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 E30 Semi-Trailing Arm Geometry
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 As a start, let us examine the camber curve for the semi-trailing arm rear suspension over the entire possible range of motion. In other words, from 0 deg. through 90 deg. - this points out some interesting traits and helps confirm that the equations are correct. The links to the graphs will open up new browser windows so that one can observe the graph and the text simultaneously: Semi-trailing arm CAMBER curves for 15 deg. and 12 deg. sweep angles. This first graph shows that the rear wheel on a semi-trailing arm suspension gains negative camber as the suspension compresses. Two curves are shown, one for the standard 15 degree trailing arm angle, and one for the optional Gruppe A 12 degree trailing arm angle. An important thing to note is that when the trailing arm has rotated all the way up, the negative camber is equal to the trailing arm "sweep" angle. A little motioning with your hands will convince you that this IS indeed the correct answer, and it is a useful check on the equations. For these curves both the gain in negative camber as well as the rotation of the trailing arm from horizontal are shown in degrees. The change in negative camber is referenced to zero. In other words, it is assumed that the rear wheels have no camber when the trailing arms is horizontal. For a real world case, any gain in negative camber would simply be added to the initial "at rest" negative camber. In terms of the effect on chassis kinematics, the primary difference between the 15 degree and the 12 degree trailing arm angles is that the 15 degree setup gains more negative camber as the trailing arm moves up in compression. ... close camber curves window Next we examine how the rear toe changes as the semi-trailing arm moves through it's range of motion. Semi-trailing arm TOE curves for 15 deg. and 12 deg. sweep angles. The basic trend is that the rear suspension gains toe-in as it compresses past the horizontal position of the trailing arms. Neither the camber curves nor the toe curves show what happens as the trailing arms move down from horizontal, but the solutions are symmetric. Thus the rear wheels would also gain toe-in (but lose negative camber) as the trailing arms move downward past their horizontal positions. As with the camber curves, the toe curves show that when the rear trailing arm is rotated through 90 degrees, the toe angle becomes equal to the "sweep" angle of the trailing arm. And once again, a little maneuvering with your hands and some 3-D visualization will convince you that this is the correct answer in the limit of full-motion. Both camber and toe approach the trailing arm sweep angle at 90 degrees rotation from horizontal. The other thing to notice is that the 15 degree trailing arm sweep angle causes the rear suspension to gain additional toe-in as the suspension compresses (vs. a 12 degree trailing arm angle). Thus the net effect of going from the standard 15 degree sweep angle to the optional Gruppe A 12 degree sweep angle is that the rear suspension experiences less camber and toe change as the wheels move up and down. This is obvious if you think about the limit of a pure trailing arm, which is basically what's on the rear of most water-cooled VW's. A pure trailing arm experiences no change in camber or toe as the suspension moves. But also note that this is not necessarily desirable. You do want some gain in negative camber as the suspension compresses to compensate for chassis roll in cornering. It would be nice if it did not change during acceleration and braking but that is tough to achieve. Some small amount of toe-in as the outside rear wheel compresses in cornering is also nice, as it adds a bit of rear-end stability. ... close toe curves window Next page »
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