death wobble with 35's ????????????

[Alfred Lord Tenniscourt]

Is it possible that someone rotated the axle to get a better drive line angle? If so, that would explain a lot...

[1971ford]

Looks real good!! When are you going to paint the hood red? I'm really diggin this whole project.

Somehow missed this reply before.
Thanks!
I'm not going to paint the hood red, because this summer i am stripping the body down again and i'll be doing some much needed body work and i'll be spraying the truck an early bronco orange color. Rolling on red on the hood would only add to the sanding time later on.

Is it possible that someone rotated the axle to get a better drive line angle? If so, that would explain a lot...

This isn't the case, pinion angle is the same as any other stock highboy.
I think I may of found the problem but no way to tell for sure. When I was re-greasing the closed knuckle (trunnion bearing) area and putting a NOS ford lockout on I noticed the lock nut was loose so that got tightened when i put everything back together.
Who knows,that might of been it.

[Alfred Lord Tenniscourt]

Sorry about the double post. I had a good one written and deleted it somehow...
I started out working on bicycles and motorcycles, so I always imagine caster in terms of a front fork. A chopper will ride steady on the highway (as long as it isn't too crazy and everything is done right), but it takes a long time to turn. On the other hand, a short track bike has a fork that sits closer to vertical, so it is a little twitchy, but can turn very quickly.
Effective caster is a tricky thing. if your tires are aired down, and the contact patch is huge, the effective caster angle is not really a definite thing. It moves around depending on how the tire is forced to conform to the pavement and how the lines of force are oriented as the tire overcomes rolling resistance and supports the weight of the vehicle. On a mountain bike, for instance, when you get into soft mud or sand the effective contact patch shifts to the front of the tire where it is breaking trail. This changes the effective caster to a very high positive value, just like having a very long fork would, which should make the bike straighten out. But, the moment of force on the tire is now oriented at a much different angle, because the tire is being pushed backwards by very high rolling resistance almost as much as it is being pushed down by the weight of the bike. This change in the orientation of the moment of force is greater than the difference the contact patch generates, and so it produces a death wobble, because the effective caster has actually gone negative.
I have two guesses about this one:
1) Someone rotated your axle to try to get a little better driveline angle, and changed the caster. On my old bump with the stock D44 and drum brakes, the pinion housing actually looks like it is pointed down a little bit. I almost rotated it myself because of that, so I can see how someone could make that mistake. If your new tires have a little more rolling resistance, that might change the effective caster just enough to cause the problem to show up.
2) Did you unload the rear of the truck? Changing the chassis angle by unloading the rear of the truck could change the caster angle just a fudge and cause it to wobble. Loading the front of the truck (like with a snow plow) doesn't have as much effect even though it changes the chassis angle because the springs, which can only pivot at the front but can forward and backwards as the shackle rotates at the back, will flatten out and rotate the axle in the positive caster direction.
Hope this helps...
ALT

*Edit* Just saw your post about the locknut after I posted this the first time. I hope that fixes it (and think it probably will!)

[Racer Z]

In bicycle and motorcycle terms, that would be "rake" and "trail". There, the steering axis is centered over the tire. For cars and trucks, the steering axis is offset at an angle just inside the wheel. If you remove the tie-rods and try to roll to car forward, you won't get very far because each tire will turn outward.

The older BMW motorcycles were famous for their "High Speed Wobble". It only happened if luggage was was on the rear. The weight of the luggage unweighted the front tire and at 75 MPH or so, the steering would start to shake.

I still think the "Death Wobble" that 1971ford is experiencing is caster related.

[averagef250]

1) Someone rotated your axle to try to get a little better driveline angle, and changed the caster. On my old bump with the stock D44 and drum brakes, the pinion housing actually looks like it is pointed down a little bit. I almost rotated it myself because of that, so I can see how someone could make that mistake.

Do people really think this? Is it common for people to not to see that the front pinion under a highboy chassis needs to point to the dirt for the U-joint angles to compliment each other?

[Alfred Lord Tenniscourt]

Oh, people think that and worse...
I had four 1/2" sockets fall out of the drivers side fender the first time I took it off. I had the wrong bearing stuffed loosely in the drivers side rear axle. I had a hacked up wiring harness for no reason besides that some drunk hillbilly decided that since he didn't understand what all the wires were for he was going to cut them up and twist some different ones together.
I could go on too.
If the guy who had my truck had ever looked at a CV joint axle setup on a monster truck, I would bet the farm that he would be out there the next day twisting that axle out of whack...
I probably shouldn't admit that the idea of rotating it myself ever crossed my mind, but I was fixing a lot of junk work around then, and it looked really wrong to me at first to see the pinion pointing down like that. My first thought was that someone had changed the axle and not bothered to line it up correctly. It took me a minute or two to get my bearings and realize it was ok...

[1971ford]

Put the race truck's 35's back on the highboy, and within a week i should have 4 of my 35' bfg ta's mounted on the highboy for good. Only problem is this wobble is still present. It pulls to the right with the 35's on too. Doesn't pull at all with the 31's.
If i put tension on the wheel to straighten it down the road, thats when the truck usually starts wobbling (can feel it through the truck and the steering wheel wobbles back and forth until i turn it either way just a tad to stop it)
Turns out the loose lock nut wasn't the only problem. I'm 99% sure it's just the front left.
I'll tear the axle down a second time and see if i missed anything. I'll swap the wheels around as well just for kicks since the tire that is on the front left has bad camber wear from being on someone elses truck.

I guess i better check my alignment. I nailed the tierod on a rock and bent it back and it still drove perfect with the 31's then i bashed the steering stabilizer in but that has no effect on how it drives (i guess the stabilizer was garbage in the first place).

