The fourth characteristic I wanted to analyze was the change in steering angle of the rear wheels as the suspension moves through its range of travel. This characteristic is called toe. The first question to ask is what benefit there is to be had in having this characteristic at all.
The answer involves understanding that the natural tendency of most chassis is for the front and rear ends to follow different arcs around a curve.
The answer involves understanding that the natural tendency of most chassis is for the front and rear ends to follow different arcs around a curve.
One need only look at wet tire tracks left behind on dry pavement to know this is true. Because of this, one end nearly always reaches it's limit of adhesion before the other. When the front end slides first, the characteristic is called understeer, and conversely when the rear end breaks traction before the front, it's called oversteer. Allowing the rear wheels to steer can change the path of the rear end and help create a more neutrally steering chassis.
For a variety of reasons the Fiero has a natural tendency to oversteer at the limit. One of several ways that GM reduced this tendency was to design passive rear toe changes through suspension geometry. How they got the rear wheels to steer becomes evident by looking at the top view of the suspension:
You'll note that the forward (green) lateral link is 26 mm shorter than the rear (blue) link. When compressed, the suspension arms angle upwards causing the shorter forward arm to pull in on the front of the knuckle faster than the rear arm does on the rear of the knuckle.
I redrew the two lateral links as they would be seen from above, once their angles had been raised to a compressed position although I've greatly exaggerated the effect on the drawing.
0.137 degrees of toe doesn't sound like much, but even slight changes in the alignment of the rear wheels cause big changes in the direction of the vehicle. Also, the flexibility of the suspension bushings contribute even more toe as well. Once again, using simulation software I was able to plot the amount of toe-in generated at any amount of suspension compression or extension, due only to the difference in length of the suspension arms:
The top half of the graph shows what the outside tire is doing in a turn, and the bottom half shows what the inside tire is up to. What the graph shows is that no matter whether the suspension is compressed or extended, both rear wheels toe-in. That works well for the outside tire on a corner, but not so well for the inside tire. Ideally, the inside tire should toe-out on the Fiero to help reduce oversteer, like the second drawing in this post. But as we've seen before, in a turn the weight transfers mostly to the outside tire so the inside tire has less influence on the direction of the car.