This chapter explains some of the methods to correct or adjust the rear suspension of your ill-handling race car. I discuss methods to improve the car’s 60-foot times. Once those are within reason, the rest of the run will be greatly simplified. Don’t try to accomplish this by yourself. Speaking from experience you might do a fairly accurate job in a 13-second quarter-mile car, but as your car gets faster it will be much easier and more accurate to track your performance and improvements with another per-son watching the car.
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The best method short of using a data recorder, (see Chapter 12) is to have the other person record video of the car from a rear corner as it leaves the starting line, focusing in on your hand on the steering wheel. Drivers make instinctive reactions with the steering wheel when the car launches and don’t even know it. Having grown up in Michigan and learning how to drive on icy and snowy roads, I have seen many occasions where the race car I was driving started to turn and I instinctively corrected that turn before I could even think about what was happening. With the front wheels straight, wrap tape of a contrasting color around the steering wheel at the 12 o’clock position so it shows up well in the video. By watching the video of your hand on the steer-ing wheel as the car launches you are able to tell if the car turns left or right (or both) as it leaves the starting line. Many of today’s popular NHRA Pro Stock drivers have a white line about 1 inch wide around the top of the steering wheel, so it’s obvious they’re watching this care-fully as well.
If you are turning the steering wheel back and forth, the tires are spinning and the car is not hooking as it should. If the car launches and turns to the right (typical, as discussed in Chapter 1), your car needs more preload. Add small amounts of preload based on how bad the car turned right and test again. Continue adding preload in small amounts and continue testing until the car launches straight and true. If you get to the point where the car launches and turns to the left, you have too much preload. Remove the preload you last installed and test again.
Having the Right Torque Converter
This discussion assumes your car is in good mechanical tune and you are leaving the starting line at or near maximum torque for your particular engine. Notice I said torque, not horsepower. The torque (not horse-power) makes the car accelerate from a dead stop at a fast rate of speed. Horsepower comes into effect after the car is well on its way and higher in the RPM range.
If you’ve seen an engine run on a dynamometer, you know that the dyno only measures torque. Horsepower is simply a mathematical function based upon torque and the engine RPM. The dyno is not measuring the horsepower of the engine but simply the torque and RPM and then using a formula to calculate horse-power.
HP = (RP M x Torque) ÷ 5,252
As an example, let’s use the engine I have in my car.
Sometimes I have an issue because I have the wrong converter in the car to launch the engine at peak torque value. It was dyno tested and found to have a maximum torque value of 633.9 ft-lbs at 5,000 rpm. Use the formula:
HP = (5,000 x 633.9) ÷ 5,252
HP = 603.5
As the engine goes faster, torque falls off. But because of the formula, the horsepower continues to rise until the torque really drops off.
At 6,500 rpm, the torque was 540.0. Using the same formula:
HP = (6,500 x 540.0) ÷ 5,252
HP = 668.3
Any dyno sheet has numbers for each 100 rpm. I have given you the peak numbers, which indicate thatthis engine needs a torque converter that stalls at 5,000 rpm, and the shift point should be at 6,500 rpm for the fastest possible run.
Since a dyno measures torque and engine speed you know at what RPM the peak torque is for your engine. Your torque converter should stall at or near the peak torque RPM.
According to the TCI Web site: “Converter stall is the RPM that a given torque converter (impeller) has to spin in order for it to overcome a given amount of load and begin moving the turbine. When referring to ‘How much stall will I get from this torque converter?,’ it means ‘How fast (RPM) must the torque converter spin to generate enough fluid force on the turbine to overcome the resting inertia of the vehicle at wide open throttle?’ Load originates from two places:
“(1) From the torque imparted on the torque converter by the engine via the crankshaft. (This load varies over RPM, i.e., torque curve, and is directly affected by atmosphere, fuel, and engine conditions.)
