Suspension Geometry 101 - Maxima Forums

Advanced Suspension, Chassis, and Braking Talk about suspension geometry, advanced handling/chassis setup, custom brakes, etc. NOT your basic brake pads and "best drop" Information.

Suspension Geometry 101

Old 10-17-2008, 12:10 AM
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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:



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.



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.
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Old 10-29-2008, 03:35 PM
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Copied to new thread and stickied. Thanks for writing this up.

An addition to the roll steer description.......


First, roll steer is simply a side-effect of bump steer. The problem is that since the car is leaning, one side of the suspension is compressed, one side is extended.

Second, the main issue that this concerns is stability. I've seen it at 30-40mph too in serious cases, especially on cars like Maximas that have a lot of body roll.
It's much more noticeable at highway speeds though, as you're travelling faster and smaller changes in alignment become much more apparent.

Soooo, as you start to turn into a corner, let's say you're going down an off ramp or around a curve on a narrow road. You turn the wheel just a little bit, and the car starts to turn in and "take a set" where your suspension and body roll positions will stay throughout the corner.

This is where bump steer/roll steer rears its ugly head. As the car begins to lean your inside wheel toes out (into the corner) and the outside wheel toes in (into the corner) This makes the car turn into the corner more.

Since you're moving at a decent speed, this causes the car to lean more as the car turns in more and the wheels turn in even more because of the additional change in body roll. It's a downward spiral.

Doing that on an off ramp or other narrow road at speed will end up with the driver sawing back and forth on the wheel very dramatically and the car weaving all over the lane- assuming the driver can keep the car on the road.

The ONLY way to drive a car like that fast is to jerk the car into the corner so the car starts to slide and then counter steer to keep it on the road. This is not a safe way to drive and wreaks havoc on your front tires, heating them up to roughly a bazillion degrees and liquefying the outer edges of the front tires.

So umm yeah. Bump steer is bad. Don't lower your car that much, and if you do, don't put "bump steer kits" on the car that only include a tie rod relocation piece. you MUST move both the ball joint and tie rod ends to solve the problem.
The easiest way to fix it is to just not lower the car that much.
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Old 04-09-2009, 08:56 PM
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Virtual Suspension

Just got this today. Easy to understand primer on pivots, roll centers, and CG.
http://autospeed.com/cms/A_111163/article.html
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Old 07-28-2010, 12:20 AM
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Don't forget axle pivot points can't be changed! And they are designed to plunge in/out a certain amount at stock ride height with OEM arcs. Custom axles =
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Old 09-29-2010, 09:06 AM
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I have a 1998 Max that I want to install coilovers. I want to adjust ride hight and handling. My question is, can a 1998 be lowered with coilovers without posing any problems with tire wear or loss of handling? I see a lot of people mentioning that they dropped their maxima and they say what spring and strut they used but is there other work and parts needed to lower them correctly or will coilovers do the trick? Keep in mind I'm only looking to lower it an inch or so. I'm mostly interested in a stiffer ride and want to replace the old springs and struts on the car. Thanks for any replies.
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Old 09-29-2010, 09:17 AM
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Originally Posted by crestmasten View Post
I have a 1998 Max that I want to install coilovers. I want to adjust ride hight and handling. My question is, can a 1998 be lowered with coilovers without posing any problems with tire wear or loss of handling? I see a lot of people mentioning that they dropped their maxima and they say what spring and strut they used but is there other work and parts needed to lower them correctly or will coilovers do the trick? Keep in mind I'm only looking to lower it an inch or so. I'm mostly interested in a stiffer ride and want to replace the old springs and struts on the car. Thanks for any replies.
Coilovers are much better than any spring/shock combo available for the Maxima. All the aftermarket struts are just not made for a lower ride height, and they will bottom out. Also, they are not match the valving to any particular lowering spring, and won't handle as good as coilovers.
With a good alignment, shouldn't have any tire issues.
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