Model A straight axle shake. WHY?

http://www.jalopyjournal.com/forum/showthread.php?t=394342&highlight=marmonherrington
It's a driver!!! One of it's few quirks is a mystery to me.
Get strollin' down the superslab, run about 60-65, no steering wheel leftright shake or wheelhop. Lay on the loudpedal a little more, hit a ripple or road change, front of the car starts jumping left up/right down and opposite hard enough to lose a radiator support rod, radiator and grille shell waving at you like Barbie. I can sit up and see the left front jumping up and down like a jackass and then settle like a worn pony. The steering wheel DOES NOT shake left/right--rules out tire separation. No steering bad feedback at any speed, just built in bump steer.
Let off the throttle, no brakes, settles down, bring back up to road speed just to below that feeling of immenent disaster, cruise for a bit.
Different road section, ribbon of smooth asphalt. Almost five miles long, bring the speed up to approx 80-90 mph for a while, knowing where the smooth part ends and potential trouble raises it's ugly head. NO DEATH SHAKE OR DEATH WOBBLE. If it was a driveshaft problem, I'd surely hope I would be the guy to recognize THAT problem.
Shock absorber dampening make more sense to me than anything else. I'm driving with the non-coil-over Pro Shocks bought from Speedway. I'm questioning if I had made the correct choice or if I have missed anything else.
Caster and camber are dead on spec for a drop axle or a Ford axle. Kingpins and bushings have as many miles on them as the engine. A picky SOB reamed the bushings for me, and know how to do it for a big truck with a 20,000 lb front axle. Toe is at 1/8" in. Tires wearing and cleaning evenly.
To reiterate how the car feels when driving.....No driveshaft vibration, no lane change under hard throttle to indicate misaligned rear suspension, no tire pull or shake to indicate a JUNK tire problem, no brake drag to cause either.
Putting on your thinkin' hats..Got mine. Whiskey Tango Foxtrot?

 

Too light a shock? Taking into account the monoleaf.

 

Good question, I have the same. What else applies with experience?
I wish I was the guy in the light blue DeSota, she was from Texas too.

 

My roadster does that, but it takes a BIG bump, and it settles pretty quick.

I just figured it was the crappy friction shocks that I have up front, and roll on.

Decreasing air pressure up front helps a lot.

Rich

 

Bump steer most likely.
No bumps or road undulations, dead smooth, uneven road, jumps around.
Classic bump steer.

 

That's no "classic bump-steer". Shocks are too light or damaged, magnifying any other problems.
Saw a woman driving down the expressway one day...the left-rear tire was bouncing down the road like a doggone basketball. Boing-boing-boing. Bad or disconnected shock and wheel out of balance.

 

Shocks too light only leads to wheel patter.
In that case the wheels just jump up and down.
But if it is going side to side, or wandering over bumps, that is a STEERING problem.

 

Yeah, something loose, or a basic steering geometry problem. Shocks may certainly help, but the basic cause is something else.

 

Still not "bump-steer". As I said, shocks are not doing their job, and magnifying other problems. What those problems are needs to be investigated, but it sure isn't typical bump-steer.

 

If the front wheels are moving up and down, and still steering straight ahead, the rod will not be "jumping left up/right down" as the original poster VERY CLEARLY stated.

Steering "left up, and right down" is BUMP STEER, plain and simple.

 

Warpspeed, Bump steer is one of the most misused terms out there in regards to early ford axle issues..you are only confusing the issue. what he describes has nothing to do with bump steer. please dont take offence, its just that what you think is bumpsteer is not.

 

Bump steer is perfectly plain and simple, and the definition easy to understand.

If the front wheels steer away from straight ahead, when the front suspension moves either up and down, then that is bump steer.

If that is not happening, why does the front of his rod move left when the suspension moves up, and right when the suspension moves down.

 

He is describing a high speed shake brought on by a combination of conditions..Bumpsteer as you correctly described is an inherent feature of the design of the supension, not an intermittant and semi-random violent shake. I just think its not much use to say "its bumpsteer" as the factors contributing to it (insufficent damping, loosenes of components,tyre pressure, caster, and who knows what else) arent going to go away if you re-engineer the car to have smaller amounts of bumpsteer.

