I love Circle Track and am sorry to hear Stock Car Racing is retiring. Keep up the great writing and thanks for encouraging me to think every once in a while.
Jeff Grimm, Field Engineer
We have discounted nothing. The roll angle balance is still as valid as ever. The location of the moment center is a large variable in the front end dynamics, as my sketches illustrate.
Notice how when the MC moves to the right, the moment arm gets shorter, quickly. The various locations can result in either a softer (left placement) or harder (right placement) front suspension, regardless of the springs installed. Once we figured that out, tuning the front suspension and balancing both suspensions was easier to attain. This process and methodology has been proven time and time again by lots of car builders and teams. It represents the true dynamics of race vehicles.
As far as software is concerned, I try to keep a separation from my articles and that business. Too many CT writers in the past have had a more personal agenda in the content of the articles. I found a method that's beneficial and I can't help but promote it. That being said, there are routines and methodology in the program that are not spoken of and/or known by the industry that make it work. If you discover those and can make your calculations work, that's great. In doing that, try to imagine all of the influences that act on the chassis in the turns. Leaving just one out will negate all of your hard work, just like in any math equation.
Thanks for writing.
Rear Metric Roll Center
I'm a longtime and happy subscriber of Circle Track magazine, and look to it for much technical info for my racing. However, I can't find out how to figure out and diagram the rear geometry for a stock GM four-link metric chassis. Can you point me in the right direction or tell me how to do it?
There seems to be unlimited information for the front suspension, but nothing for the stock GM rear. I hear that the roll center in the rear is way wrong in relation to the CG and the height of the front MC. To me, that means when the front is in a roll situation, the rear can't correctly follow suit and it results in the weight in the rear "skidding" the tires sideways. Please clarify this for me.
The Moment Center height for a metric four-link design is very high compared to other rear suspension systems. A typical Panhard bar is set at a height of from 9 inches up to around 12 inches, give or take an inch. The metric four-link MC height is upwards of 14 inches or more, depending on the ride height.
The metric MC height is the average height of the two instant centers created by extending the upper and lower arms out until each set intersects. The height of those intersections are added together and divided by two.
To compensate for that high MC, most racers need to run a softer right rear spring than the left rear spring. This promotes rear roll that compensates for the short moment arm the high MC creates and helps the rear match the front in roll angle desire.
Finding Front Roll Center
I've enjoyed reading Circle Track for many years. I was a mechanic for 46 years-ASE Certified Master Tech in cars and trucks for 31 of those years. I owned my own business for 24 years and was forced to retire after a serious injury. I did all phases of auto and truck repair and specialized in repair and rebuilding automatic transmissions, aligned many front ends, and four-wheel alignments.
I'm not connected with any race team-just an avid race fan who is very interested in race car technology. One would think that with this background I could understand how a roll center is determined. But I just can't seem to figure it out. I would deeply appreciate it if you could explain how this is done.
Thanks for your help, and keep up the good work.Steve Currier