Learning the jargon for suspension fundamentals
Type “car suspension” into a search of the book section of Amazon and you’ll get more than 600 results. Millions of words have been written on the subject, most of them directed at readers looking for a tenth of a second shaved off a lap time, or another five hundredths of a g in a corner. It can be a little overwhelming for a person just getting familiar with how everything works, who doesn’t know an Ackermann from a kingpin. Here, we’re going to give you a toehold in the physics, technology, and terminology of suspension systems to help you gain the basic understanding of what’s going on when the rubber meets the road.
You Get Sprung
A vehicle suspension has pretty straightforward design goals – support the mass of the vehicle, and keep the tires in proper contact with the ground. To meet those goals, there are three main requirements. The first is to allow some compliance to allow for unevenness in the road. This is almost always accomplished by suspension components that can move up and down relative to the body, with some sort of spring in between. One notable exception are racing karts, which have their axles fastened directly to the frame, but even they have ‘springs’ in the form of the tire sidewalls which flex to absorb bumps. Because karts are generally run on very smooth courses, they can get away with having very little ‘suspension travel’ but for other vehicles, the need to have the suspension follow the ground requires something more elaborate.
Springs can take the form of leafs, coils, torsion bars, and even airbags. “Spring rate” is a measure of how much force is required to compress a spring, often represented as pounds per inch. The higher the spring rate, the stiffer the spring. Switching from factory springs with rates that are a compromise between controlling body motion and providing a comfortable ride to something stiffer will generally, but not always deliver improved handling. A progressive, or rising-rate spring, becomes increasingly stiff as more load is placed upon it, letting designers split the difference between passenger comfort and performance.
Putting A Damper On Things
The second requirement is some sort of damping on the motion of the suspension. We’ve all seen cars and trucks with blown shock absorbers wallowing down the road, or bouncing a tire like a basketball. Without some sort of damper, the suspension will continue to compress and extend until all the energy being stored in the springs is finally lost to friction. “Shock absorbers” almost always utilize fluid being forced through calibrated openings to perform this task, and by changing the size of the openings or the viscosity of the fluid, they can be ‘tuned’ to be harder or softer. Some high performance dampers are “single-adjustable” with one control to alter compression and rebound response together, or less-often, to adjust compression stiffness with non-adjustable rebound. “Double-adjustable” shocks have separate independent control over compression and rebound. Finally, many dampers will have an internal mechanism to make them velocity-sensitive; small, slow motion will be softly-damped, but as the suspension moves faster over a wider travel, the damper will become stiffer in response.
Some high-end performance cars are equipped from the factory with electronically-adjustable “active suspension” dampers; back in the day, a servo changed the position of the mechanical adjustment valve, but modern systems rely on hydraulic fluid that can change viscosity in response to an electronic signal in a fraction of the time.
On A Roll
The third requirement is roll stiffness. Any time a car is turning, inertia is acting on its center of mass, which by necessity is above the point where the tires contact the pavement. This gives it leverage, and the higher the center of mass is above the ground, the easier it will be for lateral g forces to try to flip it over onto its door handles. One major advantage of lowering a vehicle from the stock ride height is reducing this mechanical advantage and keeping the body relatively level. This helps prevent the inside tires from losing grip and forcing the outside tires to do all the work.
Very stiff springs and a lot of compression damping can also help control body roll, but at the cost of increasing ride harshness in a straight line. Ideally, a suspension design should provide extra resistance to body roll while still allowing reasonable spring rates, and that’s typically done by “anti-roll bars” which act as springs that connect the left and right sides of the suspension. These used to be widely referred to as “sway bars” but sway is an entirely unrelated kind of motion, so “anti-roll” is now the preferred term for them.
By changing the roll stiffness using anti-roll bars, it’s possible to adjust the traction bias of the vehicle – counter-intuitively, increasing the stiffness of the bar on the front increases traction at the rear, and vice-versa. Over the years, manufacturers have decided it’s safer for cars to understeer (where the nose of the car loses traction first, and you go straight into a ditch) than oversteer (where the back end loses traction and decides it wants to be the first to reach the ditch, so the car swaps ends) so most factory suspension designs that incorporate anti-roll bars will have stiffer ones in front than in back, or even none at all on the rear axle in some cases. A typical first suspension mod with cars that understeer more than they probably should is to install a rear bar or upgrade the factory one to promote more-neutral handling.
We’ve covered the major components necessary for an effective suspension setup, and how changes to things like spring rates, compression and rebound damping, and roll bar stiffness affect how the vehicle handles. Once these broad fixes are applied, small adjustments take us the last ten percent of the way, and even on cars with no changes to the suspension hardware, the difference between a properly-aligned car and one that’s just “within factory specs” can be very dramatic.
The aspect of wheel alignment that, for better or worse, gets the most attention is camber. “Camber” measures how the wheel is tilted relative to the centerline of the car – a suspension with negative camber tips the tops of the tires inward and the bottom outward. A few degrees of negative camber is pretty common to see in suspension setups for road courses, because it helps keep the tires’ contact patch evenly loaded side to side during cornering. Of course, this ‘racing look’ got taken to extremes, and we probably don’t have to explain why stretched tires on highly-cambered wheels isn’t the hot suspension setup for anything other than the most extreme professional-level car show hard-parking.
On vehicles with four wheel independent suspension, camber adjustments can typically be made to both front and rear wheels, but our next suspension geometry term only applies to the front wheels, since they do the steering. “Caster” describes the angle at which the wheels pivot, which will always be ‘laid back’ from vertical. Caster adjustments have a big influence on how much the wheels will want to return to a neutral, straight-ahead steering position and how ‘damped’ the steering will feel to the driver.
Another important adjustment is called “toe.” This describes how the wheels relate to each other on the same axle, relative to being pointed straight ahead. Manufacturer’s specifications almost always require a bit of toe-in, with the front edges of the tires closer together than the back. This promotes high-speed stability and keeps the car pointing straight instead of wandering side to side. For race course duty, less toe-in or even neutral or toe-out adjustments are sometimes used, because this will greatly quicken steering response and cornering turn-in, at the expense of feeling ‘twitchy’ in a straight line.
While live-axle rear end designs don’t have any adjustment for it, most independent rear suspensions have some provision for setting toe. Once again, a bit of toe-in helps straight line stability, while a neutral setting or a bit of toe out will make the vehicle feel a lot more ‘lively.’ It’s also worth keeping in mind that like too much camber, too much toe can also grind tires to dust in short order, so extreme settings are to be avoided if you care about things like treadwear.
There is, of course, a lot more to suspension adjustment than what we have room to cover here. As always we’ve tried to lay out the basic terms that will let you join the conversation and understand some of the basic concepts that underlie suspension tuning. And since you stuck around to the end, we feel it’s only fair to explain that a kingpin is what a wheel pivots on when you turn it left and right, and that Ackerman describes a steering setup with geometry that makes the inside wheel pivot farther than the outside wheel in turns, so that each tire can follow an arc closer to the radius of the turn instead of scrubbing sideways slightly. But that’s a subject for another day…