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Chassis / Frame Design for the 1928 to 31 Model “A” Ford Street Rod – Part 4 

With the new steel at home in the shop it was time to formulate a plan for the construction of my new chassis.  I thought that I would start out easy and duplicate the shape of the front frame rails.  Using the original 1928 – 31 Ford Model A frame / chassis, I traced the pattern of the front frame horns onto a piece of scrap ¼” plexi-glass. This template was almost 3 feet long.  At the time, I felt it would be a good idea to have templates for every bracket and shape I designed for the chassis.  This will save me time if I would like to build another car using the same suspension concept.  The pattern was then cut out on a stationary scroll saw I have in the shop.  Having acquired a full wood shop over the years has a few advantages or benefits towards the build process of my street rod. 

1928 - 31 Model A Ford front frame / chassis template pattern to transfer to the 2" x 4" rectangular steel tubing

The template duplicating the front end of the original 1928 – 31 Ford Model A frame / chassis was then placed onto the end of each long section of the 2” x 4” rectangular tubing tracing out the pattern.  At the time I did not have my plasma cutter and I needed to figure out how to cut this shape out with some level of accuracy.  Using my angle grinder equipped with a very thin cut off wheel, I very carefully cut the shape out on the sides of the rectangular tubing.  This took a bit of time, but I had no other options for this task.  I used the cut off wheel to slowly score through the steel tubing with a final desired shape of the frame rail.  Once this was completed, I boxed in the openings on the top and bottom of the frame rails with 1/8” steel plate.  In the end, the finished corners edges had the same rounded contour shape as the original 2” x 4” steel rectangular tubing.

C4 Corvette rear suspension veiw # 1 for the 1928 - 31 Model A Ford frame / chassis. This is showing the riser or kick up of the frame rear section. A 4" kick up was perfect for my appliction. The rounded edge of the tubing allowed for a very heavy and deep weld that was ground even with the side edge of the frame for a very clean and even look.

It then came time to set up the two frame rails on the shop floor using axle stands and small blocks of wood for shims.  Both frame rails were levelled off using the laser level and balancing the diagonal corner measurements.  To help with this, I drilled small 1/8” holes in the center of the rectangular tubing at each end of the frame rails.  This became a constant point of reference to make sure everything was level and square.  Once this was completed, I used some scrap angle iron and welded several pieces across the width of the frame to keep everything in place.  My two most important measurements were the width of the frame along the centre line of the front axle and the width of the rear differential carrier mounts.

C4 Corvette rear suspension veiw # 2 for the 1928 - 31 Model A Ford frame / chassis. This is the initial view of the rear of the frame. I ended up making it slightly too long and later shorter the length of the frame so it. This change will allow the 1931 Ford coupe body to fit nicely over the frame / chassis rails.

The outside frame width at the center line point for the front axle center will be slightly wider than the stock 1928 – 31 Ford Model A frame / chassis.  The width will be the same as taken from the original C4 Corvette front cross member.  An important point to keep in mind, the design for the C4 Corvette suspension must not change.  Any changes to the suspension design will result in poor handling performance on the street rod or possible disastrous and dangerous operating conditions for the street rod and occupants.  You do not want to be on the 6 o’clock new cast. 

C4 Corvette rear suspension veiw # 3 for the 1928 - 31 Model A Ford frame / chassis. This view show the mounting for the C4 Covette stablizer rods or "dog bones". I used 4" square steel tubing cut to the shape in the picture. One side of the tube was cut off to make the tube into a channel. This give me the proper width for the stabilizer or control rods. The open end of the tube was welded to the side of the frame. A piece of 2" square tubing was used to created the final mounting bracket for the stabilizer rods. I used a metal cutting band saw for all of the metal cutting here.

After measuring the mounting distance for the C4 Corvette (1986 Corvette convertible) rear differential carrier, I decided on another deviation from the original 1928 – 31 Ford Model A chassis / frame.  The new mounting brackets for the differential carrier will be welded to the outside section of the frame rails.  I designed tear drop brackets and cut them out using 5/16” plate steel.  These brackets required a small notch to be removed from the body frame rails.  I also remove the rear floor panels and one of the rear cross braces on the body.  This will allow the 1931 Ford Model A coupe body to be easily mounted to the new chassis with the new chassis just fitting in between the body rails of the coupe body.  I will explain more on this later, in another section,  on how to strengthen the rear of the body to compensate this modification.  Using this type of chassis design, will not allow the use of a rumble seat.  Again, to make design process easier, I removed all floor and trunk panels on the 1931 Ford Model A coupe body.

C4 Corvette rear suspension veiw # 4 for the 1928 - 31 Model A Ford frame / chassis. This view shows a cross member tacked in place. It will provide a mounting location for the center differential carrier housing and add much needed strenght to the 4" kick up on the frame. It will act as a gusset for the kick up.

A few more measurements on the length of the chassis frame rails needed to be made.  I discovered that the rear of my new chassis was slightly too long.  The body would not drop over chassis frame rails.  The chassis was hitting the rear of the body frame.   I quickly shortened that rear of my new chassis making it just long enough to keep the frame nicely within the 1931 Ford coupes body  frame rails.

C4 Corvettes (1986 Corvette convertible) use an aluminum support beam that is fastened to the rear of the transmission and the front of the differential.  This would not look too good on a street rod.  Instead, I decided that two new cross members would be used on the chassis.  One cross member for the transmission and another cross member for the differential.  These two cross members would also increase the strength of the new chassis.  I used 2” x 3” rectangular steel tubing for these cross members.  I tack welded the cross member in place for the differential.  The addition of this cross member to the rear of the chassis will also form as a gusset to help strengthen the welds for the rear kick up.  The angles of the transmission and differential carrier are very important.  Both the transmission and differential cannot be mounted straight.  The transmission must be angled downward 3 degrees from horizontal and the pinion on the differential must angled upwards 3 degrees from horizontal.  This will avoid driveline vibration and help preserve your universal joints on the driveshaft.  I will include a section later on with more details on setting up the driveshaft, transmission, and differential.

The next step was to mount the upper and lower control arms to the side of the new chassis.  Now I had a bit of a problem.  The chassis was to narrow.  How would I make the chassis wider to allow the mounting of the rear control arms?  I did not want to use the original mounting brackets.  They were just too ugly and I did not have anything to attach them to.  After a bit of thought, I decided that if I used 4” square steel tubing with one side cut off, that I could mount the rear control arms tastefully to the side of the chassis.  A new set of control arm mounts were designed and constructed from 2” square tubing with the ends rounded and one side cut off.  These mounts will be welded to the 4” square tubing that was already tacked to the side of the frame rails.  Again I matched up the angle for the control arm mounts to the same angle found on the 1986 Corvette convertible donor car.

Please note:  I will be including complete measured drawings and templates for the various sections of this 1928 – 31 Model A Ford Hot Rod frame / chassis to purchase in the very near future. 

Come back next week to see how I created and started the design for adapting the C4 Corvette (1986 Corvette convertible) front suspension to the new 1928 – 31 Ford Model A chassis / frame.

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