This is part two of Clint's whitepaper on understanding the new rear suspension.
At this point I feel that there are three items of importance.
- Travel (which is covered in part I and can be read by clicking here)
- Toe change/bumpsteer (which you are reading right now)
- Rear camber control (which will be the third paper to be done soon)
The first step to understanding how this works is to understand how the suspension goes together. On this setup we have a few things:
- Upper arm
- Lower arm
- Trailing arm
- Toe arm (THIS is the one that concerns us for this paper)
I have absconded with and marked up the diagram from part I to help people understand everything. Here it is:
Before we go any further I would like to point out - I am NOT an artist. For pretty artwork we rely upon Fullerton (he does nice work, and he's cheap too!)
The part we REALLY want to look at on this is labeled the toe arm. One end of it has a bushing and goes into a bracket on the rear subframe. The other end of it has a balljoint with the stud going though a tab on the front of the rear knuckle.
In fact, it may look very much like a front knuckle in that the tab and arm are almost like the tab and tie rod leading to the steering rack.
Essentially, that's what we have sitting right here at the back of the car. Thing is they are not hooked up to a steering rack. Despite this it is very important as this is the cause of the bumpsteer and poor toe control (yeah, I said it. Keep reading and I'll explain why).
Now, all things being equal, and taking bushings flex out of the equation all the parts I've mentioned, upper arm, lower arm, trailing arm, and toe arm, all move in an arc.
The centerpoint of the arc being the center of the bolt attached to the rear subframe. The radius is described as the length of the arm.
Now, here's where things start to get confusing, but I'll see if I can make some sense of it.
The toe arm moves in an arc. As the wheel goes up and down the arm follows it's path of arc as described by the centerpoint, and with a radius of the length of the arm.
The toe arm is a different length from the upper arm and lower arm so it follows it's own rate of motion along the circumference of its own arc. Addtionally, since this toe arm is attached to the front of the knuckle it will pull in or push out on the knuckle given suspension setup and motion.
That's it in a nutshell - the toe arm will change the toe angle of the rear tire as the suspension moves.
Here's why.
At STOCK height the toe arm is parallel to the ground. This means that the knuckle attachement point to the knuckle is the furthest from the center of the arc for the toe arm. The furthest it will ever be. To see this you can think of two parallel lines perpendicular to the ground. One goes through the ball joint for the toe arm, and the other goes through the toe arm mounting bolt. The distance between those two lines is the furthest it will EVER be given the current mounting points and angles.
As the suspension compresses or extends the distance will shorten and it will pull the front of the knuckle inward (kinda like steering, but not).
I know this doesn't make much sense so I drew a picture (remember I am not an artist). Here it is:
Look at line "A" on the picture. This is the toe arm, at stock height.
Now look at line "B." This is the toe arm when the suspension compresses. Remember the inner mounting does not move. It's the outter mounting attached to the knuckle that moves.
Now look at line "C." This is the arm when the suspension droops.
Now look at the blue line that is labeled "change in distance." This is the amount of how much that knuckle at the toe arm attachment point it pulled.
Since the attachment point is on the front of the knuckle this means it toes the tire in!!
How much, well, I worked up a little spreadsheet for this. Pay attention to the pull in. Here's a screenshot:
Remember it because we'll be looking at it later as well.
So - the result:
On a car set up for STOCK ride height you get a toe in every time you compress a wheel, and you get a toe in every time the suspension droops. Seems that no matter what you do the rear tires toe in (as for number of degrees - don't know. I have not had a chance to calculate it yet).
Now, here's the bad part.
On a car that's turning you have a tire that's loaded and toes in. On the other side you have a tire that's unloaded and also toes in. The loaded tire, as it turns in will actually push the rear of the car toward the inside of the turn.
Here's what happens when that happens -
You get GREAT grip out of the rear as you VERY quickly load up the tire to ideal slip angle for maximum grip (tires need some slip angle to achieve max grip when side loaded). The toe in combined with the angles to begin with get you to that max grip quickly. Thing is the rear is fighting you on coming around the turn as it's steering to the inside of the turn on the loaded tire. So you push it a little more. What happens is that you have now exceeded your ideal slip angle given a load for max grip, and the tire starts to slide. If you're hard on it then it's el drifto time since you get snap oversteer. Hope you know to keep your foot in it and countersteer.
On the other side of the car you have the tire that unloaded. Sure it's not loaded but it is contributing a bit to overall grip. Thing is it's toed in as well which means it's trying to steer to the outside of the turn. Great, perfect, and awesome as that's a big help! Problem is that lack of load and the fact that it's toeing AGAINST slip you never acheive max grip out of that tire so you don't get much out of it.
2008 Rear Suspension Part II: Toe Change/Bumpsteer 
