Force-based roll centres vs geometrical roll centres 2

[TalonDG]

I've been noticing more and more mention of a procedure that calculates the roll centre of a vehicle from a force-based perspective, rather than a purely geometrical one.

Circle Track in particular has been running a series of suspension modelling articles, and the author is adamant that the geometric roll centre is NOT the actual centre that the sprung mass rolls around.

There is also supposed to be some benefit to tailoring the roll resistance at each end such that the ultimate roll angle (from a force-based analysis) is the same at both ends - the idea being that you don't want the car fighting itself.

This procedure is supposedly pretty new (last 10 years or so) and came as a result of comparing actual recordings of wheel travel from suspension position sensors to predicted values (for a given amount of roll) from a kinematic analysis.

I have WinGeo3 from Bill Mitchell, and while my suspension position sensors aren't on yet, visually the car does not seem to following the predictions from the software - it looks like the outside is compressing less, and the inside extending more, than would be expected. See

http://farnorthracing.com/nats2003/

for examples.

I bought SAE983033 which seems to explain WHY the geometric analysis isn't enough, but it glosses over a lot of steps involved in actually calculating the nonlinear analysis. And if the Circle track article series described it, I missed that issue.

Does anybody have a reference that describes the steps involved in a force-based roll centre analysis in detail?

DG

 

[bhart]

For those of you that are interested, I just received this as an inclusion to the newsletter that Mark Ortiz (an independant chassis consultant and author of a vehicle dynamics Q&A section in Racecar Engineering magazine) publishes each month. It seems very applicable to this discussion, however, I did not attend the lecture and have not seen the video, so I am not vouching for its worth. So far, everything that I've managed to get my hands on that was written by Mr. Ortiz seems to be well thought out and concisely presented, and he has never shyed away from confronting the details.
Should this post be interperted as an advertisement by any of the users of this forum, then I should mention that I am not affiliated with Mark Ortiz in any way.
I would prefer not to publish anybody's contact information on the internet, so if you're interested in the video, a quick Google search should point you toward Mark Ortiz the chassis consultant.

The details included about the video read:

NEW VIDEO

I am pleased to announce that I now have videos available of the presentation I gave at UNC Charlotte this past March. The one-hour lecture is entitled “Minding Your Anti: Understanding Factors in Load Transfer”. It deals with the origins of load transfer and presents a “force-based” or “lateral anti” approach to the notion of roll centers. This is original and very current thinking on the matter, and not to be found elsewhere. Videos are single VHS cassettes, and sell for US$50.00. This price includes shipping to any destination, worldwide. North Carolina residents please add 7½ % ($3.75) sales tax.

 

[GregLocock]

Fair enough tho I'd argue with "It deals with the origins of load transfer and presents a “force-based” or “lateral anti” approach to the notion of roll centers. This is original and very current thinking on the matter, and not to be found elsewhere."

As I have seen that very idea promulgated here on eng-tips, and that's how I think about it, when I have to, which is not very often.


Cheers

Greg Locock

 

[TalonDG]

I just got back from PRI.

While I was there, I had an opportunity to meet with Mr Mitchell. The latest version of WinGeo3 contains a "Vehicle Dynamics" section that allows one to specify CG location, etc, and then load the car - and then the program will calculate the roll amount and "actual" roll centre etc based on the load.

I also picked up a book written by... the name escapes me for a moment - he used to do seminars for GM, Canadian gentleman - anyway, the book is called "An Introduction to Race Car Engineering, Volume 1 of 5" and it's a couple of inches thick. Good reading, heavily reliant on WinGeo3, lots of examples.

DG

 

[JPARKER70]

DOES THIS MAKE SENSE TO ANYBODY ON HERE?


According to my GRT book you are right about calculating the roll center. Measure the height of the L side of the J-bar from the floor, add it to the height of the pinion mount from the floor and divide the number by 2.


