Roll Steer is Defined as the change of the the steering angle of the rear end housing due to chassis roll. Roll steer is present when the rear axle turns in relative to the chassis centerline as the chassis rolls through a corner. One way to picture the relationship between the rear axle and the chassis is to understand how this relationship works, as the car goes into a corner at speed the chassis will want to roll opposite to the direction of the turn, we all know this as gravity, during this movement a solid axle on its pivots does the following, Say for instance we are turning left, the chassis is rolling to the right, The right side pivot of the axle is moving upward, this action is making the pivot movement longer on the right side of the axle adding an additional lenth to the right side wheel base. The left side pivot is extending downward shortening its pivot length causing the left side pivot to move closer to the left front tire making the wheel base of the left side shorter, This action causes the rear end to slightly pivot on each side of the chassis causing it to be out of square to the chassis, and will automatically help the chassis turn to the left from the rear of the car, the rear end as you drive around the corner will want to steer to the outside of the turn, depending upon how much of a rear steer you have, this can be good or bad when you corner and drive down the straight aways, If you have under rear steering the rear end will want to drive to the inside of the race track and will most likely not want to turn in the corners. When using roll steer on a dirt car it is always best to have over steer in the rear, which helps the rear tires keep traction, not allowing the tire to break free from the surface. NO Rear Steer will do the following: When forward grip is lost the tire will slip to the outside of the corner and side bite will not be present making turning impossible. With the correct amount of rear steering the axle pivots move allowing forward bite as the car moves around the turn and side bite is present allowing the car to turn.
It doesn't take much of a change in the rear end angle to make a significant change in the handling of the car because of the changing arc movement of both pivot areas of the rear end.
There are many different ways to tame and control the actual movement to help you benefit rear roll steering. Depending upon the chassis type you use you can use the following methods to slightly change your Rear Roll Steer.
When using a 3 link setup, the mounted front links of both left and right side arms are usually mounted on an uphill angle you can very the degree of angle both sides to change the amount of steer you put into the rear end. I suggest that both sides start with 5 degrees uphill mounting angle to the chassis first, then you can taylor the amount of steering you choose for the track and conditions. I start at 5 Degrees because you get more bite out of this combination when you are coming out of the turn, Adding an 1/8" to the length of the Left rear will make large changes to cause under roll steering and by the opposite of shortening the left side by 1/8" makes an increase in the Rear Roll Steering.
When using a 4 link and rear trailing arms, either pivot on the car is prone to lengthen or shorten when using a birdcage to free float the pivot to the axle. the pivot that is facing the outside of the corner will most likely lenthen much like the 3 link suspension and the pivot to the inside of the turn will lengthen, there are adjustments you can make to the length of the links and trailing arms to tame the amount of movement you will get when the chassis rolls through corner. As a rule of thumb the 4 link and trailing arm set up use a Dynamic Roll steer which is not built into the rear end type it needs the body roll to work, This is where you need to make more body roll when you enter the turn to have the greatest effect of rear steering.
When a leaf spring suspension is used the same basic principles are used, it is always a good idea to use a sliding adjustment to correct the required amount of roll steering.
ALWAYS REMEMBER THAT A SMALL AMOUNT OF ADJUSTMENT MAKE A BIG DIFFERENCE IN THE WAY THE CHASSIS WILL TURN WHEN TUNING THE LENGTHS OF YOUR AXLES PIVOTS TO THE CHASSIS.
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