Along with this great ability to follow the surface better, there are some deficiencies associated with the basic design of the system. All IRS systems are not alike and Corvette has some unique features which can cause headaches for someone wanting good cornering performance in all conditions.
Rear Camber – One disadvantage of this system is how to rear camber is controlled. The diagram above shows two lower links called the lower camber rods. These rods are to establish the camber angle of each rear wheel. They have squishy rubber bushings at each end which flex as the car is cornered. The effect is to change length which results in the rear wheels changing the camber angle to more positive as it is hard cornered; this is not desirable. It tends to lift the contact patch of the tires and reduce traction when it is most needed. An effective solution is to replace the factory bushings with urethane bushings. One other method is to eliminate the bushings all together and use a hemispherical rod end linkage making the connection rigid thereby getting rid of camber changes. There is an additional benefit to this fix in that you can then adjust the rear camber, something the factory camber rods do not allow you to do.
Another impact can arise if there is wear in the gear train inside the differential. This allows a similar problem to occur causing camber changes. This is because of the C2-3’s use the half shafts as a kind of upper a-arm. It is fine when things are new and tight but if you introduce wear it will impact the camber in a bad way.
Controlling Toe – Toe-in or out of the IRS happens during acceleration and breaking. Once again rubber bushings are the culprit. The C2-3 years of Corvettes use a single rubber bushing trailing arm on each side of the car.
The net result is unpredictable handling caused by the constantly changing torque steering inputs during braking and accelerating. This is a result of the toe in or toe out changes as the car brakes or accelerates hard. (see figure) One solution to minimize torque steering is to install urethane bushings instead of soft factory rubber bushings. This will minimize, but not eliminate torque steer.
Transverse Leaf vs Coil Over Spring
The body roll coupling of coil spring on a fully independent rear suspension is nearly zero. That can not be said about a transverse leaf spring-like the C2-C7 Corvettes have since the spring is attached to the frame at the center and both wheels. If the frame rolls, the wheel outside the corner is compressed while the inside wheel is unloaded by whatever the body roll produces. This is in addition to the normal weight transfer of cornering. The suspension is not truly independent since the transverse spring makes them dependent on each other, and the motion of the frame/body.
By changing a few bushings and a little effort the C2 – C3 Corvette IRS can be improved significantly overstock. You will notice the difference in a more stable turn entry and exit, and more predictable handling all around. Adding a coil-over kit like the Shark Bite rear coil-over will make the rear suspension truly independent ( Something the C7 doesn’t even have!) It will significantly & noticeably improve traction on the street or track.