Project 968 Building a Better Race Car / Part 2
Part 1 of our project ended with the paragraph "The rear bushings are now complete. What remains ahead are front bushings and camber plates, front and rear sway bars, custom shocks & struts, stiffer springs, stronger A-arms, brakes, roll cage, race seat, steering wheel quick disconnect, etc. I will address some of these topics in the next section."
At this point in our project, the goal of removing all rubber components from the suspension is nearly complete. The rear of the car is done and that represents about 90% of the work involved. The front is a simple matter of removing the front A-arms so we can replace the rubber bushing in the front of the arm with a spherical bearing bushing. The Caster block assembly is also changed out at this time and this literally takes about 10 minutes.
Since this car is going to be subjected to about as much abuse as a car can be, we elected to change the front A-arms to Fabcar arms for safety reasons and this was the perfect time to do it. The Fabcar arms or Charlie arms that are available for our car do not really offer a performance advantage, however since the factory cast aluminum arms are subject to failure we decided to be safe rather than sorry.
Below left is a picture of the Fabcar arms before installation but with the front bearing cartridge already installed. Below right is a picture of the Arm installed in the car.
The caster block is the red object toward the left edge of the second picture. Missing from this area is the brake rotor, caliper, and spindle assembly to allow better viewing of the arm in place.The only remaining pivoting or rotating pickup point is the upper front strut mount. The normal factory unit is a large rubber block which incorporates a bearing to allow the front struts to turn when the steering wheel is turned. Again, our goal is to get rid of any rubber which can allow deflection so we elect to install a camber plate.
PCA club racing changed the camber plate rule last year and camber plates are now required to be pinned in the center location. By pinning these you lose one of the advantages of camber plates, that being the easy adjustment of wheel camber at the plate, however the benefit of having a zero deflection mounting point is what we are after. The pictures below show the camber plate being drilled so it can be pinned in the center location. The picture on the right shows the plate installed in the chassis (view from inside the engine bay).
With all mounting locations and A-arms taken care of, it is now time to turn our attention to installing a proper race shock and spring setup. 968s and 968CSs come from the factory with a relatively healthy spring rate of about 165 lb/in in the front and around 170 lb/in in the rear. This is up from early 944s and 944 turbos which came with rates of around 128 lb/in front and rear. Since the 968 is a heavier car these higher rates are expected.
Whenever you deviate from the factory rates, it should be understood that you are giving up ride quality, however if done correctly, performance can be significantly enhanced. What I mean by doing it correctly is just this. Springs and shocks must be replaced as a tuned system. I am aware of too many installations where 944/968 owners install a coilover kit which consists of a threaded sleeve, spring upper and lower hats and a higher rate spring. This simply does not work, or at least it does not work well. The factory designs a car's suspension system as a tuned mass system. In simple terms this means that for a given mass and spring system, a particular amount of dampening is needed (by the shock or strut) so that any energy in the way or bumps in the road are properly dampened out so that the vehicle remains stable.
When only a spring is upgraded, let's use the example of replacing a 140 lb /in spring with a 300lb/in spring, you now have over twice the energy stored in a given spring deflection. Since the shock from the factory was only designed to dissipate the energy of the lower rate spring, the higher rate spring will cause vehicle oscillations to last longer than they should, which, in the end affects stability and vehicle grip. There is much more to this subject than this simple explanation, but this conveys the general idea as to why a spring-only upgrade does not work.
So, now that we know you need to do both spring and shocks, where do we go as far as spring rates for a race car?
Well, that's not a simple answer, but higher rates are a safe bet. The idea is to increase the rates of the springs in order to prevent the car from rolling due to load transfer which occurs during cornering. When the car rolls, bad things start to happen to wheel geometry, which affects contact patch, camber, and in general, grip. With the experience we have on the track in similar cars, we elected to use a 650 lb/in spring up front with a rear helper spring and torsion bar combination that gives us a rear wheel rate of about 300 lbs/in.
After a few track sessions we found this to be too light and increased the front spring to 850 lb/in and the rear to about half that. Notice that I have talked about different things here, in the front I have said spring rate and in the rear I have said wheel rate. What is important is wheel rate. The front wheel rate of our cars due to geometry is about 94% of the spring rate and that changes slightly throughout the wheels' motion, but these are rough figures. A 650 lb/in spring has a wheel rate of about 611, a 850 lb/in spring has a wheel rate of about 800 lb/in. The rear rate is a little more complex due to the geometric relationship of the shock to the rear banana arm and wheel, but a close approximation is that the wheel rate is about half that of the spring rate. When you add in a torsion bar (required for PCA stock classes) in addition to a coilover helper, it gets even more complex.
The shocks we elected to use to dampen our vehicle are independently double-adjustable units with remote gas canister reservoirs. There are many different configurations available to use, from simple fixed valving units, to rebound only units, to rebound and bump (not independent of each other) units, independently double adjustable, triple adjustable, and even quadruple adjustable. The latter have to do with high speed and low speed adjustments for both bump and rebound. High speed would be like hitting a bump and low speed would have more to do with corner entry response. All of these systems have their place, with distinct advantages or disadvantages depending on application. Considering we are looking for very good performance, a high level of tune-ability, but we didn't want to break the bank we decided on our LEDA independently double adjustable. What is critical is the valving in the shock can handle the spring mass system.
Below are some pictures of the shocks in various states of assembly. The knob on the body controls rebound dampening, and the knob on the remote canister controls bump.
