As we saw in the previous post, wheel camber dynamics don't seem to have much benefit in straight line performance. As a Fiero accelerates, the rear end squats down causing the wheels to tilt inwards at the top and away from the road reducing traction. As we'll see in this post, that loss in straight line traction becomes negligible though when we look at the benefits that camber gain has in cornering performance. In essence, the small amount of traction lost through camber gain in bump is a necessary trade off for the much larger gain in handling performance while turning corners.
As a vehicle turns, say left a corner, the weight of the car shifts to the passenger side causing the passenger side suspension to compress, the driver's side suspension to extend, and the body to roll. If the suspension were designed without camber gain, and if the chassis rolled at say, 6 degrees, the tires would follow the chassis roll angle and tilt away from the pavement at 6 degrees as well, reducing cornering grip. Instead, an ideal suspension design causes the wheels to remain flat with the road as the chassis rolls in a turn, like the illustration below:
As a vehicle turns, say left a corner, the weight of the car shifts to the passenger side causing the passenger side suspension to compress, the driver's side suspension to extend, and the body to roll. If the suspension were designed without camber gain, and if the chassis rolled at say, 6 degrees, the tires would follow the chassis roll angle and tilt away from the pavement at 6 degrees as well, reducing cornering grip. Instead, an ideal suspension design causes the wheels to remain flat with the road as the chassis rolls in a turn, like the illustration below:
In the above illustration, the chassis has been tilted 6 degrees towards the passenger side as though it were going around a hard left turn. Notice how the strut is fully extended on the driver's side, fully compressed on the passenger side, and how the wheels are flat on the ground giving maximum traction. To accomplish this feat, the driver's side wheel had to gain 6 degrees of camber and the passenger side had to lose 6 degrees. Unfortunately the Fiero's rear suspension doesn't actually work this well.
On an actual '88 Fiero, the rear suspension design was limited among other factors by physical packaging which forced designers to compromise on ideal performance. At 6 degrees of body roll an '88 Fiero's rear wheels look like this:
Rather than gaining 6 degrees of camber, the driver's side wheel can only gain 2.548 degrees so the angle of the tire with respect to the road is 6.0 deg - 2.548 deg = 3.452 degrees. On the passenger side it's even worse since the wheel is left tilted 4.765 degrees with respect to the road surface. I've shown the worst case scenario because it's easiest to see the difference, but using the Lotus Suspension Analyzer program the camber at any amount of body roll can be found. I used the program to generate the graph below:
To get an idea how this graph works let's use a more typical body roll angle of 3 degrees, and walk through finding the wheel camber on both sides. First, on the horizontal axis find +3 degrees. Next, follow the vertical line up from 3 degrees until it intersects the blue plotted line. Then read the corresponding value on the vertical scale, which in this case is about 2.2 degrees. That's the angle the outside tire will be at with respect to the road, so on a left turn, that would be the angle of the passenger tire.
To find the angle of the inside tire (the driver's tire on a left hand turn), simply use the other half of the graph by scrolling along the horizontal axis until you reach -3 degrees. Then drop down to the plotted line and read the corresponding value on the vertical scale which is about -1.9 degrees. So at 3 degrees of body roll, the view of both tires would look like this from behind:
So clearly the camber gain on an '88 Fiero isn't perfect, but then I've left out two things until now. The first is that Pontiac decided to give the Fiero -1 degree of wheel camber when the chassis was level. So rather than having the rear wheels sit perfectly vertical at rest, the wheels are adjusted to tilt inwards at the top when a proper alignment is carried out. This has the effect of reducing all the values in the above graph by 1 degree, which helps make the outside tire flatter to the road but worsens the angle of the inside tire. That works out well for the Fiero because in a turn, most of the car's weight is transferred to the outside tire anyways, so it's more important that it be flatter than the inside tire.
The other issue is that because of tire sidewall and bushing flex, the ideal amount of camber gain isn't 1 degree for every degree of body roll as I suggested earlier. Rather, it's closer to 0.7 degrees of wheel camber gain for every 1 degree of body roll. Later when I delve into designing my own suspension, I'll revisit this in greater detail.
In these last two posts I've covered camber in bump and camber in roll. Next up is how toe changes with suspension travel... it should be simpler to understand than camber.