Bump steer?

[DEEPWOODS]

Just changed the front axle to HP F250 and 37” tires drives terrible now very touchy and has bad bump steer The steering link and track bar are about perfect parallel to each other. They are steeper now than they were before the axle swap .will the steepness cause bump steer even if they are parallel?

 

[Jdgephar]

Parallel doesnt mean much if they are different lengths. It's all about the sweep arc of the drag link and trac bar.

You would probably get better results adding a drop pitman arm.
Sent from my SM-G981U using Tapatalk

 

[DirtDonk]

What else has changed?
How high is your lift?
How wide are your wheels and how much backspace?
What is your caster? It’s probably very good with a high pinion differential, but it’s worth knowing. There might be such a thing as too much, but I don’t think it would affect bump steer. Just not sure.

You might check out “the cure” at rough stuff specialties and see if it’s compatible with your replacement tie-rod and draglink.

Did you change the steering wheel size? Did you change the steering gearbox?

 

[DEEPWOODS]

Paul its the same lift as before 3 1/2” .I checked and the length difference of the track bar and steering link are almost identical as they were before both track bars are are about 5 1/2” shorter than the steering link . The most noticeable difference is it was knuckle over and now it’s normal knuckle under because the F250 knuckle arms are so high I had interference with steering linkage with knuckle over so I had to go knuckle under .Also it now more caster than any bronco I have ever seen

 

[Broncobowsher]

When I see steering as "twitchy" I think you have low or negative caster. You did an axle swap, what did you do during that swap to address caster?

 

[DEEPWOODS]

I used the same 7 degree bushings as before it now has a lot more caster than it did have . I will check the angle at the bottom of the ball joint tomorrow. The track bar riser was much higher on the old axle so it was not as steep and it was not as parallel with the steering arm as it is now . The wheels are 9” now were 8” Backset is now 4” was 3 1/2 “

 

[DEEPWOODS]

I found an old pic of the old axle . It also now has the torque steer when you mash the gas one way and the opposite way when you let off the gas but drives perfectly straight when you coast.

 

[DirtDonk]

Torque steer can come from the steep angles. I’m sure that’s not the only thing, but it’s certainly one of them

 

[DirtDonk]

7° bushings and a high pinion front end can, as you say, have a lot of caster.
If you have more than 7° positive you might look at reducing it somewhat. But wait for others to comment on the effects of high caster.

How is your front pinion angle just out of curiosity?

 

[Apogee]

I'm sure they're new given the axle swap, but I've had "bump steer" issues caused by bad track bar bushings on my '77, as well as improper geometry when I purchased it. I fixed the geometry (they were running a track bar drop bracket on a '77 but no drop pitman arm, which is opposite what was needed for a 3.5" lift), and then found the track bar bushings when the axle still jumped around under the truck when I hit any sort of pothole or square edged bumps.

What are you running for your caster and toe? Any camber issues? Air pressure on your 37's? They look pretty good in the pic, but it never hurts to ask.

 

[DirtDonk]

What exactly was the interference when you tried to retain the TRO aspect of the tie-rod? What exactly hit what?

It's a bit hard to tell for sure in the earlier pic, but is your draglink at the slightly steeper angle than the trackbar? If so, that's probably the opposite of what you want. If you could get it more parallel, or a bit shallower than the shorter trackbar that would probably be better overall.

A shorter bar might be a bit steeper because it travels in a "quicker" arc (probably not the correct terminology) than the longer bar. At some point then the two sort of cross over. Since your trackbar is shorter than the long draglink in this setup, maybe getting the draglink to a shallower angle will be the biggest bang for the buck at first.
But getting them both to a lower angle can only help.

Can you fine tune the angle on the trackbar any further with the riser hole locations?
But no matter what, the bottom line is still this:
1. Shallower overall angles are better.
2. More parallel is (almost always) better.
3. The longer the bars the better.
4. More equal length is better.
5. Closer to the same plane is better.

With probably the most emphasis on #1 the "shallower angles" aspect than anything else.
But they're all important, so any combination of those being out of whack is going to give you more grief than just one of them.

Paul

 

[DEEPWOODS]

pics of caster and front drive shaft angle . Is the caster to much ?

 

[DirtDonk]

Pinion angle looks ok from this angle.
Caster is a lot! Not sure if that qualifies as "too much" or not, but it sure means you have plenty to play with.
If you can't figure out other things that might cause the twitchiness, you can reduce the C-bushings to 4° or even 2° versions and get away with it totally. Even if you lost 5 degrees, you'd still be at over 5 degrees positive. And that's more than most!

Speaking of positive and negative, just so we're sure we're all on the same playbook page, is that angle finder indicating that your upper ball joint is behind the lower ball joint? In other words, it's tilted "rearward" rather than "forward" correct?
Looks like it from here and I can't imagine a scenario where it would not be with a high-pinion front end, but want to be 100% sure.

What is the air pressure in your tires? Have you played with that yet?

And while we're working out the kinks, have you done the test where a helper turns the wheel back and forth just enough to move the wheels a bit while you're watching what's going on underneath and making sure nothing is loose or wearing out? Even a broken tack weld on the lower trackbar bolt will cause wandering with just a tiny bit of flex.

Paul