Drag Racing Traction: Rear Suspension – Leaf Spring

Rear-wheel-drive cars with leaf springs have the same issues as their coil-sprung competitors. They must have a means to set pinion angle, preload, and get the car to launch as fast as possible. In addition, a leaf-sprung car has to deal with the issue of spring wrap-up.

First I discuss some of the more popular “old school” methods. Many racers successfully still use these same methods today in bracket racing with cars probably having around 350 hp or less. Later in the chapter I discuss some of the more technologically up-to-date methods for launching rear-wheel-drive cars with leaf springs.

 


This Tech Tip is From the Full Book, HOW TO HOOK & LAUNCH: TRACTION MODS FOR STREET & STRIP. For a comprehensive guide on this entire subject you can visit this link:
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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 to the ground, transmission, engine, or any of the many other methods I have heard of. Those measurements are important for other reasons, but they have nothing to do with pinion angle. Before you go out and add to the pinion angle under your car, know that pinion angle robs horsepower as the engine overcomes the pinion angle. By “overcome,” I mean the rotation of the driveshaft and pinion gear attempt to straighten out the pinion angle and force however much pinion angle there is to zero degrees or flat. This happens no matter how much pinion angle is installed into the car. The more pinion angle there is, the more horsepower is needed (or robbed from the engine), which could be otherwise used to accelerate the car faster. This straightening process (flattening the “V” and removing the pinion angle to zero) creates a leverage affect at the front of the driveshaft and at the rear end. This leverage effect uses the weight of the car to drive the tires harder into the pavement.

 

Pinion angle represents the difference between the driveshaft and the pinion gear, nothing else. How much you choose to run can aid in tuning how your car launches.

Pinion angle represents the difference between the driveshaft and the pinion gear, nothing else. How much you choose to run can aid in tuning how your car launches.

 

The proper technique for checking driveshaft angle is to have the vehicle at rest on a level surface with the weight on the tires. A drive on lift is optimal. Place the angle finder on the driveshaft (as shown) and read the gauge. It’s all quite simple, but many people make it more complex than it really is. Accuracy is key. Just be sure to double-check your measurements and use good-quality tools.

The proper technique for checking driveshaft angle is to have the vehicle at rest on a level surface with the weight on the tires. A drive on lift is optimal. Place the angle finder on the driveshaft (as shown) and read the gauge. It’s all quite simple, but many people make it more complex than it really is. Accuracy is key. Just be sure to double-check your measurements and use good-quality tools.

 

You need to unbolt the driveshaft and use the pinion yoke to get an accurate measurement. Again, the car has to be at rest on a level surface with the weight on the tires.

You need to unbolt the driveshaft and use the pinion yoke to get an accurate measurement. Again, the car has to be at rest on a level surface with the weight on the tires.

 

This is the digital angle finder manufactured by Allstar. It’s a top-notch tool and makes finding pinion angle easy and repeatable.

This is the digital angle finder manufactured by Allstar. It’s a top-notch tool and makes finding pinion angle easy and repeatable.

 

Always run the least amount of pinion angle necessary to get your car to hook, but not so much as to waste 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. I’ve seen up to 7 degrees for cars running a large shot of nitrous—maybe in the range of 1,000 hp with combined engine and nitrous horsepower. When you measure your driveshaft angle (e.g., 1.5 degrees down) and the pinion angle (e.g., 3.5 degrees up in the opposite direction), adding the two together-represents 5 degrees total negative pinion angle. Make all settings with the driver or something of equal weight in the driver’s seat and the car race ready. A reasonable amount of fuel should be in the tank (typically half-full). The car must be setting on all four tires inflated to race pressure. In most factory production cars, to get longevity of parts, I have seen a positive number to near zero degrees pinion angle depending upon the age of the chassis components being used, especially the springs. It is very important to get yourself an angle-measuring tool then check and correct your car’s pinion angle. To change the pinion angle, wedges can be installed between the spring mounts and the leaf springs, thus tipping the pinion either upward or downward depending upon the shim being used and its direction.