(2) From inertia, the resistance of the vehicle to acceleration, which places a load on the torque converter through the drivetrain. This can be thought of as how difficult the drive-train is to rotate with the vehicle at rest, and is affected by car weight, amount of gear reduction, tire size, ability of tire to stay adhered to ground, and the stiffness of the chassis. (Does the car move as one entity or does it flex so much that not all the weight is transferred during initial motion?)
The primary thing we want to remember about torque converter stall speed is that a particular torque converter does not have a ‘preset from the factory’ stall speed, but rather its unique design produces a certain range of stall speeds depending on the amount of load the torque converter is exposed to.”
My car’s torque converter is stalling at 3,850 according to my Sports-man Data Recorder from Racepak. At 3,900 rpm, the torque was 509.5 ft-lbs. The point is that the torque converter needs to match the peak torque RPM of the engine for the car to launch as hard and as fast as possible. In this case, the car is launching at 1,100 rpm below its peak torque at a loss of 124.40 ft-lbs, resulting in 60-foot clockings about .05 second short of where they should be. The car is still consistent; however, it could be faster. That .05 second removed from the 60-foot clockings could result in a .10 to .15 second quicker ET, and could be a bigger thrill in the ride and probably bring the front wheels up 6 to 12 inches farther.
However, as mentioned previously, if a car is standing on its back bumper it may be getting maximum traction off the line, but not necessarily getting maximum forward acceleration motion. Since the car is pivoting on the rear tires, the front of the car is actually going rearward in order to get to that height and making the car much harder to push forward because of the rearward momentum.
Adjusting Pinion Angle
Pinion angle is measured in reference to the angle of the pinion gear compared to the driveshaft. As I said, to the driveshaft, not the ground, transmission, engine, or any of the many other methods I have heard of. While these measurements are important for other reasons, they have nothing to do with pinion angle.
A pinion angle topic discussion has many opinions. Some agree that pinion angle helps a car’s launch, while others say it has no effect. To those who feel it has no effect, I always propose the question, “Then why does a car hook harder and have a quicker 60-foot time with more pinion angle?” No one has ever been able to dispute this.
Before you just go out and throw additional pinion angle into your car, remember that pinion angle robs horsepower as the engine over-comes the angle. The rotation of the driveshaft and pinion gear straightens out the angle and forces it to 0 degrees or flat. This happens no matter how much pinion angle is installed on the car. The more pinion angle there is, the more horsepower needed (or robbed) from the engine (which could be used to accelerate faster).
The effect of this straightening process (moving the pinion angle to zero) creates a leverage affect at the front of the driveshaft and also at the rear end. Using the weight of the car, this drives the tires harder into the pavement.
Cars using no-hop bars or lift bars must have the pinion angle reset to compensate for the change from installing one of these kits. It is very important to get yourself an angle measuring tool, then check and correct your car’s pinion angle.
If you have a factory four-link suspension system, pinion angle can be changed with adjustable upper control arms. By making both arms longer in unison the car loses pinion angle. By making both arms shorter (in unison) the car gains pinion angle. If you have a factory three-link system (torque tube) then the torque tube must have an adjuster to make the changes. With the adjuster at the bottom, you need to lengthen it for more pinion angle or shorten it for less pinion angle. If the adjuster is at the top, you need to do the opposite.
Now you can see why all of my adjusters are double adjustable, meaning no bolts need to be removed to make adjustments. Just loosen the jamnuts to make adjustments as the adjuster has left-hand threads on one end and right-hand threads on the other end. Be sure to look at the threads when loosening the jamnuts to be sure you are not accidentally tightening them. Single-adjustable adjusters require one end to be removed from the car to make adjustments making the process more difficult and more lengthy.
With factory leaf-sprung cars, wedges can be installed between the axle housing tubes (where the leaf spring mounts) and the leaf springs themselves, thus tipping the pinion either upward or downward depending upon the shim being used and its direction.
Ladder-bar-equipped cars need double adjusters at the rear of the bar, just like a torque tube. With the adjuster at the bottom, you must lengthen both ladder bar adjusters in unison for more pinion angle, or shorten both ladder bar adjusters in unison for less pinion angle. If the adjuster is at the top then you need to do the opposite.