I drove an old hotrod (Squeek Bells' bellbird special) last year and it did exactly what was described above, very weird and scary, now that car was built nearly fifty years ago and I put that down to a multiplication of worn bits and poor design all contributing, but yours sounds a litttle more hard to trace..Id start with differnt shocks and add a leaf in your spring pack , not necessarily becuase its too soft, more that the current combination of components may be setting up some sort of harmonic vibration (for want of a better term) and changing a few variables might make it go away, or at least move to a speed range you dont often use or cant obtain. Thats all I can offer..

 

What is the caster setting?

Bob

 

I added a panhard bar to my A and it is like a completely different and wonderful car.

 

Makes sense. I'd add this: Bump steer can (and should) be felt as some form of feedback at the steering wheel resulting from forces fed into the drag link. When the wheels try to steer the car off center, momentum instantaneously resist the change in direction and can throw the force into the steering box. It's curious that he's not feeling any feedback at the steering wheel. Just sayin'.

Bob

 

I'm surprised this is the only mention of a panhard bar. That was the first thing I thought of.

 

To give a little more clarification, the car does have a little bump steer. Bump steer causes the car to dart left or right if you try to fight it by holding the steering wheel still. If you follow the steering wheel, go with it, the car goes straight. You can jump on one frame horn, both sides move equally, can watch the steering wheel turn a little with suspension motion.
I have let go of the steering wheel at 50 on straight section of road, no hands for better than almost a quarter mile. The kingpins, heims, and tie rod ends ALL have less than 250 miles on them and double check good for slack.
What I'm trying to describe is almost like having the front axle on a seesaw. At the correct speed and road influence, the seesaw motion is very harsh and does not cause the steering wheel to shake left/right. It makes the radiator wave left/right just as harsh as the tires jumping like a jacksass.
Tire pressure could be an influence too. Keep thinking guys! Thanks!

 

What kind of tire are you running?

Some of the Cokers are known to be bad out of round.

When I first put my car on the road, the front tires bounced HORRIBLE above 50 MPH.

I had them "road force" balanced, where they take into consideration any out of round for the tire, and any out of round for the wheel, and mount the tire at the best location on the rim to counteract.

Do a search on "road force balance"

Rich

 

If all else fails, you might want to re-check the wheels and tires. I bought a coupe with a similar problem. When I would hit 50 mph, the front end would start shaking violently. As soon as I backed off, it would smooth out immediately. I found a tire shop with a balancing machine that had a roller which applied pressure to the tire surface as it was rotated (simulated road travel). The machine indicated a problem with the tire (bias ply), even though the tire was in good condition and had even tire wear. After changing the tire, the shake calmed down a lot, but was still evident. I later discovered that the steel wheel was slightly center-bent. After I had the wheel straightened, the problem disappeared completely.
When you buy tires, always check the date code stamped in the side-wall of the tires. You'll be surprised at the results. Some new tires sit on the shelves of the manufacture and dealers for years before they are sold to the consumer.

 

Tires could be the cause. I'm on 165R15 radials that have been mounted for about four years and just getting road miles.
I have trouble believing it's a tire problem because what I've learned to troubleshoot at work with tire problems have a typical steering wheel shake when the tire induced vibration occurs. But I also know not all symptoms are the same.

 

Thanks Patrick, that is what I thought.

One other thing that nobody here has yet mentioned is front wheel rim offset or backspacing.
If the front has a significant amount of scrub radius, then steering forces will be produced on each side in proportion.
On a dead flat road, these forces will balance and cancel out from side to side. On a bumpy road, the scrub forces will increase with more suspension download, and decrease as weight comes off that wheel.
Over one wheel bumps these steering forces become unbalanced, and the car can dart sideways very suddenly.

The original beam axle was designed to have large diameter and narrow tires. When you fit very much smaller diameter wheels and tires, the kingpin axis moves in relation to where the tire pivots on the road where it steers. Wider tires also increase the steering scrub forces associated with all this.
Just a thought....

 

I'm surprised that this thread got to the 22nd reply before anyone got close. Assuming the king pins are good and slop free, to solve the problem check the caster/camber. On a very light car, you need to lean the top of the king pin back twice what the book says. They talk degrees, you need to talk 1/8 inches. About two to three should solve the problem. Then get the center line of the tire in to meet the center line of the king pin at the ground. When this is done, there will be no need for a panard bar. There will still be a small amount of bump steer that is easy to live with.

