Leaf Spring Suspension

This style of suspension has been around since the creation of automobiles, the first race cars used this type of suspension and still use it today with a lot of popularity. This type setup can be very forgiving at the race track requiring little fine tuning, it can also be a night mare if not set up properly. We can sum up the workings of the leaf spring with the following statements.

Two mounting positions are possible for the leaf spring, above the axle and below the axle, both positions work very well, if you are mounting the spring above the axle you may want to use a restraining chain or strap to keep the rear end from coming free if something were to happen in a crash.  spring shackles are most always mounted above the axle, no inverted spring setups are talked about here.

The Leaf Spring has spring rates that are defined by it deflection between the spring eyes.  Normally both ends are confined to motion by the eyes, the front eye is commonly fixed, it can be secured higher or lower to the rear eye, this plays a factor in the way the spring performs.  The axle that is mounted on top of the spring helps keep the roll center lower, when the axle is mounted on the bottom of the spring helps raise the roll center.  With that said, a lower roll center is used on asphalt tracks and a higher roll center is for dirt track racing., if the roll center of the right side of the rear is too high, the body will not be able to roll.  The roll center can be determined by drawing an imaginary line between the eyes, and then drawing a vertical line up from the center of the wheel base, where the lines connect is your roll center for that spring.  Pinion angle is generally governed by the shackle mount and may have to be shimmed to correct to desired angle.  The pinion angle generally stays at the determined angle through the range of motion.

Factors that change characteristics

There are several factors that can change the characteristics of a mono leaf suspension,  here are a few.  The mounting brackets that hold the axle to the leaf spring will create a dead zone for the spring rate, this dead zone will add as much as 5 percent to the spring rate of the leaf  . As the leaf spring is tightened down on the axle it will twist slightly, the amount of spring rate change from this happening will vary by the amount of twist. Either fabrication, shims, or special brackets will be needed to correct the given amount of twist.  if the eye alignments are not straight and parellel this will cause a pinching affect on the spring rate, the more bind or pinch the more rate added to the spring.  The rear shackle motions and movements also play a roll in how the spring works, when the spring is compressed the shackle is going to want to move, as the spring compresses it forces the rear eye upwards(vertical)un loading the spring decreasing the body height, if the rear eye moves downwords (vertically) this loads the spring causing the body to lift or raise. This is useful in setting up your car for body movement, its pretty much controled by the length and amount of movement by the rear shackle.  If the shackle is short the movement will be faster, and loading or unloading of the spring will happen faster, so by this you can control how fast body roll can happen,  You can fine tune your setup by having shackles with multiple mounting locations on it.  The use of sliders on the rear spring eyes eliminate the up and down motion of the rear spring eye making the rate more consistant, it also tends to see less binding then a shackle. If you do use shackles start with a 90 degree position of the shackle when the car is on the ground and weighted,  when the spring changes position, moving the shackle forward by 30 degrees the rate of the spring will stiffen, when moved to the rear by 30 degrees the rate will soften, by these motions multiple mounting positions can be used to help with the way the car handles on a given track with tight or loose conditions. These must be looked at by moving the suspension in simulation of race conditions to avoid an ill mistake by the leaf spring movements.

 Spring eye bushings are another factor in how the spring operates.  If you are using a stock rubber bushing, the bushing actually dampens some of the spring rate of the leaf spring as it works, most performance cars switch to polyurathane bushings as a way to firm up this area to gain performance. As a racer you are not worried about a feather soft ride, so I would incourage you to use a solid steel greasable bushing which takes firmness to the next level, there is no give and the spring maximizes it spring rate.

The front spring eye, location, location.....where to put it. Start with this fact, if you mount the spring eye lower in the front then the back, the handling will tighten the car up, give you less size bite, and lower your roll center, mounting the front spring eye higher than the rear will loosen the cars handling, gain side bite, and give you longer bite out of the corners, also raising your roll center.

With all this said, the leaf spring suspension is great when used on race tracks of over 3/8 mile. The suspension can be very forgiving so you wont have to change spring rates through out the year as seasonal changes happen and the track changes, fine tuning can be done by raising or lowering the front eye to rear eye location, and also by mounting the front eye more inboard or outboard of the rear eye, remember that shock ratings are only a clock, measure time for the action of the spring to take place, a lower rated shock, not so stiff will allow the suspension to work much faster than a stiffer shock.