My name is Dale. I am a expert in the Classic Car Category. You may check my credentials if needed. I will try to help you diagnose the condition.
Slow Speed Shimmy on acceleration means on moderate acceleration usually means that when the vehicle is in what called the neutural load status for the suspension that the right front and left front tires are not staying paralell with each other either vertically or horizontially and/or a combination of both. You are on the right track in your attempt to find the problem. It would be so much easier if I was right there but will try to find it anyway.
I am trying to remember if there is a drag link on the Model A. I think there is. It would be connected to the tie rod between that and the steering gear. A few questions? How are the shocks? The Model A has a transveres spring set up. Are the bushing at the end of the spings OK ? Is the center support tight and centered properly? Have you checked the "TOE IN " ? That would be the distance at the center of the front tires side to side in the middle "horizontal mesuarement" to the distance side to side at the rear of the tires. It should be toed in at least 1/8 to 1/4 of an inch. It is difficult to explain how to check it but here it goes. Have a friend hold a tape measure exactly at the center of one of the front tires and pull the tape "to the other side" at the same spot in the center of the tire. write down the measurement. Do the same thing at the rear of the front tires and take down the measurement. subtract the last measurement from the first and that should be 1/8-1/4 " . In other words the measurement in the front should be shorter than the back measurement. If not it must be adjusted with the tie rod.
Let me know if this helps and if you need further explanation. Good Luck with your "A"
Thanks for the reply, unfortunatley all your answers have been checked, the toe in is 6mm so it fits within your 1/4 to 1/8 guide. The shocks are modern, the springs are tight and all checked by two seperate model A drivers. So as you can see I am at my wits end trying to find the problem. The steering is really good with vertually no play at all!
Did I miss something or is the shimmy in the front end of the vehicle. Is it possible that the shimmy is coming from the drive train. ie: engine/transmission/differential ?
I had a similar experience with a Model A when I hit a bump. I found the front spring was 'sticking'. After I gave it a good lube between the leaves, no more problem.
If the spring perches are in reverse this will make the car very hard to control in a slalom pattern. This will cause a very unstable vehicle after having redone the kingpins. I did not know that the perches were installed wrong by someone long ago.
Regarding the perches, they are specific to each side. When you look at them, you will see a flat, horizontal spot with a hole or dot cut into it. That dot must be toward the back of the axle to keep the caster correct. Also, before dismantling the front end, look at the spring shackles and see if they are worn, evidenced by a gap in the bushing area where they pass through the perches and springs. If there's a gap, replace the bushings and shackles together. So, a quick rundown of our suggestions: Check Kingpins and bushings Check tie rod ends Check correct installation of perches Check perch and spring bushings/shackles Check bearings in front wheels (you never know!) Check ball end of wishbone radius rod Check steering Check tires for pressure Check toe in/out