Advanced Chassis puts the adjuster at the top to create less stress, causing the suspension to pull on the adjuster rather than push. Never use bolt-on ladder bars added to the stock suspension, as this creates a binding situation due to the two different suspension systems having two different arc travel patterns. Bolt-on ladder bars added to the stock suspension definitely get rid of wheel hop, but does so by binding up the rear suspension. They do not allow the suspension to move freely, and this is not ideal for the best-possible traction.
If you have a factory four-link suspension system, pinion angle can be changed with adjustable upper control arms. If preload is adjusted into the car suspension, then the preload must be removed before set-ting the pinion angle. Notice how many flats must be lengthened in the upper control arm to get it to neutral, then after setting the pinion angle, the passenger side upper control arm can simply be shortened the same number of flats, and the car will have the same preload it had before adjusting the pinion angle. Neutral can be felt as you are turning the adjuster to get rid of the preload. There will be a spot that the adjuster can be moved back and forth a small amount with your hand using no wrenches.
The bolt should be able to be pushed in and out freely. Also, if you have an antiroll bar, one or both ends of the anti-roll bar must be unhooked to remove any preload from the anti-roll bar. Remember, all adjustments must be made with the car race ready, setting level on all four tires, and with the driver in the driver’s seat or some-thing of equal weight. By making both arms longer in unison the car loses pinion angle. By making both arms shorter (in unison) the car gains pinion angle. Once the pinion angle has been set, the preload can now be set, and then reattach the anti-roll bar in a neutral position.
Cars with lower-horsepower engines and even some cars with higher-horsepower engines may hook satisfactorily and consistently as they are. For the car that is not giving you the desired 60-foot times or not high enough wheel stands or not consistent enough 60-foot clockings, you should look at your cars pinion angle. With the right settings, pinion angle can make the car hook harder by planting the rear tires with greater force.
Factory pinion angle which is near 0 for longer U-joint life (100,000 to 200,000 miles) needs to be modified. To make pinion angle adjustments, the car needs to be race ready and sitting on all four tires with the driver or someone of similar weight sitting in the driver’s seat. Race ready means all tires are the correct pressure, the driver in the driver’s seat, all liquids at the proper level, the gas tank at the proper level, etc.
Using an angle finder, measure the driveshaft angle and then the pin-ion angle. Once you have the driveshaft angle you may need to unbolt the driveshaft to get the pinion angle using the differential yoke that the driveshaft bolts to. Other-wise quite often on the GM rear ends there is a flat spot on each side of the rear cover where the axle tubes are pressed in. This should be machined 90 degrees to the pinion. The drive-shaft and the pinion angles together normally make the shape of a very flat letter “V.” If so, add the two angles together to get the present pinion angle. Pinion angle is only the difference between the pinion and driveshaft.
From the factory, your car probably has next to 0 degrees of pinion angle. Each car needs its own setting. Too little pinion angle and the car won’t hook well. To much pinion angle and the car will hook, but at a loss of horsepower. I suggest a starting point of 2 degrees negative pinion angle for cars in the 400 hp range and maybe 4 degrees for cars in the 600-hp range and up to 7 degrees for cars running a large shot of nitrous maybe in the range of 1,000 hp or more of combined engine horse-power and nitrous horsepower off the line. Very seldom does any car need more than 7 degrees of negative pin-ion angle.
When you measure driveshaft angle and pinion angle, adding the two together represents total negative pinion angle.
If you are into time savings and convenience a digital angle finder from Allstar Performance is well worth the extra dollars. Not only is it digital making it much easier to read (1/2-inch-tall LED numbers with two decimal places compared to 1 degree increments on a dial angle finder) it will also rotate the display if the angle finder is needed in an upside- down position so you can easily read the display. It also has a button to turn on a screen backlight and arrows on each end of the display to tell you which end is up and which end is down.
