Technical Adjusting caster with four link?

I agree, but I'd like your opinion on wobble..

If it really has only 1 degree positive right now, with a P-bar that actually does inhibit wobble in some cases, could this car get a wobble as soon as the bar is removed?

just a thought, in case the OP never experienced wobble before
.

 

I will do this, thanks Fuzzy. I did read on either Pete and Jakes or speedway that a four link tube axle setup NEEDS a panhard bar, but I am going to take it off and see anyway. After that I'm going to tear it apart and figure out what spring and bushings I need, and fix the angles on the drag link and panhard bar.
Thanks guys.
Aaron

 

Aaron D. wrote: “the car jumps around when the front end hits a bump in the road, and it over steers pretty easy.”

First thing, make sure your kingpins, tie rod ends, steering box, shocks, etc. are not worn out. Also, kingpins that are too stiff, typically due to years of inadequate grease, will also make the car handle poorly. In this case, when the car hits a bump the wheel turns a bit and will not “auto correct” because the force that keeps the wheels straight, due to caster, cannot overcome the friction of the partially seized kingpin. This will cause the car to veer left or right and this has to be corrected by the driver, making it a pain in the ass to drive. Check camber, which I assume is good, ie the wheels stand straight up and down when viewed from the front of the car, and toe, too much toe will cause problems as well as zero toe.

If those are good:
Stock caster is 7 degrees and that works really well, but it's not set in stone.

For clarity of the following discussion, positive caster is when the king pin is leaned back, towards the rear of the car and negative caster is when the king pin leans towards the front of the car. Positive caster causes the car to want to go straight down the road, negative caster…well that is why it’s hard to drive backwards in a straight line at any appreciable speed, when driving backwards, the wheels have “negative” caster and the car doesn’t go too straight.

The important thing is that you don't go into negative caster when the axle moves down relative to the frame. With wishbone or radius rods this can be a problem, especially with only 1 degree of caster. It will become evident if the car tends to get "squirrely" more from a uniform, across the road bump, ie speed bumps, railroad tracks, road work, where both wheels move down relative to the frame vice just one wheel hitting a bump. The way I check for negative caster at downward suspension travel is simply to jack up the car by the frame the same amount in the front and rear to keep the frame at the same angle relative to level ground as when the car is sitting on the ground. Once the front wheels are off the ground, check the caster. With an initial ride height caster of 7 degrees, the caster at full travel should be greater than 0 degrees and not negative.
However, with a 4-bar front end, this should NOT be happening; unless your 4-bars are not parallel or not similar in length.

4-bars, or more aptly, parallel bars, counter the above effect of split wishbone and radius rod type suspension. Incidentally this is a side effect of why they were introduced in the first place. Parallel bars were introduced because you can’t use wishbones split to the rails with tubular front axels because the split wishbone/radius rod suspension will cause the caster on one wheel to be different than the caster on the other wheel when only one wheel hits a bump or whenever the axle is not parallel with the frame, viewed from the front. We can get away with this on an I-beam axle because the axle will twist, however, a tubular axle will not twist and you end up with welds that snap as you’re driving down the highway…bad day, so they came up with parallel bars. This is why parallel bars became popular about the same time that aftermarket tubular axles came on the market. The side effect of preventing the axle from twisting is that the caster stays the same throughout the entire suspension travel. Unless, they’re not parallel or not the same length, which on a tube axle, depending on the amount of “un-equalness” could cause other problems, ie a suspension that will bind. On an I-beam axle there wouldn’t be any suspension binding because the axle will twist, however, you still have to ensure it doesn’t go into negative caster during travel, up or down depending on how the 4-bar is un-equal.

If your bars are parallel and are similar in length and if the car goes straight on a smooth road, then:
1. I'd still set the caster to 7 degrees (you can get away with less on parallel bars because of the above reasons, but I’d still go with factory 7 degrees)
2. You probably have a different problem....bump steer. If you are using a crosslink steering set up, do you have a panhard rod? If not, I just posted a description of bump steer and how to set up a panhard rod at the link:

http://www.jalopyjournal.com/forum/threads/panhard-on-model-a.995360/
Not running a panhard rod with cross link steering will also cause under / over steer, especially at freeway speeds, as the frame shifts sideways relative to the axle.

If you are running a drag link type steering, meaning the link between the steering box and “front driver’s side wheel” is in-line with the frame, then the same basic concept applies as for the crosslink and panhard rod set up. See the attached for a diagram of how NOT to do it. The blue arc is how the end of the drag link, where it attaches to the steering arm, will move as the suspension travels. The red arc is how the axle will travel. In this example the two arcs are not the same, therefore, there will be relative fore and aft movement between the king pin and the end of the drag link as the axle moves up and down. This relative movement will cause the wheels to turn left or right when you hit a bump, hence the term “bump steer.”
Ideally, the draglink would be parallel to, and the same length as, the 4-bar. Also, the point where the drag link bolts to the pitman arm would be mounted directly above where the 4-bar mounts to the frame. In practice, this is not 100% achievable. So do the best you can, just keep in mind, the more you deviate from the ideal, the more bump steer you will have.

Someone mentioned a steering stabilizer. They have their place, but all too often they are used to cover up a worn out or poorly set up front end. Where they are appropriate is if you drive on a lot of rough roads and are tired of the manual steering box ripping the wheel out of your hands when you hit pot holes. Otherwise, I would find the problem instead of putting a band-aid on it, at least that’s my 2 cents.

Kipp

 

Thank you Kipp, you have confirmed what I have been reading during my research on the problem. I think I have a good plan now to correct the problem. I will keep you guys informed on my progress and corrective actions. Thanks again, Aaron

 

I disagree

 

Now ya done it!

 

Well that escalated quickly. Haha. New spring, shocks and tie rod ordered. Time to pressure wash, prep, and repaint everything. Panhard bar mount will be modified after everything is back together.
Aaron

 

really cool hot rod!...I really like the old hot rod vibe, full fendered with dropped head light bar, looks like something built in 1975. Pls keep us updated on you progress.

Kipp

 

My eyes are telling me that your tie rod doesn't look straight. If it's not then it's probably flexing which will cause assorted random issues and should be replaced with a beefier, straight version. Or it's the camera angle, or my ageing eyes!
Chris

 

Brake hose exits frame w/o a clip? See finger. If temp. for repair, okay.

 

Be careful about giving out "expert advice" like that.

It depends on whether there is coils or a leaf spring front.
And with a cross-leaf it is also depending on the angle of the shackles.

You won't see a modern straight axle sprint car without a panhard/watts linkage

 

Yup, tie rod was bent. I have a new one now. All the parts are repainted black and curing. I'll post pics when I get home next week.
Thanks

 

Update: I replaced the spring, shocks, and tie rod. I also modified the panhard mount, so now the panhard bar is horizontal. Repainted and reinstalled everything, set the toe in at 1/8" and took her for a drive. The car handles so much better now. It handles so much better now that I don't even care what the caster is.
Aaron

 

Thanks for the update . Glad things worked out , enjoy !!
dave

 

Well that's a happy outcome, thanks for the update.

 

Well done and now you know.