Nissan Spec-V part 11; Lessons in Geometry
9/27/2006
The Nissan Sentra Spec-V handles well but the suspension has a few problems. We will have to re-engineer the suspension geometry to try to make up for the car’s shortcomings.
Unlike cars with sophisticated multilink suspensions, the Nissan Sentra has a basic beam rear axle suspension and an independent but simple MacPherson strut set up in the front.
When the front suspension is significantly lowered for better handling and aerodynamics, the camber goes positive when the car rolls over under cornering. This is less than optimal to get the most traction.
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, which is the distance between the car’s center of gravity and the roll center, increases. Simply put, centrifugal force has a bigger lever arm to roll the car over more. This is not good for handling.
When the front is lowered, the bump steer becomes extreme. Bump steer is when the steering linkage tie rods and the lower control arms go through different arcs when the suspension is compressed. This causes the wheels to steer even when the steering wheel is not moving, causing 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. This big disparity in roll center heights makes the car roll diagonally onto the outside front wheel during a turn. This gives the car an unstable, tipsy feeling. It also overloads the outside front tire-causing extreme understeer at the limit.
To fix the camber loss under roll and roll center height issues, we dropped the outer control arm pivot location 3 inches 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 that of the stock car. It also prevents the loss of negative camber when the car rolls over in a turn.
To correct the bump steer, 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. This puts the steering linkage much closer to being in line with the lower control arm in its new position. We then used shims to adjust the steering linkages position until we were able to eliminate much of the bump steer. These changes should give us much more front bite and reduce body roll under hard cornering.
To keep the stock rear suspension from flexing sideways under load, Nissan uses a device called a Scott-Russell linkage. The linkage does reduce flex but the big soft rubber bushings needed to keep it from binding up hinders it. The linkage causes binding and uneven wheel rates from side to side.
The Scott-Russell linkage has a very high rear roll center. To eliminate these issues we removed the linkage entirely. We replaced the linkage with a panhard rod. The panhard rod keeps the suspension from flexing sideways and lowers the roll center by as much as 6 inches from stock. 
We designed the rod’s mounting points to adjust by inserting the pivots in different holes and sliding the mount. This way the roll center height can be adjusted to help control the amount of rear roll. Ideally, we want the rear to roll slightly less than the front. With the panhard rod in place, we have reduced the cars tendency to overload its front outside wheel.
The lower control arm’s front mount hangs out unsupported. This causes the lower control arms to move under hard cornering load causing steering inaccuracy and tire contact patch loss. 
We fixed this by building a brace that ties the front mounting point of the lower control arms to the engine cross-member. 
Since the lower engine cross-member is mounted in rubber, we replaced the rubber bushings with bushings made of solid aluminum. To further reduce flex we machined Delrin plastic bushings to replace the soft rubber bushings holding the steering rack in place.
A removable aluminum brace was fabricated to tie the upper shock towers together so that flex could be greatly reduced up top as well. At this time we also mounted the remote reservoirs of our KW coil-over shocks to the firewall where they could easily be reached in a pit stop for fast adjustment.
We mounted our rear reservoirs just inside of our sway bar mounts for easy pit access.These modifications are very difficult and require a great deal of engineering, machining and fabrication but they should give us an edge.