Axle vs. SAI adjustment for camber?
 

This is probably a question for Austin, but lets see what we come up with.

As I figure it, there are to ways to adjust what we commonly refer to as Camber.

1. Adjust the angle of the axle relative to the strut at the base by use of camber bolts, etc.

2. Adjust the angle of the strut itself by use of camber plates.

I am trying to figure out the pros and cons of each from a geometry perspective.

Obviously camber plates are easiest to adjust, and the combination can be used to optimize tire clearance issues, but what are the basic geometry considerations to each.

Is one more likely to cause bump steer, or some other negative effect?

I am going to keep researching, but perhaps Austin or someone else has already done so...

Here is some basic alignment stuff to get you started if you are not familiar with these terms.

http://www.familycar.com/Alignment.htm

I think strut angle is better because you get the same static camber but a better curve under roll.

I would have said the opposite.

Taking an extreme example.

With a 0 degree strut angle (vertical) and 88 degree angle at the axle, camber is -2 degrees at both zero and 90 degrees of strut rotation.

With a -2 degree strut angle and 90 degree angle at the axle, camber is -2 degrees at zero degrees of strut rotation but 0 degrees at 90 degrees of strut rotation. (The tire has rotated the 2 degrees, I think.)

Okay, expanding. Aside from the better camber curve you get from tipping in your strut tops 2 degrees, you get a better castor curve too. Leave the ridonkulous extreme non-real-world examples out and Dean. ;)

I need to sit down and expand my thinking on it whne I haev more time tonight but it seems at first glance to me that by reducing the angle between strut and axle, you slow down the camber loss as the strut is shortened under compression.

You will have to explain the "better camber curve" as well as the caster impact. as caster is 90 degrees from SAI, I would think they are completely independent. They both affect dynamic camber, but not each other as far as I know.

OK, so you guys talking about this got me to sit down and make a model of this. Do any of you have any measurements of the Impreza front suspension? Like Control Arm length, strut length and travel etc... (also distance from top of the strut to control arm pivot) I have a working model but have no clue the sizes of anything.

Offhand, with silly guesses for dimensions, I can actually get camber _gain_ if the strut angle is right, even with the control arm up past horizontal. However I may have some dimensions way off. :huh:

1. (Spindle/strut adjustment): I would say this is the most decoupled camber adjustment. Adjusting the spindle/hub relative to the base of the strut will have the least effects on the front view geometry. SAI will stay the same. Instant center locations & roll center height will change a bit as the strut body angle changes. As that spindle/strut body angle increases (so the strut body itself swings closer to vertical) you will get some more side loading on the strut & possibly more wear/play in the shaft bushings over time.
2. (Camber plate adjustment): Has multiple side effects on SAI, instant center locations & roll center height. It's also way easier to adjust than #1. If you're just using it for fine adjustments at the track (within ~2-3*) it shouldn't have noticeable bad effects.
3. (Lower control arm): If you have adjustment in your LCA chassis pivot (like with an eccentric), or an adjustable length LCA, you can also adjust camber by pushing the lower balljoint in or out. This will also affect SAI, but probably won't change the instant centers & RCH much if the LCA is near horizontal. With an eccentric I believe it would also affect bumpsteer a bit since the pivot center would rise & fall a little as you adjusted camber.

Off the top of my head I don't believe any of them have a large effect on bumpsteer. If you change caster with caster/camber plates though it will, since the tie rod height changes as the spindle tilts back/forth.

For practicality reasons, I would keep the camber plates roughly centered in their range of adjustment and use #1 & #3 as your gross adjustments in the shop to get the camber close to where you want it. Then use the camber plates as your fine adjustment for final alignment & at-the-track adjustment. You don't want to have to monkey with #1 at the track if you can avoid it.

SAI has noticeable bad effects on dynamic camber when the wheels are steered, but it's a more important consideration during design of the suspension than when the suspension's already been built and you're just trying to align the car. Making sure you have enough range of adjustment in your camber plates is more important IMO than trying to absolutely mimimze the SAI by keeping your upper strut bearings as far outboard as you can get them. You never know when you're going to want/need to make a camber change in either direction.

does anyone know how much anti dive/lift is built into our system? #3 as described above changes dive/lift too, and i haven't seen it mentioned.

I guess technically it would since any elevation change in the inner LCA pivots affects the side view geometry as well, but I think any rise & fall in pivot height from an eccentric adjuster would result in really miniscule changes in your side view swing arm length, not enough to notice any effect in the anti's.