Ed Piatek, program engineering manager for the CTS-V, elaborates on the steps he and his team took to eliminate the dreaded axle hop: "Cars with independent rear suspensions and a limited-slip differential are naturally prone to an oscillatory motion between the two half-shafts [which lead from the differential to each wheel hub]. They're like springs that you wind up, and because they have the same rate [of movement], as you unwind one, it transfers torque to the other, which sets up a ringing effect (resonance). By adding a larger, stiffer half-shaft to the left side, it stops the ringing, or the oscillatory windup, that leads to axle hop." How much larger is the left half-shaft? It is 2.2 inches in diameter, versus the one on the right, which is 1.4 inches in diameter. In the rear-suspension cutaway that Cadillac showed us today, the difference was immediately obvious. "The left half-shaft," says Piatek, "is twice as stiff as the one on the right."
Cadillac engineers also established separate suspension algorithms for braking and accelerating modes to further prevent axle hop, or tramp. "At low speeds," explains Piatek, "we let the rear end take a set, but then we stiffen the rear dampers [using the Magnetic Ride Control] in rebound so the car stays down. We've also established an algorithm for the ABS that looks for differences in wheel speed. When we see that, we send a brake pulse to one of the rotors. It doesn't slow the car; rather, it merely slows one wheel, which prevents unwanted oscillatory motion."
Several tire-smoking, gear-banging launches of a manual-transmission CTS-V with Mikels behind the wheel indicated that all of these efforts have borne fruit. The CTS-V catapulted forward with grace, with very little in the way of lateral body motions, as we blurred the scenery on our way to an indicated 150 mph, a speed at which the car was barely breaking a sweat. And the 2009 CTS-V absorbed freeway-style expansion joints far more easily than you would expect from a hardcore performance sedan.
Mikels made two high-speed passes of the ride-and-handling loop, which is riddled with big dips, rough pavement, off-camber surfaces, chatter bumps, railroad tracks, and other real-world road conditions that wreak havoc on improperly tuned chassis. During our first run, he set the CTS-V suspension to sport mode, and the car soaked up every obstacle without a hiccup yet with a fair degree of suspension compliance. But even when Mikels set the suspension to comfort, the CTS-V handled all of what the engineers call "suspension events" easily.
Our rides in the 2009 CTS-V today came in the midst of what chief engineer Lyon calls "our final exam stage." Over the next week or so, Cadillac is driving a CTS-V for the equivalent of 24 hours at racing speeds, making sure the car is truly ready to take on the M5 and the E63. From what we saw, heard, and felt by the seat of our pants today, it looks like it surely will be.