Shortend IRS

[PTAddict]

I didn't explain this very well, here's another try. Think of an IRS as a front suspension whose ball joints are replaced by some sort of "rigid" linkage. Of course, no linkage is perfectly rigid, especially when it uses bushings which are also subject to deflection. So if, for instance, you attach a three foot long bar to the hub, sticking out from the side of the car, and then push forward on that bar with a hundred pounds of force, the hub/carrier/linkage/bushing assembly will deflect slightly, toeing in toward the center of the car in plan view. Just how much it deflects depends on the design of the linkage and bushings, but even a fairly small degree of deflection toe-in is enough to make the car "hunt" back and forth under hard acceleration as first one wheel, then the other gains and loses traction over bumps and surface irregularities.

Of course, there is no three foot long bar applying toe-in torque on a real car. What there is is a moment arm whose length is determined by the distance, from rear view, between the center of the rear contact patch and the effective axis of rotation of the rear hub carrier (the effective axis of rotation is not easy to figure, and will depend on the exact design, but a first approximation for a double lateral arm setup is to draw the axis through the upper and lower attachment points like they were ball joints on a front suspension). With a high offset wheel, this moment arm can be kept to near zero length (this moment arm is called scrub radius on a front suspension, and maybe on rear suspensions too - I'm not sure). With deeper dish wheels, the moment arm rapidly increases. For example, we have a deep dish wheel that makes the effective moment arm 4 inches long. Under hard acceleration with sticky tires on a 3500 pound car, we can easily get a thousand pounds or more of accelerative force at the contact patch. Working through a 4 inch moment arm, this creates approximately the same toe-in twisting force as the three foot long bar with 100 lbs force I used as an example above.

Just how much of a problem this will be in a given real world design is hard to say - all you can say is that the suspension was designed and modeled by the OEM using high offset wheels, and by changing those assumptions, you may be creating a bit of a problem. Computer modeling of such a situation is fairly complex, but it should be pretty easy to rough out a real world test with a long bar and an alignment gauge.



Hope this makes sense.