 

Preload and Spring Wrap

Preload and spring wrap-up can be controlled using traction bars or slapper bars, which are the most common methods used in lower-horsepower cars. 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 types of traction bars for leaf springs. A gap is left between  the snubber on the front end of the traction bar and the spring. The gap can be different on either side before slapping the frame (or the front of the spring depending upon the style in use), thus creating preload.

Moving the driver-side traction bar snubber closer to the frame creates preload, helping to keep the car from turning right. If you need more preload, adjust either the passenger-side snubber (first choice) closer to the frame or back the driver-side snubber (second choice) away from the frame. If your car turns right upon launch, you need more preload and should adjust the passenger-side snubber closer to the frame. If your car turns left upon launch, then you need less preload and should adjust the passenger-side snubber farther away from the frame. Remember, the more distance the snubber is from the frame (or spring) the longer it takes before the car reacts to your settings.

For those of you running rear-wheel-drive GM cars with leaf springs, Landrum Spring Company makes what they refer to as a Parabolic leaf spring. The Parabolic spring is a mono-leaf design that is thicker in the middle and tapers as it moves closer to the ends of the spring. This thinner leaf at the end allows the car to plant the rear tires quicker and harder. The thicker middle part of the spring helps prevent spring wrap-up. You still need some sort of slapper bar or a set of CalTracs traction bars to stiffen up the front half of the spring.

 

These leaf spring wedges are used to adjust pinion angle on vehicles equipped with rear leaf springs. The wedge is mounted between the spring mount pad and the leaf spring itself. Doing so changes the angle the axle housing sits at.

These leaf spring wedges are used to adjust pinion angle on vehicles equipped with rear leaf springs. The wedge is mounted between the spring mount pad and the leaf spring itself. Doing so changes the angle the axle housing sits at.

 

These bolt-on slapper bars from Competition Engineering are typical and affordable. They are easy to install and can be used to adjust suspension preload while stiffening the front half of the spring. Most leaf-spring-equipped street-based cars can benefit from having them, and if they can be used to help tune the suspension, that’s even better.

These bolt-on slapper bars from Competition Engineering are typical and affordable. They are easy to install and can be used to adjust suspension preload while stiffening the front half of the spring. Most leaf-spring-equipped street-based cars can benefit from having them, and if they can be used to help tune the suspension, that’s even better.

 

This is my former Olds Omega drag car. I used Chrysler’s proven super stock leaf springs under the rear of it with great success. The car didn’t know they were Chrysler parts.

This is my former Olds Omega drag car. I used Chrysler’s proven super stock leaf springs under the rear of it with great success. The car didn’t know they were Chrysler parts.

 

This is Dan Zrust in a 1968 Plymouth Barracuda. The car competes in the notoriously tough Super Stock ranks and is powered by the venerable 426-ci Chrysler Hemi engine. This shot was taken at the 2009 NHRA U.S. Nationals in Indianapolis, Indiana.

This is Dan Zrust in a 1968 Plymouth Barracuda. The car competes in the notoriously tough Super Stock ranks and is powered by the venerable 426-ci Chrysler Hemi engine. This shot was taken at the 2009 NHRA U.S. Nationals in Indianapolis, Indiana.

 

9) Here is a classic shot of legendary Mopar racer Ray Mancini from 1969. He’s shown piloting a 1964 Dodge, powered by a 426 Hemi engine at the NHRA World Finals in Dallas, Texas.

9) Here is a classic shot of legendary Mopar racer Ray Mancini from 1969. He’s shown piloting a 1964 Dodge, powered by a 426 Hemi engine at the NHRA World Finals in Dallas, Texas.

 

Chrysler muscle cars have the option of Mopar super stock springs. They have a heavier spring rate on the passenger side than on the driver side, thus creating preload. Additionally, they are much stiffer in the front half of the spring (due to the axle not being centered on the spring) than the rear half, creating a rear suspension that works like a ladder bar setup. I have used this style of springs in a GM Oldsmobile Omega race car that I once had with a mild 455 Olds engine, and they worked great helping me win the IHRA Milan Northern Nationals.

