Roll Camber

Could someone help me understand Roll Camber? In Milliken, it is said to be to be wheel camber angle over chassis roll angle. But I am confused as to what this means. Is this the wheel camber you want over the chassis roll that is desired?

Roll Camber is simply the amount that the camber changes due to the roll of the chassis as the vehicle goes around a corner. in roll, the outboard suspension compresses and the inboard suspension droops. if the suspension had zero camber gain in compression, then your roll camber on your outboard tire (measured in degrees) would be equal to your static camber angle plus whatever positive camber resulted from the kinematics of your particular suspension design as the chassis rolled.

if you knew (from your kinematics and your spring / bar rates) that your chassis would roll one degree in a 1g turn, and you knew that your outboard suspension would compress one inch in that same 1g turn, you'd probably want to design your suspension to gain at least one degree of negative camber in that first inch of travel, to offset the positive camber change that your tire will see due to the chassis roll.

there may be other trade-offs to consider in this analysis for your particular application. but that's roll camber in a nutshell.

Roll camber is what happens as the car body rolls to the outside during a turn. If both arms were the same length, the ball joints would both describe the same arc as the body rolled. This means as the car rolled to the outside of a turn, the top of the tire would lean outwards relative to the road surface, causing positive camber relative to the road and leading to a loss of traction as the inside of the tire lifted off the road surface.

In most cases, the upper control arm is shorter than the lower control arm; as the inner mount pivots for both arms move down relative to the ball joints when the body rolls, the upper arm has a shorter arc than the lower arm. This will cause the upper ball joint to move closer to the car's centerline relative to the lower ball joint, thus increasing the negative camber as measured from the pivot points. This can be adjusted via pivot and ball joint placement to have the tire wind up vertical (no camber) as measured from the road, or with the top of the tire leaned inward, meaning negative camber. Negative camber moves the load on the tire toward the inside, since the tire sidewalls 'give' this generally means more rubber on the road which equals better traction.

The camber changes as the body rolls, this is called the 'camber curve'. The amount of camber gain needed throughout the camber curve is dictated more by the tires than anything. In extreme left/right turn situations, the caster angle throws a monkey wrench into the works as well.

So yes, the roll camber is a combination of the body roll and the steer knuckle's camber angle. General rule: the longer the upper arm, the less aggressive the camber curve, the shorter the upper arm the more aggressive the curve. On cars with, say, 4 or 5 inches total wheel travel as measured at the rim, the upper arm should be around 70% of the length of the lower arm, this gives a reasonably aggressive camber curve. If the wheel travel is a lot more than this, the 70% rule probably won't apply.

On really old cars with wooden spoked wheels and tall skinny tires, positive camber put the center of the tire directly below the kingpin, making for easier steering (zero scrub radius) and lessening the side loads on suspension parts of iffy metallurgical quality.

So if I was designing a suspension that would roll 1.5 degrees under 1 g and was aiming for a camber of 1.5 degrees static, my roll camber would be 1.5 over 1.5 degrees of camber. My only problem is that this results in a roll camber of one and the equation for the front view swing arm length is (t/2)/(1- Roll camber). This would result in something divided by 0.

Edit: I am only working with 2 to 3 inches of travel.

Hmmm. 1.5 deg body roll per G? 1.5 degrees static? 2 to 3 inches of travel? Add enough negative camber in that distance to maintain 1.5 deg negative?

You are going to need a really short upper arm to accomplish that. It would be so short, it would then become a 'virtual swing axle'. If it's going to be a 'virtual swing axle', then skip the middleman and a lot of fabrication and make it a plain ol' swing axle.

You might even consider making it a stick axle with a center pivot with 1.5 degrees negative camber built into your knuckle pivots. Sometimes the simplest solutions are the best.