Renderings by Filip Troja...
Renderings by Filip Trojanek
Here's the thing about solid-axle rear suspensions; fancy parts and exotic housings designed for track use aren't much good if we're not getting it to the ground. Hooking ain't just for drag racers; the quicker and harder we can get back on the throttle when exiting a corner, the harder we can accelerate and the faster our lap times will be. That's a consideration that becomes increasingly important as horsepower and torque levels rise, and with what we've got in store for project Max Effort, we knew we needed to do our homework.
While torque-arm rear suspensions in general are great at transferring power and controlling axle rotation, there's more to it than just bolting one on-lower control arm angle and torque arm length both have a direct impact on forward bite. There's no single perfect setup that's best for all conditions, since vehicle specifics and track conditions vary (wet, dusty, temperature), but they can be overcome by fine-tuning, provided you've got the ability to adjust. There may be times when more forward bite trumps perfect roll geometry and makes the car much faster. Other times the track surface will be ideal, so emphasis can be redirected to optimize roll geometry to increase lap times. What's important is the ability to deal with changing conditions, and understanding how the adjustments affect the car.
For a torque...
For a torque arm suspension, the instant center is located on a vertical line that passes through the chassis mount of the torque arm. When a line that passes through the lower control arm is extended to the vertical line determined by the torque arm, the point at which these two lines intersect is the instant center. Extending a new line from the contact patch of the rear tires through the instant center all the way up to the front of the vehicle determines the value we need to determine the antisquat. This new line determines a point on a vertical line passing through the center of the front tires. The height of this point divided by the center of gravity (times 100 for a percent) of the vehicle is the antisquat value.
To cope with all those variables, Max's rear suspension system is getting the upgrade to CorteX Racing's Ultimate LCAs that are designed to allow quick and accurate geometry adjustments that will dramatically alter the performance. To minimize the roll oversteer, our LCAs are growing several inches in length. Why? Long LCAs are beneficial because for a given change in height of either end of the arm, the angularity changes less than with a short arm. This keeps the suspension geometry more optimal in bump and roll modes.
As far as the considerations on the chassis, the main variables we're concerned with are center of gravity (CG), height, tires, wheelbase, driving style, and track condition. By using a properly designed torque arm rear suspension system it's possible to set up a car with between 60 and 100 percent antisquat and still achieve excellent roll-steer characteristics. That means we can theoretically create a rear suspension with the best of all worlds: amazing turn-in, high mid-corner grip, and also exceptional bite when exiting a corner, allowing the driver to get on the throttle hard and early. Get all that right and lap times drop radically.
For reference, these are the...
For reference, these are the standard CorteX Racing LCAs we used during the rear suspension install to temporarily connect the rearend to the subframe connectors. The geometry is good, but adjustment is limited.
As viewed from the side, the...
As viewed from the side, the upgraded CorteX Racing Ultimate LCAs allow a great deal of angle adjustment. The steeper the angle, the more forward bite will be available when exiting a corner, at the expense of adding unwanted roller oversteer.
The assumption in getting...
The assumption in getting the exact value of antisquat is the center of gravity (CG) of the vehicle. From experience, as well as creative use of wheel scales and jacks, we approximate the CG of Max Effort at 18 inches from the ground with an error of +/- 1 inch. That directly affects how long the torque arm and lower control arms ideally should be, since what really matters when evaluating the antisquat for tuning purposes is the relative effect of changing the angle of the lower control arms. Since we'll have so much adjustment available, a good approximation of the CG is more than adequate for our purposes.
Here's Ryan Kertz's sketch...
Here's Ryan Kertz's sketch of our plan of attack on Max Effort. This LCA design is a slight variation of CorteX's standard Mustang Ultimate LCA to account for the wheelbase change on a Cougar.
To begin, Kertz took chassis...
To begin, Kertz took chassis measurements to determine the race height for the rear suspension, then dropped the lift to double-check. Accounting for the angle from the unloaded front suspension, we're at 4.5 inches subframe to ground.
To get the length and geometry...
To get the length and geometry we need, neither the subframe mount nor leaf-spring pocket will do. We'll be cutting out the floor of the original torque box and fabricating a new mounting point.
With the floor removed, it's...
With the floor removed, it's now easy to see how much room we have to work with without altering the Cougar's inner floorpan.
To create our mounting point,...
To create our mounting point, Kertz welds in a flat 3/8-inch plate for the floor of the torque box and to rebox the rear subframe. We'll explain that large hole later.
To create the longer LCAs,...
To create the longer LCAs, Kertz disassembled our previous short ones to reuse the rod ends and threaded bungs and TIG'd them to a longer section of chromoly tube.
With the LCA tacked together,...
With the LCA tacked together, the next step was to create the pocket that will capture our rod end. Joe Chatterton created this simple bracket to do the job out of the same 3/8-inch plate.
Here it is tacked into place;...
Here it is tacked into place; now we've got a far stronger mounting point for our LCAs as well as room for adjustment.
Here's the completed LCA....
Here's the completed LCA. Note that now we've got a great deal of adjustability on both ends of the arm that should allow us to dial in Max Effort for any surface or weather condition.
Remember that hole in the...
Remember that hole in the torque box floor from earlier? It extends all the way through the floor inside Max. Inside the tube welded to the floor is a 7-inch bolt that runs all the way to the lower LCA bracket. This Grade 2 bolt is for mock-up only; we have the appropriate strength on order.
On this end of the LCA, adjustment...
On this end of the LCA, adjustment will be as easy as rearranging this stack of spacers. As it stands, we've got roughly 2 inches of adjustment, which is plenty for our purposes.
|CENTER OF GRAVITY = 18 INCHES