Muscle car suspension, Dirt Simple Basics 1
It seems like everyone`s talking about performance suspension lately. There are plenty of cool looking parts and references about improved geometry and improved cornering etc. But what does it all mean? What`s improved geometry and why should you care? What`s wrong with the cars in the first place that anything needs to be “improved”? Well, it all comes down to a few basic principles and a little geometry and physics but it`s not exactly rocket science. With a good grasp of the basics it`s easy to tell what`s good, what`s bad and what to do about it.
The suspension dynamics of every car are dictated first by the pivoting points of the suspension also known as pickup points. These include the pivot axis of the ball joints, tie rod ends and all of the suspension bushings. Lines drawn through these pivot axis determine many of the factors that govern how the suspension behaves such as the roll center, the camber curves, anti dive, lateral roll center migration etc. These factors all interact and thankfully they tend to follow trends ie. when a few factors are bad, others tend to be bad, fix one and the others tend to get better too. This is the real foundation the rest of the car is built on. If the pickup points are in the right places (AND assuming the springs, shocks, tires etc. are selected correctly as well) the car will inspire confidence and really perform well. If the pickups are placed poorly (and if the springs, shocks etc. are off) the car will drive like an old shopping cart.
First let`s take a look at the front end and what goes on there. Most of our classic cars use a Short / Long Arm or SLA suspension. They use a short upper A arm, and a long lower A arm with ball joints joining them to the uprights, also known as knuckles or spindles (the spindle is technically the round shaft the wheel bearings fit onto). The pivot points of these components and their orientation to each other determines largely how well the front end will work. When the suspension moves up and down all of these parts will swing in their own arcs and interact with each other. For example if the lower A arms are fairly level and the upper A arms angle down hill toward the ball joints like most classic muscle cars do (especially true of early GM and Ford muscle cars) then as the suspension compresses (jounce or bump travel) the upper ball joints will follow the arc prescribed by the A arms and swing up and also out in that arc. The lower arm being much longer and relatively level swings in a milder arc with much less change and it`s ball joint moves more or less only vertically. That tilts the spindle out at the top and gives us positive (+) camber gain. In this case positive is a very bad thing because when the suspension compresses on the outboard side of the car in a turn the tire tries to lay over on it`s side wall! Tires aren`t meant to take load in that way so as a result the sidewalls distort and deflect badly. In extreme cases we`ve actually seen white lettered tires with the top edge of the letters beveled from contact with the road! This is NOT the fast way around a corner!
To add insult to injury the intersecting lines between the pivot points of this same suspension put the front roll center very low, usually a few inches below the ground (!). The roll center or RC is the geometric point about which the car tries to roll. Having the RC very low induces unwanted body roll. It does so in part because the front moment arm between the CG and RC becomes quite long. We`ll discuss this in detail in a future tech article.
Simply put, the above factors (and many more we haven`t touched on yet…) contribute to cars that generally go around corners like a cow sitting on a bar stool. The traditional “fix” has been to…well…not fix it at all and go straight or go slow. Of course if you wanted t go slow you wouldn`t be building a hot rod… To make matters worse we`ve all been spoiled by new cars. What seemed fine 40 years ago just doesn`t cut it today. Which brings us to the practice of modernizing and re-engineering the classic muscle car, some call it Pro-Touring or whatever, I just call it common sense. Some gains can be had by tuning the antiquated chassis with sway bars, springs and such, but it amounts to putting bondo over a rust hole or icing on a cake made with sawdust. The best way to go about it, the right way, is to fix the cars basic design issues and then tune it like a modern performance car. This method yields a car that generates great performance numbers but also drives and rides great too. It`s this line of reasoning that brought about our SC&C Street Comp suspension packages.