How important is a steering stabilizer? I have one on the truck but apparently it wasn't doing anything because after i bashed it in it drives the same. Except that i can't turn full lock one direction since its bashed in.

[Racer Z]

How important is a steering stabilizer?

The steering stabilizer is a shock absorber of sorts. It prevents things the tires hit from jerking the steering wheel out of your hand. Not needed on smooth roads. With extra wide tires on a rough, rutted road, is when it is needed. Keep in mind that it also makes the steering hard, you have to overcome the stabilizer to turn corners.

Extra wide tires also put an extra strain on bearings and such. Tie rods can get bent easily with extra wides and big rocks. If you don't need the extra traction, a slightly narrower tire does have it's advantages.

[fomocoguy]

Ryan, I don't know as much about a leaf spring front end but I can tell you that death wobble is a big problem on coil spring Cherokee front ends. Mine did it once after I put my lift on and it went crazy. The steering wheel was jerking back and forth wildly and it wouldn't stop until I brought it down to about 5 mph. It scared the crap out of me. After that I did some research and the consensus seemed to be that lowering too high a tire pressure, a good steering stabilizer, and well balanced tires would usually do the trick on a Cherokee. So I bought a $75 stabilizer from Old Man Emu, had my tires balanced, and started running them at about 30 psi (the Cherokee only weighs about 3500 lbs). It hasn't done it since.

[BigBlueF250]

Jack up the right front tire and check for a bump in the tread. Sounds like a separated tire.




Steve

[1971ford]

Thanks for the info guys. I'm trying the ideas out.

It's not the tires, it does it with any set of 35's i put on.


Today i checked the toe with a tape measure... i hate doing this because it's too dang hard to get a good measurement. It was anywhere from toed in 1/8" to toed out 1/4". So i decided i'd experiment. I painted a line across the sleeve and threads so i can return it to where it was if it doesnt get better (counting how many times i pass the mark and what direction as well, right now i have it 1 revolution counter clockwise than where it was).

I turned it half a revolution counter clockwise (looking at drivers side from passenger side) to bring the toe in a tad just in case it's toed out a little too much. Test drove it and it drove the same, although i only drove it above 30mph for a couple hundred feet.
Then i turned it another half revolution, now at one full turn of the sleeve to toe it in, and above 30mph it was 100x worse.

That's where i'm at now. I'm going to back it off to where it was and try again the other way, assuming maybe it was toed in too much to begin with.


Another thing to add that i noticed. On BOTH front tires, the inside 1" of tread is still dirty. 95% of of the tread on both tires are black from being driven on except for the 1" of inside tread on both tires. Thoughts? almost like it has too much positive camber. That makes me think it's toed in too much, causing the tires to go into positive camber. I know thats how it works on i-beams but i have no idea if its the same principle on a solid axle.

[Racer Z]

My first thought is "Take it to an alignment shop and have it checked and set."

But, if you want to set the toe yourself, this is how you can do it. I've done it this way myself.
Please note that it helps to have a helper.

1) Jack up the front tires and cover the tread area with white chalk. Try to find a smooth area without tread.
2) Scribe a line on the tire. Do this by rotating the tire and holding a nail. Now you should have a black line in the white chalk. Scribe both tires.
3) Set the truck down.
4) Roll (push) the truck forward (not backwards) to get the suspension settled. Probably about two truck lengths. Do not use the brakes, this will upset the toe. Do not roll backwards, this will upset the toe.
5) Use a tape measure to measure the distance between the scribe marks. You need a helper now. Pull the tape tight, it can't bow or you will get a false reading. You MUST have the tape the same hight from the ground on both left & right as well as on the front & rear of the tire. The higher up the better.

Adjust the toe as needed. Trucks should be from 1/8 inch to 1/4 toed-in. 1/4 is a bit much for my taste.

The true measurement is supposed to be taken on a line level with the axle it the diameter of the tire. This is impossible to do with a tape measure.

The caster/camber can't be checked without special tools and Ford didn't provide an adjustment anyway.

[1971ford]

I made sure i drove it straight a good 15 feet before i checked toe, but i did use the brakes. I'll re-try tomorrow by pushing and not using brakes. I'll draw the line too. I was measuring from a certain tread point to point and double checking from inside to inside of tire at a line that is in the tires mold. The line on the tread sounds best.
THanks

[1971ford]

I set the sleeve back to where it was then about 1/4 of a turn clockwise to toe it out just a tad. Then I aired the tires down to 40psi (they were at about 48) and put the steering stabilizer back on (just took it off earlier today to try and get a new one, which they didnt have, but this one still takes a lot of force to extend/compress).

Then i went on my test route and so far so good... got up to about 45 mph (where it would of wobbled with every other setting of the sleeve so im going to hope i have cured it.

It would make sense, because the sleeve is slightly bent so toeing it out just a tad would bring it back to where it used to be.

[1971ford]

I found a d44 LP axle on craigslist (pulled from a bump highboy) for $50... I need to grab the np435 the guy is selling to replace mine and considering grabbing the axle too instead of chasing mine around trying to find out about the wobble. But then i have to do lots of work to get the new axle nice and pretty and swapped under.
So i want to see if i can finally fix this wobble.

My tie rod is slightly bent from apparently trying to move a boulder with it once, the i bent it back, and it's pretty close to straight but definitely not dead on straight. Could the tie-rod be deflecting slightly as i drive causing the death wobble? Are the tie-rods .250 wall?

And could it also be because my front springs are sagged? They just barely have any arch to them, i'd say i've lost an inch or more of ride height since the truck was new. Maybe this is an issue?