Front Suspension Geometry Explained | Caster Camber Toe
Front Suspension Geometry
The earliest cars were basically horse-drawn carriages, plus an engine and minus the horse. Their suspension systems were pretty simple carryovers from those horse and buggy days. But as drivers began demanding more performance, comfort, and reliability from their cars, suspension design has advanced by leaps and bounds. In fact, the various types of front suspension have advanced to the point where many of us have trouble understanding exactly what’s happening as they travel up and down over bumps and around corners. Below is a 101-level primer on front suspension explained, to help you not only understand what’s going on with an independent front suspension system, but also know what to change in the interest of better performance.
Camber Caster Toe | Car Suspension Diagram
Camber, Caster, and Toe are the “big three” terms when it comes to front suspension geometry. We seem to hear these terms the most because just about every independent front suspension has some means to adjust these, whether it be a coil spring suspension, IFS with coilover shocks, or front torsion bar suspension. These settings can have a huge impact on performance and tire wear. But just what is caster and camber? What is toe? Before we get into the explanation, here is a simple car suspension diagram that shows what each of these terms is referring to:
What is Camber in a Car | Camber Angle Adjustment
What is camber angle? Consider camber explained simply as the vertical angle of the tire when viewed from the front or back. Camber is measured in degrees, and negative camber occurs when the top of the tire tilts inward toward the center of the car. Positive camber occurs when the top of the tire tilts outward, away from the car’s center. Camber is typically adjusted by adding or removing spacers between the upper control arm’s cross shaft and the framerail.
Camber affects the tire’s contact patch both while the car is rolling down the road and when the car is going around a corner. Camber also tends to change as the suspension goes through travel. This is particularly important to remember when considering that the suspension on the outside of the car will compress as weight is transferred under hard cornering. In this situation, you want the tire to stick to the road, so creating the largest possible contact patch is critical. As such, many performance suspensions make adjusting camber angle easy, and many have camber gain built in, which causes negative camber to increase as the suspension compresses. This is also why you see static negative camber dialed in on many racing suspension setups. Look at the front of a Formula 1 car sometime and you’ll see lots of negative camber.
What is Caster in a Car | Caster Angle Adjustment
Because they both refer to critical alignment specs, caster vs camber are often confused, though they refer to very different angles. Caster refers to a vertical line that runs through the two points about which the spindle rotates when turning (typically the ball joints). Caster is adjusted by changing the angle of that vertical line. Negative caster occurs when the line is rotated forward, positive caster occurs when the line is rotated backward.
Caster is what causes your steering wheel to return to center when going around a corner. Adding positive caster also tends to increase the car’s ability to track straight going down the road. More positive caster is generally a good thing, but it also causes an increase in steering effort since you’re essentially lifting the weight of the tire and wheel as you turn. Note that caster is ofter hard or impossible to adjust on stock suspensions. Many aftermarket performance suspensions offer added caster or provision for caster adjustment.
Toe In vs. Toe Out Explained
Toe is the angle of the tires relative to the car’s centerline when viewed from above. Toe-in means that an imaginary line running through the centerline of the tires will intersect the car’s centerline some distance in front of the car. Conversely, toe-out alignment means that same imaginary line through the tire centerlines will intersect the car’s centerline behind the car.
Simple enough, but how does toe effect handling? Most baseline setups for rear-wheel-drive street cars start with a small amount of suspension toe-in. This inward angle of the front tires compensates for any slop in the suspension from things like control arm bushings, allowing the tires to be parallel to each other when the car is travelling down the road.
The question gets more complicated when you consider toe as a tuning tool in a performance car. There are lots of variables to consider here, but generally an increase to toe-in will help reduce oversteer (the rear of the car sliding out) and stabilize the car at speed. Increasing toe-out will typically reduce understeer (the front tires “pushing” on corner entry) but can make the car feel unstable and “darty” at speed.
What is Ackerman Angle
Ackerman is a simple concept that is often misunderstood, but is in fact quite simple and dates back to the days of horses and buggies. Imagine yourself and another runner on the track at a running event. If you are in the inside lane, you will have less distance to run than your friend in the outside lane. The same is true when your car goes around a corner. The inside front tire is turning on a tighter arc than the outside front tire. So what is Ackerman Angle? Ackerman is the geometry that allows each front tire to travel in the proper arc without scrubbing.
Ackerman is achieved by creating an angle between the point where the front tie rod ends connect to the steering arm and the point about which the spindles rotate, typically the ball joints or kingpin. Imagine looking down at a car from the top. Draw a pair of lines from approximately the center of the rear axle through the ball joints or kingpins. On a rear-steer car, the tie rod ends will meet the steering arms on those lines behind the ball joint and to the inside of the car. On a front-steer car, the rod ends will be on those lines ahead of the ball joints and to the outside.
What is Bump Steer
Bump steer is a dangerous condition caused by the front suspension steering itself as it travels up and down. This is caused by improper geometry in the steering linkage and its relationship to the suspension’s pivot points. If you’ve ever been in a car and felt the steering wheel being pulled in your hand when you go over a big bump, then you’ve experienced bump steer. Bump steer can also present itself as an unexpected change in direction as the car passes over a bump without steering input from the driver.
You might think of bump steer as an issue that only shows up in old hot rods with drag link steering. However, bump steer can be even more dramatic in more modern rack-and-pinion equipped cars. The short distance from the pivot point to the rod end on most rack-and-pinion systems means that incorrect geometry will present itself as violent bump steer. Always take care to match any rack-and-pinion upgrade to a suspension system that was designed to use that rack.
Anti Dive Suspension Explained
If you look closely, most independent front suspensions with two control arms per side mount the upper control arm at an uphill angle. This creates what’s known as anti-dive, and it helps keep the front of the car from “diving” under hard braking. The principle here is that the rotational torque of the rotor pushes forward on the upper control arm assembly when the brakes are applied. Because that control arm is at an angle, some of that force is then transferred to the spindle as a downward force, pushing the tires down into the surface and pushing up on the chassis of the car to counteract some of the forward weight transfer caused by the braking force.
Adjustable Suspension Systems
Most independent suspension systems have some means of adjusting caster, camber, and toe. Dialing in these settings allows the car to handle predictably and safely under normal daily driving circumstances. However, most stock suspension systems are designed to function within reasonable parameters, essentially limiting the amount of customization and tuning available to the user. If you want to be able to set up your race car, hot rod front suspension, or drag car front suspension for more extreme performance driving or a modified ride height, you’ll need to add some adjustable suspension components or a performance front suspension kit. Check out our article on adjustable suspension systems to learn more about what’s available and learn how to drop car suspension and dial in more performance.
