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Ranger Mike
Ranger Mike is offline
Sep27-11, 08:46 AM
Sci Advisor
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Roll couple distribution is the relative roll stiffness between the front and rear of the car, and the left and right of the car. The front and rear roll centers are points that are designated by the vehicles mechanical suspension linkages. When we connect them with a line we have a roll axis. It is not necessarily parallel to the ground. Weight distribution, and roll coupling distribution can create a roll point at the front of the car which is lower to the ground that the roll point of the rear of the car. This creates a sloped line. The angle of this line has influence on how much weight is transferred, and where it goes.

Kinematic Roll Center. The most commonly used definition is the geometric (or kinematic) roll center; the Society of Automotive Engineers uses a force-based definition.

The location of the geometric roll center is solely dictated by the suspension geometry, and can be found using principles of the instant center of rotation. The SAE's definition of the force based roll center is, "The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll". from wikepedia..

The lateral location of the roll center is typically at the center-line of the vehicle when the suspension on the left and right sides of the car are mirror images of each other as in road course cars..not for the circle track folks.

The significance of the roll center can only be appreciated when the vehicle's center of mass is also considered. If there is a difference between the position of the center of mass and the roll center a moment arm is created. When the vehicle experiences angular acceleration due to cornering, the size of the moment arm, and angle of this arm, combined with the stiffness of the springs and anti-roll bars (anti-sway bars in some parts of the world), dictates how much the vehicle will roll. This has other effects too, such as dynamic load transfer.

If you remember nothing else about the roll center, remember this -
High RC = means more Camber gain per inch of suspension travel
Low RC = means Less Camber gain per inch of suspension travel

Let us consider 4 RC Heights:

1) Roll center height ( RCH) = center of gravity (CG)
No pivoting here, it means that there is no roll. Its like trying to spin a door applying force in the hinge. The car is turning, lateral force is applied, but there is no roll. This makes the car as hard as a brick bat since we arte not using the spring/shocks. BAD setup.

2) RCH between CG and the ground. Depending the percentage of that height you distribute how much force goes through the wishbones and how much through the spring/shocks. The range between 15% and 30% of RCH compared to CG is the most common place to locate the RC HEIGHT. Most of the race cars I've seen have front RCs within 1 inch of ground and the rear RC slightly higher - this is mainly to get front and rear roll in phase - basically making the rear load transfer happen a little faster than the front to compensate for the later development of slip angle on the rear axle vs. the front. A full blown super late model round track car with fabricated front clip has a 1.5 inch RC, the "stock clip" late models have 2.125" RC ( both are for 13 to 18 degree banked paved tracks).

3) RCH = ground height. All the lateral forces passes from the chassis to the tires via springs/shocks. You are not passing any force through the wishbones in a pure lateral load condition.

4) RCH below ground. More force than what's actually transferred passes through the spring/shocks, so that the wishbones are loaded under "a negative" force. This means outer top wishbone for example is not under compression, but under traction.
This is the case of Sedan Road racing cars that have to maintain the suspension geometry from the original street car when you reduce their ride height, there you have to find the best compromise between what you gain from aero and reduced CG height and what you loose for poor suspension geometry. Here you don't have jacking, but the contrary. Also it is the case of heavily "tuned" street cars. The Ford Falcons used to have the front roll center below ground, not because of a choice, but because that's where it lays when you lower the car with the wishbones pick up points that the rules stated. That was later "corrected" and now they are all above the ground.

Formula 1 Crespi XXV Formula Renault chassis. Front RCH -1 inch (that's underground ) Rear RCH - 1/2 inch (13mm) (underground). These cars are in a whole different ball game. The suspension of a Formula car has push/pull rod mechanism with springs attached between the rockers & the vehicle ("corner" springs) & a spring attached across the rockers to allow symmetrical movement, but to resist differential rocker movement , the sway bar (anti-roll bar). It is also possible to attach a spring between the rockers to allow differential movement, but to resist symmetrical movement (heave/pitch spring). Many F1 vehicles use all three types of spring at both ends of the car. Way too advanced for this post or this author for that matter so I think
Bob Hahn summed it up best.

