Tire PSI and Wandering

[GSP9]

I have been fighting both a wobble and a wandering issue in our 1970 with a 3 inch lift and 30x9.5x15 BFG All Terrain KO tires. Front tires didn't look to bad but when I moved them to the rear it got rid of most of the wobble. Ended up replacing those two tires with basically the same tire, now the KO2 tire. Wobble is gone but it still wants to wander. The front end was aligned, new trac bar bushings, drop pitman arm, etc. I bought a trac bar drop bracket but haven't installed it yet.

The two new tires were put back on the front and the tire shop put 34 psi in them. It wandered pretty good. I have slowly decreased psi and the fronts now have about 25 psi in them and the rears about 28 psi. It's starting to get considerably better.

My mechanic was the first to mention that he thought I should lower the psi into the 20's for these types of tires. I thought he was crazy when he said 25 psi but he was right on. When the tires had 30+ psi in them and the Bronco got up to about 50 mph it was not fun to drive. It would wander back and forth so much that you were two hands on the wheel and thinking about driving 100% of the time. Tonight with the lowered psi, I got up to 60 mph and it wandered a little but I would feel comfortable driving it for say 20-30 minute trips. Much more than that and it would tire you out. So my questions is, how low have you guys gone on tire psi for street use? Any thoughts?

 

[bronconut73]

Mine is about the same as yours maybe a smidge lower.

You can test for perfection by putting a chalk mark on your tires across the tread then drive a little and see how much gets worn off. Keep lowering pressure until you are wearing the whole chalk line across the tread off. Then you have matched the weight rating of the tire to your vehicle's weight. This would be different front and rear, ie heavier in the front.
Just be sure that you still like the way it handles with the extra low pressures.

If you have too narrow of a wheel for your tire it may crown your tread no matter how low your pressure is.

 

[texbronc]

I'm running 33x12.50's and settled on 28 psi all around. I'm not sure how the extra width vs yours affects it, but mine is happy there.

 

[B RON CO]

Hi, Trac bar drop should help. Do you know what the caster angle is? The alignment shop checked it, maybe you have the report. A little more caster angle might help. Short wheelbase and high center of gravity does not help. Good luck

 

[Broncobowsher]

Lots of factors go into tracking/wandering. Yes, tire pressure is one of them. As is tire design, rim width, alignment (caster and toe are big ones), track bar angles and bushings, steering geometery, and many more going all the way into the rear suspension. Sounds like you are finding the easy one to be a big factor in your truck.

 

[hucklburry rev2]

Yes, my 34x9.5 like about 24. It was a short process to dial it in.

 

[Steve83]

Lowering the tire pressure is just masking the problem; costing you gas, and tires. Frank ran 35psi in his 33s, and it drove like a new vehicle after we got all the bugs worked out. Steering box, track bar drop, alignment...

 

[GSP9]

Not sure what the caster angle is but it goes back in Tuesday for some other minor items so I'll ask. He's going to go back thru the front end and try and get it dialed in. Fortunately, he's a Ford guy thru and thru and he has a 68 Bronco so he knows these things really well.

When I first got this it was almost undriveable from the wobble and the wander. As I said earlier, it has a 3 inch lift and they never added a drop pitman arm. So that was the first thing I did and it helped. It went in to get aligned and he thought it was pretty good but he made some minor adjustments. Then I replaced the trac bar bushings and that helped a little more. Then I replaced the two bad tires and started fooling with the psi and that has helped considerably but there's still a little work to do. I just wasn't sure how low you guys were dropping psi in your rigs but looks look I'm close to as low as I should go.

I'll fool with the psi a little more, take it to him on Tuesday and if we haven't gotten it 100% then I'll go ahead and add the trac bar drop bracket.

 

[B RON CO]

Hi, the trac bar drop is a weld in piece. I would bring it with you and get it done, or else it is another trip to the shop. This will bring your trac bar more in line with your drag link.They should be parallel. This will also help center your front axle. You may notice more rubbber under the fender on the drivers side.The only easy adjustment is toe but caster and camber can be checked. Good luck

 

[DirtDonk]

Exactly. Don't experiment further until you correct the angles. With stock steering setups, a tracbar drop bracket ALWAYS gets installed with a dropped pitman arm.
Well, almost always. But that's another story.

With your tire size you should not "need" a riser bracket, and a drop keeps your stock geometry nearer to true.
Once you bolt that on, as mentioned go back to the shop and see if they can weld it for you. Then experiment with your tire pressures and other stuff.
It's all one big system and everything has to work together. And one of the basic key aspects to a properly set up front end is that the trackbar and draglink are as close to parallel as you can get them, and as close to the same length as you can get them (if you were fabbing up your own or they were adjustable) and pivoting in as close to the same planes as possible.