 

I just recently started having issues with my 39 Ford. Very similar action. I have a stock axle, a mono leaf spring and coker 5.60 x 15's. The thing is I have had it like this for a couple years with no problems and but around six months ago I had taken it to a funeral service here locally of a fellow hot rodder and on the way there I had cut the wheel to had and hit the median in the middle of the road. The nice thing is it was a beveled one the bad thing is I jumped the drivers side wheel up in the air.I pulled over and check things out visually and they appeared to be good. So I decided to run up the highway and as soon as I hit a bump in the road and this is at 60 mph my car started to shake violently so I slowed right down and went between 40 - 50 the rest of the way. When I got back home I decided to order a new drag link and both new tie rods for it I have installed them and a new pair of shocks on the front but I had also just added a SBC with the old cragar adapter to the car as well. again I took it down the road and right around 60-65 mph it wants to shake the shit out of me. I am going to change the king pins next and add a panhard bar hopefully this will cure the issues. If not then I will change the entire suspension out of it.

 

WHAT TIRE PRESSURE????????????????????

Those skinnys bounce like superballs with anything over 25#

 

I'm interested in this one. My T bucket did exactly the same thing with 5.20 (VW size?) 15 inch tires on the front. Get up around 65 and it was time for a laundry change when you got it slowed down. I had a pair of British car lever shocks on it that may not have worked as they should have.
I always figured that I needed better tires on it but would sure like to find out some real causes.

 

Typical tire force variation (first or second harmonics) are felt in the 60 to 65 MPH range. Your chassis has natural frequency that lines up with the tires' frequency, and tire forces are setting everything off. Changing system stiffness (springs, shocks, frame, tires) can shift or break up those harmonics.

You can't change the force variation of those tires. Manufacturers measure individual tires, and the OEM offsets the wheel mounting circle slightly to compensate (called match mounting). Some tire store mounting machines will measure the assembly R1H (radial first harmonic) and rotate the tire on the rim to minimize it, using the wheels non-uniformity. If all 4 tires were the same size, try front to rear swaps. Vary tires pressure to slightly change the tire stiffness.

Stiffen the shocks, springs, or chassis (e.g. panhard rod) to try and shift or break up the harmonic frequency lining up. Alignment changes most likely won't effect it.

Good luck.

Steve

 

My money is on the CASTER you need to have at least 7% on the axel check you shackle bracket bolts and see if there is any mis-alinement, to often front cross members are welded in flat and wish bones are tweaked to make up the difference......................IMHO

 

165SR15 Pep Boys tires on ugly ass Jeep Wrangler 15x6 wheels with 4" backspacing. Tire pressure is 25 psi. Caster is 7 degrees when measuring on top of the bare axle at the kingpin boss. Camber is top of tire out 1/2 degree Toe is 1/8" in.
I work at a shop that aligns front ends on Honda cars to semi trucks, and have helped troubleshoot many tire/powertrain/driveline vibration problems over the past 22 years. All of y'all are doing a very good job with trying to help me think this one out, thank you.
When the front end goes into jackass mode, the steering wheel is not trying to shake left/right in my had at all. For this reason, I don't think a steering damper will have any effect. I'm using a Mustang steering box that has rebuilt or at least disassembled, rebearinged, and resealed at a real deal steering sector remanufacturer. A panhard bar is needed more by a car that has cross steer than one with Mustang push/pull steer.
On an uninterrupted smooth stretch of road, I can open the car up and drive comfortably 15 mph faster than where the jackass motion starts on a road with bridge and expansion joints. That's why I'm really thinking shock absorber dampening instead of any steering related problems.
New kingpin set, reamed CORRECTLY, and shimmed correctly. New heims on the tie rod tube. New ends on the drag link. New shackles and bushing on the monoleaf spring. Oldest "COULD BE" junk is the tires, but the problem is not CONSISTANT. Only with road irregularity influenced jackass mode.
Keep thinking and talking, please. I know some of y'all have missed the problem, but at least you're thinking out loud of what other problem potentials are. Thanks!

 

I had no steering wheel shake at all and the vehicle tracked straight.