Roll center debate

[Vega V-6]

Just looking for ideas on what roll center specs some have been using on their full bodied cars. I seem to see numbers for the front at 3 to 4 inches, and rear at 7 or 8 inches. Have people been able to achieve that front roll center on their street cars? Guys running some of the big autocross events, what roll center have they achieved? I'm building a tube frame v6 turbo'd Vega and have found out why you don't use a drag chassis kit for a autocross car. To get the roll center, caster camber where it needs to be i needed to change alot about the chassis, tubing bends and all. Like several others, they have found out the Mustang ll front end design leaves alot to be desired. I still missed my numbers i was trying to get for the front roll center. after the fact i wanted to raise the car 1 inch and lost some a arm angles i was shooting for and now i have about a 12" roll center. Is that too high to expect any good results? Just hoping to pick some minds and get some opinions on this subject.

[ironworks]

It's not just a matter of achieving a certain roll center number. You want will work best with your cars center of gravity and intended use. I know race car guys that run an instant center that is below the surface of the track. There are lot of different variables that go into a roll center number. Shoot if you turning left the roll center is not even in the middle of the car anymore. It is off to one side sometimes.

[mfain]

Just a couple of observations. In the past, low front roll centers (2-3 inches, or lower) and higher rear roll centers (or moment centers, if you prefer) closer to the rear axle centerline (typically 12-13 Inches), resulted in a roll axis that sloped up to the rear of the car. This resulted in a requirement for a stiffer front roll resistance than the rear to produce a somewhat balanced set-up (typically 400-550 # front springs and and 200-300# rear springs). If you look at the later model stuff (Corvettes, Vipers, and even stock cars with low mounted panhard bars), you will see that they have lowered the rear roll centers with a corresponding, significant increase in rear spring rates. Lower roll centers generally require stiffer springs to control the roll. The sway bar sizes have also increased to add more roll stiffness, mostly due to the increased tire width. The primary objective is to produce a balanced set-up -- one where the front and rear roll resistance results in both ends of the car trying to reach the same roll angle under a given cornering condition. Lots of variables -- CG, track width, tire width, etc.! There are several good suspension programs that will allow you to experiment. Just remember, when you change one thing, you change lots of others. Lowering the roll center, for example, will generally reduce negative camber gain with suspension droop. Good luck.

[JRouche]

Originally Posted by mfain

Just a couple of observations. In the past, low front roll centers (2-3 inches, or lower) and higher rear roll centers (or moment centers, if you prefer) closer to the rear axle centerline (typically 12-13 Inches), resulted in a roll axis that sloped up to the rear of the car. This resulted in a requirement for a stiffer front roll resistance than the rear to produce a somewhat balanced set-up (typically 400-550 # front springs and and 200-300# rear springs). If you look at the later model stuff (Corvettes, Vipers, and even stock cars with low mounted panhard bars), you will see that they have lowered the rear roll centers with a corresponding, significant increase in rear spring rates. Lower roll centers generally require stiffer springs to control the roll. The sway bar sizes have also increased to add more roll stiffness, mostly due to the increased tire width. The primary objective is to produce a balanced set-up -- one where the front and rear roll resistance results in both ends of the car trying to reach the same roll angle under a given cornering condition. Lots of variables -- CG, track width, tire width, etc.! There are several good suspension programs that will allow you to experiment. Just remember, when you change one thing, you change lots of others. Lowering the roll center, for example, will generally reduce negative camber gain with suspension droop. Good luck.

UMM!! I dont have anything to add. You pretty much nailed it. Great explanation... JR

[Tony Arnold]

That was a really good explanation on where RC is located. But there are also some areas you also need to consider that will play a roll with RC. One is the location of each instant center which is the point at which the upper control arm angle and lower control arm angle intersect. In todays Stock car or even road racing front end geometry the instant centers are in the center of the tire patches and as low as you can get them. Another thing to look at is not where the RC is staticly, but where it goes under dive and roll. You may have it in the are you want it but going thru a corner it maye move left, right, up or down. The rear RC being located low with high sprig rates does a couple things. It will allow the rear of the car to roll more which transfers more weight to the LF or RF depending on the way your turning. This makes the car tight or push. The way that is corrected is big rear springs. The springs will help the car rotate thru the turn will freeing the car up. The low trackbar will also make ALOT of forward bite. The big Front sway bars do the same thing. They control roll and transfer weight very fast which make forward bite. There are a few things you can do with the front geometry to allow you to run very soft front springs while still giving a feel to the driver of the tires into the trakc or road. Stiff springs may push the tire into the road very hard but not for a long time where soft sprngs push the tire into the road longer but not very hard so there is a happy medium to be found there and that is by building jacking force into the front end geometry. A very good front end geometry program to look into is performance trends RC calculator. you can play with RC, instant centers, pick up points, springs, all kinds of stuff.