Pinion angle adjustments affect other aspects of the car. Now with a greater bite in the rear, the front of the car should lift higher. If your car squats in the rear, it should squat harder. If it lifts in the rear if may lift even harder. Therefore, any shock adjustments you had previously made may also need to be changed. The point is to get the car’s chassis to hit the rear tire as hard as possible and then control that hit to your desired needs with shock settings. That is why I strongly suggest quality double-adjustable shocks front and rear to provide you the ability to set your car’s chassis to where it needs to be for your particular combination.
Set the front shocks in about the middle to a loose setting for extension. While testing, keep loosening the extension setting until you reach the desired front-end lift. Remember, you can get a car to lift too far in the front by creating a rearward motion. This rearward motion is similar to the front of the car trying to back up, thus making it harder to move the entire car forward. The compression side adjustment must be stiffer so the car doesn’t come down so hard it bounces, thus loading and unloading the rear tires repeatedly.
The rear shocks need to be set differently depending upon whether the car lifts or squats in the rear upon acceleration. For a starting point, a car that squats in the rear should have the rear shocks set fairly loose on compression to allow as much squat as possible, yet fairly stiff upon extension to help control wheel hop. A car that lifts in the rear will be opposite, but remember that even though the car is trying to lift in the rear you also want to get weight transfer (pitch rotation) and too stiff of a compression setting will not allow this to happen.
Also take into consideration that when setting the car’s rear shocks, a car that lifts in the rear (since the rear of the car is lighter than the front of the car) can lift before the front, thus making it impossible for the front to lift, resulting in no weight transfer (pitch rotation). With quality adjustable shocks you can control this and make it work to the car’s advantage.
Adjusting Chassis Preload with Rear Coil Springs
In cars with rear coil springs using a factory four-link setup (two upper control arms and two lower control arms), preload can be accomplished in many different ways. One way is to install air bags inside the coil springs. A typical starting point for air pressure is 25 psi in the passenger side with 5 psi in the driver’s side. If the car moves to the right upon launch, more air is needed in the passenger-side bag. If the car moves left upon launch, less air is needed in the passenger-side bag.
Custom rear coil springs are offered by many manufacturers, offering a stronger spring in the passenger side than the driver’s side. There is not much you can do if the springs are not right for your car’s application except find a spring the same size with a stronger or weaker pounds-per-inch rating for the passenger side of your car. These advertisements lead you to believe that any 1970 Chevelle with a big-block engine needs the same spring as another, regardless of torque and horsepower.
That is simply not true. A combination of both custom springs and air bags may be used. The disadvantage to either of these methods is that by preloading the passenger-side rear corner with an air bag, a taller spring, or both, you are also raising that same corner and pushing down on the opposite (driver’s side) front corner. This hinders the maximum weight transfer possible, but should correct crooked launches.
A better way to set preload in cars with rear coil-springs using a factory four-link setup (two upper control arms and two lower control arms) is once the pinion angle has been set adjust the passenger-side upper arm shorter than the driver-side control arm. This preloads the chassis getting rid of the typical GM right turn off the starting line.
This method allows you to install preload into the chassis, and also gives you adjustability of preload without greatly sacrificing ride or chassis movement as air bags or trick springs will. This method allows for no air bags and the same spring to be used on both rear corners and does not eat up suspension travel as pre-loading with a heavier spring on the passenger side or heavier air pressure in an air bag. Shorten the passenger side upper control arm one to two complete turns for preload. Start with one turn (also referred to as six flats because of the six-sided adjuster).
If the car now turns right upon launch shorten the passenger-side upper control arm more. If the car now turns left upon launch lengthen the passenger-side upper control arm. Work up on it slowly until the car launches in a straight line. Don’t run any more preload than necessary or the car will not drive in a straight line. Now you can see why all of my adjusters are double adjustable, meaning no bolts need to be removed to make adjustments. Just loosen the jamnuts to make adjustments as the adjuster has left-hand threads on one end and right-hand threads on the other end. Be sure to look at the threads when loosening the jamnuts to be sure you are not accidentally tightening them. Single adjustable adjusters require one end of the control arm to be removed to make adjustments making the process more difficult and more lengthy.
