The proper way to determine...
The proper way to determine the moment center height for a leaf spring system is to measure the height of the two eyes from the ground, add them, and divide by two. This method has been proven to be true and accurate through the use of dynamic simulation software.
The reason why the leaf spring design is so good for higher g-force conditions is because the design has a very wide spring base. The wider the springs are mounted apart, the less roll tendencies there are. Also, the moment center height can be fairly high with a leaf spring design. This shortens the moment arm in the rear and that also produces less roll.
As the g-forces increase with a tighter track, the rear roll increases at a higher rate than the front. It’s important to control this increase in roll rate in order to provide better balance in the setup and more overall grip.
There is much less rear steer when going over heavy bumps or holes in the track with the leaf design and so the rough conditions that upset a three- or four-bar car are welcomed by the leaf-spring car.
The overall cost of a race car is less when using leaf springs mostly because you don’t need many of the mounts, linkages, and other hardware that must be used with three- or four-link rear suspensions. And, maintenance is less not having to worry about rusting Heim joints or broken mounting bolts that are in constant, high stress, high shear conditions.
If the spring blocks that...
If the spring blocks that are attached to the axle tube are not parallel, then when you bolt up the springs, there will be a binding effect that will cause a redistribution of the loads.
Also, a leaf spring can be used in conjunction with a coil or coilover spring to enhance the spring rate adjustability function for chassis setup balance. Teams will often put a high rate leaf spring on the left rear and a low rate mono-leaf spring on the right rear along with a coilover spring and shock. That way they can change the spring fairly easily on the RR to adjust for changing track conditions. The LR spring also controls most of the axle wrap-up and is about half as stiff in that regard as would be two stiff leaf springs.
Disadvantages of the Leaf Spring
There are pluses and minuses for every suspension system. The leaf spring system has a few disadvantages when compared to a three- or four-link system. Although the typical bar-link-type of system doesn’t handle tight, tacky, and rough dirt conditions as well, the money race is usually run once the track has become slick. So, at the end of the night under dry conditions, more times than not, the suspension that favors a slick track will win out.
The steel leaf springs may tend to lose shape and that means the car may loses ride height. If the “sag” is uneven between the two springs, it can alter the amount of bite, wedge or cross weight in the car which changes the handling. Some teams are said to go through three or more sets of springs per season. Spring sag also changes the rear steer characteristics of the car because the height of the front eye changes as the spring loses height and that changes the axle-to-mount angle.
There is very little, if any, adjustment for rear steer characteristics with a leaf spring system. Some builders will put several mounting holes at the front bracket and some only put one hole keeping with the theory that less adjustment means less rope to hang yourself with, setup wise.
The installation of the leaf...
The installation of the leaf spring must be done with care to avoid binding of the springs. Note the angle of the spring pad to the spring from a side view before tightening both sides. Also observe the relationship from a front or rear view and make adjustments to the spring chassis mounts accordingly.
To change rear steer, you would have to make height changes to the front mount and the same amount of change to the spring spacing between the axle tube and the spring to maintain the original ride height.
It might also be possible to have multiple height adjusting holes in the front mount as well as in the shackle. If the distance from the axle tube to the front mount and to the rear mount are different, then the hole spacing in the front and shackle must be different in order to maintain the original ride height with rear steer changes.
With the bar cars, the teams have a choice of adding lift bars, pull bars, push rods, front or rear spring mounting options, a wide range of rear steer adjustability, and variable moment center height adjustment. Available for the leaf spring cars is a mono-leaf floater that can be clamped or left open. This allows for a lift arm or pull bar to be added to the leaf system to control acceleration forces.
The rear moment center height is not easily adjustable on leaf-spring systems and the RR bite effect of angling the Panhard bar toward the RR tire contact patch can’t be done with the leaf spring system like it’s done with a four-bar system.