It would not be cost effective for anyone to try to keep track of all the changes chassis builders make to their cars. Some builders keep their cars basically the same year after year and some make changes weekly it seems.
I have to believe that every chassis builder makes changes based on the belief that the car will be better as a result. It only makes sense. And I know for a fact that many car builders have embraced the concept of moment/roll center design and have updated their cars front ends as a result.
Another thing to remember is that racers are constantly searching for new setups and making changes to their shock package and spring package. So, the chassis builder must accommodate those trends with a chassis that will work best for what the racer is doing setup wise.
As far as running an older dirt chassis, I can tell you that dirt tracks are rough on racing chassis and after a few years, the metal is fatigued, bent, broken, and worn out. The cost of a newly constructed Late Model or Modified chassis is not prohibitive. If I were starting out, I would buy a good used chassis at a good price with lots of good parts on it, then replace the chassis and bolt on all of those cheaply acquired parts and go racing.
Moment Centers For NE Dirt Modifieds
I finished reading your article on moment centers in the Dec. '11 issue. I find your articles interesting. I have a question comparing your recommendations for moment centers on dirt cars and what are run on northeast DIRT Modifieds. These cars run straight front axles, the Panhard bars are mounted in the center of the axle, sometimes 1 or 2 inches below. The front coilovers look like they are mounted slightly to the left on the axle.
From what I understand about the moment center on a straight axle this would put the moment center at about 9 inches high and just left of center. This seems high for a car with an engine set back so far (the rear machined surface of the engine block is 56 to 66 inches back from the center of the axle) with 64 to 66 percent rear weight and 52 to 54 percent left-side weight.
I know they usually run a stiffer LF spring by about 100 pounds. Could you please explain or do an article explaining how they get the front ends of these cars to work so well or maybe an article on the geometry of straight axle front suspensions? I know they are antiquated but they are out there. Thanks for all your great information.
- Fred Ehlert
When I speak of front moment center location, I'm always talking about a double A-arm front suspension. For a straight axle front end, the moment center height is the average height of the centers of the ends of the Panhard bar.
As for lateral location, the car ôfeelsö the MC midway between the top of the spring mounts. It doesn't matter where the Panhard bar is mounted, this lateral MC location is related to the spring locations on a straight axle suspension be it front or rear.
Teams like Brett Hearn and others make the front end work by balancing the setup in the cars. If you noticed, Brett drives in straight, through the middle straight, and off the corners mostly straight. The left front wheel is almost always in contact with the racing surface and carrying load. It's not really magic, just knowing how to arrange your spring rates to accomplish a dynamic balance. We have run numerous articles on that process. Go to www.circletrack.com to find past articles that will help you with your setup.
Trouble Cornering, Bite Off
I have a Chevy Monte Carlo with a 9-inch Ford Moser rearend (pinion 6 degrees) Late Model asphalt oval track car. I'm having an issue with cornering and no bite off the corner. I figure it's a roll center issue since we've tried everything else that we can think of. We recently switched to the Ford rearend and have had the issue since the installation.
I need to know what type of angle the upper control arms on the rearend should be because of the height of the Ford 9-inch or any other info you might be able to give us.
- Charlie Wilkins
It's a bit difficult to understand your issue without more information. But, from what you've provided, you might have a problem created when switching to the Ford rearend and it could be related to the rear moment center height.
If this is a car with the metric four-link type of rear suspension, and from your description you refer to upper control arms, plural as if there were two, then we might be on to something. If the rearend mounts were higher on the Ford differential than the Chevy, then that would raise the rear moment center and cause the car to be looser than it were before.
So, if for instance, the car were neutral before the swap, it would now be loose with the new higher rear MC. The solution is to try to lower the rear end mounts to the height of the Chevy mounts, or you might need to soften the right rear spring rate and/or stiffen the left rear spring rate to compensate for the higher and stiffer rear MC.