Rear camber plate install (large pic)

[gramercyjam]

A while back some folks wanted to see how I modified my rear strut towers to accept ground control camber plates as on my '73, the strut top bends down considerably greatly reducing adjustability. I got a new camera so, here it is:

 

[jeromio]

That looks pretty good, but

 

the strut top bends down considerably greatly reducing adjustability.

 

I'm trying to understand what this means.

 

I'm also a bit curious about the range of adjustment possible with plates. Most of our cars are lowered relative to stock, so the camber gain is wrong. In my car, the primary problem I have is loss of traction - increasing static camber is gonna make that worse. But decreasing it will most likely screw up the cornering. It's a puzzle.

[zlalomz]

Thanks for the pic. I might do something similar up front for a wider track .

Keep that new camera working. I saw some pictures on that other website but would love to see more.

[gramercyjam]

That looks pretty good' date=' but

 

the strut top bends down considerably greatly reducing adjustability.

 

I'm trying to understand what this means.

 

I'm also a bit curious about the range of adjustment possible with plates. Most of our cars are lowered relative to stock, so the camber gain is wrong. In my car, the primary problem I have is loss of traction - increasing static camber is gonna make that worse. But decreasing it will most likely screw up the cornering. It's a puzzle.

 

What I mean't was after significant lowering, too much camber is a problem. The camber plates were installed to put the camber back to about -1 degree, the correct amount for my tires (the tires looked pretty much like this / , way too much camber after lowering). In order to get back to that amount of camber, the tops of the shocks need to be moved out quite a bit further than the camber plates will allow if installed without modifying the tops of the strut tower to move the camber plates out as well.

 

When you say the camber gain is wrong, do you mean camber gain from compression of the suspension in a corner? That camber is going positive?

[jeromio]

When you say the camber gain is wrong, do you mean camber gain from compression of the suspension in a corner? That camber is going positive?

 

Yeah - although I am by no means an expert. My understanding is that once the arms are parallel to the road, any further compression results in positive camber gain. (the arm<->strut angle gets more obtuse since the strut gets shorter and the arm<->pivot length and arm stay constant length)

 

Although, the rear susp. has the strut tops much farther inboard (than the fronts), so, the actual camber trade-off point may be a bit past horizontal.

 

Perhaps John Coffey will notice this thread and post in.

 

I'm definitely with you on correcting the / tire action. I just don't know what the best way to do it is. Certainly having more ways to adjust has got to be a good start though. Now that I look back at your pic, I see how you added metal to the outboard side of the strut top mount.

[gramercyjam]

I suppose the more suspension travel you have, the more it matters. Perhaps that is one of the reasons why the fast autox Z's run high spring rates.

[JMortensen]

Jeromio- I probably led you astray on the camber thing. For the longest time I thought that when the control arms pointed up, the camber got progressively more positive. Apparently, as JohnC and Terry have pointed out to me, this is NOT the case.

 

What does happen, though, is side load on the suspension will compress the suspension when the arms point up, and not when they point down.

 

Sorry!!!

 

Jon

[jeromio]

Maybe the issue is that due to the geometry, the camber gain is negligible.

 

However, it is fairly easy to visualize. The angle of the wheel (camber) is fixed relative to the strut. So, if you disconnect the bottom (ball joint) of the strut, you can get crazy with the camber by moving that bottom point in towards the car (positive camber) or out, away from the car (negative).

 

Since the lower control arm is what locates this bottom point, it determines the camber. Imagine a straight line made by the inner pivot point and the strut top, but have it continue above and below. If you stick the lower arm straight out from the frame of the car, that ball joint is at it's longest point away from that line. At all other angles, the ball joint is closer to the line. So that position, where the lower arm is 90* right angle to the strut top<->lower pivot line, is the max camber position of the suspension.

 

If negative camber is a good thing, then ideally, that would be the point where most of your cornering occurs. So you'd want your arms to be at a greater than 90* angle at static ride height. Part of the reason for relocating the pivot point upwards is to alleviate the bad effects of lowering the car to the point that the arms actually point up at static ride height.