Don’t forget that just as 1,000 hp is an arbitrary point at which a car needs the imaginary intersection point out much further in front of the neutral line it can also be used as an arbitrary point that a car can turn left at the starting launch instead of right. The need for preload is based upon power with the more powerful cars needing the passenger side arm longer and the less powerful cars needing the passenger side shorter. It seems like cars in the low-9- to high- 8-second range can be run without preload. Slower than that, they need the upper passenger side arm shorter. Faster than that, they need the upper passenger-side arm longer. Adjusting the passenger-side control arm longer is the same as adjusting the driver-side arm shorter. Adjusting the passenger-side control arm shorter is the same as adjusting the driver-side arm longer.
Adjusting one of the upper control arms, shorter to create preload, cocks the rear end a little out of square under the car. Depending upon how tight the fit is for the rear tires in the wheel well openings this can cause an interference between the tires and frame or body. Just as pinion angle goes away under acceleration so will the preload set using this method. Even with the rear end being square under the car bias-ply drag tires will grow (get narrower and taller) at the far end of the track. Be sure this is not an issue for your car.
Bias-ply tires also get shorter and wider as they become more used thus not only changing effective gear ratio but also again changing tire clearance. Sometimes sacrifices must be made. I have my car setup with the biggest tire physically possible to get under the car. When new the tires fit fine. As they become more used they start to rub a little on the 3-inch tailpipes while turning corners. Rather than run a smaller tire I choose to let them rub. When I get new tires I just repaint the tail pipes and start over.
While not necessarily recommended, and depending upon how bad they rub, the same can be done if your car’s tires rub from setting preload. However, depending upon how your upper control arms are made you could use some hardened shims and try to relocate the rear end to one side. If both sides are rubbing you may have to go to a narrower tire but the sacrifice will be well worth it for a good straight line launch. Another way to correct it without going to smaller tires would be to install an anti-roll bar. Remove enough preload to allow the tires to clear and then add additional pre-load with the anti-roll bar.
Many of the late 1960s and early 1970s GM performance cars came from the factory already having the passenger side rear corner higher than the driver-side rear corner. You could switch springs side-to-side and still have the same stance. Springs should be installed and used for a few days to settle in before determining if you need to make any adjustments. If you have this problem, or are using either air bags, trick springs, or both, this stance can be mostly eliminated by cutting 1/3 to 3/4 of a turn off the passenger front coil spring.
Never use heat to lower a spring, as it ruins the spring by changing the temperament of the metal. Instead, use a high-speed cut-off wheel. This is a lot of work, since you need to carefully remove the coil spring each time you cut it, but it’s worth the effort once you’re done. Take small amounts at a time. If you cut too much off at once, you’ll be buying a new spring. This can be a very dangerous job and should not be attempted unless you have experience and the right tools.
Cars with rear suspension setups that lift in the rear (like no-hop bars or lift bars) should never use the air bag or trick-spring method. Doing so uses up most of the upward suspension travel in the rear suspension, leaving none left for the launching of the car. Also, never use air shocks in a drag car unless you have a 600-pound tool box you plan to leave in the car while racing (not allowed or recommended).
Adjusting Chassis Preload with Rear Leaf Springs
For cars with leaf springs, there is more than one way to accomplish preload. Traction bars (sometimes called slapper bars) are the most common method used. A slapper bar gets its name because it bolts to the existing spring U-bolts, replacing the plate under the spring and extending forward. There are many different versions of these traction bars for leaf springs. A gap is left between the snubber on the end before it hits the spring. The gap can be different on both sides before slapping the frame, thus creating preload.
Also available for most brand leaf spring cars are Chrysler Super Stock springs. The name came from the fact they were developed in the late 1960s by Chrysler for NHRA Super Stock race cars. They have a heavier spring rate on the passenger’s side than the driver’s side, thus creating preload. Additionally, they are much stiffer in the front half of the spring than the rear half, creating a rear suspension that works like a ladder-bar setup. I have used these style springs in a GM Oldsmobile Omega race car and they worked great. See more about these springs in Chapter 2.
