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Roll centers and Pitch Centers

The Jacob's ladder's approximate roll center can be found by intersecting the center lines of the two straps. When you change the holes where the straps mount, it changes the roll center height and/or the roll center side to side location. The panhard bar's approximate roll center is located in the middle of the panhard bar. One major difference between the two designs, the Jacob's ladder's roll center goes up when the car rolls right, the panhard bar's roll center moves down when the car rolls right.

The real roll center of each is found by using the point found in the above method and drawing a line from there to the center of the contact patch of the right-side tire. The roll center is the distance between where that line crosses the lateral plane of the center of gravity and the ground. If you just move the intersecting points found in the paragraph above method to the left, the actual roll center height moves down.

Effects of roll center height: A higher roll center resists roll, we learned in Rethink Dirt that resisting roll in the rear actually increases weight transfer, increasing weight transfer in the rear will make a car looser. Lowering the rear roll center in the rear will prevent weight from transferring making the car tighter. If any of you are baffled by my statement, or think that I just stated it backwards, please read the Rethink Dirt paper under the Tech Department/Chassis Setup. It is just like soft vs. stiff right rear spring rates, a softer right rear spring rate will transfer less weight to the right rear making the car roll more and make the car (generally) tighter.

The reason it makes the car tighter is because maximum traction is achieved when the rear tires are equally loaded. We know this because of the tire efficiency principle.

Bottom line: Lower your roll center in the back to make the car tighter. Raise the roll center in the front to make the car tighter.

There is always a tradeoff. There are two types of weight transfer, geometric and elastic. Geometric weight transfer is weight transferred through the linkages (panhard bar or Jacob's ladder) and Elastic Weight Transfer is weight transferred through the shocks and springs. As we raise the roll center, we are increasing the amount of Geometric Weight Transfer and reducing the amount of elastic. The issue with geometric weight transfer is it happens instantaneously; with elastic weight transfer it happens slower because we have to wait for the shocks and springs to compress. As the roll center is raised, the ride will become more harsh and the car can get a tendency to hop and also does not absorb the ruts and bumps as well. The geometric weight transfer does not have the advantage of going through the shocks and springs. The weight is instantaneously applied to the undamped spring known as the right rear (or right front) tire. This is what causes the chassis hop. Some call this a traction hop. I suppose they call it a traction hop because it happens when the car gets a lot of traction which is true. When there is more traction, more weight is transferred causing the right rear tire to get into the hopping state very quickly. Most of the time, high traction is achieved because of track conditions. On a slick smooth track, you will almost never see a traction hop. Keep this in mind.

F1 cars keep their roll centers at ground level and control the roll by an anti-roll bar because of this effect and the fact that they have independent suspension which nicely puts the roll center really low.

A true Jacob's Ladder is one that has the spread of the straps mounting points on the frame twice that of the length of the straps themselves. So a Jacob's Ladder with 5" straps and a spread of 10" between the frame tabs is known as a 5x5x10 Jacob's Ladder and has the perfect geometry to create almost no horizontal movement (Lateral) of the rear axle as it moves up and down through the suspension travel. If we change the strap lengths to something other than 5" on this ladder design to say 4-5/8" on the top and 5-3/8" on the bottom, we will be deviating from the perfect Jacob's Ladder design. Hence we will incur some lateral movement of the rear axle. However, depending on how much you deviate, the lateral movement is very minimal. 5x5x10, 6x6x12 & 7x7x14 are all common Jacob's Ladder sizes. As we change the strap lengths we can move the roll center higher or lower. Try to subtract from the one the same amount as you add to the other to minimize the lateral movement and to keep the bearing carrier in the same place.

All this explanation of course if for lateral traction considerations only (or side bite, I hate that word) For longitudinal traction (forward bite, I still like that word) the roll center height makes almost no change. For longitudinal traction, we need to look at a concept called Pitch Centers which are similar in concept to roll centers but affect the car in front to back movement instead of side to side. The pitch centers are determined by the pick up points of the front and rear radius rods and in the case of a Z link rear (modified Watts link to be technically accurate) the rear torsion arm angle and upper radius rod angle. There is also a pitch center for the left side and one for the right side just like we have front and back roll centers. The rear linkages control pitch centers when the car is accelerating forward, and the front radius rods control where the pitch centers are for decelerating or braking.

If we raise the mounting points of the front radius rods on the frame it will raise our deceleration pitch centers and keep the nose of the car from diving under hard braking load. Again, the weight will be transferred more geometrically and can cause front wheel hop under hard braking. This is hardly ever an issue on dirt but can be on asphalt.