 

I don't actually know what angle the arms are relative to the road when they're at 90* to the strut<->pivot line, but I strongly suspect that once the arms (on the front) are past the horizontal, the angle is less than 90*.

 

Much respect to JohnC and TerryO - I'd really like to see the threads where they state that the front wheel of a lowered Z does not gain positive camber during compression....? I'm tempted to go jack up my car, loosen the spring perches and play this out with an angle finder and a digicam. Well, not all that tempted - I think I'll just go to bed instead... 8)

 

Anyway, that is a loooong winded way of saying that I'm fairly (not 100%) confident in my understanding of camber gain in the front. I dunno about the rear though. Strut tops are much farther inboard in the rear. So though I'm sure the effects are the same, the relative angles and measurements are not.

[tube80z]

If negative camber is a good thing, then ideally, that would be the point where most of your cornering occurs. So you'd want your arms to be at a greater than 90* angle at static ride height. Part of the reason for relocating the pivot point upwards is to alleviate the bad effects of lowering the car to the point that the arms actually point up at static ride height.

 

Actually you want the control arms to be very near level to work best. This will help to reduce the jacking forces and scrub the tires less. If you want to adjust camber it's best to add it statically as you can't really get much back from the suspension without compromising other items worse.

 

Cary

[blueovalz]

The control arms only reach the "peak" on the "negative camber" curve once the control arm rises (e.g. the car is lowered) to the point in which the arm is perpendicular with the strut tube. Being the strut tube is laid inboard at the top by 13 degrees (in general), then the top of the "negative camber" curve is when the arm is 13 degrees to the horizon (road), which means the car is pretty low at that point. The amount of continued negative camber gain at this close proximity to the peak of the curve means very little negative camber gains will be realized as you continue to lower the car, but none-the-less, the geometry cannot be argued with. Don't get confused between camber effects, and the position of the wheel as the wheel moves inboard and outboard with the arm. The kingpin angle (so to speak) of the strut tube is what mucks all this up.

Look at it from the strut's point of view. If the strut were perfectly vertical, then the horizonally perfect arm would then be at the top of the negative camber curve, because the arm is perpendicular to the strut tube, and further lowering of the chassis would indeed cause the camber to pass the top of the curve and start going toward the positive direction. BUT, this would also mean that anything other than a horizontal arm in either direction (up or down) presents inself as inducing positive camber. Because of the 13 degree inclination though, this geometry allows the increase of negative camber throughout the complete "probable" range of strut compression, and not just part of it.

[jeromio]

Okay. If I read that right, you're not disagreeing with me, just adding more precise data - yes? In other words, camber starts going positive after around 13* of pointing up.

 

Even with my pivot hole mod, my arms are still a bit upwardly pointed. Springs are stiff though, so I don't get much roll though, so there probably is little chance of seeing this positive camber effect.

 

So, the effects are probably similar in the rear. The struts angle is probably about the same - the tops are more in board but the arms are longer too.

[Drax240z]

Since the lower control arm is what locates this bottom point, it determines the camber. Imagine a straight line made by the inner pivot point and the strut top, but have it continue above and below. If you stick the lower arm straight out from the frame of the car, that ball joint is at it's longest point away from that line.

 

If I am reading this right, you are assuming that the line created between the inner pivot point and the top of the strut (which is the kingpin inclination angle by the way, or KPI) is parallel to the frame... uh, not true!

 

With the macpherson strut setup, you will gain POSITIVE camber as the control arm travels higher... which is why macpherson struts aren't optimal for racing.

[JMortensen]

Drax, I thought MacPherson wasn't good because of the camber change with suspension movement. Upper and lower control arms do a better job of keeping the camber consistent. That is the disadvantage to MacPherson struts AFAIK.

 

Jon

[jeromio]

If I am reading this right, you are assuming that the line created between the inner pivot point and the top of the strut (which is the kingpin inclination angle by the way, or KPI) is parallel to the frame... uh, not true!

