measured the roll center of front suspension
 

Over the last year or so, I've been doing some reading on suspension geometry and wanting to do some measurements of the roll center and the center of gravity of the SVX front and rear suspension. Been too lazy up to this point, but the Aug, 2005 issue of SCC has a nice article on measuring suspension geometry, so I finally climbed under the car to measure the front suspension pivot points. Turns out the SVX front suspension has a very typical MacPherson strut geometry. The roll center of the front suspension at factory height is about 2.75" off the ground. That's kind of meaningless all by itself. What's more important though is that now that I have the suspension geometry, I can determine what effect lowering the car has on the suspension geometry. Turns out that lowering the car 0.5" has the effect of lowering the RC by about 1.25". Probably sounds great, but actually, this is bad. Because the RC drops more than the CG, the roll couple increases, and a larger roll couple causes cornering forces to make the car lean more.

For you folks who want to get the most handling out of your lowered SVX, the way to stop the increase in roll couple when the car is lowered is to move the A-arm ball joint further from the spindle. I think this might actually be easy with the SVX because the ball joint is pinch-clamped to the spindle. If the ball joint can be lowered 0.5" in the spindle, then the CG can be lowered without adversely affecting the RC (for a car lowered 0.5"). I'm thinking that some kind of spacer might be needed too, but otherwise, I think its possible.

In a day or so, I'll post some drawings showing my suspension measurements.

and some drawings as to how we could fabricate up such a spacer? ;) :cool:

I'm not experiencing much lean at all with the sway bars and Konis that I'm using in conjunction with your springs....but then again, I do understand the engineering behind what you're saying :D

-Bill (still trying to find the time to adjust his OEM rear bar in order to reduce the 4" inner rear tire lift seen in recent autoX photos....)

p.s. did you do your own measurements to determine the scrub radius?

Mychailo,

I'll be getting your sport springs this upcoming Xmas (due to unforseen wheel bearing and 4EAT replacement this Spring/Summer). As I live in the country and drive on several imperfect roads, I'll probably go with the custom springs you've made for a few customers that retains the stock ride height, but increases stiffness.

I'll still be interested in seeing your findings...

-Chike

I don't really know your suspension setup, but I would assume that your springs and sway bars are stiffer than stock. So, while the moment about the roll center may have increase, you may also have a larger reaction force from springs and sway bars to prevent the body from actually rolling more.

Accurate explanation or no?

Yep, that's correct, but if the increase in the roll couple can be reduced by lowering the ball joint, then more benefits can be had from lowering the car. Reducing the roll couple at the front will also have the additional benefit of removing some of the understeer characteristic from the car.

Last night, I looked at the ball joints that I took off my car last summer, and it should be possible to drop the ball joints about 0.25" without much difficulty. Any more than that, would require an adapter that would probably result in dropping the ball joint by an entire inch. For the 0.25" drop, all that's needed is to cut a radius into the shaft of the balljoint so that the pinchbolt can get past the ball joint. I'll post pics on that soon.

Bill, yes I was able estimate the scrub radius from the measurements. My estimate is that with stock wheels, the scrub radius is about -20 mm which is pretty close to the value of -27 mm that UberRoo got. So it seems that the scrub radius is indeed a bit negative. Kinda interesting.

But if you alter the location of the pinch bolt won't that make the ball joint stud susceptible to breakage? The slot relies upon the clamping pressure to make it secure and dropping the slot below the pinch bolt could prove hazardous. Another consideration is the unsupported length of the stud is increased. Seems to be akin to taking a few whacks of an axe to a tree trunk...

Yep, I agree that there are limits to dropping the ball joint stud below the OEM location. My feeling is that it could be dropped up to 1/4" without significantly weakening the connection. If I do it this way, I'll make a collar for the exposed portion of the stud, and I'd probably make a little post to drop into the pinch bolt area so that the clamp doesn't get deformed because there is nothing to grip. The other option would be to make an adapter clamps to the stud and then has its own stud that goes into the pinch bolt. The adapter would need to be pretty long though, and I imagine that it would drop the ball joint by at least an inch which would most likely be too much of a drop.

This is by far the most interesting thing I have read in quite a while. Does any body have pics of the said ball joints/mouting points? And one last thought how hard would it be to apply the selected method to the rears for even or near even dropping pourpose (I'm only assuming its the same)?

Sounds like we need some Aerospace-quality material... :cool: :cool:
(Drawings! I need drawings!! :D )
-Bill

If you were really desperate, you could remove the hub and disassemble it, clean everything throughly, and then braze the ball joint into the socket at whatever depth you want. It's also possible you could simply braze (or weld and re-temper) an extension onto the ball joint. Proper brazing can produce joints good for over 100,000 PSI. I figure the surface area on top of that joint (unless there's a cap or recess) is more than one square inch. Think a hundred thousand pounds is enough?

Another possibility is that you could acquire a short piece of tubing that fits nicely into the socket in the hub. If you turned down the ball joint to a diameter that fit into the tubing, you could mill a couple of holes into the side of the reduced ball joint and some corresponding holes in the tube extension into which you would drive a couple of pins to keep the joint from sliding out. The tube (with ball joint installed) would be inserted into the socket in the hub just as before, except the recess for the clamping screw would only be on one side of the extension instead of going all the way around as it does [did] on the ball joint.

Me thinks it possible.

Am I understanding this correctly? Are you trying to increase the distance between the hub assembly and the contol arm?? Or are you trying to get the ball itself further from the hub assembly??

Why not just mount the ball joint on top of the control arm, rather than under? Without measuring, I guess that would drop the arm about 1/2" to 3/4".

The actual joint itself is inside the hub assembly because the ball joint housing is inside the hub assembly. Lowering the arm doesn't change the effective geometry. The joint must be moved.

The ball joint need to be located closer to the ground. It has to be the ball joint because this is the lower-outer pivot point of the suspension. Lowering the ball joint changes the slope of the line between the ball joint and the inner control arm mount which in turn raises the roll center and reduces the roll couple. This is apparently a very common mod on MacPherson strut cars used for racing. If you get a chance, pick up the Aug, 2005 issue of Sport Compact Car. Lots of good pictures.

It seems to me that this mod will also allow for more camber compensation when the car leans.

The stud coming off the ball is what attaches to the hub via a pinch bolt. The socket part of the joint bolts to the control arm with three through bolts, right? So, it seems to me that the joint is not inside the hub assembly, right?

Seems like those would work, but I'm hoping there is some way to do it without disassembling the suspension or having to weld on it and potentially mess up any heat treating.