The Death Wobble Thing, Brakes

D

DHZIEMAN

Senior Member
Death wobble on my Model A is started by applying the brakes. One front wheel seems to brake in spurts. I have to replace the steering sector bearing at a minimum, also, but at this time, I am concentrating on front brakes that start the death wobble and there really is no sense putting it back together as it is.

What is known, I need the brake shoes relined and have taken the drums to my local machine shop to get them cleaned up for more than rivet grooves. Sure enough, the Right drum was out of round. (braking in spurts) To clean up previous rivet tracks and get the drums surface round again took about .028, which is not much. Slightly under 1/32 inch!

Here is the question……….I got to watch the local guy turn the drums. They are not cast iron, but steel. The tip off was how the drum cutter trimmed off a long running piece of steel ribbon, kind of like drilling steel and getting the same long ribbon. The manager of the local shop did the turning at no charge. Pretty neat guy!

Here is the question.... Can steel drums be turned .028 and be OK to use or, was the out of round a tip off of not enough drum strength to start with?

I am of the opinion, these drums have not been turned before as the brake shoes seem to fit fully, and are not shimmed in any way.

Opinions and experience on this welcome!
 
Steel drums should not be turned that much. That said, many have been.

A side note, your 'death wobble' is not caused by your brakes. There is something loose in the front end. But, I think you knew that.
 
I do model a brakes for friends in my north east texas area. Done lots of them. My rule is if the steel drums measure .120 or less in thickness. i will not turn the drums again - for that matter wont even work on the brakes short of starting over wiht new drums. Original steel drums were .140 to .144 in thickness. Problem is less than .120 thicknesss they heat up VERY quickly and as they heat up, expand, then fade, then warp again.


Check the thickness at four places at 90 degree intervals.
 
Patrick L. & kenparker- Thank you both. Based on your comments,I will do the measure at 4 points yet. Something pointed out is these drums warp and when they get thinner they warp more and I have already got one drum warped and turned to clean it found, and I believe, based on your comments and one local fellow here, these two steel drums if reinstalled will continue to warp and now that they are a bit thinner may warp more. These two steel drums I believe will become trot line weights, and I will bite the bullet and do cast iron while the car is on jacks. (and then work on the sector shaft bearing and any other front end issues) Thank you both!
 
A good starting place to get the death wobble under control is to have someone sit behind the steering wheel and move it left and right just enough to bump it back and forth to see where the slop is in the steering linkages etc...
Have someone do this while you are under the car looking things over. Start at the steering box and move on to the drag link and then to the tie rod. Also don't forget to examine the assembly at the end of the wish bone where it attaches to the flywheel housing. Remove all of the slop by tightening things up and then set the toe in. Good odds that once you have all the slop out and the toe in set then you have a good chance of getting rid of the death wobble. It it doesn't go away after that jack up the front wheels and check the front wheel bearings to see that they are adjusted right and check for looseness in the king pins.
 
I've posted this before and the question to me with "death wobble" is do you want to fix the symptom or do you want to fix the root problem? Ever wonder why one Model A with everything worn and loose in the front end does not wobble, yet another A with one little thing slightly loose shimmies like crazy?

To fix the symptom just make sure everything in the front end is as tight as possible. Then keep your fingers crossed. You are only holding tight a wheel that wants to shimmy.

To fix the actual problem so the wheels do not want to shimmy in the first place there is really no mystery. If it shimmies something is usually BENT affecting caster. It may be only on one side or both. Tightening everything up to new spec certainly improves steering and tracking but does not address geometry errors that induce resonance like shimmy. If you have a bent axle, 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 you do not have the factory spec caster. Simple visual inspection likely will not reveal a bend of only a degree or two. You need to carefully measure.

Think of a supermarket 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".

80 years of pounding pot holes and RR tracks bends both the axle, shackles, wishbone, frame front cross-member, and spring "U" bolts in the negative direction. Reducing caster just one degree from the factory spec 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? Same goes for the spindles the wheel bearings ride on. They get bent too and that affects everything!
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Mike K. - First off I just enjoy this sentiment you put on your posts Mechanical engineering 101!

And I appreciate your information on all the things I need to get into.

So far, I know I have a very loose worn steering sector bearing. I have had my wife turn the steering wheel and have quite a bit of slop movement at the steering sector bearing. All other front end joints and king pins are solid. And the steering worm gear turns as the wheel is turned, just lost movement with the slop of the bad sector bearing. The ball joint at the Transmission has been fixed as best I can with a kit. So far, that leaves the sector bearing and all the more techie stuff you are presenting. Thank You!
 
