Bad Shake over 40mph

My father just gave me his 1941 Buick special but it needs a lot of TLC. The first major issue I need to iron out before I can go through the car is at about 35-38mph the car gets into a nasty shake or harmonic. It's so bad that it feels and sounds like the car is getting ripped apart. Once the shake starts it will even carry if down til 30mph then dissipate. It seems from the front end. What could cause this? I put if on the lift and nothing seems broken or really lose. I am not familiar with old cars, I work on the new stuff all the time. The only component that I can move comes from the left knuckle to a rod that's hooked to the pitman arm. It has an off joint with a plate on it that translates. Idk if that is even the issue but out of all the parts I can move that joint. Also the front seat bar bushings are shot. Any ideas? Thanks tom

I had that trouble with a 27 Pierce Model 36 EDL. Car drove fine up to and past 40, but later after driving, slowing down, speeding up, and finally about 40 the car just shakes like it is about to fly to pieces. It has tall wood spoke wheels, to, adding to "The Skeer". My mechanic said that the tires, expensive Lesters, had flat spots on them from long storage, and he thought that when the wheels turned just so [a kind of harmonics set in]. It might take some driving around before the front wheels get back to "just so", then the shakes happen again. They keep up with the same ferocity until the car stops completely. "Dang". I said, or something similar. It sure would be an expensive experiment to buy two new Lesters, even if we COULD find them. Then the waters parted before me and I remembered the spare! That sure had no flat spots. So we changed one of the front tires and the problem disappeared. I just knew the whole front end was trash, and the front wheels broken up. Exact same thing happened with a Model T. Swapping the tires around solved the awful shakes.

There is another thread called 'death wobble' Check it out.

I had that trouble with a 27 Pierce Model 36 EDL. Car drove fine up to and past 40, but later after driving, slowing down, speeding up, and finally about 40 the car just shakes like it is about to fly to pieces. It has tall wood spoke wheels, to, adding to "The Skeer". My mechanic said that the tires, expensive Lesters, had flat spots on them from long storage, and he thought that when the wheels turned just so [a kind of harmonics set in]. It might take some driving around before the front wheels get back to "just so", then the shakes happen again. They keep up with the same ferocity until the car stops completely. "Dang". I said, or something similar. It sure would be an expensive experiment to buy two new Lesters, even if we COULD find them. Then the waters parted before me and I remembered the spare! That sure had no flat spots. So we changed one of the front tires and the problem disappeared. I just knew the whole front end was trash, and the front wheels broken up. Exact same thing happened with a Model T. Swapping the tires around solved the awful shakes.

Thats good to know. I was thinking it could be the tires but I am not sure. Like u said it could be expensive to just try new tires. I will try moving them around

There is another thread called 'death wobble' Check it out.

I checked the other thread out. I could not come to a conclusion as to what it could be. Anything from tires to bushings to alighnment. So I am still stumped, car is on the lift and I am not sure what to do

I am not an expert on this at all. I can't quite understand what part that you found that easily moves but I am pretty sure that a part on the front end that is that loose is not a good thing. I suspect it might be your problem. A good old fashioned alignment guy is probably who you need to look at it. You can generally find whoever is still in business and specializing in large truck suspension/alignment work knows how to work on cars of that vintage. The front end of big trucks still closely resembles the front end on our antique cars.

I am not an expert on this at all. I can't quite understand what part that you found that easily moves but I am pretty sure that a part on the front end that is that loose is not a good thing. I suspect it might be your problem. A good old fashioned alignment guy is probably who you need to look at it. You can generally find whoever is still in business and specializing in large truck suspension/alignment work knows how to work on cars of that vintage. The front end of big trucks still closely resembles the front end on our antique cars.

My may have to do that. I will see if I can snap a photo and upload

My '29 Plymouth had the front end shake, but not as bad as you described. New king pins and tie rod ends made a world of difference. Also adjusted worm gear in steering box. Just a few thoughts on what worked for me. Good luck

Do you mean the tie rod has a loose end?

Best way to check steering slop is to put it on the ground, have an assistant turn the steering back and forth a bit while you look at all the steering connections. You will see what is moving when it should be and what is sloppy and movement. Drive-on ramps are the story for this job. You can also check the wheel bearings and king pins for movement when the wheel is off the ground by trying to rotate the wheel top out-bottom in and vice versa. And then turn the steering by turning the road wheel by hand. It should turn freely and evenly with no change in resistance lock to lock. You can also check suspension bushes and so on by putting the wheel just off the ground and levering it up with a pry bar under the tire. These tests are all done as part of our legally required Warrant of Fitness (i.e. safety) check. Much movement anywhere and work is required to get a pass to allow us to legally operate the vehicle on public roads. Just wiggling it by hand with no reaction load will not be enough.

So after 10 minutes you will have a pretty good idea of the condition of your front suspension and steering. If all is OK, perhaps change the tires around as stated above. If you have a friendly tire shop, a balancing machine should show in a few seconds if there is a flat on the tire. Just be careful that they use a dolly on the studs rather than a mandrel on the centre - I understand the centres are not necessarily concentric with the bolt pattern on old wheels.

First and easiest, rotate the position of the tires, and see if it makes any difference.

My '29 Plymouth had the front end shake, but not as bad as you described. New king pins and tie rod ends made a world of difference. Also adjusted worm gear in steering box. Just a few thoughts on what worked for me. Good luck

So it looks like kingpins have small amount of play. Wheel bearings are good. Tie rod has a little play on the one end, but the steering box also has some play. How do I tighten that up? I can replace all the parts in the front end but it looks like a lot of work to do the control arms and bushings