 

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I always liked to see the Chrysler Super Stock cars leave the starting line. You could tell how good they were, planting the rear tires, by the amount of separation visible between the leaves in the back half of the springs. Eventually, spring clamps were used to eliminate this separation and to create even better 60-foot times, as the lesser amounts of separation caused the tire to be pushed into the pavement quicker.

The Mopar super stock spring name came from the fact they were developed in the late 1960s by Chrysler for its NHRA Super Stock race cars to give them maximum traction with minimum research and an advantage over the GM and Ford Super Stock cars of the time. The Big Three car manufacturers were heavily into drag racing and lived by the motto “Win on Sunday, sell on Monday” in the hope that customers see a car win a race on Sunday and want to buy one on Monday.

Some of the more notable Chrysler Super Stock racers were the Mancini brothers, who produced a string of successful Chrysler factory-backed race cars. Mancini Racing used that race experience to grow into a parts business that sells suspension parts for Chrysler race cars.

Whether you are using Chry-ler super stock springs or not, the ride height can be controlled by  re-arching the existing leaf springs. Most large cities still have businesses that re-arch springs for truck or trailer applications, and should be able to do your car springs.

 

Rise or Squat

Should the rear suspension rise or squat upon launch? The location of the front mount (height in the chassis) of the rear-end leaf spring deter-mines whether the car lifts in the rear or squats in the rear when leaving the starting line. If the mount is above the neutral line, the chassis lifts in the rear. If the mount is below the neutral line, the chassis squats in the rear.

I have tried to drive the point home that a car which lifts in the rear will plant the rear tires harder. This is true to a point. I am assuming the average reader of this book will not be driving/building a 7-second quarter-mile car the first time out. You first must understand the basics.

However, there is a point in horse-power-to-weight ratio that some cars with higher horsepower levels (800 to 1,000) need the front of the leaf spring mount to be lowered. Thirty years ago most full-bodied cars benefitted from a higher front leaf spring mount. With today’s horsepower the mount needs to be much lower. Using the drawing on page 18 the fact of whether the car lifts or squats in the rear depends upon where the front spring mount is in relation to the neutral line.

Lower-horsepower cars can use a higher mount thus making the car lift in the rear upon launch. As horse-power levels (800 to 1,000) increase, the mount needs to be lowered. The reason for the necessary lower mount is high-speed stability. A car lifting in the rear plants the tires harder but with that same lift at the top end it loses traction. With today’s tires lower-horsepower cars have sufficient traction. Now as the horse-power gets higher and higher (more than 1,000) the car hasthe ability to break the tires loose on the top end. For safety and to be able to complete the run the front spring mount needs to be lowered.

However the opposite occurs at the starting line. Most 1,000-plus-hp cars today have some sort of power adder (NOS, blower, supercharger) and can adjust the starting line power to compensate.

Every car/chassis is different. There is no exact measurement.  What works just right for one car won’t necessarily be right for another car. The idea here is to experiment. As long as you have the knowledge, you can make it work through trial and error.

Put that same no-hop bar on a Mustang (also a factory 4-link suspension) with the same horsepower-to-weight ratio and it will hook for about 6 inches and then blow the tires off. The Mustang has the engine much closer to the rear end than the GM intermediate thus giving the rear control arms a completely different leverage affect. Today’s 7-second quarter-mile ET nitrous oxide cars running on a 10.5-inch tire will not use a no-hop bar. In fact they will either raise the rear of the lower control arm or if permitted lower the front of the lower control arm to almost level and leave the upper control arm in its factory location (different class rules allow different modifications).

For high-speed acceleration, the intersection point needs to be much further out in the car for top-end stability. A car that lifts in the rear will hit the rear tire harder. You can hit the rear tire too hard and go into tire spin or tire shake. There sometimes needs to be a compromise. Less hit at the starting line (where power can be brought in slowly) for more traction at the far end (so the car can develop more miles per hour and still go straight).

