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Top Bow
Laminates
While working on disassembling the top of my 1914 Runabout, the top bows, like most parts of a 92 year old car, needed to be repaired or replaced. Of the three top bows, one had retained the original shape (it seemed to be in the best shape), however, it was filled with nails, staples, cracked some, and the grain was eroded from weather or wear. I checked with the dealer catalogs for top bows and the cost was $80 for steam bent bows and $270 for the laminated bow, plus or minus some. So I figured I could save some money by building them myself. The two methods of bending wood is by steaming it or by laminating it. I know there is some "spring back" in steam bent bows, so the bending jig would have to be designed to compensate for that percentage, and drying time would be a factor in controlling spring back. I didn’t have the experience with steam bending but I have done some wood lamination. The spring back in wood lamination in negligible, so I decided to use this method. Laminated wood will not try to straighten and is exceptionally strong. Of course, Henry’s steam bent bows has a proven track record, but as I said, I am not too familiar with this method. The type of adhesive I used was "Titebond III made by Franklin International. It is water proof and working time is about 7 or 8 minutes, which gives you time to clamp up the laminates if you move fast. I generally apply the glue a little heavy to extend the working time. I used oak wood for the laminates. The oak came from a tree I cut down during the construction of our house. I air-dried it for approximately 12 months until the moisture content was about 7 or 8%. I tested the wood for strength by gluing a couple of pieces of oak together and tried to break it along the glue line. Since it held together, I thought is was ok. While the top irons were on the car and the old bows removed, I measured the center to center distance from top bow socket to top bow socket and it was 51 inches. That gave me the basic dimensions of the bending jig. The dimension from opening to opening on the bending jig (the greatest inside dimension) worked out to be 52 1 /2". That is 51" plus half the thickness of the finished bow added on each end, 51" + 3 /4" and 3 /4" = 52 1 /2".
Designing the bending jig using the best of the original 3 top bows. Original bow dimensions: 1" thick, 1 1/ 2" wide, inside radius 5 1/ 8", outside radius 6 1/ 8"(based on the original bow). Center to center dimension:51" (to match the center to center top irons dimension). Jig will have a slight crown to duplicate original bows. Lay out full size drawing on paper first to work out any design problems.
Bending jig: made of two 3 /4" pieces of plywood wood screwed together. Finished dimensions were 1 1/ 2" thick, 21" wide, 64" long. Top section is 6 1/ 2" wide. End sections are 6 1/ 2" wide.
Eight laminates per bow: 1/ 8" thick X 1 5/ 8" wide (trim to 1 1/ 2" later after glue is dried) X 76". I cut these on the table saw. Short pieces on bending jig (16 - 3/ 8" X 3 / 4" X 6") are used to keep laminates from slipping around after glue and clamps are applied. The half-round pieces are used in the radii of the jig to apply pressure to the laminates.
Applying glue to laminates. Make sure you have lots of glue and paper towels on hand. I applied glue to one side of the laminate only. Have all clamps ready to go. It is a good idea to clamp the laminates together dry first to see what kind of clamping problems you will have. Things never go perfectly smooth when gluing laminates up, but generally it will work out.
Beginning to glue laminates together. Curved blocks of wood on radius of jig eases the radius of laminates into corner of jig. Tighten clamps slow to allow the wood fibers to begin to compress. Note: distance from end of jig to beginning of radius is 7 7/ 16" (the tangent point)
Approximately 20 clamps. Begin to clamp at crown of jig and work towards the ends of laminates. I had to borrow some clamps to make sure I had enough. There should be no gaps between the laminates.
Use as many clamps as possible in the radii of the jig to close all gaps. A "pad" of wood is used on the inner side of the jig at the crown section and near the straight ends to distribute the pressure of the clamps.
After tree days, release the clamps. Spring back should be about 1% or less. You could release the clamps after one day, according to the directions on the glue bottle, but I wanted to make sure the glue was completely dry. There is some stress in the wood laminates as you bend them and the laminates try to straighten out.
Trimming one end of the bow to remove excess glue using the table saw. Note the extended fence on the table saw to keep the cut square (approximately 12" to 14" high and as long as the table top).
Trim about 1/16" off each side so the finished thickness is one and one half inches. Sand bow with 150g and then 220g in preparation for routing and finishing.
Run a 1/ 4" radius router bit along the edges of the bow for a finished look. Begin to fit the ends of the bows to the sockets of the top irons. After all bows fit the top irons, apply a finish to the wood. After fitting and finishing, secure the bows in the sockets with #6 X 1" woodscrews. I followed the height dimensions of the original bow to get the over-all height of the finished product.
Begin to install restraining straps and other hardware.
Retraining straps in place and bows adjusted to proper locations. Ready for upholster.
This is the finish product. The body is now ready to drop on the chassis. |
