Roll steer/ LCA's converging question

[Eric Howell]

Is it possible to have LCAs pointed up slightly and converging in plan view and end up with roll understeer?
My LCAs rise about 2 degrees going towards the front and angle in about 9-10 degrees towards the front. It is an offset 3 link that also uses a frame mounted Watt's link.
Here is a screen shot of the number I came up with...
(clickable)
https://static1.pt-content.com/images/noimg.gif (http://s177.photobucket.com/albums/w235/87wildside/Fab/suspension/?action=view&current=ScreenHunter_02Aug240852.jpg)

What do you guys think? Is going to have roll oversteer or roll understeer?
Any help is appreciated.

[Eric Howell]

Bump.

[parsonsj]

Eric,

My 3 link does that: slight up angle from housing to chassis, and slight convergence in plan view.

jp

[Eric Howell]

When you figured out the geometry did you come up with roll understeer? If I drop the RCH down to 11" it has 0 roll steer at ride height. The problem is the spreed sheet I used doesn't have anything for bump. Would like to keep it as neutral as possible.
I found this post at CC but since this is my first suspension I have my doubts that I could get that lucky.
http://corner-carvers.com/forums/showpost.php?p=348547&postcount=21

[parsonsj]

Katz and I worked to a neutral roll steer (or as close to it as possible), but he did the analysis, not me. I don't have the actual numbers. Just the fab measurements.

[Mean 69]

To find the roll axis for the suspension system you describe, look at the LCA convergence intersection in Plan (top) view, determine how far away from the axle centerline this intersection is. Now, take that measurement, and project the LCA's in side view to that dimension, and determine the vertical position. This represents the forward point that the roll axis line is constructed on. The rear point is the intersection of the front point, and the Watt's center, projected onto the rear axle assembly (which will be essentially the same as the actual Watt's height, unless the Watt's is way far fromt he axle, or the LCA's have a crazy inclination, which yours doesn't. The beauty of this setup is that you can alter the roll axis indepent of the "anti's" which is pretty slick in my humble opinion.

With convergent LCA's in plan view, and an offset upper link, you might want to convince yourself that you won't have any assymetries in left/right roll, etc. I looked at this years ago and hadn't gotten to the point where I was convinced it was NOT an issue, but never finished the exercise. The offset upper is a neat trick for cancelling torque reactions from the engine (i.e. drag racing), but I am personally not so interested in that as a primary goal (road race applications). Might be okay, might not.

Mark

[parsonsj]

Mark,

How does the use of a PHB change that analysis? Just use the mid point between the chassis and housing pick up points as the roll center?

jp

[Mean 69]

Yes John, exactly (assuming that the mounting points of the ends of the PHB are symmetrical to the chassis, or at least nearly so). Just project the two points (front convergence, and rear height of bar) to create an axis, the actual roll center height is the plane of the axle itself. The angle of the axis defines roll under/over/neutral steer.

Keep in mind that all of these are "kinematic" assumptions, and that angles and such change as the suspension articulates. In the case of a PHB, the angle of the bar will change a bit when the suspension moves up and down. This can be minimized by using a vey long physical length PHB, which I know your car has. Longer is better, it reduces the relative amount of movement/effective length change as a function of suspension movement. This applies to pretty much all links in a suspension system, within practical limits, longer is better for the same reason (special case apply where you'd deliberately want dynamic changes...).

The reading is a bit dry, and the book is expensive, but there are some good principles in a book called "Race Car Vehicle Dynamics" on this and many, many other topics. Milliken is the author (actually, "they are the authors..." pair of brothers wrote the book, it's the current reference for next level learning of this stuff....).

Mark

[Eric Howell]

Thanks Mark. If I understand correctly the roll axis would be what determines the roll steer then and that would mean the roll steer can be changed by moving the Watt's, correct?

The main reason for the offset link in my application is because of the cast center on the axle.

[Mean 69]

That's correct, Eric. Additionally, if you build in adjustment to the LCA inclination, you can also effect roll steer in that manner, but I'd warn to rely on this too much as you'll need to make some compromises that might not work in the overall picture's favor.

Cast center, gotcha. One important point that many of the guys who sell four link kits downplay, incorrectly... If you weld on an axle tube, you will induce a bend in the overall axle assembly. Fact, period, end of story. This can be compensated by heating/cooling in the opposite direction with a torch, but you will need to be able measure pretty accurately when doing so. Many ways to accomplish that. Be safe!

Best of luck, enjoy.
Mark