For high-speed acceleration, the intersection point needs to be much further out in the car for top-end stability. A car that lifts in the rear will hit the rear tire harder. You can hit the rear tire too hard and go into tire spin or tire shake. There sometimes needs to be a compromise. Less hit at the starting line (where power can be brought in slowly) for more traction at the far end (so the car can develop more miles per hour and still go straight).

With today’s tires a car running in the low 10-second ET in the quarter-mile which lifts in the rear after the front end has lifted (pitch rotation) can use the harder hit off the line successfully. As the car gets further out and the chassis settles down, today’s tires still provide enough traction for the car to stay hooked and drive straight all the way to the finish line. Keep in mind that if a car is lifting in the rear and the front is not lifting, the car is actually transferring weight to the front (reverse pitch rotation) and removing weight from the rear tires and having less possible traction.

Now look at a 7second NOS quarter-mile ET car with more than 1,800 hp. The front mount needs to be even lower for top end traction, which causes a loss of traction at the starting line. Timers add the nitrous in stages with each stage farther down track. There is only one stage or no nitrous at the starting line.

Since the car does not have a lower front spring mount it will have better leverage down track and will not be trying to lift in the rear down track and therefore have more weight pushing on the rear tires providing better traction during the 175+ mph runs.

Just as 1,000 hp is an arbitrary point at which a car can turn left instead of turning right at the starting launch, it can also be used as an arbitrary point that a car needs the front leaf spring mount much lower in the chassis.

 

The Neutral Line

The neutral line is a reference point used when considering weight transfer (pitch rotation) in suspended cars. It begins at a point above the front spindle centerline as high off the ground as the center of gravity of your car, and continues to the rear tire contact patch directly below the rear axle centerline. If the front mount of the rear-end leaf spring is above the neutral line, the rear of the car raises upon launch. Chapters 3 and 4 explain how to get the pivot point farther rearward in the car. The opposite is true with most rear-wheel-drive cars with leaf springs if the car is making a lot of horsepower, as they already have a pivot point too far rearward. The closer the front mount of the rear-end leaf spring is to the rear axle, the harder the suspension hits the rear tires (of course, you can overpower a tire). If this same front mount of the rear-end leaf spring is below the neutral line, the car squats upon launch.

A car that lifts in the rear pushes down harder on the rear tires. You can‘t lift 100 pounds without put-ting 100 pounds of extra pressure on your feet. However, moving the front mount of the rear-end leaf spring too close to the rear end causes the car to hit the tire too hard. It’s possible to overpower a tire causing it to wrap up like a rubber band. When it can’t wrap any more, it unwraps (like a spring), allowing the tire to unload and lose traction. Too hard of a hit on the tires may work some of the time at some tracks but not all of the time at all tracks.

 

The adjustable pinion snubber limits how far the rear axle housing is allowed to rotate. These are popular with Chrysler racers and this one is available from the Mopar experts at Mancini Racing.

The adjustable pinion snubber limits how far the rear axle housing is allowed to rotate. These are popular with Chrysler racers and this one is available from the Mopar experts at Mancini Racing.

 

For the Chrysler cars, Mancini Racing is now offering an adjustable pinion snubber. This unit fits to the center front of the rear axle housing. It helps control axle wind-up by adjusting the height of this snubber to the floorpan, depending on the individual requirement. This provides a more precise adjustment and locking than any other snubber on the market today. Snubber height is obtained by turning the threaded shaft up or down. When the bumper is at the desired height, it is locked in place by screwing the lock nut down tightly onto the tube top. This unit is CNC-machined to ensure manufacturing consistency and quality throughout.

If you could lift a car off the ground at its center of gravity the car would not roll in any direction—neither sideways nor front to rear. Center of gravity calculation is a very complicated and mathematical procedure. Solid spacers must replace the shocks. Two 10-inch-tall blocks and a set of scales are also needed. First weigh the front of the car, then lift the rear of the car 10 inches by setting the scales on the 10-inch blocks and reweigh the front of the car. Then plug all of this information into a mathematical formula. Instead, use the camshaft centerline as the center of gravity when figuring the neutral line.

