Several months back in How it Works, we had notable suspension industry experts chime in on the proper way to set your ride up with four-wheel scales. There was a ton of great information about finding the front-to-rear weight distribution, crossweights, and sway bars, and the effects on the static balance of the vehicle. With any vehicle that turns left and right it’s the goal to load all four tires equally and achieve 50-50 weight distribution front to rear. Many of us have swapped in the biggest, nastiest engine we could for power, ignoring the vehicle’s weight balance. You’ve seen the all-iron big-block Chevy stuffed into Vegas or Monzas. I’m not talking about race cars—these are street cars. Those things probably had 60 percent of the weight on the front end and 40 percent on the rear. They would burn the tires until you lifted. Also, they wouldn’t turn if they had to. They would plow through the turns, melting the outside edge of the tires no matter how much positive camber we screwed into the alignment.
After reading it, I hope you can appreciate the importance of equal tire loading and setting up suspension for different disciplines of racing. What I noticed from the story was that there was very little information about my favorite application of power: drag racing! As you all know, a while back I completely rebuilt my Davis roadster. During this rebuild I fabricated all new four-link suspension bars, replaced all the rod ends, rebuilt my Koni coilovers, and had the rear springs tested for pressures. The rear suspension needed to be completely set up for pinion angle and suspension preload to (hopefully) have the car launch straight when applying 800 lb-ft of torque. One final variable to throw into the mix was that I had never run the car with this much power. I had a really good handle on the setup for 600 lb-ft of torque, but never this much twisting force.
With four-link suspensions, you want to set a minimal amount of preload on the right rear tire. When the torque hits the rear suspension, the twisting moment of the driveshaft on the rearend pinion lifts the right rear tire off the track and plants the left rear. Weight transfer from the launch helps plant the load back on the right rear, but you may need to preload an extra few pounds on the right rear tire at rest to make the car go straight.
Well, I wanted to set my car up neutral, which means the weight on the rear tires is equal with the driver in place and all the fluids you’d have to race the car. Unfortunately, I don’t own a nice set of scales to measure this accurately. The old-school way to set this baseline is to set all four of the bars to the same length and, with the car race ready, adjust the upper right four-link bar until you find that it is free to turn, and no load is on the rod ends. From there, if you wish to increase the load on the right rear tire you would shorten the top right bar several flats (1 flat equals 1/6 turn).