Adjusting Rear Rise or Squat
A car that lifts in the rear upon acceleration plants the tire harder than a car that squats in the rear. As I stated earlier, you can’t pick up 100 pounds without putting 100 pounds of extra pressure on your feet. A car transferring weight from front to rear (pitch rotation) and then lifting in the back puts extra pressure on the rear tires when compared with a car that squats in the rear.
For cars with rear coil springs using a factory four-link setup (two upper control arms and two lower control arms), the imaginary intersection point of the upper and lower control arms (if extended forward) can be adjusted to make the car lift or squat in the rear. If the imaginary intersection point of the upper and lower control arms is above the neutral line, the rear of the car pushes up on the body upon launch, therefore pushing down harder on the tires, planting the rear tires more effectively and eliminating wheel hop. The closer this imaginary intersection point is to the rear-end housing the harder the chassis hits the rear tire.
Keep in mind that you can hit a tire too hard. If the imaginary intersection point of the upper and lower control arms is below the neutral line, the car squats in the rear and therefore is not pushing down on the tires as effectively.
For cars with rear coil springs using a factory four-link setup (two upper control arms and two lower control arms), the imaginary intersection point of the upper and lower control arms can be changed by altering the angle of the upper or lower control arms. Cast-iron no-hop bars have been around for years that raise the back of the upper control arms and greatly shorten the imaginary intersection point of the upper and lower control arms bringing it on the top side of the neutral line.
Thirty years ago full-bodied cars with much less tire bite and lower horsepower levels responded well to these changes. By causing the car to lift in the rear you not only planted the tire harder but you eliminated wheel hop. The original-design no-hop bars are too tall with today’s horsepower levels and cause the suspension to hit the tire too hard. I have spent more than 30 years perfecting no-hop bars and have achieved the right height with my shorter plate steel no-hop bar that work with today’s much higher-horsepower cars. Being shorter, the upper control arm is not changed so radically and does not hit the tire as hard.
You can overpower a tire and set it into spin mode if you hit it harder than the tire is engineered to take. Any time you switch to no-hop bars of any style you must add adjustable upper control arms to reset the pin-ion angle or it will be farther from where it should be than it already was with factory control arms.
For a short time there was a company that made lift-bars. These lowered the rear of the lower control arm. This method moved the intersection point above the neutral line using the same principle as the no-hop bars, but they did not intersect at the optimum location. They worked well on some cars and at some tracks, but not all the time at every track. Because of the different imaginary intersection point created, these bars were too drastic and also hit the tire too hard, just like the taller cast-iron no-hop bars. Today, they are no longer being made nor is the company still in business. I tried them when installing non-GM rear ends in GM cars. Now you can get a weld-on no-hop bar when installing non-GM rear ends in GM cars, leaving the lower control arms in the factory location.
Several aftermarket rear-axle sup-pliers have raised the upper mount location; however, it is raised less than 1 inch. This is not enough to make a significant difference, and a no-hop bar is still needed. If possible, have the upper mount attached at the factory height (based upon the axle tube centerline) and add weld-on no-hop bars.
For cars with rear coil springs using a factory three-link setup (a torque tube and two lower control arms), the shorter wheelbase changed the imaginary intersection point of the torque tube and lower control arms to a point that the car already rises in the rear and works great as is.
Front mounts for ladder-bar suspensions normally have multiple vertical adjustment holes. Squat or rise can be adjusted using the neutral line drawing, which is used for a reference point when considering rear rise or squat in suspended cars. The front mount for the ladder-bar car is the reference point to be used when making the necessary adjustments to achieve rise or squat.
Rear leaf-spring-equipped cars need to follow the same procedure as ladder-bar cars when making adjustments for rise or squat. They may be adjusted by either changing the front spring mount or rear spring shackle/mount. This is discussed further in Chapter 2.
Adjusting Anti-Roll Bars
Notice this section is titled adjusting anti-roll bars, not sway bars. There is a difference. Sway bars (both front and rear) should be removed completely from pure drag racing cars.