While not as accurate, it serves to use in calculating the neutral line for reference to see if the car will rise or squat. Having the exact center of gravity is not important, understanding the principals involved in this discussion is. No two cars respond the same with the same settings. One front leaf spring mounting point that works the best in a certain car may be completely wrong for a another car. Do your homework and determine what is best for your car.

 

Ladder Bars

Ladder bars are a great asset to making a rear-wheel-drive car with leaf springs work. This is because they relocate the pivot point farther forward in the car by having a lower mounting point than the front mount of the rear-end leaf spring, therefore making the mounting point closer to the neutral line. By having a lower pivot point, the car isn’t as violent and doesn’t over-power the rear slicks. You can also lower the pivot point by lowering the front of the spring mount hole in the frame. If you have class rules not allowing this modification, an old trick is to weld the old hole shut, grind it off flush so it doesn’t show, and relocate the hole lower.

 

This Advanced Chassis ladder- bar setup has double adjusters for setting pinion angle and suspension preload. Ladder-bar setups are great on the drag strip, but not so great on the street. They tie the control arms together, which is fine for straight-line acceleration but detrimental to cornering.

This Advanced Chassis ladder- bar setup has double adjusters for setting pinion angle and suspension preload. Ladder-bar setups are great on the drag strip, but not so great on the street. They tie the control arms together, which is fine for straight-line acceleration but detrimental to cornering.

 

Subframe Connectors

Most rear-wheel-driven cars with leaf springs are unibody designs with subframes. This means they don’t have a full ladder-type frame from the front of the car to the rear. Instead, the frame and body are assembled as a single unit. The front subframe (engine bay) is connected to the floorpan as is the rear sub-frame (rear suspension area). Structural integrity is designed into the vehicle’s integral body/floorpan/stub chassis assembly.

 

These subframe connectors are typical for a unibody car and were designed for use under a Chrysler product. Subframe connectors tie the front and rear of the car together to make it stronger and less likely to flex. They are equally popular with drag enthusiasts and corner carvers.

These subframe connectors are typical for a unibody car and were designed for use under a Chrysler product. Subframe connectors tie the front and rear of the car together to make it stronger and less likely to flex. They are equally popular with drag enthusiasts and corner carvers.

 

These unibody designs proved to be adequate for production car use and cost less to mass produce. They typically result in a lighter-weight car, so fuel economy concerns were addressed as well. Under race conditions, unibody cars tend to flex a lot more since they don’t have the full-frame chassis to prevent twist under heavy loading. To correct this, subframe connectors that connect the front subframe rails with the rear subframe rails should be installed. This greatly stiffens the car and helps reduce body twist, making the car respond better to hard-launch conditions.

These handy prefabricated connector packages let you tie the front and rear subframe longitudinally. Most frame connectors can be bolted or welded in, depending on individual preference and the design of the connector. When installing your roll bar or roll cage, be sure to tie the front subframe, rear subframe, and subframe connectors all together.

 

Calvert Leaves

Having spent most of my years drag racing in muscle cars with coil springs, I decided to contact John Calvert (from Calvert Racing Sus-pensions) to get some ideas on the latest technology in making rear-wheel-drive cars with leaf-spring rear suspensions hook, even though the same old issues still exist of setting pinion angle, preload, getting the car to launch as fast as possible, and  controlling spring wrap-up. Here is a summary of what John had to say when I asked him about how to get a leaf-sprung car to hook:

“The short answer is, ‘With a balanced approach and a lot of testing.’ Several areas can contribute to the success of quick, efficient, and consistent 60-foot times. Some of the areas have a great deal to do with success and some hardly affect it at all. Many cars are sensitive to certain settings or components and not so worried about others.”

 

An excessive wheelstand creates rearward motion, which is not the way we’re trying to go. Upward motion is good, but rearward motion can be counterproductive. Lifting the front end a few inches is just about as good as it gets.

An excessive wheelstand creates rearward motion, which is not the way we’re trying to go. Upward motion is good, but rearward motion can be counterproductive. Lifting the front end a few inches is just about as good as it gets.