From his notes " In more laymen terms, I believe you are asking about using migrating roll centers to tune your platform.

I've looked at your sketches and descriptions and see you are still using "above ground" roll centers (RC). The detriment is the further inside the RC travels the more roll is induced, and geometry is less effective.

We've been using migrating roll centers to tune suspensions for 15 years...way before force based software became available. We found locating static RC just below the ground, and migrating to ground level, on center with the outside tire to be most effective.

You'll find weight doesn't transfer to the outside, but actually to the inside via kinematics, ie causing the inside tire to load and the outside tire to unload...slightly. Yes sprung mass is still loading the outside tire but total wheel rate is less than conventional design.

This in turn not only allows more total grip, but also absorbs steering input disruption much easier, thus upsetting the vehicle less on turn in. This allows a deeper turn in, and quicker response manipulation in the long run.

Again, for the laymen, imagine, for each instance, a solid rod connecting each tire contact patch to the RC, and another solid rod from the RC to the CG. Imagine the cg as an extremely dense cannon ball, and the RC as the anchor point.

With the RC above ground, and traveling to the inside, there is a "pulling " force countering the CG. Gravity pulls the sprung CG down, with nothing to support it but springs, the platform rolls.

As the action proceeds, the RC "pulls" against the contact patches, at a decreasing angle, but extended arm which does cause a leverage effect, which in turn accelerates weight transfer, due to "lack of resistance".

The angle of "anti-force" or "anti-percentage" is paramount to weight transfer side to side, as it is under braking and accelerating in a longitudinal fashion.

When the RC is located below ground level, the applicable forces ALL happen in reverse. The CG is above ground the contact patches are "between" the CG and RC, in a 2 dimensional view, which should be understood before looking at a top view and visualizing 3 dimensionally.

With the RC below ground level, and migrating to the outside, the forces from CG to RC are in compression, In order for the cannon ball to move outwards, it must travel upwards due to the angle of the anchor point, the RC. Consequently, gravity as it is, the CG down force locates on the inside tire instead. Anchoring the RC at outside tire patch optimizes anti forces. Locating RC outside the outside tire patch, and above ground, actually causes the platform to reverse roll.

In more simple terms, if the RC is above ground, less movement is better. When static RC is located below ground, locating the RC under the outside tire patch under load is optimum.

Migrating roll centers add another dimension to setting up your platform, and in fact, considered "voodoo" by all but the upper levels of racing, who can afford the engineers, software programs, and testing and thus rarely used in the lower ranks. Most people simply lower the vehicle as low as the rules allow, and go with what they have. I've even seen vehicles raised, (by top teams)in order to prevent RC movement.

BTW, if you really want to upset the car during a "steady state turn", locate the RC just above ground in static, and have it below ground under load. Also, If you're not running below ground level RCs with your FWD're backing up!

...just my humble opinion...

Hope this has helped, or at least stirred the pot a little

Bob Hahn

Some day I hope to be near a mart as this guy..I'm still in 2D.

You can also change a cars response characteristics with roll center position, you are really tuning your elastic to geometric weight transfer ratio by where you position the rc.

A car with the rc closer to the cg will have higher % of geometric weight transfer that occurs, geometric weight transfer is instantaneous so you can have a car that points well but doesn't have as much mechanical grip compared to a car with a lower roll center which will have better mechanical grip (using your springs more) but poorer response (but is really dependant on tire side wall stiffness and spring and shock rates)

Jluetjen describes what happens when you have a Roll Center too high and it flips over the CG.

1) Roll centers are not static. As was mentioned earlier, they can move around (in 2 dimensions) quite a bit depending on where the car is in its roll and bump travel. Having a roll couple that suddenly doubles or triples in size can cause an awfully weird handling car, especially if the roll-couple were to suddenly reverse so that the Cg was below the roll center!

