More on suspension - rear toe and bump control - Hyundai Genesis Forum
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post #1 of 5 Old 06-03-2009, 12:05 PM Thread Starter
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More on suspension - rear toe and bump control

This is a paper I did a while ago addressing the new rear suspension on the 2008 STi.

While I did write it to address the STi, the GC has a very similar setup, and the same principles apply.

For example - on the GC we have and upper arm, a lower arm, and a toe control arm.

The toe control arm is what we're going to look at here.

Paper is as follows:

At this point I feel that there are three items of importance.
  1. Travel (we covered this earlier)
  2. Toe change/bumpsteer (which you are reading right now)
  3. Rear camber control (which will be the third paper to be done soon)

There's actually a fourth which is bushing flex. I'll get to that one later.

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.

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post #2 of 5 Old 06-03-2009, 12:06 PM Thread Starter
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Toe control when you lower the car:

Now that we've established the toe characteristics on the stock car lets talk about what happens when you lower the car.

Once you lower the car the toe arm is no longer parallel to the ground. The outboard side (the ball joint on the knuckle) is now higher than the inboard side.

Let's take a look at a simple picture of this here:

"A" is the car just sitting there not doing anything
"B" is the suspension compressed
"C" is the suspension at droop

There are two things that happen when you lower the car.

The first thing that happens is that toe change happens FASTER than at stock height. Take a look at this (we've seen it before), but this time pay attention to the graph.

Point number 1 is the starting point at the stock height. Lets say you enter a corner and exit a corner. Your dynamic toe change goes from point 1 to point 2 and you're done.

Now, lets say you're on a lowered car. Well, you're going to be STARTING somewhere around point 2, and end up around point 3 and back by the time you're done.

Now, here's the important part - the further the suspension MOVES (either in bump or droop) the faster the change happens toward the end. In other words, the toe change is not linear, and as you get more into it.

The second thing that happens, and this is a big one is that the car will actually dynamically steer against you in a corner.

The loaded tire will quickly toe in (more quickly than stock - remember point 2 to point 3). The unloaded tire will now toe out because the toe arm is approaching parallel to the ground. This will remain the same state until the suspension droops to the point that the arm is parallel to the ground. At that point the tire will toe in.

What this means is that you get passive 4 wheel steering. VERY VERY cool, BUT, and this is a big one, it's in the wrong direction!!!! It's steering the loaded tire into the turn, and the unloaded tire into the turn, but it's at the rear of the car!!! Dangit!!!

This means we're back to the same problem as before in that you'll be fighting it until it lets go.

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post #3 of 5 Old 06-03-2009, 12:10 PM Thread Starter
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In part I we looked at travel, and lowering the car. Some solutions exist for this in the fact that you can work with a shortened strut body and find a good balance between bump and droop.

In this one there is no good conclusion. A shortened strut body is not going to help. Sure, lowering the car is going to make it worse, but truth is it's not that great to begin with it. Sorry, but the truth hurts sometimes.

In the end you have all sorts of dynamic toe, and with the way the car is currently set up you can't really use it to help you.

Can that be changed? Absolutely. What's it going to take? Well, quite a bit actually, but we're working it. In fact, I would hope, really really hope we're not the only ones working it.

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post #4 of 5 Old 06-03-2009, 01:05 PM
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good to know. i'm glad i'm not a fan of the lowered/slammed look... especially if it makes handling even worse.
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post #5 of 5 Old 06-03-2009, 01:43 PM
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You must just have a way of explaining things, as that was one of the most understandable suspension geometry reads I've ever read. You kept indicating it might be hard to follow & I was expecting some difficult read that wouldn't make sense in my head. Therefore, I thought I was going to have to read it several times, but I understood it the 1st time through.

Now, what do we do about it?

I'm intent on lowering my car, if only an inch.

Keep us posted...

Sponsored By: Columbus Body Works / ARK Performance / Modern Automotive Performance / Forged Performance

Last edited by Tufast; 06-03-2009 at 01:46 PM. Reason: punctuation
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