 

Weight

“We [Calvert Racing Suspensions] deal with a wide variety of drag cars. We might see a car that weighs around 3,300 pounds, runs a big-block engine that puts out 500 to 600 hp, has a 3-speed auto-matic transmission, uses a transmission brake, has a pair of 29×10-inch slicks, and runs 10-second ETs in the quarter-mile. We may see a 1,200- to 1,500-hp car running nitrous oxide, a 2-speed automatic transmission, running on drag radial tires, deep into the 7-second range in the quarter-mile. Even trucks, small ones, full-sized, and Diesels.

“We offer a suspension package for these vehicles that was developed over the years, first with the introduction of our now-famous CalTrac traction bars, then a unique split monoleaf spring, an adjustable rear shock, and finishing the package with a 90/10 front shock. It seemed each step brought out a new obstacle to address.”

Center of Gravity

“In the beginning there was a trash can. Yes, I remember reading an article which explained how pushing a car was like pushing a refrigerator or kicking a trash can. This was (and still is) a very important thought. So, go ahead, kick away and see where you have to kick to get that can to slide right across the floor. Now how do you kick a car?

“First, lift it like an airplane. Hold it like you would to find the center of gravity (CG). It’s normally nose heavy, so maybe around where the steering wheel is, draw a plane (not an airplane) to divide the car in half with the line. Now put the car on a rotisserie and find out where the points need to be in order to spin it to be balanced. Draw another line through the car. Where the line intersects through the plane is the point it needs to be balanced. Now push with a vector that starts at the rear tire patch and aims through the front attaching point of your leaf spring (the spring eye) and see where it goes.

“If it’s a normal leaf-spring car, it goes too far back. If it’s a traditional Mopar with 20-inch front half-leaf springs, it goes way too far back (too much down pressure on those tires). This is why ladder bars were developed—to move the attaching point forward and correct the pushing vector.

“An NRHA rule for Stock Eliminator cars states: ‘No part of the traction system can extend forward of the front spring eye, and it has to be a bolt-on device.’ Lower the attaching point. Now the pushing point has been corrected and you can really put the power to the car, pushing the car with the tire. With the CalTrac traction system we are controlling leaf-spring wrap-up (this is mandatory) and correcting the pushing point (or at least making it better), and giving some other benefits like side-to-side preload and total preload on the rear springs. This gives some adjustability to the rear suspension. I love adjustability. I know no two cars are alike even if they are alike. The pushing point and adjustability in the back is very important.

 

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“Leaf-spring suspensions are like all others in drag racing in that they need a coefficient of friction between the tire and track surface to allow the suspension to be a player. There are a large number of considerations when discussing leaf-spring cars. Whether the car’s purpose is to work well on the street, street and strip, or strictly on the strip, the more drag race you want, the less street you get. The instant center of a leaf-sprung vehicle is typically too far back, resulting in excessive down pressure on the tires and a requirement to employ methods to control it. This is how we do it.”

Shock Absorbers

“Rear shock absorbers are one of the main items to consider. Control of the rear end is paramount to success-ful launches. Shock absorbers control the separation in the back. Typically, the rear is always trying to separate so an adjustable rear shock is essential. No control in the back of a leaf-sprung car results in great initial traction, but terrible secondary traction.

“A certain amount of violence can also be diminished with a good set of rear shocks. Good-quality shocks on the back are needed. That doesn’t mean they have to be expensive. They just need to be adjustable for rate of separation. The forces trying to push the rear tire down on a leaf-sprung car are tremendous. Cheap little shocks don’t do unless your car runs in the 15-second range, and then your car doesn’t need anything. But as the car gains in power and you improve in its ability to get a hold of the track (slicks), you need better things.

 

CalTracs bars versus slapper bars. Notice the better instant center location (being moved forward) due to the lower mounting point of the CalTracs bar when compared to a traditional slapper bar. CalTracs bars cost more, but they do more, so it’s justified. It’s no surprise CalTracs bars are very popular in all the classes where they are legal upgrades.

