Dear readers: I recently had the honor of having a wonderful interview by Kevin Tetz, co-host of the TRUCKS! TV program. Kevin is doing a number of podcasts for The Eastwood Company as part of the Shop Talk series, and I was fortunate to be one of the first people interviewed. Kevin had some great questions and comments, and in the 50-minute interview, I talk about how I first got interested in cars, how I learned about metal shaping, who my mentors were, how I started my business, what it was like to have one of my cars win the AMBR award at the Grand National Roadster Show, how I developed my workshop and DVD series, and toward the end of the interview I discussed my current personal project, which has a completely hand-formed aluminum body and scratch-built chassis. You can listen to this interview by going to my website: www.covell.biz, and clicking on the player there. It's a great way to get some background on the life and times of Professor Hammer.
Q. I have a 1957 Chevy pickup that I have installed coilovers and a four-bar setup on. I was thinking of using 4-inch OD .250 wall tubing to form the notch, along with boxing plates. Will this be OK, or is there another approach? The upper coilover mount is right behind the rearend, on the outside of the frame.
Via the Internet
A.There are several things to consider when C-notching a frame. Most people don't realize that when you cut a notch in a piece of tubing, you decrease the bending strength tremendously. You might think that if you take a piece of 2x4-inch tubing and cut a 2-inch notch in it, it would still have half the bending strength left. In fact, a notch this size would reduce the bending strength by 80 percent! I know that most trucks have a frame made from channel, but since the frame is generally boxed, at least in the area of the C-notch, my comments will address a tubing structure. While filling the notch with heavy steel and using fish plates will help to a degree, even if you used ¼-inch plate for the reinforcements, you would still be giving up more than 60 percent of the bending strength with a 2-inch notch in 4-inch tubing! Trucks are tremendously overbuilt, since the manufacturers know that people do all kinds of crazy things with them. Most classic truck builds will never see anywhere near the loads they were originally designed to carry. Nevertheless, it is prudent to think about the loads the frame has to deal with. Most of the truck's weight is supported by the springs, and different spring configurations load the frame differently. Leaf springs feed their loads into the frame at two points, usually a couple of feet ahead of and behind the rear axle. With leaf springs, the frame is much more likely to flex when C-notched, since the widely spaced spring mounts have a tremendous amount of "leverage" on the notched section.
Since you are using coilover shocks, the loads are being fed into the frame much closer to the notch, and the bending loads on the chassis are far smaller. This means that your suspension design can tolerate a larger notch than a truck with the stock leaf springs.
It's hard to tell you the exact amount that it's safe to C-notch your frame, since there are so many variables involved. Still, I'd encourage you to take a cautious approach. The potential problem is that when a notched tube does bend, the bending forces are concentrated in the center of the notch, and if it flexes too much for too many cycles, the steel can fracture! If you start looking at trucks like yours with frames that have been C-notched, and talk with some of the owners, you will start to get a feel for how large a notch is acceptable.
You can email your questions to Professor Hammer at firstname.lastname@example.org
, or mail to Professor Hammer, c/o CLASSIC TRUCKS Magazine, 1733 Alton Pkwy., Irvine, CA 92606. You'll receive a personal reply! We'll print your name and city unless you request otherwise. Ron Covell has made many DVDs on metalworking processes, and he offers an ongoing series of workshops across the nation covering all aspects of metalworking. Check them out online at www.covell.biz
or call for a current schedule of workshops and their free catalog of DVDs. Phone 800-747-4631, or 831-768-0705. You can send a request by mail to: Covell Creative Metalworking, 106 Airport Blvd. #105, Freedom, CA 95019.