trivia question? C3 Ralley wheel bump??

[dick foehringer]

ok, while garnet blasting my 69 ralley wheels i noticed a bump about the size of a BB on the face of the wheel right on the outside rim. i at first thought it was a piece of debris but it wouldnt blast off. so then i found the same bump on all of the wheels. so my guess is that it was the welding machine testing its setting?? its not in the same place on each wheel but is on every wheel.
OK gurus, why is it there??
regards dick

[Terry M.]

[Jack H.]

Add-on

Also, not all rally wheels had the 'bump' indicating the low point.... Some, by random chance happened to be 'perfect' with respect to being in round when they left the supplier's line! And, the trick of having the tire mfgr mark the sidewall of the tire with a dot of color to indicate its high point is still in use today. Eyeball some new cars at the dealership and you'll ocassionally find a color dot on the sidewall of tires here/there....

Bottom line, both the wheel mfgr and tire supplier had to spin/test their products, so marking hi/low spots was essentially free and GM got the benefit of using less balancing weights/wheel as a result.

[John H.]

Re: Add-on

That was (and still is) the industry's solution to what Chevrolet called "Smooth Road Shake" back in those days - a wheel and tire that balances with no problem, but has radial runout or radial force variation that causes suspension disturbance at freeway speed in spite of behaving perfectly on a dynamic balancer. In the aftermarket, this gave rise to the Hunter GSP9700 machine which not only balances dynamically, but diagnoses both lateral and radial runout and radial force variation in an otherwise properly-balanced wheel and tire assembly.