C3 Frame Stiffening

[Brad T.]

I am doing a frame off restoration on my '69 coupe. The frame is blasted and epoxy primed at this point.

Are there any recommendations for adding additional gussets, welding or other methods to stiffen my frame?

[Dick W.]

Re: C3 Frame Stiffening

Unless you are dealing with a seriously compromised frame or plan on racing it, you will gain very little by stiffening the frame.

[Paul D.]

Re: C3 Frame Stiffening

Brad,
In our segment of the Corvette hobby at least, I'm afraid extra welds and gussets would send up a red flag to judges and probably even the casual observer. However, if you have other plans for the car (racing), the older Chevrolet Power books (late 80's) had a section in it that advised how to do it. Unless you need to stay within a sanctioning body's rule set to be "period correct", better performance can probably be achieved with one of the newer updated chassis that utilize C5/C6 componets. Chip.

[Duke W.]

Re: C3 Frame Stiffening

GM already did this for you. In the mid-sixties they added a triangular gusset between the side rails and the no. 4 cross member, and I believe it was '69 that a stiffener was added to the side rails at the kickup.

As others have said, for normal road use no additional stiffening is needed - just a good inspection and treatment of any corrosion. The kickup region is the most highly stressed part of the frame, and it also seems to be the area that is most subject to corrosion damage.

Duke

[David H.]

Re: C3 Frame Stiffening

Here is a link to some information:

http://corvette.wikia.com/wiki/Corvette_Wiki

There are several articles under "Suspension" that may be of interest to you. (Dick Guldstrand article)

The aforementioned "Chevy Powerbook" link (8 megs) is under "Misc Overall" about midway down the page.

[Bill M.]

Re: C3 Frame Stiffening

In Star Spangled Sports Car there is a paragraph (in the 1963 chapter, p.141 in my copy) where a GM engineer relates that they tried a stiffer frame for the '63 and the ride got much worse.

[Michael F.]

Re: C3 Frame Stiffening

It's already built like a truck frame. You can tow a 20ft boat with it. Much stronger than a 3000lb car would warrant.

Dicks suggestion hit the nail on the head.

Some compliance is needed.

[Duke W.]

Re: C3 Frame Stiffening

In Star Spangled Sports Car there is a paragraph (in the 1963 chapter, p.141 in my copy) where a GM engineer relates that they tried a stiffer frame for the '63 and the ride got much worse.

I recall reading that and found it odd.

Nowadays engineers want as stiff a frame as possible within their weight budgets. Basic stiffness is specified as the first mode bending or torsional vibration frequency, the higher the better, and 25 Hertz is considered a good target and is in the ball park of what was achieved with the C5 design. The C4 was considerably less stiff - maybe only about 15 Hertz as I recall.

Theoretically an infinitely stiff frame would be best, so all compliance is in the suspension. Stiff frames can get away with lower ride frequencies (which is a function of spring stiffness and unsprung weight) to achieve a good ride without compromising handling.

The C2 frame is considerably stiffer than the C1, and in particular, the '63 Coupe is the stiffest of all because the solidly mounted body makes the birdcage and integral part of the frame. The C2 base suspension ride frequencies are also lower than the C1.

It is probably a matter that as the years passed engineers gained a better understandiing of the relationship between frame stiffness and ride/handling, and today's philosophly is nearly the polar opposite of 50 years ago.

BTW, the C5 SAE papers in the SAE Corvette paper book provide excellent explanations of why frame stiffness is important, why the C5 target was 25 Hertz, and how it was acheived.

Duke

[Clem Z.]

Re: C3 Frame Stiffening

most frames are "stitch" welded so if you do a total weld you will stiffen up the frames. when we had to use stock frames for race car we always total weld all the seams

[Dick W.]

Re: C3 Frame Stiffening

You stiffen a C-3 frame and then you have to work on the suspension to match it to the lack of frame deflection. I reinforced a Jeep frame one time and found out (the hard way) it was designed to flex. I would almost get stuck in a parking lot.

[Michael M.]

Re: C3 Frame Stiffening

It really depends on what you are going to use your car for. Judging purposes just leave the frame alone. If you are going to race the car you MUST strenghten your frame. Boxing your front frame rails, continuously welding the frame where the factory skipped welded, adding gussets, and even adding a roll cage are all beneficial when you race. When a customer tells me he wants a frame for racing the first thing I do is to continuously weld the seams. Before welding the seams if you take a hammer and hit the frame it sounds like ting, ting, ting. After welding the seams and you take a hammer and hit the frame the sound would change to clunk, clunk, clunk.