We Toast the C5 Corvette's Final Year with a Test of the 2004 Corvette Z06 - Page 2 of 6
by Hib Halverson
text and images ©2004 Hib Halverson
No use without permission
What basic qualities make a C5 handle well?
- A combination of a perimeter frame (using C5's now-famous, hydroformed rails) a closed, load-carrying, drivetrain tunnel and, on the coupe, a "halo" bar above the seat backs, results in a structure which was stiffer than that of any sports car on the market when C5 was introduced in 1997 and which, today, remains one of the stiffest around.
- CNC-machined, cast aluminum suspension crossmembers which provide a rigid and accurately positioned mounting for front and rear control arms.
- Suspension with minimal toe change and optmized camber curves, even near extremes of travel. This allows ample wheel travel for a civilized ride while retaining suspension geometry optmized for handling throughout that travel.
- Control arm and bushing designs which decouple ride and handling, allowing optimized handling with reduced cost to ride.
The Z06's "FE4" suspension adds specific, higher rate springs and stabilizer bars, unique, fixed-valve Sachs shocks, slightly different alignment, wider wheels and Goodyear's outstanding Eagle F1 Supercar, ultra-performance radial tires.
Wanting C5 to go out with flair, back in spring '02, Dave Hill sent his ride and handling Engineer, Mike Neal, to The Nurburgring, the famed German test track. The mission: make the 2004 Z06 even better.
"Even in the final year," Mike Neal told The Corvette Action Center, "we took pride in making improvements. We sat around and had a pow-wow-'What are we gonna do for the final year.' The easy thing was to carry over. We wanted that last one to be the best one yet which is the opposite of what I think a lot of companies might do in a final year."
The Nurburgring is not like a test track you'd find at GM's Proving Grounds and there's no race track like it, either. It's really long and more like a public road running through rural foothills than a dedicated facility for racing or testing. In fact, at times, Nurburgring is open to the public. Anyone driving anything from motorcycles to tour busses can pay their 14 Euros and take a lap. Though the road was built in 1927, it's not in disrepair as are highways in infrastructure-challenged states in the U.S. like Michigan or my own People's Republik of Kalifornia. What it does have, as a result of the rolling countryside and highway engineering practice common early in the last century, is lots of undulating, uneven sections, many blind curves, frequent elevation changes, basicially...its the Gravity Games for cars.
"It's not a race track." Neal said, "It is a road-a country road on which you're allowed to drive flat out. I believe there is a thousand foot elevation change. There are 157 turns (other descriptions put it at 176 or 177). It's long-I've always heard 14 miles."
"Every corner...you're not just getting lateral acceleration. You're in a corner at max lat with a big heave in the middle. You crest the hill, the car really unloads and you wonder where your grip is. A moment later, it's in a huge compression. You are crushed into your seat at the base of a hill and, man, it sticks like crazy."
"You have to learn the track twice. Once in plain view...to know where the left turns are, where the right turns are, which ones are fast, which ones are slow. Then, you start bringing your speed up and, now, what you weren't noticing before at slower speeds were these gigantic vertical accelerations. There's four places at Nurburgring where a Corvette is airborne and you're well over 100 miles an hour. You gotta relearn it because, now that you're goin' faster, you gotta change the way you pick your lines to take in account the way the car is loaded or unloaded in corners."
"Nurburgring really pulls out your weaknesses. You're going to learn a lot of stuff, but we don't optimize our tuning for there only. We'd have way too stiff of a car for anything in the states and people will hate it, so you have to temper everything. We learn what we can, then we come back here (GM Proving Grounds and on-highway testing) and apply what we've learned there to development and tuning we do here. Then, we go back and see how the whole combination works at Nurburgring."
"We were over there three different times with the Z06. I was there in April, 2002. I had done tuning here. I went over there and did some. Then, I came back here and did even more. When I finished, I sent it back over there on the next trip-I didn't go, but Heinricy happened to be over there so I sent him sets of shocks. He loved this valving compared to anything else. I would say, we didn't just develop these shocks at Nurburgring. We used a combination of both places. In the end, we validated it over there on the last trip. John said it was perfect."
The cornerstone of shock absorber tuning is the "force/velocity" curve. It quantifies how much damping force is exerted for a given suspension velocity. Mike Neal made a significant change to the force/velocity curves of the '04 Z06.
A conventional hydraulic shock uses hydraulic fluid, forced thorough small orfices in the shock piston as it moves to provide damping. Flow through these orfices is contolled by disc valves located on the top and bottom of the piston. The way guys like Mike Neal "tune" the shock is by making changes to these "valve stacks".
"For most of C5," Neal said, "all I had to play with was disc thicknesss, diameters and the number of discs in each stack. There was a fixed pre-load for all those discs. The disc center is lower than the disc seat so the discs already have some bend to them. The amount of bend was fixed for most of C5. Then Sachs came out with an adjustable preload so I could shim that center up or down and put a different curve in those discs. I was able to take some preload out and make up for it by doing more things with disc thicknesses and so forth. Adjustable preload gives me more flexibility."
"That became available for '02. We made a rear shock valving change that year because I had gained that flexibitiiy where I'd been frustrated without it before. It was a quick improvment because I didn't really have a lot of time after it became available just before the '02 release. Since then, I've worked it more and came out with the '04 valving."
"The new shock valving results in less body movement and what movement still occurs is more poised and controlled. You want the body poised and the suspension doing the moving. Sometimes that means you need more damping, but sometimes it means you want less. In this case, we needed more in one area, less in another and more in a third."