Roll center is half way between the 2 rod ends in a straight line. If you move the bar up on the frame you are actually raising the roll center by 1/2 of what you move it up on the frame. So why does this add tighten the racecar? Raising the rollcenter is suppost to loosen car, right? This can be hard to understand sometimes, so I'll try to explain as easily as possible.
If the panhard bar is flat, most of the side force on the car is pushing straight sideways on the rear end, trying to slide the RR tire. If the bar has alot of angle in it, the car will try to use the bar as a pole vault. Which causes some of the force to push down on the rear end instead of just sideways. This adds traction to the rear tires and tighten racecar. Once you get a certain amount of rake in the panhard bar, the added rake creates more downforce on the rear tires then the slight raising of the roll center takes away. So the car is tighter even with a slightly higher roll center.

Example. Lets say the panhard is perfectly flat. If you raise the bar on the frame 1 inch this will most likely loosen car. Once you keep raising the bar on the frame it will get to a point that the rake will start tightening car. Most cars start with enuff rake that raising on frame will tighten car up, but if you fairly flat it may loosen car.

Body roll is a fuction of the heigth of the roll center in relationship to the heigth of the COG (center of gravity). Let say your rear COG is 16" up from the ground and your rear roll center is 12" up. If you want more roll, you can lower the rollcenter (i.e. panhard bar) down on both sides one inch. Now your at 11" on the rear roll center, the car will roll more. Or you can raise the COG. This is done by moving wieght up (lead, fuel cell, battery, or anything with weight). Rake in the panhard bar mechanically jacks the car as the car is trying to polevault over it.

The shorter the bar the faster the bar will gain angle when the body rolls. This means usually the shorter the bar the less angle needed to do the same job as a longer one. The shorter it is the faster this happens, which can make the car very erratic and hard to be consistant in.

Alot of times you will be using the panhard bar to make the car start getting up so the car will climb the bars for a tighter exit.

Left side pinion mount verse Right side!!!
When the panhard is mounted to the left side of the pinion, the downward force by the bar is closer to the LR tire so it puts more of the weight on the LR then the RR. This causes the car to be slightly looser in but much tighter coming off. The same with the right side mount, its closer to the RR tire (7&quot that more of the weight is on the RR. This is tighter in and losser off then a left one.

There is no real set rule on panhard bars, but generally :
1. You can use more rake on banked tracks
2. Too much rake on a small tracks (1/4) can create a loose condition off as the car has a loose rollsteer and you are chasing the back end up the racetrack and can't stay on the bottom.
3. The heavier or faster the track the longer ones are "usually" better.
4. Stop and go tracks like left side bar (if you like left bars)
5. Faster momentum tracks like rake but don't need to be as low- keep the rake your using just raise both pinion and frame. This keep most of the bite but keeps the car from rolling so bad and carring the LF.

 

[jb56]

I've been researching and testing this topic very heavily with NASCAR type stock cars. I've found that a combination of gRC and fRC helpful in both design and prediction of the race car.

When I design new component sizes such as spindle heights, pin angles, pin heights, a-arm lengths, etc., I calculate the gRC but I'm more concerned with the location of the Instant Centers and camber effects. I then run through some calculation of the forces involved from the wheel through the linkages, especially the lower a-arm. I found the angle of the lower a-arm where it connects to the frame to be very important in calculating wheel travel and roll more accuratley. Along with other lateral type anti forces of course.

I also put together a program that has so far been very close for predicting the balance of the race car. Granted the testing has been limited to one particular type of car but it's based on formulas found in books from Milleken, Gillespie, Rowley, and others.

If anyone is interested it's located at

http://www.BrownPerformanceEngineering.com.

I've used Bob Bolles software and found some interesting anomolies. Bob's software doesn't seem to work well with anti-roll bars larger than 1.25" in diameter. It also didn't calculate the rear wheel rates accurately for me. It doesn't show dynamic wheel loads on the tire contact patch. It also doesn't give any suggestions or ideas for spring/wheel rates - other than balance out the roll angles. It continually suggested high cross weight values (which didn't seem to really deviate much from car to car) and very high rear roll centers.

The WinGeo software from Mitchell seems to work well it just takes a while to become accustomed to it. The latest book from Rowley really helps in using the WinGeo software.

This has just been some of my experiances in trying to learn about roll centers and software that supposed to help us - atleast in the circle track racing side of things.