If'n the rest of the front end is up to snuff, fixing the sector shaft bearing won't fix the shimmy.
Don't forget to check the spring mounting and shackles.
 
I've dealt with enough Death Wobble to know something about it. It can be caused by almost anything loose in the front end. One partial cause was the spring shackle being worn out. It later turned out I had to completely rebuild the steering box too. On another car, it was the flattened out steering balls and a loose pitman arm. One thing I learned, it can be more than one cause. Get everything fixed up that you think may be the cause, take the car out and the wobble is still there, Frustrating! Maybe not quite as easy to initiate, but it's still there! Some times it's as easy to diagnose as being loose wheel bearings. But this cannot be diagnosed at a distance. There's too many possible causes. Then I've heard too, that sometimes a simple alignment can cure it. Good Luck!
Terry
 
DHZIEMAN said:
. . . So far, that leaves the sector bearing and all the more techie stuff you are presenting. Thank You!
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DHZIEMAN, Model A's tow quite nicely with the drag link disconnected, proving that nothing 'loose' in the steering linkage is related to the root cause of shimmy. Again, that is only a fix the symptom, not the cause item.

If I were you, before I drove myself nuts chasing everything down the 'tighten everything up' rabbit hole, I'd get some real diagnostic numbers from a Hunter Alignment Rack. Then get a copy of the Ford Service School 1936 little red booklet publication: "Wheel Alignment". About 60 pages of good reading and illustrations.
 
On my roadster I had the death wobble and everything appeared tight and good so I set the toe in. Drove great until I went over a "speed hump." Wobbled badly and I found the toe in to be wrong again, so I reset it. Good to go until I got on some cobblestone spot. Then way out of adjustment. Disassembled the front end very carefully and at about the second thing I took apart I found a strange grease zerk that showed some funny wear on it. This was on the passenger side tie rod end and so I replaced that zerk, adjusted the toe in and it was cured. The zerk was too long and would lock up the spring inside the tie rod end, allowing me to make the correct adjustment, but when I hit any bump a bit hard, the spring would jump over the zerk, making the adjustment more than 1/8 out of specs again with the return of the wobble. Sometimes it is something very simple that baffles us.
 
Start with wishbone. Many are worn or loose.
There should be virtually no play up down fore aft.

If good work forward.
 
You should have 6 - 7 Degs. of Caster. You can do a quick check with a bubble level on the face of the axle on each side. Make sure the car is sitting on the ground. bounce the front and rear end to make sure it's settled. Then put the bubble level on the vertical face of the axle. and see where the bubble is sitting. every 1/16" is equal to 1 Deg. So 6 Degs. should be 3/8" from level with the top of the level leaning back. Check both side they should be equal. This is only a rough check but it works. Usually a wobble is fron not enought "Caster" JP. (Little quick checks you learn from Racing.)
 
This may sound extreme but I would recommend dropping the front end and taking it apart completely.

Not a big job. In this way you can once and for all make sure everything has been done right.

These cars are now over 90 years old. Most have had many different people, with unknown skills, working on them.

Steering is important to say the least. A shimmy can be deadly. Get it done right. Enjoy.

Don’t forget the front spring!
 
T.B., Ok so the caster angle is only 3-4 degrees and you want to increase it to 6. How is that done? By heating or twisting the front axle? Suddenly the need for a tight (No slop) spring shackle bushings becomes really apparent. Are shims available for the top of the springs? Are there some adjustments I don't know about? When you say "The face of the axle", the axle is a pretty rough forging and from what I've seen, the face is not a good point to get a reading. Am I missing something?
Terry




Tinbasher said:
You should have 6 - 7 Degs. of Caster. You can do a quick check with a bubble level on the face of the axle on each side. Make sure the car is sitting on the ground. bounce the front and rear end to make sure it's settled. Then put the bubble level on the vertical face of the axle. and see where the bubble is sitting. every 1/16" is equal to 1 Deg. So 6 Degs. should be 3/8" from level with the top of the level leaning back. Check both side they should be equal. This is only a rough check but it works. Usually a wobble is fron not enought "Caster" JP. (Little quick checks you learn from Racing.)
Click to expand...
 
I don't think caster is much of a big deal with these ole monsters. Ford's spec is 5º+ as mentioned and is not easily adjusted. I tend to prefer a bit less caster as I like the steering wheel to return easier. I would doubt caster is the cause of the wobble.
 
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