You can email your questions to Professor Hammer at covell@cruzio.com, or mail to Professor Hammer, c/o CLASSIC TRUCKS Magazine, 774 S. Placentia Ave., Placentia, CA 92870. We'll print your name and city unless you request otherwise. Ron Covell has made many DVDs on metalworking processes, and 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 and fine-quality metalworking tools. 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.
A rare bumper is shown at...
A rare bumper is shown at the rear, with a reproduction-in-the making in the foreground. Read below to learn how the ends can be shaped.
Q.
Hi Ron, we've got a bunch of your DVDs and have learned a lot! We are presently restoring a rare old vehicle. It has had a lot of metalwork and we're really happy with how things have turned out. Haven't had any luck finding bumpers that aren't destroyed.
There is a local metal rolling facility we have used at work, and they are able to roll 0.140-inch-thick steel with a similar crown, and the side-to-side arc. The problem is that at the ends of the bumpers the arc tightens up, and they can't accommodate that on their rolling equipment.
Do you have any suggestions for shaping the new bumper ends to simulate the original ones?
Sheldon
Via the Internet
A.
Steel over 1/8-inch thick is pretty hard to work with-it takes a pretty hefty hammer blow just to make the stuff move. If I had to shape the bumper ends, I'd probably heat the metal to red heat with an oxyacetylene flame, and then it would respond to hammering with reasonable force-this is how blacksmiths work with heavy stock.
To create the shape you need, you can either stretch the center, or shrink the edges, or both. Stretching is easier, and this can be done by hammering on-dolly. I'd probably use an anvil to hammer against (or a plate of steel 1/2-inch thick or more), and you'd hammer on the inside surface of the bumper against the anvil. As you stretch the center of the strip, you'll probably see waves or ruffles form on the edge. When you flatten these ruffles out (at red heat), you will actually be shrinking the metal a little. Working in this way, you should be able to duplicate the shape of the stock bumper ends. A little sanding or filing should let you regain the smoothness on the outside of the bumper.
Q.
I'm an avid reader of ClassicTrucks and have subscribed for years. I have a '52 Dodge pickup, and I want to do a frame notch to install a 9-inch Ford rearend. I'm using 3/16-inch steel plate for boxing the frame, and 1/4-inch steel for the notch. How big of a notch can I put in the frame? I was thinking of cutting the notch 2 1/2 inches. One-quarter inch of the notch would be filled by the notch material, leaving me with a 2 1/4-inch notch. Can I make the notch bigger?
Dave
Via the Internet
A.
Hey Dave-glad you enjoy the magazine! Your question is good, but a little hard to answer directly.
The bending strength of a frame is governed by the smallest section. Obviously, when you notch a frame, the notched area creates a section smaller than the original framerail. You'd like to think that if you cut a notch out and reinforce the notch with really thick metal, it will put a lot of strength back into the frame, but the percentage of strength you put back in this way is deceiving.
In rough numbers, the bending strength of any beam is proportional to the cube (third power) of the height. To use round numbers, if your stock frame was 4 inches tall, and you cut a 2-inch notch out of it, you are not reducing the bending strength by half, you are reducing it by about 80 percent, and using heavy plate to fill in the notched area doesn't really give you that much more strength!
To further complicate matters, the biggest forces that are put on the rear of the frame come from the springs attached to the rear axle. Some trucks, like early Fords, have a transverse spring, which attaches to a crossmember that is just a few inches behind the area that would be notched. Your truck has two longitudinal springs, with one spring attachment point in front of the axle, and one roughly 2 feet behind the axle. The closer the load is applied to the notch, the better the notch is able to withstand the bending force. Since the stock spring attaches quite some distance behind the notch, it puts a pretty heavy bending force on the notched area. If you are planning to use coilover shocks, which will attach either to the crossmember or to the framerail sides within a few inches of the notch, the bending forces are a great deal less.
Now, we've all seen frames notched even more than what you are proposing, but just because it has been done doesn't mean that problems can't happen. A further complication is that when a frame is notched, all the bending forces tend to concentrate right at the notch, and after many thousands of cycles, if the metal flexes too much, it can fatigue, and possibly cause a crack!
I'm not a structural engineer, and it's beyond the scope of this column to give specific advice on mechanical design issues, but I do want to make you aware of some of the issues at stake with this kind of modification. Again, if you use coilover shocks instead of the stock leaf springs, the loads on the notched frame will be a LOT less! Another option is to kick up the rear framerails so no notch is needed. Of course, this will raise the bed, too, unless you rework the floor. Now we are getting into a "ripple" effect where one modification can lead to another, but I want all our readers to give safety the highest consideration when building their classic trucks!