Project Nissan Sentra Spec-V part 5; Lessons in Geometry

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Unlike Honda whose small cars have sophisticated multilink suspensions which helps to ensure good handling when those cars are built into race cars, the Nissan Sentra has a crude beam rear axle suspension and an independent but simple McPherson strut set up in the front.

The front suspension has many faults.  First off when the car is significantly lowered to reduce the center of gravity height for better handling and aerodynamics, the camber of the tires goes positive when the car rolls over under cornering.  This is less than optimal to get the most traction out of the tires.

Next, when the car is lowered, the roll center, which is the geometric point in space that the car rolls around, drops.  Although this sounds good, the roll couple now increases. The roll couple is the distance between the cars center of gravity and the roll center.  Simply put, centrifugal force now has a bigger lever arm to roll the car over in turns even more.  This is not good for handling.

Finally when the front of the car is lowered, the bump steer becomes extreme.  This is when the steering linkage tie rods and the lower control arms go though different arcs when the suspension is compressed causing the wheels to steer even when the steering wheel is not being moved.  This causes instability under braking, cornering, hitting bumps and acceleration.

On the Sentra the rear suspension has an extremely high roll center that stays high with lowering but the front suspension’s roll center drops with lowering, below the ground in our case.  This big differential in roll center heights makes the car roll diagonally onto the outside front wheel in a turn.  This feels bad to the driver, giving the car an unstable tippy feeling.  It also overloads the outside front tire causing extreme understeer at the limit. Since we are lowering the car a large amount 3-4”, we need to address these issues if we expect the car to handle well under the hardcore use we expect it to see.

To fix the camber loss under roll and roll center height issues, we dropped the outer control arm pivot location by 3”.  We did this by modifying our lower control arm to use a spherical bearing instead of a ball joint.

This enabled us to use a long shank and a spacer between the bearing and the spindle to drop the pivot point.  This restores the roll center location to a point close to the stock car.  It also helps prevent the loss of negative camber when the car rolls over in a turn.

To correct the bumpsteer, we eliminated the tie rod on the steering linkage and replaced it with another spherical bearing.  We then drilled out the taper on the steering spindle arm and bolted the bearing directly to the spindle arm on the opposite side as stock.  This puts the steering linkage much close to being in line with the lower control arm in its new position.  Then we used shims to adjust the steering linkages position until we were able to get rid of much of the bumpsteer. These changes should give us much more front bite and reduce body roll under hard cornering.