Dear Guys, I nearly bought a '69 Chevelle convert a year ago, among the very small objections was the car had a funny little almost imperceptible shimmy at around 10 mph. after that it was pretty okay, then I bought a '69 convert and found that the car I now own has a similar annoying shimmy between 20 and 30 mph...at 65 she's rock solid, so I figured to ignore it, frame is straight, wheels and tire assembly were balanced before installed, about the alignment, I dunno, so then I see this show on American Hot Rodder and a guy is showing a '68 Camaro convertible with something he calls a "cocktail shaker" in the back, a cylinder with weights, shot I guess, and says that when you have the roof off a car it sometimes has strange vibrations...is this so? What the heck?, is that what I got?, is there a fix?...boy I was flabbergasted...anybody know about this phenomenon? thanks in advance, R. B.

Sounds like what we used to call "radial shake". Tires squirm a little at slow speed. Don't know if today's tires still do this, don't notice it any. Could be something like this.

tom3 echoes my first reaction. Had several radial tires with separated belts that exhibited the same characteristics. A slight side to side shimmy at certain speeds (typically slow). The only way I could tell it was the tire was by running it on a lift. The wobble was noticeable then. It's been my experience that it usually isn't the tire you think it is either. More often ends up being the opposite corner.

The "cocktail shakers" (AKA jiggle jugs) are a cannister filled with oil and a weight that is located as far rearward and outward as feasible. It's intended function is to counter act the torsional twisting in convertibles. The weight doesnt move as the body twists away from it, the oil dampens it and it eventually makes contact with the opposite end of the cannister, attempting to halt the movement of the body. Your car has a base bending frequency in the high teens or low twenties, that is to say that its base bending mode is about 20 mhz. A world class car today is around 35 mhz. 60's cars were designed as "high load path" cars with much of their overall rigidity in their roof. Convertibles typically have more material in there rocker section, frame and floor pan, but are never as rigid as a fixed roof car. Remember doors ad little or no strength to a car in terms of torsional twist. As your tires or the road surface or driveline vibrations "excite" the vehicle at or near its base bending frequency (your 20 mhz) the chassis will torsionally twist at that frequency. The jiggle jugs, although not entirely effective, reduce the modal bending. They were invented at GM's Noise and Vibration Labratory by a guy named Richard and I believe were first used on '67 Camaro drop top.
An easy way to tell if your shake is inherent torsional shake is that your cowl and windshield pillars will move side to side as the vehicle twists torsionally at the frame and floor pan. And it will be repeatable at a given speed on a given section of road. Individual cars can be better or worse than others of the same model, depending on cold spot welds, and various tolerence stack-up throughout the vehicle. Don't know if any of this is why your car shakes, but there's the story behind jiggle jugs. It would make sense that it would shake at low speed where road input is at a lower frequency, closer to your cars base frequency. And similarly would not shake at higher speeds where road input frequency is much higher and farther from your base bending frequency. Have a great day.

Dear Gary, thank you for taking as much time as you did with an obviously knowledgeable and scientific answer....if you or anybody else can just confirm one more thing, then if the engine rotates clockwise as the driver views it ( that may be wrong, I'm working from memory ) then the body wants to twist the opposite way so I could try to stabalize this car to some extent by putting a jiggle jug in the passenger side, back corner of the trunk...simple stuff like this often confuses me...is that correct based on what Gary said? Thanks again....R. B.