Well, not really. The kernel of the example is that the ball joint is farthest from the car center line when the arm is straight out. But when the arm<->strut tower angle is 90* is the camber change point - north of this point the gain is positive. (by strut tower, I mean this line between strut top and lower arm inner pivot) In my view the interplay of these 2 points (arm straight out and the arm/tower 90*) is the camber curve sweet spot.

 

With the macpherson strut setup, you will gain POSITIVE camber as the control arm travels higher... which is why macpherson struts aren't optimal for racing.

Well, yes and no. The main point is that the camber curve is non-linear sigmoid. There is a (narrow) range where camber change with compression is increasing negative. But then it flops and the camber change switches to increasing positive. In my version of the story, this point is arm<->strut tower angle = 90*.

 

There's a long thread over at cc.com where this is all described by people (engineers, much smarter than me) in terms of the arm<->strut angles - but I had a hard time relating to that explanation.

[Drax240z]

Ah yes, Jeromio you've got it right. In my last post I was assuming that you were starting from the point where the strut line/control arm was 90 degrees, which isn't necessarily the truth in practice.

 

Jon, you are also correct. I guess it's sortof a toss up as to what is worse, inconsistant camber gain or postive camber gain in bump/droop. Though I'd prefer inconsistant negative camber gain to consistant positive camber gain I think. (it might be harder to drive, but it should ultimately be faster)

 

Oh yeah, sigmoid? A new word for me.

[johnc]

Are we talking about the front or rear suspension in this thread?

 

Grammercy runs bias ply racing slicks in his FP car if I'm not mistaken. Those tires don't like a lot of camber with 1.5 negative being the absolute limit. DOT-R radials and radial racing slicks like a lot of camber and I run 3 to 3.5 negative front and 2.5 to 3 negative in the rear depending on the track. When we had Goodyear 23 x 8.5 x 15s on Bryan's 240Z we ran 1 degree negative camber all around.

 

The camber gain (more negative camber) in bump on the front a 240Z is mostly a result of caster, although the control arm/strut design does give a little more negative camber in bump, but body roll tends to negate this. Once things start getting modified, then all bets are off. In all cases at static ride height your front lower control arm must at least be level with the ground or, better yet, pointing down from inboard to outboard to some degree. The more the better while still maintaining the ride height you desire.

 

On the rear, you get more camber gain in bump until just before the control arms go level with the ground. In all cases at static ride height your rear control arms should have a down angle of some amount from inboard to outboard. The more the better while still maintaining the ride height you desire.

 

All of this is assuming solid bushings everywhere in the suspension. Bushing deflection (especially the stock isolators at the top of the strut) add geometry changes under load.

[blueovalz]

Hopefully this illustration will help this seemingly confusing subject. In this illustration, the initial position of the "grey" control arm is horizontal (but as John pointed out earlier, this is not necessarily the best set-up). This control arm (CA) is attached to the strut (thick black line) at point "A" which will be the spindle pin. The spindle pin follows the "red" arc as the wheel moves up and down through it's range of travel. As can be seen here, negative camber increases as the wheel moves upward along the arc only until it reaches point "B", which is perpendicular to the line connecting the strut mounting points. At this point the camber stops becoming increasingly negative, and reverses itself to start in a positive direction. The thin blue line shows the increased negative camber resulting from the CA movement between points "A" and "B". What confuses most people though is that even though the camber is still going negative as the spindle pin rises from "A" to "B", the same pin is also moving inboard at the same time. This is initially counter-intuitive because most folks think that if the wheel is moving inboard, it must be going positive in camber, but not so! Now if the strut were perfectly vertical, then yes, this would be true, but the kingpin inclination is what mucks this up, but only for movement between points "A" and "B". The relationship of inboard/outboard movement and its related positive/negative camber connotation holds true everywhere else along the compliance arc (red arc) except for this narrow range. One last thing to add is this is for the rear only. The front suspension holds these same principles as well, but then you add caster and turning angle, and then everything gets mucked up even worse than it already is here.

One of the reasons John stated that a drooping CA is good is because the lower the pin is in relation to the arc on initial set-up, the greater the rate of camber change for the same compliance, along with the roll center advantages.