Technical Adjusting caster with four link?

My 1928 Tudor has a super bell 4" drop tube axle (spring over axle)with a four link system. I have found that my caster angle is 1 degree at ride height. I would like to adjust the caster to around 6 or 7 degrees. Do I just adjust the four bar links longer on the bottom links and shorter on the top links? Do I need to worry about the angle of the top of spring in the crossmember? Are there wedges that go in between the spring and crossmember? Sorry if these are basic questions, but this is my first Model A. Thanks, Aaron

 

Why 6 or 7? you don't just adjust to a random number. What is the car doing wrong, that you want a huge change?

Yes you need to match the angle for the spring to mount.

the lower bars get moved forward(longer), but also the uppers need to go forward(longer) only a tick, just to keep the total spring pack lined up to the front crossmember mounting point.

Yes you can use an angled shim if needed, during a caster change. "if needed" means we don't know if the angle is OK before you start. It could be in a bind because the front crossmember should be set for more than 1 degree, when built. IMO

 

Just my $0.02 worth..but , be careful if you lengthen the bars as to the amount of adjuster thread left in the bars . Don't want to screw them all the way out...

Can't answer to the spring angle tho...

 

Speedway has adjustable spring perch.Crossmember should hav been installed with correct angle.

 

F&J, the car jumps around when the front end hits a bump in the road, and it over steers pretty easy. From the research I've done, most model A owners prefer 6 to 7 degrees positive caster. The front crossmember seems to be the original one, its still riveted on. I don't know what angle Ford originally installed them at.

Turbo26T, thanks, I did check the length of the rod ends and made sure there was sufficient threads inside the rod. There are 20 threads on the rod end and I turned them in 15 full turns.

Mopar, the crossmember may be installed correctly and my spring could be tilted back as it is now, I don't know. Is there a way to measure the crossmember angle?

 

It may have been.
Then someone throws a rubber rake on the car and now its quite different.

 

You should post up some pictures of your cars front axle and associated bits.
There might be some issues that you overlooked and others might pick up on...

 

Is your frame stock? If so I would recommend the adjustable perch's. Pete & Jake's recommend 6-8*

 

Aaron. ...
Don't know your location. ...but...
I work with solid axle stuff a bunch ( midget race cars)
And a hot rod or 2...
I I can help you out shoot me a message. .
I'm in the monterey bay area in the middle of California.

Caster should be fairly easy to adjust. .as said earlier. .you might need a wedge or adjustable perch at the crossmember...
No problem. ..

 

On a full fender it may be tough to view, ... but if a spring mount angle is not correct for the 4 bar setting you choose, the spring eye ends of the spring, viewed from the side, will look twisted, compared to the stack of leaves.

it would not be easy to measure on a done car, with fenders radiator etc

You don't need to follow other random caster settings. You may find that with your rake and your cross member, that you can only adjust to 4 or 5 before the mount angle is wrong. At that point, test it on the road.

caster is not random number for "any A" or "any Ford". You are affected by tire belt type and sizes, the cars weight bias, the ratio of the steering box, etc etc.

 

Stock Model A crossmember is set to 7 degrees.

 

....provided the chassis is dead level..

so, if the OP has 2 degree chassis rake, and if he adjusts the bars so that his spring is fitting the stock A crossmember exactly, then his true caster, theoretically should be at 5. At this point, all the old wives tellers would say shim it to get 7 or 8. Why do that without a test drive?

back to this random 7-8 stuff. Take a look in Motors Manuals for a typical 60s rear drive US car, You will see in the alignment specs, that caster is more on power steering option, versus the same car with manual steer. That should make it very clear that the car itself, does not indicate a fixed caster setting, rather it is determined by the sum of it's parts.

 

You say it 'jumps around when you hit a bump...'? You might need a dampener, P&J have one for 4 bar front end that work good. The caster thing gives stability on flat highway, it won't 'wander'.

 

On a 4 link you seldom adjust the lower bar much once you get your axle located, the lower bar locates the axle and the top bar sets angle. Sometimes when you move the top bar a ton you have to make minor adjustments to the lower bar but not very often.

If you are running a cross leaf you are going to have to do more then just adjust the 4 link if you are making a big change in the caster angle, if you got very far you are going to bind your spring, so you will also need to shim the spring in the mount to get the proper angle on the spring as well as move the axle.

Here is something to think about, back when we were building these old cars we seldom changed anything when we changed ride height. If we dropped it we dropped it and went on with life, that does not make wanting to achieve a specific caster angle wrong, but if you can drive it and it does not have any handling ills just drive it.

 

Dirt T, the frame is boxed and Z'd in the rear. Thanks for the recommendation.

Dave, thanks for the offer, it would be a long trip. I'm in near Tampa Fl.

Fuzzy, thanks for the useful info.

OJ, thanks for the useful info.