2) Suspensions with high roll centers are prone to jacking. The classic example is the rear suspension of a early Triumph Spitfire (see pic). Basically, because the roll center is relatively high in relation to the contact patch, there is a tendency for the chassis to be jacked up and over the contact patch. Not a big deal on 50's sports cars with non-sticky 50 series tires, but this can be a very big deal with the high loads induced by today's super-sticky rubber. If the roll center goes over the Cg, the chassis will actually "jack down" and start to roll to the inside of the turn.

Roll Center location should be placed at the vehicle center line for road course cars turning left and right.
On circle track cars turning left, RC location relative to the center line gives you a jacking force which is a component of the vertical load. If you have a jacking force of a certain amount the spring carries proportionately less load therefore deflects less - if you want soft springs for grip, but need to maintain ride height for aero reasons, I'm sure a little bit of jacking force could be useful. With "reasonable" RC heights, the net jacking forces wind up being pretty small. BUT..the location of the RC relative to the vehicle centerline can provide a huge jacking force to the point you start to carry the left front wheel when accelerating out of the corners. Most asphalt cars have the RC three inches to the right of center line. Dirt cars run 4 inch to the right RC. Imagine a point on the outside edge of the right front tire to pavement contact point. . Think of a pole vaulter sticking the pole in the ground and that is the anchor point. If we draw a line from this point to the RC we have a moment arm. When we enter a left hand turn the car tire sticks and thus the car tries to move up through this RC. Mean while a whole lot of transferred weight is coming from the left rear, and right rear and even the left front and counter acts this force. It all happens pretty quick. The total steady-state load on the car can only come from gravity and aero. That's just basic physics. Normally, the sprung mass is just supported by the springs. When the tires generate lateral force, the sprung mass is either pulled or pushed through the links. In order to reach equilibrium again, the springs have to either take up more or less load, relative to how much the tires are gripping.

A car with RC to the left of center line will push going in and be loose coming of because there is not a lot of down force on the right front to stick the tire and there will not be enough lift on the left front to plant the right read tire coming out of the turn. I had a hard time figuring this out until I read the attached paper by Mitchell. His analogy on how this pole vaulters thing worked is to use a shop floor push broom ( the kind with the long handle ending in the Tee shaped row of bristles). If you pick the handle up from the shop floor about 3 inches and push it glides pretty easy. If you raise the broom handle to shoulder level and push it, it takes some effort. RC on left side car does not have enough angle to do the job.

Boring technical summary

The lateral load transfer distribution (front vs. rear, how much each gets) is a big influence on chassis balance.
For the most part, total load transfer is a function of CGH and track width.
Of the total load transfer, a portion of it (proportional to distance from CG to roll axis) is a rolling moment, and is taken up by the springs and bars.
The remaining portion (proportional to distance from roll axis to the ground) is non-rolling overturning moment, and is taken up and split by the roll axis inclination.
The relative amount of front spring and bar, to rear spring and bar, decides how the rolling moment is split as load transfer to the front and rear suspensions
The roll axis inclination decides how the non-rolling moment is split as load transfer to the front and rear suspensions
The slope of the line from the contact patch to the RC indicates the proportion of jacking load on the sprung mass, to cornering force of the tire.
Roll axis inclination is not static, it will change when the car pitches, yaws in the turn. And Rc moves all over the place during cornering, acceleration, is squirrelly as hell..

The bottom line is that you should know where your Roll Center is and know how it effects your car at each track your racing. Toady's computer programs like Performance Trends circle track analyzer or Suspension Analyzer are as important as the tire pyrometer in today's racing..You absolutely must use it. and don't forget..its all about tires, Tires, TIRES....

High RC = means more Camber gain per inch of suspension travel
Low RC = means Less Camber gain per inch of suspension travel
Attached Thumbnails
Roll center (tune to win).jpg   rc Jacking.jpg