2008 Pontiac G8 GT: Suspension Walkaround
May 08, 2009
I know I've done a lot of these lately, but the 2008 Pontiac G8 GT is this week's COW, so I couldn't possibly let it slip by. Besides, these wheelwells actually contain a few interesting tidbits to talk about.
But that's not apparent from this overall view of the front suspension. It looks like a garden-variety front strut setup from here.
First and foremost, the G8 has a dual-pivot setup, just like the BMW 135i we saw last month. The difference here is the use of steel instead of aluminum, a less-costly choice that contributes to the G8's lower price.
This snail's-eye view shows the two lower ball joints (yellow), the forward-mounted steering arm (green) and the bottom of the strut (white). Such a dual pivot arrangement moves the virtual steering pivot out to the intersection of the projected links, which allows for a zero (or even negative) scrub radius.
Here's how it looks when you turn the wheel from lock-to-lock. Don't mind the squeaking -- it's not present when the car is on the ground with the suspension properly loaded.
Yes, the virtual pivot point moves around a little as the angle of the links changes. Try not to think about it too much.
Dual pivot setups tend to look like a bowl of spaghetti, with tortured links required to keep all the bits from hitting each other as the suspension turns and moves up and down. Here the forward link (yellow) loops up to miss the steering tie rod (black) at full lock, and the stabilizer bar (blue) needs a massive bend to keep it away from the tie rod when the suspension is compressed in turns.
We can also see that the strut (white) connects to the knuckle (green) with two large bolts--a very common arrangement.
But the G8's struts have something not often seen: a set-screw that allows front camber adjustments. The uppermost large bolt hides a slotted hole or a reduced-shank upper bolt (I didn't disassemble it to see which -- no alignment rack at my house). Simply loosen the big bolts, tweak the set screw as needed and tighten things back up. But this job is only conceivable as a DIY by those who have a camber gauge and experience. And toe-in must always be reset after doing this. For the rest of you: Don't try this at home.
Here is another view of the aluminum 2-piston sliding brake calipers. The black arrow points to some cooling fins. There are more on the other side. We can also see that the lug studs (white) have a few threads removed and a tapered nose to reduce the chance of cross-threading, just like the Cadillac CTS. Neat.
The G8's rear suspension is a multilink with an upper arm (white) and three links. The upper arm is L-shaped, with the "L" curving off behind the spring, in the dark shadows beneath the body. The green and black links (the black one is beneath and behind the spring) approximate a lower arm, while the yellow link handles toe control. The G8 has a coil-over spring/shock assembly.
Like the front, everthing is steel to keep the price of the car in reasonable territory. Also like the front, the rear brake caliper, a single-piston slider, is made of aluminum and the rotors are vented.
Here's another view of the toe link (yellow). Toe-in is reportedly adjustable, but we can't quite see if there is an eccentric on the inner mount. Black indicates the forwardmost lower link.
I've talked about arm ratios and efficiency, and here's what it all means. Don't worry about the math: I'll do it for you.
The length of the lower arm is AB. Point A is the pivot point, and it has an eccentric for camber adjustment. Point B moves in lock-step with the wheel. The spring and shock mount to point D and the stabilizer connects at point C. Therefore, the stabilizer bar motion ratio is AC/AB and the spring/shock motion ratio is AD/AB.
The stabilizer bar motion ratio is around 0.4:1. That is to say, 1 inch of wheel motion results in 0.4 inches of stabilizer bar movement. Less motion means the bar won't twist very much, so it has to be thicker to get the job done. But less twist also means lower internal stress, so the engineers can select a less-costly grade of spring steel. It's a classic trade-off.
The spring and shock share the same motion ratio in this coil-over design, and it's 0.77:1 if you go strictly by the arm lengths. But the spring and shock lean inward (I'm guessing 25 degrees when the car is on the ground) and the ratio is worsened by this angle. The math boils down to an overall motion ratio of about 0.7:1. In other words, 1 inch of wheel travel produces around 0.7 inches of spring and shock movement.
A 1:1 ratio is preferable for shocks because they're more precise and less affected by internal friction when they move a greater distance, especially over very small road imperfections. But there isn't always room for everything, and the G8's setup is by no means unusual, especially for a coil-over.
The rear coil spring in the G8 GT is a progressive-rate spring. The coils are closer together at the top and more widely-spaced at the bottom. It's relatively soft initially, but it stiffens progressively as the close-set upper coils begin to bottom out. The white arrow shows a witness mark from such coil contact. Plastic sheaths (yellow) prevent that contact from being metal-to-metal in a bid to minimize noise and reduce the chances of paint scratches that can lead to rust and, in extreme rust environments, spring failure.
Some conservative carmakers refuse to use progressive springs for these very reasons, plastic sheathing or not. The aftermarket uses them extensively because lowering kits would be harsh indeed without them. But aftermarket companies don't have the same long-term warranty exposure as the original carmaker, and the aftermarket doesn't always employ countermeasures like these sheaths.
The G8's L-shaped upper arm has two pivot points on the chassis side. The one we can't see at the end of the "L" (back and to the left) is a rubber bushing, but the one at the crook in the "L" (white) sits directly inboard of the rear tire, and it takes the brunt of the lateral cornering forces. In our FE2-suspended car this one is a ball joint instead of a rubber bushing for increased lateral precision.
All of this works quite well. They've done a good job spending their available money and development time in the right palces. Almost everyone here appreciates the way our Pontiac G8 GT steers, rides and handles. And you saw how evenly the tires have worn a few posts ago.
It's really too bad this thing has to die.
Dan Edmunds, Director of Vehicle Testing @ 20,802 miles