Corolla tuners are strange people, weirdos obsessed with making 22-year-old cars go fast. It's not much different from trying to retrofit an old black-and-white TV with Dolby 5.1 surround. Why don't Corolla tuners just buy modern cars? Because they're strange. And often cheap. Many Corolla tuners try to find speed with a budget that would shame a 15-year-old high school student.
As much as we'd like to save money, using half-ass budget parts won't cut it for a car that has to make up for two decades of advances in automotive technology. Now the car is running reliably, we're addressing the most critical issue: keeping the tires planted. Even though the AE86 Corolla chassis is known for its balance and user-friendliness, the suspension has some inherent drawbacks when compared to modern designs. Take the front set-up. The MacPherson strut lay-out has minimal camber gain, especially when the car is lowered significantly-one of the first modifications executed by Corolla owners. For this reason, most tuned Corollas run an obscene amount of front camber in order to maximize the contact patch in cornering. The front suspension also combines the spindle with the bottom of the strut housing, meaning any suspension updates involve removing and disassembling the entire front wheel and hub, so the stock spindle and strut housing can be sent off for modification.
The rear suspension is a solid live axle with four unequal-length lateral links, combined with a Panhard rod that constrains movement solely to vertical motion-a similar thing is found in most muscle cars, new and old. Two short lateral links connect the chassis to the top of the axle casting on each side, while two longer links connect the chassis to brackets protruding from the bottom of the axle. The springs sit farther inboard on the axle than the dampers.
While this design is great for putting up with gobs of torque with no camber change as the car squats, it is an immense lump of steel-unsprung weight that can potentially bind as the axle rotates when the car rolls. These links also have a tendency to resist squat the more the rear suspension swings up. Not good when planning to lower the car a couple of inches.
Add all this to the fact that the rear camber and toe are non-adjustable, and you end up with a car that should thoroughly suck in terms of road racing. But it doesn't for some reason, and racers still love this nimble cult classic.
Picking Your Rates
The foundation of our suspension update is a set of custom coilovers built by Ground Control around some Koni race dampers. We started off with spring rate selection using existing coilover springs available through Swift Springs. Most Japanese Corolla tuners will recommend 8kg/mm (447lb/in) and 6kg/mm (335lb/in) spring rates for the front and rear, respectively, for an aggressive street/track set-up. Assuming our gutted Project Corolla weighs a somewhat light 2200 pounds and has a distribution of 54 percent front and 46 rear, these spring rates would be appropriate for a weight distribution range of 57/43. This is a good ratio, since a little less rear spring will keep the rear suspension better planted.
However, we're not in Japan. Our roads are definitely not the same. Keeping in mind that Project Corolla currently still runs stock pizza-cutter wheels and street tires, we decided to proportionately downgrade as best as we could and settled on a combination of 6kg/mm (335lb/in) and 4kg/mm (220lb/in) springs in front and rear, respectively. This corresponds to a weight balance of 60/40, not bad considering the nature of the tires and that most of the stripped-out weight came from the interior's rear. Assuming a motion ratio of 1 for the front and soon-to-be coilover rear suspension, this gives us a natural ride frequency of roughly 2.4Hz for the front and 2.16Hz in the rear. Most regard 2Hz as a good ballpark for a well-tuned street car. Ferrari Challenge cars have a ride frequency of roughly 2.5, so Project Corolla will have a rough ride to the track for sure.
Just to be sure, we did some back-of-the-book calculations to see how much each corner would compress with the assumed vehicle weight at front and rear. Some suspension tuners will use a factor of 1g to 1.5g of vertical acceleration on top of the weight already supported by each corner, to simulate extreme suspension inputs and ensure adequate compression travel for the spring rates chosen.
Assuming each rear corner supports roughly 440 pounds when static, if you take out the unsprung weight, 1g of compression force would mean 880 pounds of force, which would compress the spring four inches from its free length, a significant amount of travel for a lowered car. The fronts would compress 3.3 inches in total or 1.6 inches from the car's static ride height. This will give us a ballpark figure for the amount of compression stroke needed in the dampers we choose.
Proper Damping
Ground Control (GC) worked with Koni to pair up the appropriate dampers and the required spring rates. We chose Koni for its wide range of monotube and twin-tube motorsport dampers. Depending on the model and the money you're willing to spend, Koni offers both single-rebound only or independent compression- and rebound-adjustable shocks valved with linear, digressive or progressive damping profiles.
Taking overall stroke length into consideration, sufficient compression and droop travel is absolutely critical when building a custom set of coilovers. A good starting point is to add the required amount of compression travel (from the calculation we just made) to the desired amount of droop travel. Add the bumpstop length on top of that to get the total stroke length.
The optimum amount of droop travel is often a matter of opinion. For example, some front-wheel-drive race car drivers like minimal droop in the rear suspension to lift an inside wheel off the ground in cornering. This will make a typically understeering car rotate better. Having a good deal of droop on rear-drive cars is a good thing, because rotating a rear-drive car is a whole other ball of wax, and you want to keep all the wheels on the ground at all times.
Jay Morris, the mastermind behind GC suspension, picked a single-way, rebound- and compression-adjustable, twin-tube damper for the fronts (Koni part number 8641-1453 SPGC. The GC in the part number means that it's only available through Ground Control). Morris worked with Koni to have the damper valved in three digressive phases for low-, medium- and high-speed compression damping, while keeping the rebound profile fairly linear.
The new rear coilover (Koni part number 30-7325), was derived from a proven GC design used on the Datsun 510. The four-position, rebound-adjustable only, high-pressure monotube damper has a total stroke of seven inches and was originally intended for circle track and road racing. According to Koni, the linear damping profile is well suited for shorter-stroke suspensions with motion ratios near 1.
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