What is the Guldstrand Mod?
Sometimes called the G-Mod or GS-Mod, Guldstrand Mod offers better cornering performance with no other changes to the stock suspension. The Guldstrand Mod is the process of relocating the stock upper control arm pivot by less than an inch, a modification that requires drilling four holes and very little else. It works for 1967-69 Camaro and Firebird, as well as 1968-74 Chevy II/Nova models.
The procedure is named for Dick Guldstrand, "Mr Corvette," who was involved with developing, engineering, and racing Chevrolet models throughout his career. His simple front suspension modification was a huge improvement to handling and helped Camaro and Firebird dominate the Trans Am racing series.
What does the Guldstrand Mod do?
In the general sense, the Guldstrand Mod offers better cornering performance.
Technically speaking, the Guldstrand Mod maximizes the tire contact patch under cornering by moving the upper control arm pivot points down 3/4", raising the roll center and improving the camber curve. It also makes the steering more responsive because it relocates the spindle 1/4" towards the rear, causing positive caster.
Where to start?
Guldstrand still offers templates, which can also be found all over the internet. Many pro-touring forums have several discussions on the topic.
Many suspension kits for 1967-69 Camaro and Firebird, as well as 1968 to 1974 Nova, incorporate the Guldstrand Mod into the steering geometry.
This 2006 article from Popular Hot Rodding has more on the Guldstrand Mod.
Why didn't Chevrolet use this geometry from the beginning?
Automotive engineers in those days deliberately set up suspensions so that when traction was lost, the vehicle would tend to understeer, rather than oversteer. This was done in the name of safety.
Understeer means that the car does not turn enough when you turn the wheel. Oversteer means that the car turns too much.
Understeer can be corrected by hitting the brakes, which is what most people do automatically in emergencies.
Oversteer must be managed with careful steering and throttle control, which requires experience with driving at the limit of traction that most drivers lack.
Chevrolet wasn't the only manufacturer promoting understeer. In fact, classic Ford enthusiasts looking to improve handling have a Guldstrand Mod of their own, called the "Shelby Drop."
Where can I find more information?
If you're not sure what suspension setup is right for you, or if you'd like to learn about roll center and camber curves, check out the book How to Make Your Muscle Car Handle by Mark Savitske. How to Make Your Muscle Car Handle defines, explores, and reviews the complete suspension system, and teaches you how to best choose your upgrades for maximum effectiveness.
There is a lot of room for improvement in the classic designs, whether they are front suspensions relying on coil springs or torsion bars, or rear suspensions built upon leaf springs or wound coils. The essential information is here, backed up with geometry and artwork to convey the message clearly.