Maxima Forums - Suspension Geometry 101

So, dynamic alignment issues.

The idea here is that the front end of the car isn't ideal, and the LCAs, strut arms (everything connecting the upper strut mount to the lower ball joint) and tie rods move in different arcs as the suspension compresses and rebounds, and the front end is engineered for the arcs to match when the car is at a certain ride height; rather, they don't match perfectly but it is "close enough" to do the job pretty well.

The A32 was not designed to ride with the suspension compressed an extra 2" (lowered 2"), in fact that's about where it bottoms out normally. So they don't really care how the car acts when the suspension is that compressed as it will very rarely happen. So, you can see that when compromises are made as they must be with a limited budget, limited space and a macpherson strut design, this realm of travel of the suspension is ignored. When your car is this low you will generally have problems, whether you notice them or not depends on how perceptive you are.

First, camber and toe curves.

The camber "curve" refers to a plot representing dynamic camber of a wheel depending on suspension position. I mspainted up about what it looks like for an A32:

http://img.photobucket.com/albums/v6...ambercurve.jpg

So as the suspension is compressed, it gains negative camber until the curve levels off at about 1.5" compressed, then starts gaining positive camber.

Now, I have referenced it at static camber being lined up with stock ride height, but the curve is all relative, so if you set the camber at 1" lowered to be 0 degrees and then raise the car back up to stock ride height, it will be at +1 degree of camber. Think of it as you being able to move the y axis (or the camber axis) up or down to set the camber to a certain value at a certain corresponding suspension position, but the x axis and the curve stay the same.

So, with all that being said, let's discuss the stock geometry. As the suspension compresses, negative camber is gained. This is good as the outside wheel will go under compression when you turn, and you want that wheel to have as much negative camber as you can.

Now take a look at it when it's 2" lowered. At that point you gain POSITIVE camber/lose precious negative camber as the suspension is compressed. Now it's not going to compress the same amount since a) the springs you're running should be significantly stiffer and b) you will probably run into the bump stop if you're on stock length struts. But the fact remains, the behavior is the exact opposite of what you want to happen. Your outside wheel loses negative camber as the wheel is compressed, which is when you need it the most. Not good.

Toe curve:
So you get why they call it a "curve" now, and the toe curve is just the same thing as the camber curve--a plot of relative toe in/toe out based on suspension compression. This changes a bit as you change toe since you're basically changing the length of the tie rod and that means that the tie rod arc changes, and it changes when you change camber, but it's a similar effect: basically, around stock ride height the toe curve is pretty straight, so when you compress the suspension you get about the same toe as stock, and when you extend the suspension you get about the same toe. However, at 2" lowered the curve has a positive slope, so when the suspension is compressed, you gain toe-in and your tires scrub. When the suspension is extended, you gain toe-out which translates to a lack of immediate steering response.

You may think "big deal, my toe changes which I don't really notice anyway" but consider this: left and right suspension are not always in the same position. So, if you hit a bump with the left side of the car and not with the right, as the left suspension compresses the left front tire will point inward, effectively steering the car to the right. Two things happen here: a lot of this energy goes into jerking your steering wheel to the left, but the car steers to the right as well. This is called bump steer and it's no fun.

Roll steer is another problem, basically a positive reinforcement loop on your steering wheel when you are traveling at high speed. When you turn left, the body of your car rocks over to the right--standard body roll. The left suspension extends and the right suspension compresses. Can you see what's coming? The left wheel gains toe out, pointing outward (left) and the right wheel gains toe in, pointing inwards (left). So you steer left, and as the car rolls to the right it starts to steer itself more and more to the left. I have only noticed this when doing naughty things at highway speeds, because the car rolls about the same amount and the wheels turn themselves about the same amount, but you don't notice it if you're only going 30mph since you're already turning the wheel so much. If you're going 100+, you are turning the wheel just a tiny bit to corner hard and all of a sudden that smallish "self-steer" toe angle becomes very noticeable.

Ackermann: ackermann is another property of the steering, it's basically a curve of the toe versus steering wheel position. This is necessary basically for the same reason a differential is necessary. When you're in a tight turn, the left and right front wheels are turning at different speeds and are turning different radiused arcs--the inside wheel is turning slower because it is traversing a smaller radiused, or a tighter arc. This means that the inside wheel must have a greater steering angle than the outside wheel because it's turning a tighter circle.

http://rockhuggers.com/images/currie5.jpg

It is sort of hard to see here but the inside wheel is turned a bit more than the outside to compensate for running a tighter circle. The funny thing is that most race cars run anti-ackermann, that is the outside wheel turns in further than the inside wheel. This is because when cornering hard, most of the weight is on the outside wheel, and a tire with more load on it can take a higher slip angle before it starts to slide. Thus, if you have a race car that keeps its tires parallel (parallel steer) as it turns, the inside wheel will start to scream, losing grip, while the outside tire does not as much. Anti ackermann is more effective on fast tracks with less of a steering angle though, typically closer to parallel steer or even ackermann steer is effective on tighter tracks so the wheels don't scrub.

Now, intuitively it seems to me that the effect of lowering on ackermann is nil, as long as you keep the toe the same. However, I have noticed my inside front screaming for help on hard cornering when I had my softer Progress springs that lowered the car about 1.8-2". I have not noticed the same effect with my current setup although it is entirely possible that the ackermann on our cars just wasn't set up for performance driving (less ackermann, closer to parallel steer) but instead for economy (less tire scrub around slow corners, more ackermann)

The best way to add ackermann to a setup by angling the steering arms inwards or towards the center of the rear of the car (as they are in the picture), whereas anti-ackermann is added by angling them outwards or towards the center of the front of the car. This is not something we can adjust at will on a street car.

The only way to get rid of these problems if you insist on being low? Custom extendable ball joints AND tie rod relocators. Bump steer kits with just tie rod relocators do not solve the problems as toe is very sensitive and the camber curve won't be affected without extendable ball joints, thus still throwing off the toe curve (possibly in even more funny ways). You must recreate the stock ride height geometry in order to do it right.

So, Matt, if you're out there, make those damned custom LCAs. :mad: