Project Toyota Corolla GT-S AE86
Building A Coilover
Building adjustable-height coilovers for the AE86 Corolla is not easy. As mentioned earlier, the front axle spindles are part of the strut housing. It's no wonder suspension manufacturers rarely offer off-the-shelf coilover kits, since the stock front suspension housings are no longer made. Those that do typically require a core exchange or cost significantly more than your average MacPherson strut adjustable suspension. We temporarily borrowed a set of front strut housings from a fellow Corolla-phile and sent them to GC, along with the Swift springs.
In order to give the new sport dampers enough space for their full stroke length, the strut housings have to be shortened by up to two inches. This requires two precision cuts: one to remove the top portion that has the machined threads and ridges for the cap holding the inserts in place. And a section cut out to shorten the tubing. Then the top must be concentrically welded back on. GC also hacked off the stock spring perch and welded in a seat for the threaded coilover sleeve to sit on. The spindle was also reinforced with a thick steel plate, welded in to distribute the bending load from the axle. Top it off with a CNC'ed pillowball adjustable camber and caster plate, plus a coat of bling-tastic red paint, and you have a set of coilovers worth more than the car.
Setting It Up
You haven't done a real suspension install until you've done it on a Corolla. Half the effort involves pulling off the brakes, rotor and bearing assembly-just to free up the bottom end of the front suspension. We also had to slot the bolt-holes in the strut tower sheetmetal, so the camber plates (originally intended for another application) would fit properly. By the time you read this, GC should have a proper adaptor plate available.
The rear suspension is much easier to bolt in. Pick up the rear with a floor jack on the rear axle and support the car with jack stands. Unbolt the bottom of the shock from the axle and drop the jack. The rear springs will pop right out.
The new GC coilovers, which also come with custom-turned polyurethane bushings, eliminate the stock springs by concentrically mounting the new spring on the dampers. This is the easiest way to obtain rear height adjustment for corner balancing.
The drawback is that the single-layer sheetmetal used for rear strut towers was never designed to support the rear of the vehicle's weight. Nor were the dinky studs on the axle that the bottom of the damper bolts to. Some Corolla tuners assure us this is safe, but we'll eventually reinforce the lower mount and strut towers by tying in some type of crossbrace or roll bar-even though the mounting points have shown no signs of fatigue after several track days. We also kept the rear springs, just in case the shocks ever decide to rip through the strut towers.
Another advantage of GC's rear suspension is its modular component design, making it easy to disassemble when changing springs, or even shocks. Corolla tuners divide into distinct camps, some preferring to use conventional lowering springs in the stock location, others swear by rear coilovers. Either way, we're covered. If the new geometry doesn't work, we'll just take the springs out and use replacements in the stock location.
After fumbling all these parts onto the car, we needed to sort out a ballpark ride height that allows the suspension to compress and droop properly. Initially, we set each spring perch at a free-length height of the spring, which avoids both pre-loading the spring and also utilizes as much droop travel as the spring will allow. It's a good starting point for setting any coilover's ride height, but this will limit compression travel if there is insufficient total stroke.
We can figure out total stroke by measuring the length of exposed piston rod between the top of the shock housing and the bottom of the bumpstop. Using zip ties wrapped around each piston rod and pushed all the way down to the top of the insert body, you can measure how much the suspension compresses from its free-air position. When the car is lowered, the zip ties will be pushed up to the static ride height. Lift the car up again and measure how far the zip tie has moved up. This will be the amount of droop travel. Everything above the bottom of the zip tie and under the bumpstop is the compression stroke.
To make sure the car doesn't hit the bump-stop in compression, we picked up each corner of the car so only one wheel supported the weight of the car and jumped on that corner. Based on where each zip tie ends up, the spring perches might have to be raised to get enough compression travel to clear the bumpstops.
Our original free-length settings still pushed the zip ties into the bumpstops, so each corner was raised appropriately. Considering there's a finite amount of total stroke, it's probably better to compromise droop to maximize compression stroke. It's far better to have a wheel lift off the ground in cornering than having the suspension bury itself into the bumpstop. The car is still lower than we like, but does the job on track for now.
The dampers give a total useable stroke of only two inches in front and three inches in the rear. Given that we had to compromise one way or the other, we settled on 1.5 inches of compression and the remaining 0.5 inches for droop in front.
In the rear, we ended up with 2.25 inches of compression and 0.75 inches of droop, not nearly as much stroke either way as we'd like. It will work for now as we consult Koni about alternatives that give us more damping and stroke, while still fitting in the existing parts. We're also contemplating a last resort of trimming the progressive bumpstops that came with the suspension.
Fixing A Lowered Car
With Project Corolla sitting almost two inches lower than it was ever designed to, we now have to deal with all the issues that plague an overly lowered car. On the front end, the lower control arms are now pointed up by around five degrees. This moves the roll center a good deal lower than stock, which makes for a much longer roll couple acting on the front of the car. The roll center for the solid axle geometry doesn't change with lowering, which makes for a sharply angled roll axis.
Relocating the front roll center is a simple fix with roll center adjusters (RCAs) now readily available. In the case of the unique Corolla front suspension, it's just a simple two-inch aluminum spacer that shims between the lower control arm and the bottom of the strut housing, pointing the lower control arm back in the right direction.
Techno Toy Tuning (T3), the tuners that brought out a Honda S2000-powered Corolla for our Junkyard Dogs shootout (SCC, Jan 2007) supplied its RCA kit. A nifty perk of the Corolla's quirky front knuckle is that the steering arms are attached to the lower part of the upright. So with the RCAs installed, not only is the front roll center returned to its original height, the steering arms retain their original geometry, eliminating any bump-steer issues. This is why these RCAs are also called bump-steer spacers.
T3 also makes a negative-camber RCA, which, unsurprisingly, adds negative camber and widens the front track. We passed on those to keep a consistent scrub radius in front and rear, and also because the GC camber plates already provide enough negative camber adjustment.
Track Time
Being able to control and feel each tire's motion makes a night-and-day difference when compared to the tired stock suspension and cut springs. With the front camber set at minus 2.5 degrees (for even tire wear) and zero toe, Project Corolla now pulls a consistent 0.91g on each end of our figure-eight test and runs the course in 27.7 seconds, a couple of tenths shy of a stock all-wheel-drive WRX, which has two more drive wheels and twice the power. Numbers aside, the biggest difference is the dynamic response on track. The spring and damping rates we chose are nicely compatible with the Hankook Z212 street tires currently on the car, and also keep the front and rear roll response well synched. Dive is practically eliminated, but we still do have some issues with the rear suspension squatting, and its laggy response on account of the unsprung weight and uneven lateral links. Next time, we'll further update the suspension with adjustable anti-roll bars, a temporary fix for the rear lateral links, and other handling tricks we're still experimenting with.
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