Good luck. And definitely get a caster and camber reading for future reference. Even if the guy says you don't need them because they're "not adjustable" he's just wrong and you do need them for your own point of reference if nothing else.

Paul

 

[GSP9]

The reason I haven't installed the drop bar bracket to this point is getting the original bolt out of the current bracket. I ground the weld down on the head of the bolt but it just didn't seem like it wanted to come out so I figured instead of breaking it off in there, better to wait till it goes back to the shop and have them try and get it out. I have a welder, so no issue there as far as welding the new one in.

 

[DirtDonk]

Ah, ok. So the PO welded the bolt in for some reason. Wonder why, since the upper doesn't really need that typically.
Probably had some wollowed out holes letting it move around.

If it's just welded on the head, can you grind the weld without cutting the bolt? Your holes won't be needed for any hard work after that, since the new bracket has new holes in it just a bit lower down.

And that is a bit of a long bolt. Normally you don't get too much length because it could hit something in the steering. As long as yours clears though, it's not an issue.

Paul

 

[GSP9]

Have an update. Trac Bar drop bracket has been welded in and really no difference, it still wants to wander. But my mechanic started looking some more and noticed that the passenger side C bushing appears to be in upside down and he's pretty sure they are the original 2 degree C bushings.

So it looks like I need to order some C bushings so my question is this. This Bronco has a 3 inch lift on it (3" blocks in the back and correct springs up front). I've read that for a 2.5" lift to use the 4 degree bushings and for a 3.5 inch lift to use the 7 degree bushings. Since I'm right in the middle, is it best to go one way or the other? Also, at some point and it might still be a few years away, I plan on installing the 2.5" lift kit with the Bilstein shocks from Wild Horses which would use the 4 degree bushings. But I just want to get this thing trucking straight down the road right now so which direction should I go........4 degree or 7 degree? Thanks.

 

[Timmy390]

My 2.5 SL drove like crap with 4 degree bushings. I went to 7 degrees and it really cleared things up.

Each Bronco is different......

Tim

 

[DirtDonk]

Well that's why it's nice to know what your caster is now. So you can make an informed decision. I don't suppose the mechanic gave you those numbers the last time, did he?

There were no "original" 2° bushings. Originals were zero offset rubber. Only poly bushings came in 2 degree offset. Or did you mean original, as installed by the previous owner?

Either way, a '70 front end can benefit from as much caster correction as you can. I hope you have power steering so you won't notice the change in effort it takes to turn the wheels, but other than that, more caster is generally better. Until you run afoul of a bad pinion angle that is.
But that's why 7 degrees is the most you can get from a bushing. Any more and you start to run up against bad u-joint angles.

Since you're a contributor, any chance we can see a shot of the front end components? Did we see some in another thread maybe? Either way, it would help to see some here too, in this context. Maybe we can see something still out of whack.
Too bad we can't see a bad tire with the camera unless the tread is whacked.

Did you continue to play with tire pressures?
Did you happen to get the alignment specs on paper from the mechanic?
I would also play around with toe-in adjustments now that it's set to factory specs. But since they use angles now instead of inches, a change in tire diameter will change the end result and you might find some benefit to fiddling with the toe settings yourself.

Good luck!
But let's see some pics too.

Paul

 

[Justafordguy]

Go with the 7 degree bushing for sure. It made a world of difference on mine with 2.5" SL.

 

[GSP9]

I'll see him tonight so I'll check on the castor and alignment specs. I stopped playing with the psi until I got the drop bracket on and it does have power steering. Any particular area I should get pictures of?

 

[DirtDonk]

Just a general across-the-front shot showing the full tie-rod would be good. Basically tire-to-tire is what's needed.

If that opens up any questions, we can ask for more detailed shots at that time.

Thanks

Paul

 

[Digger556]

I will add that knowing your Caster, Camber and Toe numbers are important. Even with other things like tire pressure, trackbar angles, etc setup properly any of those three alignment numbers being off can cause wander.

Generally speaking, if Caster is too low or negative, you will wander or get death wobble. Camber or Toe being too close to 0.0° will cause wander. Toe out will make the truck dart left or right when turning. The alignment numbers that are programmed into most alignment machines are the original factory targets and may need to be altered for wide radial tires. I usually shoot for 3-5° of caster, 0.25° camber, and 0.25° total toe.

Also keep in mind that while most people, including myself, don't have any issues with BFG ATs, they have a very flat tread and can cause a marginally dialed-in truck to wander or death wobble.

If you can, get a 4-wheel alignment and post a picture of the printout. There are aftermarket solutions for adjusting camber and caster if need be.

 

[B RON CO]

Hi, X2 on better than factory specs. You can improve it quite a bit. Your Bronco is not stock any more