Shimmy after 55 mph
 

hi guys
Have a bad Shimmy after 55 mph. Tie rod ends are good . Drag link adjusted.
Replaced steering box. Switched front wheels and tires front to back. Checked king pins , tight. Any ideas?

Re: Shimmy after 55 mph
 

Wish bone. Drive slower.

Re: Shimmy after 55 mph
 

I have never driven any of my A's 55 mph so it would not be a concern to me. Out of curiosity why would you drive an A over 55 mph? Wayne

Re: Shimmy after 55 mph
 

Even 1/8" side run out in a wheel, could possibly cause a shaking.
Bill W.

Re: Shimmy after 55 mph
 

Pitman arm very tight on gearbox shaft? Wheel bearings? Toe in?

Re: Shimmy after 55 mph
 

A front wheel will not shimmy even if everything is loose if the caster is within spec. Yes, you can tighten everything up and stop it or add a hydraulic damper, but that is actually just another 'bandaid' way of addressing the problem!

Think of a shopping cart. When the front wheels get slightly bent back from slamming curbs and parking lot divots, you get a cart with a front wheel that goes into the dreaded "death wobble".

A loose pitmann or anything else in the steering linkage should have no effect on initiating shimmy. A Model A actually tows quite nicely on all fours with the drag link disconnected. Absolutely no shimmy if the caster is correct. "Toe" should not have any effect either. If changing it initiates or stops shimmy that is a clear sign the caster is wrong.

A bent axle, cammed out king bushings, bent forgings at either end of the wishbone, engine misalignment (the wishbone bolts to the fly cover casting at the back) or frame twist and bend are the primary culprits in throwing the caster out of spec in an "A". Simple visual inspection likely will not reveal a bend of only a degree or two. You need to carefully measure.

80 years of the 'ol gal pounding pot holes and RR tracks bends both the axle, shackles, and wishbone in the negative direction. Reducing caster just one degree from the factory spec positive 2 degrees will make wheels want to shimmy.

There is a lot of info out there about how to straighten an A axle, but not much about the wishbone. The front forgings that hold the kingpin actually bend rather easily compared to the axle. Perhaps someone has the Ford print for the wishbone and can throw in a few angles and dimensions?

http://www.vrperformance.com/mt/dia%20c.jpg

Re: Shimmy after 55 mph
 

Thanks for your quick response .

Re: Shimmy after 55 mph
 

Shimmy at speeds above 55 mph is characteristic of unbalanced tires, whether they be on the front or rear. On the fronts you feel it more in the steering wheel; on the rears you feel it more in the seat. Depending on how many tires are unbalanced and how they are distributed, switching them around may or may not make a difference.

Re: Shimmy after 55 mph
 

my recommendation is have the tires balanced.

Re: Shimmy after 55 mph
 

MikeK's comments are well taken and are descriptive of what should be to be correct. However, one can have a variety of things out of spec and still have a car that handles well, perhaps because the errant geometries cancel each other out.

My current front end has 1940 Ford spindles & brakes, a kinked wishbone, +9 degrees of camber on the passenger side, and a bent axle, yet it steers and drives well at all speeds (!?). I plan to replace it with a stock Model A axle that I am re-building from parts acquired at swaps and from Brattons. All the dimensional stars are coming into alignment but I still worry that my new perfect front end may not perform as well as the tweaked old one, kind of like the fully rebuilt engine that has a knock or oil leak or shakes like crazy.

Re: Shimmy after 55 mph
 

It's definitely about caster,camber and toe in if everything is tight.

Re: Shimmy after 55 mph
 

My coupe would barely do 55mph, but it got a death wobble riding thru a bumpy parking lot. After much fooling around checking this and fixing that, I settled on the steering box. I rebuilt and it was fine....... for a while! Then again I drove thru that same parking lot and the wobble was back, It was much less, my other work had lessened it considerably, but it was still there. Again, back underneath! This time I found something I had missed. The spring bushings and and spring perches were worn out and there was lots of slop in a joint that should have none. I corrected it and the wobble was gone.
Terry