When I got my 2013 Corvette ZR1, I noticed it thumped very noticeably when making tight turns, for example, in my driveway or in a parking lot.
Long ago, I experienced thumping or binding from the back of my ’67 Corvette and learned that it came from the rear of the car and was a symptom of binding or sticking of the Positraction clutches in the differential. The fix was a special oil additive for the rear axle lubricant. I had a similar problem with a brand new 2008 Corvette and the special lube fixed that one, too.
Club member Steve Arnold experienced this thumping or binding in his ’68 Corvette. You can read his article about the project here: 1968 Chevy 327 Rear Differential Rebuild Information.
He tried the special posi lube but it didn’t help.
He ended up having the differential rebuilt to replace the posi clutches to finally resolve the problem. The posi clutch plates are embossed with a dimpled pattern. Apparently. the dimples hold oil to lubricate the clutch plates when they slip as you go around turns. If the dimples are worn off, the clutches don’t hold oil between the plates and don’t slip smoothly.
When I bought a C6 ZR1 Corvette, I soon experienced a different sort of thumping. From the front, not the back. When turning sharply after backing out of my garage, the steering feels like it’s binding up. With loud and strong bump-bump-bump maybe every six feet. I wondered if it was from the wide, sticky tires. I eventually discovered the cause, but didn’t really understand it until now. I think it was former Corvette Chief Engineer, Tadge Juechter, who said something about reducing or eliminating the Ackermann Effect from the Corvette steering geometry for increased cornering ability at speed.
What is the Ackermann Effect? When a car goes around a tight turn, the inside wheel goes through a smaller radius than the outside wheel – due to the width of the car. The spindles and the steering rack can be set up to accommodate this – it’s been done since horse and wagon days.
So why would you want to eliminate that helpful feature? You wouldn’t, if all turns were slow and tight. But we like to drive faster and on much larger radius, sweeping turns. And with modern tires, we can corner at higher G’s. That means more weight transfer to the outside wheels. With more weight on the outside front tire, more cornering force can be generated. However, to maximize the outside front tire cornering force, with more weight shifted onto it, the outside front tire must be steered to a sharper angle.
The result is just the opposite of what Ackermann Effect causes. So, steering linkage is revised to eliminate the normal Ackermann Effect and, in the highest G-capable cars, negative Ackermann Effect provides greater cornering capability.
This article provides a clear explanation of this in more detail: Ackermann Steering Geometry
Two of the photos in the above article show an open wheel, Formula 1 race car in a turn and you can clearly see that the outside wheel is steered inwards far more than the inside wheel!
So, the price we pay for our Corvettes’ aggressive cornering capability is thumping in tight driveway and parking lot maneuvering.
Some say the C7 tight-turn thumping was worse than in the C6 ZR1’s. I have both generations, with the same Michelin Super Sport tires and really haven’t noticed a difference – they both thump and chatter. Some say the C8, with its Michelin 4S tires, is less bothered than the C6 or C7. Maybe the C8 can use a more conservative steering geometry and still meet Chevy’s performance goals. And, I wonder if that chatter/thumping comes back in the C8 Z06 and C8 ZR1 fitted with the optional, stickier Michelin Cup 2R tires.
Your observations?
