looking at the tire temperatures:
Left Front Right Front 76 83 116 78 98 126 Left Rear Right Rear 80 92 105 95 100 108
The right-front tire is working harder than the right-rear, especially when consideration is given for the rear-tire driving forces. The temperatures also show that the outside edge is working much too hard, which confirms our photo evidence of positive camber during hard cornering.
Lap times with the car in this condition were 17.89, 17.46, 17.75, 17.47; with an average of 17.69 seconds. Notice that the times vary. The 350 Chevy engine was just starting to get into valve float at the very end of the straightaway. Bob said the tach was reading 6500 rpm at the start/finish line so this would be the expected rev limit with the stock valvetrain.
To test our idea on reducing the weight during cornering of the right-front tire we installed softer-rate front springs by cutting one coil from stock Z28 front springs. This achieved an effective rate of about 500 lbs-in with the proper preload to trim the car at its original ride height.
An added benefit is that we could get rid of the hidden spring adjusters. These adjusters seem to get in the way of the shocks on this car and have damaged at least two of Bob's shocks. The cut-down Z28 springs were installed with the same setup as before: no front anti-roll bar and stock rubber control arm bushings.
On the track Bob's Camaro showed more neutral handling and the lap times improved to 17.07, 17.01, 17.17, 17.19, 17.21; with an average of 17.13. The softer front springs reduced the cornering weight transfer at the front so both of the right-side tires, front and rear, more equally carried the load. When both right side tires work equally hard, total traction is increased and the car will corner faster.
Because the roll stiffness is less on the front with the softer rate springs, the roll angle increased. Even with this larger roll angle, lap times improved about 0.50 seconds.
To reduce the roll angle we installed a front anti-roll bar from Matt Adams VSE for use on Street Stock Camaros and Firebirds. This bar differs from the stock Camaro front anti-roll bar in that it is larger in diameter and uses Heim joints on the links, which connect the bar to the lower control arms.
These links eliminate the lost motion that occurs with the stock rubber-ended links. These differences from the stock bar result in a much more effective package, which improves steering response as well as reduces body roll. The lap times when the front bar was installed were 17.20, 17.29, 17.18, 17.28, 17.30; for an average of 17.25.
This indicated about a 0.10 second loss in lap times. Bob reported that the car now had slight push again (understeer), which explains the loss in lap times. Using a front anti-roll bar increases push because the front bar causes more of the right-side weight transfer during cornering to be absorbed by the front tire. Use of the front anti-roll bar made a large reduction in the roll angle, so the resultant right-front camber change was diminished. Bob did say that he felt the use of the front anti-roll bar made the car feel more stable and improved steering response.
Compensation for the slight push caused by using the front anti-roll bar could be effected by: a) using higher rate rear springs, b) adding a rear anti-roll bar or c) using steel front control-arm bushings. These fixes either increase the amount of cornering load the rear tires would carry relative to the front tires or, in the case of the front control-arm bushings, would increase the cornering power of the right-front tire.
We removed the stock front control-arm bushings on Bob's Camaro and replaced them with new ones supplied by VSE. These bushings use a nylon sleeve that is supported by a precision-machined inner sleeve and outer sleeve. These heavy-duty, durable parts press into the stock control arms. Each bushing has a zerk fitting so they can be greased occasionally.
When the front control-arm bushings were installed on Bob's Camaro there was a marked improvement in cornering. The lap times improved to 17.13, 17.03, 17.21, 17.00, 17.14; for an average of 17.10. These lap times could have been even better but the higher cornering speeds led us into faster straightaway speeds and the engine was getting into valve float halfway down the straight. Because the engine could not accelerate any faster on the straight, the lap times were not as good as they could have been. Cornering speed was increased dramatically enough that a gear change would now be beneficial.
The steel front control-arm bushings improved the cornering at the right front because they limited the loss of negative camber because they do not deflect inside themselves. Better camber angle on the right front gives the tire more cornering capability because it puts more rubber on the track. There is less positive camber during cornering with the steel bushings, which confirms the reason why lap times improved. The tire temperatures also indicate that the steel control-arm bushings reduce the positive camber gain on the right front:
Left Front Right Front 76 86 102 92 102 119 Left Rear Right Rear 86 100 103 93 100 106
These temperatures show that the side-to-side difference across the right-front tire went from 48 to 27 degrees. When the temperatures across a tire are equal, the whole tire is being used, which means there is more cornering force available.
Further confirmation to improved cornering force was that Bob indicated no understeer in the corners. More importantly, Bob reported a dramatic improvement in the car's ability to hold a line. The car went where he aimed it and stayed there. With neutral steering and more precise handling, Bob felt he could put the car anywhere he wanted on the track and his line became closer to the track's inside white line compared to all earlier tests.
When the car returned to dirt action the new setup was quickly pronounced a success. The softer springs allowed the suspension to work when the car hit potholes, ruts, and bumps. The anti-roll bar kept the car from pitching side to side, and the steel A-arm bushings made cornering much more precise. This was evident when Bob went from 12th on the starting grid of one main event to 4th in 20 laps.
He used the low line when other cars were bunched up high, he used the middle line when it seemed best, and he drove on rails on the high line when the track was clear. Small steering input and corrections were now capable of being effective, and that adds to driver confidence. We've watched the #37 car at Ventura Raceway and it runs smoother and stays flatter on the race course than anything else out there. Our testing was a success in the most important category; under actual race conditions.
Baseline testing revealed understeer. Notice the right-front wheel being forced over into positive camber, while the left-front tire is barely touching the track
Kern County Raceway Promoter Ron Hall made his 1/4-mile, slightly banked oval asphalt track available for our comparison testing. Anyone can rent the track and learn a great deal about their car.
An important part of any testing is knowing what you're starting with. Here, Herb Adams uses a simple "bar stock on blocks" method to check the toe on the front end.
The toe of the rearend is also important to know. Have you ever measured yours? Adams also checked camber on all four wheels.
Changing springs requires a moderate level of exertion. You have to knock the bottom ball joint loose to get the lower A-arm down and the spring out.
Stock Z28 front coil springs are cut one coil shorter. Some object to this technique but Street Stock racers have been doing it for decades with good results. One step to remember is to heat the newly cut coil about a half turn from the end, and then press that end against the ground to flatten the end of the spring.
Matt Adams, owner of VSE, hooks up the steering linkage after installing the new springs.
After sufficient laps the car is taken apart again. This time we're adding a front anti-roll bar.
The VSE anti-roll bar is thicker than a stock bar and the ends are all connected with Heim joints for precise control of the car.
The steel A-arm bushings are a critical component to Street Stock handling. These bushings use a nylon sleeve and can be made to look like stock bushings if your rules require.
The bar kept the car's attitude flat but the springs were freed up to work the bumps. Tire temperatures were monitored after every test session and the results backed up the drivers seat-of-the-pants feelings and the observations of the crew.
After installing the steel A-arm bushings Bob commented that the car was much more predictable, stayed low in the turns, and held its line better. Notice that the right-side wheel is no longer deflecting to a positive camber position. Also note that the car is hugging the inside line extremely tight.