Sway bars (either front or rear) help with body roll under hard cornering. On a mild-horsepower car (300 hp or less) with a relatively unmodified system, a sway bar may help since its basic purpose is to mask the fact that there is a problem on one corner of the car. It does this by transferring 50 percent of the problem to the other side. It’s a bandage, not a fix. Anti-roll bar kits are installed on the rear of the car and are welded to the frame, with upright arms connecting to the rear housing. Reacting similar to torsion bars (see Chapter 8), anti-roll bars use a twisting motion to apply pressure through the frame to keep the front of the car level for more even weight transfer to the rear tires. Addition-ally, it helps stop the body from twisting permanently under severe conditions. More weight transferring evenly to the rear tires provides better straightline traction by giving equal bite to both rear tires. Because of the twisting they are forced to endure, anti-roll bars should be made from chrome-moly material due to its ability to twist and untwist better than mild steel.
Anti-roll bars can be installed in front of or behind the rear axle, but the upright bars must be as close to the outside (left and right) of the rear-end housing as possible. For those who don’t have enough room under the car to install an anti-roll bar, trunk-mounted anti-roll bars are available that have the uprights going down through the coil springs. This leaves room underneath the car for a large-diameter exhaust system and a factory gas tank. If you do not have a chassis roll bar mounted in the car, other versions are available with a tripod setup that bolts every-thing into the trunk.
Anti-roll bars should be adjusted only after all other suspension set-ups are complete. The driver (or equal weight) must be in the driver’s seat. The car must be resting on all four tires, inflated to race pressure. There should be no preload set into the antiroll bar. It should be installed with no preload. Lengthen or shorten the upright bars until the bolts slip fit through the heim joints and through the tabs welded to the rear-end housing. If any suspension changes are made (including preload), the anti-roll bar must be checked again for a neutral setting.
Adjusting Front and Rear Tire Pressure and Diameter
Front tire pressure as well as tire height can affect reaction times. If your reaction times are not what they should be, you can try different tire pressures and/or different tire diameters. These influence the rollout of the car before a red light occurs.
Rear tire pressure is different for each car combination and may require adjustment based on the track conditions and temperature. Normally, a good starting point for bias-ply racing tires is in the range of 11 to 13 pounds, while drag radial tires prefer the added pressure of 18 to 22 pounds, which also helps create a stiffer sidewall. These pressures are starting points, since there are many variables.
A low-horsepower car may need more tire pressure to keep the tire from sticking too much and bogging the car down. A high horsepower car may need less pressure to make the tire work and stick harder. A high-horsepower car may like tubes to stiffen the sidewall even more.
Wheel width also affects the optimal tire-pressure point. A wider wheel can cause the tire tread to cup in the center by separating the side walls farther apart, therefore needing more air to flatten out the tire tread again. A narrow wheel can cause the tire tread to bulge outward in the middle from bringing the sidewalls closer together therefore needing less air to flatten the tire tread again.
After racing, check the tread depth to be sure of even wear from side-to-side. If it’s not even, there may be a tire pressure issue. Too much wear on the outside edges indicates that the tires need more air pressure. Too much wear in the middle indicates that the tires need less. I am talking small changes here in all of these examples—1 to 2 pounds.
Scaling a Car
Cars are scaled on all four corners to get the best traction possible. Traction is the friction developed between the drive tires and the surface they are trying to grip. The best traction possible needs both drive tires to grip the surface equally.
First make sure that the surface you are using is flat and level (shim the scales to be level if necessary). Next, just as with setting pinion angle and preload, make sure that everything is in the car and where it will be when you go down the track (fuel, water, nitrous oxide, batteries, and driver’s weight). Do any kind of alignment on the front or rear before you weigh the car, as well as any ride height adjustments that you want to make. Check all tire pressures and set them to the race settings.
To be sure your scales are level you can use an old trick taught to me many years ago by a contractor while hanging a suspended ceiling.
