Reviewing some interviews on the development of the car, an engineer described how their target to lower the center of gravity allowed the use of softer springs while keeping body roll minimal. In an episode of Jay Leno 's garage featuring his Lotus Elan race car build, he explained that Colin Chapman directed his engineers to give a generous amount of spring rate for the car, to maximise the contact patch on British b-roads.
An important element to the car's fun to drive character stems from the suspension needing stroke in order to transfer the weight to rotate the car, while maintaining the right amount of grip. Here is an excellent video that describes the Toyota's approach. I have two different setups that I have developed, the Street Alignment and Race Alignment.
Factory Suspension Setup:
(Spring Rate: 2.3 kg/mm Front , 3.3 kg/mm rear) :
1. Spring Rates
The main objective here is to keep maximum stroke and compliance by choosing a spring that is not too stiff. Many people look at the spring rates (2.3kg/mm F, 3.3kg/mm R) and automatically conclude that Toyota wanted to create a tail happy car with the stiffer rear. The truth is a little more complicated.
Spring Rates selection need to consider the multi-link rear suspension setup. This affects the motion ratio, which means that the rated spring rate at the rear of the car is not truly reflective of the spring rate that you experience.
To get the "wheel" rate of the rear spring, you multiply the spring rate by the square of the motion ratio. A simple way to do this is to multiply the rear spring rate by 0.75. Therefore, the true spring rate for the car is (2.3kg/mm F, 2.47 kg/mm R) which means the car is only a touch stiffer in the rear. From that starting point, we can study the popular coilover kits:
* RR actual is the rear spring rate (RR) x 0.75
I decided to go with the stock dampers matched to the RCE Yellow Springs which drops the car around 0.5". It is a little more of a drop than I would have liked, but the price was a deciding factor. If money was no object the KW Clubsports would be on the car.
Camber is non-adjustable on the stock GT86 / FRS / BRZ unless you install camber bolts, plates or adjustable bushings. The Whiteline Camber Bolt Kit can add up to 1.5° of negative camber and I combine it with a camber plate with the objective of improving turn in and reducing understeer.
Caster Is not adjustable on the stock GT86 / FRS / BRZ. Given this car's street oriented usage, it is not something that you would usually adjust unless you were looking for more braking stability under high loads.
Toe is adjustable via factory rear eccentric bolts and front tie rods, and a touch of 'toe out' will help increase steering feel and turn in while slightly compromising steering stability at higher speed.
Personally while I can attest to the advantage of running the car with slight toe out on the front, it is a great contributor to tire wear, so I have made the change to go to a 0° setup.
5. Anti-Roll / Sway Bars
Given that the car now has stiffer springs, I'm still testing if adjustments need to be made to the anti-roll bar setup. For now, it will remain stock.
6. Tire Pressures:
I run 20% lower pressures in the rear in order to maximise grip on corner exit. Therefore, trail braking technique becomes very important to lighten the rear of the car during entry. For an in-depth guide on how to adjust tire pressures for competition, I would highly suggest reading Kenny Brown's excellent article.
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Amuse Suspension Setup - They dropped the height of the car but increased the stroke to ensure that the car copes well with a 'backroads' environment. Note that they use 8kg spring rates front and back (same as HKS Hipermax SP) with 4 degrees of camber in the front and a light rear.
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Camber: 0 +/- 45' (0 +/- 0.75°)
Caster (Reference): 5°54' (5.90°)
Toe (Each Wheel): 0°00' +/- 0°11' (0.00° +/- 0.19°)
Camber: -1°12' +/- 45' (-1.20° +/- 0.75°)
Toe (Each Wheel): C + D: 0°10' +/- 0°15' (0.16° +/- 0.24°)