CalTracs bars versus slapper bars. Notice the better instant center location (being moved forward) due to the lower mounting point of the CalTracs bar when compared to a traditional slapper bar. CalTracs bars cost more, but they do more, so it’s justified. It’s no surprise CalTracs bars are very popular in all the classes where they are legal upgrades.

 

The light is green and the Mustang is off. Note the differences compared to the photo below left both the front and rear suspensions are raising. The nose comes up due to rotation, and the rear of the car comes up as the suspension works to push down on the rear tires. The wheelie bar is planted, limiting the car from rotating rearward. You can bet this car is fast since it’s using all of its power to plant the rear tires equally and move forward.

The light is green and the Mustang is off. Note the differences compared to the photo below left both the front and rear suspensions are raising. The nose comes up due to rotation, and the rear of the car comes up as the suspension works to push down on the rear tires. The wheelie bar is planted, limiting the car from rotating rearward. You can bet this car is fast since it’s using all of its power to plant the rear tires equally and move forward.

 

A bit too much rotation and you’re on the back bumper. Not only is energy being used to push this car rearward on this particular launch, but how aerodynamic do you figure this is? A slight adjustment is all it takes to make this better, and since this was during time trials it was the right time to experiment.

A bit too much rotation and you’re on the back bumper. Not only is energy being used to push this car rearward on this particular launch, but how aerodynamic do you figure this is? A slight adjustment is all it takes to make this better, and since this was during time trials it was the right time to experiment.

 

This new Cobra Jet is only moving forward. The front tires are tippy-toeing on the pavement, with just enough raise in the front end to load the rear tires heavily. The rear slicks are biting for all they’re worth. This is a great launch with no wasted momentum.

This new Cobra Jet is only moving forward. The front tires are tippy-toeing on the pavement, with just enough raise in the front end to load the rear tires heavily. The rear slicks are biting for all they’re worth. This is a great launch with no wasted momentum.

 

“Rear shocks come right after traction bars. Now you can start con-trolling the rear end and how much down pressure you want it to have. Or, if you are observant, how much down pressure the car can handle. Keep in mind the down pressure theorem: ’You can only give as much down pressure to the rear end as the front end will react to.’”

Leaf Springs

“Leaf springs fit right in to the ‘as the power increases’ need. As more power is developed, a higher spring rate is needed to control it, and the platform needs to be reinforced so aftermarket springs become important. Every conceivable combination has been used/tried over time—mono-leaf, multi-leaf, a stack of leaves on the front and only one out the back and then incorporate coil-over shocks, thick multi-leaf, add a leaf to the multi stack, and finally the split-mono design.

“The split-mono design because it’s lightweight and it emulates the parabolic mono-leaf, but allows a certain amount of adjustability. By overlapping the front half to the back half it can be used at different thicknesses front and rear, change the arch on the back to accommodate ride height, and experiment with a number of combinations looking for that perfect spring for each application.

“Control of leaf-spring wrap-up used to be the job of the traction bars. Redirecting the pushing point of the car is now accomplished through the connection point and preload adjustments on our systems. Additional power is controlled and used more efficiently. In 2010, Al Jiminez became the first leaf-sprung car driver to make a 200-mph pass in the quarter-mile. One of the advantages of the four-link and ladder bar suspensions used to be their ability to change the instant center (pushing point) of the car. A bolt-on traction system for leaf-spring cars can now make that correction.

 

These are Mopar super stock leaf springs. Note that the front half of the spring is shorter than the rear half this stiffens the front of the spring and doesn’t allow it to flex.

These are Mopar super stock leaf springs. Note that the front half of the spring is shorter than the rear half this stiffens the front of the spring and doesn’t allow it to flex.

 

This 1968 Charger got a good bite at the initial hit of the throttle enough to pull the front tires off the ground. However, it then went into tire spin. Time for some tuning. This pass won’t be optimal, and it’ll show up on his time slip from the 60-foot time to the final quarter-mile elapsed time.

This 1968 Charger got a good bite at the initial hit of the throttle enough to pull the front tires off the ground. However, it then went into tire spin. Time for some tuning. This pass won’t be optimal, and it’ll show up on his time slip from the 60-foot time to the final quarter-mile elapsed time.