1. Get a piece of clear plastic tubing about 11 feet long. Fill it (except about 3 inches from each end) with a colored substance (e.g., water with an ample amount of red food coloring). Plug each end to keep the liquid in the tube.
2. Make two wood bases, about 4 x 4 inches. Nail an upright (1 x 1 inch, and 6 inches tall) from the bottom side of the base to the base. Mark the uprights starting at the base in graduated increments (e.g., 1/8 inch) on both uprights making sure the marks on both uprights are measured the same from the base up.
3. Fasten the plastic tube to the uprights, even with the top mark on the upright, being sure both ends of the tube are fastened on the same corresponding mark as the other end.
4. Place one of the base-and-upright combinations on one scale and the other set on another scale. The liquid centers itself just like a bubble in a level. Reading the liquid on each upright tells you if the scales are level or if they need to be moved up or down to be level with the other scale.
Dan Bowers from Advanced Chassis says, “The object of scaling a car is to counteract the natural forces (preload) that work on a car’s suspension to make it turn right or left on the starting line. In cars with less than 1,000 hp (an approximate arbitrary number or point at which the car’s chassis reactions change), you are adding weight to the right rear tire to counteract the driveshaft rotating the rear-end housing. The pinion gear rotation puts a down-force on the ring gear fastened solid on the driver’s side of the rear-end housing, and the twisting motion tries to lift the passenger-side wheel while planting the driver-side wheel into the pavement. The result is more traction on the driver’s side, making the car want to turn to the right.
“A good starting point for 800-hp cars is around 30 pounds heavier on the passenger tire than on the driver’s side. If the car still goes to the right add more preload (weight). If the car goes to the left, take away some preload. A strange thing happens when you approach 1,000 hp and above, the clockwise rotation of the engine toward the passenger side overcomes the forces of the driveshaft turning the corner, and now lifts the driver-side wheel—planting the passengerside wheel into the ground. This makes the car go to negative, or reverse, preload. This means you actually want to make the passenger-side tire weigh less than the driver-side tire.
“How do you actually make tires heavier? Cars equipped with four link-style rear suspension systems are generally the easiest cars to make pre-load adjustments to, as these cars are usually based on a full chassis with all of the components easily accessible.
“To add preload, you unhook any anti-roll bar (just one side), or you can ‘keep up’ with your adjustments by re-centering the link after each adjustment so there is no tension on the anti-roll bar. Just about all of this can be done by simply shortening the top passenger bar on the four-link to add preload or lengthen it to reverse preload.
“The trick is to get your preload without having your car all jacked up on the passenger’s side. If you find this condition, you can also add or take away preload by jacking up the left front (driver’s side) spring on the car to add weight to the right rear. This accomplishes the same thing without your ride looking funny. You can also lower the right front (passenger’s side) spring to accomplish the same thing depending on what the car ‘needs’ to look right.
“It doesn’t really matter how you do this, the car doesn’t care how you put the weight there. The only thing that really matters is that you get the weight differential between the two rear tires.”
For drag racing purposes, the rest of the weights are inconsequential. It is ideal to have more weight on the rear tires and less weight on the front tires. A front-heavy car is hard to keep hooked up. A rear-heavy car can do excessive wheel stands.
The more weight there is in a car, the more weight there is to transfer (pitch rotate). Therefore, a heavy car is easier to wheel stand than a lighter car. That’s why heavier cars probably like tubes in the slicks as well as they should like drag radials.
To be completely accurate, cars should be rolled onto scales. The rear could be jacked up and the scales slid underneath, but when you jack up the front the front tires arc (with camber), and when placed back down there is a side load on the sidewalls of the tires tipping the scales.
Making the right tuning adjustments can make the difference between a very competitive car and an average car. Just like fine-tuning your engine on the dynamometer can usually result in a easy 20- to 25-hp gain, fine-tuning your chassis on a set of scales allows the car to make use of that extra horsepower. After all, what good is that extra horsepower you just built (or purchased) if the chassis can’t use it?
Written by Dick Miller and Posted with Permission of CarTechBooks