I'd like to get some opinions & predicitions on how the suspension geometry would change if the mounting points for the UCA were moved down.

I would think this would add more static negative camber, and increase the camber curve, or whatever you want to call the rate that camber dynamically changes as the suspension compresses. Is this accurate?

 

Yes. Toe will also change.

 

Assuming you mean "moving down" as in, using spacers or something between the shock tower and UCA, then here's what I think:

This increases the inclination angle of the UCA. This causes a steeper camber curve (which is the camber per unit of wheel travel; you can get rate of dynamic camber change from it) since the arc causes more lateral displacement of the top of the wheel per inch of suspension travel.

 

it will also lower your front roll center. depending on how far you "lower" the uca, and how far the rc moves due to that, you may loose any gain in dynamic camber change due to increased chassis role. due the math and some modeling, figure out how much the rc moves vs. gains in the camber curve and then determine if it's worth it.

nate

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x &raquo;</TD></TR><TR><TD CLASS="quote">it will also lower your front roll center. depending on how far you "lower" the uca, and how far the rc moves due to that, you may loose any gain in dynamic camber change due to increased chassis role. due the math and some modeling, figure out how much the rc moves vs. gains in the camber curve and then determine if it's worth it.

nate</TD></TR></TABLE>

According to the diagrams in Puhn, it would raise the roll center, not lower it.

 

where can I read up on roll center theory, I feel like its the one aspect I just can't wrap my brain around.

Would I want a lower or higher roll center anyway?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Agent Smith &raquo;</TD></TR><TR><TD CLASS="quote">According to the diagrams in Puhn, it would raise the roll center, not lower it.</TD></TR></TABLE>

are you sure? i might have to do some rough sketching to fully visualize this, but i'm pretty sure rc height is reduced relative to cg height. unless the lca is is at a pretty steep angle too.

nate

 

I imagine it would have a detrimental effect on CV joint longevity.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Greyout &raquo;</TD></TR><TR><TD CLASS="quote">where can I read up on roll center theory, I feel like its the one aspect I just can't wrap my brain around.</TD></TR></TABLE>

I can't say I fully appreciate all the nuances associated with roll center, either. Of the books I've read, the one that has the best discussion of how to find the roll center is Fred Puhn's How to Make Your Car Handle. It has a bunch of diagrams that show how to find it.

The first diagram on this page shows how to do it for our cars. As you can see, if you lower the inner end of the upper bar, it moves the intersection of the two red lines closer to the center, which will raise the roll center.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by solo-x &raquo;</TD></TR><TR><TD CLASS="quote">
are you sure? i might have to do some rough sketching to fully visualize this, but i'm pretty sure rc height is reduced relative to cg height. unless the lca is is at a pretty steep angle too.
nate</TD></TR></TABLE>

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by maxQ &raquo;</TD></TR><TR><TD CLASS="quote">I imagine it would have a detrimental effect on CV joint longevity.

</TD></TR></TABLE>

adjusting the angle of the upper control am by moving the mounting points up & down will have no effect on ride height.

It will only change the static camber, the dynamic camber curve, and after some reading, it seems the roll center will be raised a bit. I guess my questions are: A) are these 3 things true and B) are the last two a good thing.

 

Nah, it'll lower it. Just think about it. Lowing the UCA pivot point will "point" the intersection lower, nearer to the ground. The only way it could raise it is if the lower arm inboard pivot is lower then the outboard end, which is unlikely.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kb58 &raquo;</TD></TR><TR><TD CLASS="quote">Nah, it'll lower it. Just think about it. Lowing the UCA pivot point will "point" the intersection lower, nearer to the ground. The only way it could raise it is if the lower arm inboard pivot is lower then the outboard end, which is unlikely.</TD></TR></TABLE>

because the line drawn is intersecting with the lower control arm line, it won't so much as move down (if at all) as move it closer to the center of the car. This will cause the lines drawn from the opposite side contact patch to get steeper, and THOSE lines will intersect higher.

my brain hurts maybe I'll draw a picture...

 

The IC (instant center) will move closer and lower, so you can't tell where the RC will move without taking measurements, which I sure as hell am not gonna do.




Modified by GSpeedR at 5:00 PM 8/3/2004

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Greyout &raquo;</TD></TR><TR><TD CLASS="quote">

adjusting the angle of the upper control am by moving the mounting points up & down will have no effect on ride height.</TD></TR></TABLE>

It's not the ride height that's the problem. It's the change in dynamic camber that'll possibly kill it.

 

What is an IC?

Wouldn't this all be subjective (whether these spacers would move the roll center up or down) based on the ride height of the car to begin with?

 

IC = instant center

Added in above post.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Greyout &raquo;</TD></TR><TR><TD CLASS="quote">

because the line drawn is intersecting with the lower control arm line, it won't so much as move down (if at all) as move it closer to the center of the car. This will cause the lines drawn from the opposite side contact patch to get steeper, and THOSE lines will intersect higher.

my brain hurts maybe I'll draw a picture...</TD></TR></TABLE>

But both sides are being changed, right? So the intersecting point at the center-line of the car will be lower.

BTW, THE books to have is "Race Car Vehicle Dynamics" by Milliken and Milliken. Like $85 but it is extremely complete and technically intense.

 

Books on Suspension Theory and Practice:

Don Alexander "High Performance Handling Handbook" (2002) gives a reasonable account for the non-technical person, and it does not go into enough detail - 1 star

Fred Puhn "How to Make Your Car Handle" (1981) is a bit dated but he handles the subject at a technical level where you could understand most or all of the suspension code words e.g. roll center, roll stiffness, roll couple distribution, ect. I recommend this to most people, including those who are not math whizzes - 3.5 stars

Milliken and Milliken "Race Car Vehicle Dynamics" (1995), the most complete engineering based book on the subject, with lots of historical info. (if you are interested in that sort of thing) includes good information on aero. It is a mathematical treatise on the subject. - 5 stars (especially if you are a math whiz)

Carrol Smith "Tune to Win" (1978), an excellent book that delves into every nuance of suspension theory, but of course from the late Mr. Smith's very direct and opinionated style. I still like this book since I believe the information is excellent - 4.5 stars

Paul Van Valkenburg, "Race Car Engineering and Mechanics" (1992) another excellent book written by an engineer (and a personal friend of mine) who goes into great detail. He worked with Mark Donohue in the 60's and early 70's during Mark's total domination of Can-Am, he knows his stuff. 4-stars

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Greyout &raquo;</TD></TR><TR><TD CLASS="quote">because the line drawn is intersecting with the lower control arm line, it won't so much as move down (if at all) as move it closer to the center of the car. This will cause the lines drawn from the opposite side contact patch to get steeper, and THOSE lines will intersect higher.

my brain hurts maybe I'll draw a picture...</TD></TR></TABLE>

A picture? Here ya go:



Stock roll center is where the blue line intersects with the green line. Altered roll center is where the magenta line intersects with the green line.