 

This early Fairlane has a solid bite on the track. The front wheels are up a bit, and the slick is being pushed down and loaded up. You know this was a good pass.

This early Fairlane has a solid bite on the track. The front wheels are up a bit, and the slick is being pushed down and loaded up. You know this was a good pass.

 

“A note here on spring perches. A universal perch allows you to sandwich the leaf spring between the perch and the shock mount. If you would like to experiment with a change in pinion angle, just loosen the U-bolts and slide in a wedge. The main problem with the GM-style perch is that they were designed to be used with a rubber insulator and they lack a method of interfacing the leaf-spring center pins to the perch. Additionally, to remove the insulator, spacer plates are needed to allow the spring to be properly clamped. In order to use the universal style you need to reinforce the back side to keep it from bending. We designed a perch that incorporates a better interface (more contact area) with the axle tube and it has built-in reinforcing tabs. We market it as a heavy-duty perch.

“The bigger the tire, the better the bite. Most leaf-spring-equipped cars rarely get the luxury of bigger tires. Normally, the need to make way for the tire means the springs get moved inboard. Since there is a lot of fabrication going on, a four-link or ladder-bar set up is installed with coil-over springs.”

Starting-Line RPM

“One of the areas of experimentation when dialing in a suspension is the starting-line engine speed (RPM). Be conservative in the beginning to analyze what is going on in the rest of the suspension, and then slowly adjust to find the most the car will take and still be successful. If you are foot-braking the car, you are limited to leaving the starting-line RPM to no more than the RPM just before the car’s suspension starts to react. Otherwise you will use up part of the cars suspension travel before ever launching the car and not leaving as much as you could have for the launch.”

Coefficient of Friction

Coefficient of friction between the track and tire depends on tire pressure that is at least close to where it needs to be, and that usually means to the 1/4 pound. Normally on a light-weight, tube-chassis-equipped car with wide tires, that would be about 6 pounds. On a foot-brake, full-bodied car, tire pressure could be around 12 pounds, and adding drag radials it will go up to around 18 pounds.

 

This Nova’s back tires are moving, but the front ones are not. That means tire spin. Unless you’re in the burnout box, that’s bad news.

This Nova’s back tires are moving, but the front ones are not. That means tire spin. Unless you’re in the burnout box, that’s bad news.

 

You thought all a wheelie bar could do was stop the car from flipping over backward, but the smart guys at Calvert Racing figured out how to harness this energy and use it to push down harder on the tires. Innovations like this make people wonder why this wasn’t thought of a long time ago.

You thought all a wheelie bar could do was stop the car from flipping over backward, but the smart guys at Calvert Racing figured out how to harness this energy and use it to push down harder on the tires. Innovations like this make people wonder why this wasn’t thought of a long time ago.

 

Here’s another look at a CalTracs setup. There’s a lot to see here if you know where to look. Notice the minimal mono-leaf setup, the heavy-duty hardware, and the simple adjustments that can be made without removing the arm from the car. The length of the arm can be changed by loosening the jambnuts and twisting the arm. Nice and easy.

Here’s another look at a CalTracs setup. There’s a lot to see here if you know where to look. Notice the minimal mono-leaf setup, the heavy-duty hardware, and the simple adjustments that can be made without removing the arm from the car. The length of the arm can be changed by loosening the jambnuts and twisting the arm. Nice and easy.

 

This is the split mono-leaf spring setup. It’s a lot lighter than the factory spring stack it replaces, and less likely to wrap up as well. Many different spring ratings can be had to suit the needs of just about any car or truck.

This is the split mono-leaf spring setup. It’s a lot lighter than the factory spring stack it replaces, and less likely to wrap up as well. Many different spring ratings can be had to suit the needs of just about any car or truck.

 

Although John and I explain the same thing in different ways, the end results are the same: to give you enough information about rear-wheel-drive cars with leaf springs so you can put it to use and make your car hook the way it should for your application.

 

Written by Dick Miller and Posted with Permission of CarTechBooks

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