In an offset chassis, even...
In an offset chassis, even if we install the same spring rates on both sides in the front, we will have different wheel rates due to the different motion ratios at each wheel. This is because the lower control arms are different lengths. This formula will calculate the wheel rate for any installation ratio.
We need to square that number. The "installed ratio squared" number is then multiplied by the square of the Cosine function of the spring angle measured from vertical, and that answer is multiplied times the spring rate in pounds per inch. The result is the wheel rate at that corner of the car.
If you have a racing buddy and want to run his setup, make sure you know the relationship of the motion ratios and spring angles when comparing the two cars. His motion ratios and spring angles may well be very different than yours and the setup may not translate well.
Rear Spring Installation In the rear for a solid axle suspension, the car "feels" the spring base as the distance between the top of the two springs. This is all the car knows and it is as if the chassis were sitting on a pair of springs that are resting on the ground. The rear solid axle assembly is a solid base for the springs to sit on and the car does not know, nor does it care, where the wheels are located.
Overturning moments acting through the center of gravity are effected by the resistance to roll created by the spring base width and spring split, the rear moment center height, and the rates of each spring. To effect changes to the rolling tendencies of the rear suspension, we need to look at altering the spring base (distance between the tops of the rear springs), the installed spring angle, and/or the moment center height. The narrower the spring base, the greater the tendency for the rear end to roll for a given set of springs and moment center height.
A swing arm rear suspension...
A swing arm rear suspension is designed so that the spring is mounted onto the trailing arm and moves a shorter distance in relation to the wheel. This results in a motion ratio much like that found in the front suspension. The rate felt by the car is much less than the installed spring rate because, as the car rolls, the top and bottom of the spring moves. For every inch the chassis moves vertically, the spring compresses less. Finding the installed rate is just like finding wheel rate at the front. You have a motion ratio combined with a spring angle.arm
A narrow spring base can be a real problem for some types of cars. The four-link, Dirt Late Model cars sometimes have the springs installed at high angles with the top of the coilover positioned well inside the framerails. This severely limits what we can do to eliminate excessive rear roll in the car.
Many teams have moved the top shock/spring mounts out closer to the wheel to increase the rear spring base. The swing-arm types of dirt car have a very wide spring base by virtue of being mounted straight up, but suffer from another variable-motion ratio. The spring is mounted directly on the trailing arm, similar to the front spring mount, and that reduces the rate that the car "feels."
We can do a calculation similar to the front wheel rate calculation to find the effective spring rate. In most cases, we see about half the effective rate as the installed spring rate. So, if we install, say, a 200-pound spring in the RR, the car will react as if there was a 100-pound spring mounted directly to the rear end.
Stock classes, as well as sedan touring cars based on stock dimensions such as the USAR Pro Cup, Camping World Trucks, Nationwide Series, and Sprint Cup Series all suffer from a too-narrow rear spring base. To overcome this deficiency, some teams have deviated from conventional spring rate layouts and gone to a stiff RR spring. For this very reason, the BBSS setups tend to favor these cars. This "excess" spring split greatly reduces the roll tendencies of the rear suspension and caused a more balanced setup in the car, especially when using a large diameter sway bar.
Now, we see this same method used in many more series combining softer front springs with a higher rear moment center (Panhard bar). What these teams have done is work out a solution to their balance problems associated with the narrow rear spring base. The key objective still remains to balance the roll tendencies of the two suspension systems.
Make sure that you know your true spring rates, replace bent or fatigued springs, and always know your wheel rates when altering the mounting points or transferring the setup from one car to another. The car basically rides on four springs and the more we know about how they affect the setup in our cars, the more accurately we can develop a winning setup.
On a Z-link four bar type...
On a Z-link four bar type of rear suspension, there is an indexing effect as the chassis travels that will tend to rotate the birdcage and, along with it, cause changes in the height of the bottom of the spring mount. In this example, as the chassis moves down, the arms straighten out and that causes the birdcage to rotate. When the spring is mounted behind the axle tube, the indexing stiffens the rate felt by the chassis on that side. You can mount the spring in front of or behind the axle for different effects. If the RR spring is mounted to the rear of the axle, then as the car goes into the turns, crossweight will be reduced because load will increase for the LF and RR tires. Mounting the spring in front of the axle will increase bite, or left rear weight.
This car is built with the...
This car is built with the springs mounted to the birdcage in front of the axle and the top of the spring mounted inside the framerails. A unique feature of this design is that the spring can be mounted either in front of the axle or behind it. We try to reduce the angle of the spring by moving the top as far out as possible.
This example is typical of...
This example is typical of a stock rear suspension where the springs are mounted on top of the axle tubes or on top of truck arms. Either design results in a narrow spring base that is hard to compensate for when trying to lessen the rear roll tendencies.