Leaf-spring rear suspensions...
Leaf-spring rear suspensions adapt well to various conditions and are very common in dirt-track racing.
Leaf Spring Science
I have a couple of questions I am hoping you can help me with. I know that rear-spring shackle location in reference to its height above or below the leaf spring's solid front mount can make a difference in roll steer, under both acceleration and braking. I've also seen a sliding shackle used instead of a swinging one. Can someone enlighten me more on shackle placement and design in relation to handling characteristics?
Another question that relates to leaf springs is what are the benefits and/or drawbacks of a locating pin offset from the center of the spring length-that is, the front sections of the spring being shorter than the back half. Chrysler products seemed to use this quite successfully on drag-race cars in the '60s to control rear spring wrap-up. These cars seemed to rise up in the rear during initial takeoff, instead of squatting and transmitting more weight to the tires. Does it make any difference whether the front or rear section is the shorter length?
Jeanne Worthy - Evaro, MT
I am going to assume you are talking about multileaf springs and not mono-leaf springs. With that as a base, here is my answer.
Think of the front half of the leaf spring as being a trailing arm and a torque arm. That is the portion of the spring from the locating pin to the front frame mount. Most of the travel that occurs in this portion of the spring is the result of torque through the rearend housing, because the leaf spring is the only thing that prevents the rear housing from rotating.
The angle of the leaf spring from the rear housing to its frame attachment point at the front is what causes the rear of the car to rise or squat on acceleration. The more the front portion of the leaf runs uphill to the frame, the more lift or anti-squat you have on acceleration. When you raise the front mounts, you do induce roll steer. The amount of steer depends on the length and angle of this portion of the spring.
By stiffening the front portion of the leaf spring, more lift or anti-squat will be induced on acceleration. This stiffer front rate transfers engine torque to the frame faster than a softer rate does.
Think of the rear portion of the leaf spring as being the spring. This rear portion of the spring has no effect on roll steer or the roll center of the car. Notice how the shackles or floaters attach to the frame. They can move front to rear, so they do not induce steer-only the fixed front mount does.
The rear shackles should be mounted exactly parallel to the angle of the leaf spring (when viewed from overhead) to prevent bind. Personally, I prefer the sliders. One of the benefits to this type of mount is that as the body rolls over, the shackles pivot, and this affects the diagonal weight or wedge in the car. The rear mount in this application is used to set ride heights and the diagonal weight.
The point at which the leaf spring contacts the rearend housing determines the roll centers in any leaf-spring car. You measure this point to the ground on each side, then add them together and divide by two. The sum is the rear roll-center height. Measure the distance between the two locator pins on the leaf springs (side to side) and divide by two. This will give you the lateral roll-center location.
The point at which these two intersect is the true rear roll center. When lowering blocks are used, the rear roll-center height is calculated by measuring halfway between the rearend housing and the leaf spring to the ground. In other words, if you add a 1-inch lowering block to the rear, you lower the rear roll center by 11/42 inch.
The front of the leaf spring should be shorter than the rear so it can function as described above. The Chrysler products were among the first to incorporate this design. This design was very effective on dirt late-models in the '70s and early '80s. However, when the front of the leaf spring was as long as the rear, the resistance to torque from acceleration and braking was too violent and caused considerable amounts of wheelhop.
If the questions you asked were to be applied to a mono-leaf-spring car, the answers would be basically the same with a few minor exceptions. The mono-leaf does not hold up the car. That is accomplished by coil springs, usually coilovers. The torque arm usually controls the torque from the rearend housing. Everything else is pretty similar.
Any type of leaf spring used in racing should have some sort of a damper shock-mounted horizontally or both horizontally and vertically (on a torque arm) to the rearend housing to help control wheelhop under acceleration or braking.
The problems inherent to the leaf-spring design, such as lack of roll-center adjustability or rear-steer adjustability, have led to their demise in most forms of racing. I still believe they have a place on limited classes (such as Street Stock) on dirt. They do afford a certain measure of adjustability over four-link passenger-car suspension.
As a side note, I would recommend you contact Landrum Spring Services, (404) 622-9348, a good source for both mono-leaf and multileaf applications.
Mark Tutor General Manager
America Online Racing, Mooresville, NC