Pork, thanks for the useful info. So when setting up a four bar, do you mount the spring and adjust the lower bars to align the axle to the spring, then slightly adjust the top bars for correct caster? It makes sense that if the frame is at or near 0 degrees and the front crossmember is at 7 degrees, the axle would be at around 7 degrees. It then would be minor adjustments to get the caster right.
I was thinking about putting a new spring and shocks on anyway, maybe I can figure all this stuff out then.
Thanks, Aaron

 

Many original A crossmembers were cracked and repaired at the mount over the years. If they cut it out and put a new plate where it was cracked, the angle could be changed.

If it still is at 7 and the axle is known to be at 1, then there should be obvious bind in the spring. Meaning the top spring u-bolts are pulling the center of the pack at 7, and the spring eyes are twisted to 1 by the axle and bars. They might have an angled shim at top, or modified the crossmember angle.

You need to adjust all 4 bars to get the caster and location correct. Let's say you make your first major bar adjustments to align the spring and get the caster close, but find that you need more caster to match the crossmember, then you still need to adjust all 4 bars...IF the spring was aligned.

If the spring was aligned perfect and then you only adjust one pair of bars to add caster, then the spring can't still be aligned perfectly now.

 

This is how I did mine. Original 32 Ford chassis, Boxed with a big X member and a 6 inch Zeee in the rear. Motor and Tranny in chassis, Body on frame, Gas tanks in. Placed car on floor with out 4 bars attached to axle. Took angle finder and placed it on top of King pin. Saw that it was at the required 7 degrees and then attached 4 bars and rechecked. If it would have been out I would have dropped the bottom bar and tightened the top bar a little, read 1/2 turn, at a time until it was correct. Then adjusted bottom bar to fit.
Good luck.

 

Well after reading on Pete and Jakes website , I may have found some of my problem with the jumpiness of the front end when I hit a bump. My panhard bar is not parallel with the steering drag link. This would create a bump steer when I hit a bump. The pic is kind of hard to see but the angled bar is my panhard bar.

 

Judging by flatness and height of the spring eyes over the axle, it looks like it is too short. May be just the picture; but it also looks like it is bent on the R/H side possibly as a result of it being short. Any chance that you have a better picture showing the whole front. Hard to see behind the panhard bar and tie rod; but it appears the drag link is running uphill to the right, sorta of in a conflicting angle with the panhard bar. Maybe take the panhard bar off and see if it makes any change in bump steer.

 

I notice a couple of things. First the arc that the axle is being pushed by the severe angle of the pan hard bar could be a lot of "bump steer" issues. Second, perhaps it's just the angle that the picture was taken at, but the passenger side shackle bolt looks way off center thru the spring. Check the slop in the shackle. Like Rich B suggests you may be wearing out the bushings because of the pressure the spring is putting on them. You may need new spring shackle bushings. Are those scrapes on the axle because of bottoming out?

 

Pretty much, use your lower link to center your axle and make minor adjustments with the upper, fine tuning so to speak.

 

That will bind the spring.

When you adjust the top bars after the spring was aligned to the crossmember mount, that will push or pull the top of the spring off it's centered location.

you cannot do caster with only 2 bars, unless the spring was misaligned to the top mount to begin with, and if the caster change of two bars luckily brought it back to alignment

 

Woodie, I agree about the bump steer. I think the shackle thing is just from the picture. The scratches are from a floor jack, just paint.

Rich, I agree the spring looks to flat and bent. I don't know if the guy that had the car before my "rearched" the spring to get the front end lower, but like you say, it may be to short and spread to far. The drag link is running at a slight angle also, I may be able to slide the pitman arm higher on the steering box shaft, not sure but I'll take a look at it.

Thanks guys, Aaron

 

It looks like it was sitting too low at one time and a spacer block added between the spring pack and crossmember. What I'd be doing is taking the whole thing apart and rebuilding it. Your problem will be a combination of things that others have pointed out and worn out parts. If you make another adjustment to correct the effect of several problems you will be adding a complication. It is the winter and nows the time to tear the whole thing apart and do it right.

 

Well I wouldn't know.

 

Guys....it could be the car sitting on uneven ground...or camera distortion.. But the shackle angles are not matched??

Because it has P-bar, does that shackle mismatch mean that the bar is actually pulling the axle over to one side? That is what it looks like to me.

If the bar really is wrong length, that must really act spooky over bumps?

Any opinions on that would be good to hear
.

 

With the panhard bar running downhill & the draglink running uphill , 1 gets "shorter" when the other gets "longer" , this makes bumpsteer twice as bad as if there was no panhard...figure out a way to get them at nearly the same angle [level would be best]
dave

 

First thing to do is remove the pan hard bar and see how it drives. NO straight axle car needs a pan hard bar!! Period!!!!

 

I agree wholeheartedly ! But his reversed angles are making the situation worse than if they were parallel...
dave