UCA Shim vs Caster

[basstrix]

I'm looking for info on the relationship on shim thickness at the upper control arm (UCA) vs caster angle. I'm envisioning a trial and error / characterization exercise to get the front end dialed in.

Going way back, I had trouble getting the caster set properly...the alignment shop said they weren't able achieve positive caster....they had the strut rods pulled so tight the bushings were bigger than the backing washers.

The last time I had the front end aligned was right after I had a chassis shop true up the unibody. Since that time, the left lower control arm has migrated forward at the chassis bushing such that the front edge of the arm is rubbing on the inner surface of the mounting bracket/cross member. I assume that's a combination of a crappy LCA and too much pull from the strut rod.... It's obvous the forward strut rod bushing is also more compressed than the rear....

Going forward, I intend to use shims under the UCA to get positive caster, rather than pull on the strut rod so much.

If any of you have this info or know where I can find it, please post it up.



Disclaimer: I thought I posted on this topic last night, but I must have forgotten to complete the post.

 

[Sheriff41]

If you're asking how many degrees per shim thickness, I don't think I've ever seen it listed. Back when I was doing alignments it was pretty much trial and error refined by experience. I'd be interested if you do find a reference, though.

 

[basstrix]

Sheriff, that's exactly what I'm looking for....whether it's something documented or through experience. I'm going to try and construct a kinematic model in CAD, but those can be difficult to obtain accurate measurements. I think I can get a general sense from a simple model, but one never knows how good the model is until you put it into practice.

Since you have experience doing alignment, I'd like to pick your brain. I have the rear axle supported on jack stands. I built a set of wheel cribs and put steel wheels on them so the cribs can move in and out as the suspension articulates (from raising/lowering on jack). On top of the cribs, I have turn plates. I used a laser level to set the ride height, front to rear, the same as when it's sitting on the ground.

I'm using the Longacre digital caster camber gauge. I hope to be able to make some basic adjustments with the vehicle supported as described. In your opinion, will I be able to make any adjustments without pulling the front wheels? I'm pretty sure I can do camber, but concerned caster may require a bunch of disassembly.

 

[Sheriff41]

You shouldn't need to remove the wheels but you will probably need to raise them to take the load off so you can reach in and r&r the shims. Also, after jacking you need to make sure the suspension is settled. On the old Bear rack we would roll the car back then forward to do this.

 

[basstrix]

The cribs with wheels on them is to allow the suspension to settle after raising/lowering.

 

[Hemikiller]

The 66 shop manual shows 1/32" of shim affects caster by 1/2°.

 

[cjstaci]

Give a call to street or track, LLC. It’s been a long time since I’ve rebuilt my front suspension, if I remember correctly, I used an 1/8” shim and the alignment shop was able to get 4 degrees castor.

 

[Stanglover]

I'm interested in this too. My alignment guy was able to get 3 deg of pos castor and I feel it needs about 4 to 4.5 degrees. With other minor suspension upgrades, heavier sway bars, Boss 351 (Eaton) rear springs, KYB Excel G shocks, the car doesn't seem to want to center as quickly as I'd like.
I talked to my alignment guy and he has experience with shimming, so when I can get around to it, I'll do it, but if others can get some better idea of shim thickness ahead of time, that would be very useful.
Also added to this, with radial front tires, what is the preferred toe-in? Mine is currently 1/16" per side. I feel 1/8" is more where it needs to be. Thoughts?

 

[basstrix]

Thanks Hemi, that's exactly what I'm looking for.

 

[jomar1957]

I don't think 71-73 used shims,camber was adjusted on lower control arms, caster by strut rod, keep in mind that cars had bias belted tires that expanded when rolling that created more caster when rolling. Radials Don't expand therfore caster is lacking and difficult to adjusted for what is needed to stop the wandering. The best answer is global west upper control arms with 3 degrees of positive caster built in and it does away with the crappy upper control arm bushings found in stock application and good ball joint as well . Best thing I did to my car along with roller spring perches

 

[Sheriff41]

Also, make sure your UCA shafts are centered in the arms. I've seen some that were installed way off center.

 

[rcadd1ct]

I just tried to do this. Personally I don’t think you can get enough sJim’s in to make 3 degree. If you do the pivot shaft would most likely bend or other bad things due to pressure and a non flat mounting surface due to angles.

here are the stock vs the global west MNR-733 upper control arms

it moves the balljoint way farther than I thought.

I added 3/32 in shims No where enough. Then the issue above so I went gw.

like I said doing this now….so if you have questions let me know.

 

[rcadd1ct]

OBTW.

if you do get enough caster from the lower strut rod you might have the tire hit the fender when turning. I was out of adjustment on my opentracker racings heim joint strut rods and I only had 1/4” gap between the tire and fender. That was only 2.5 degrees of caster.

 

[basstrix]

I just tried to do this. Personally I don’t think you can get enough sJim’s in to make 3 degree. If you do the pivot shaft would most likely bend or other bad things due to pressure and a non flat mounting surface due to angles.

here are the stock vs the global west MNR-733 upper control arms

it moves the balljoint way farther than I thought.

I added 3/32 in shims No where enough. Then the issue above so I went gw.

like I said doing this now….so if you have questions let me know.

How far over is the ball joint on the GW arm vs the OEM?

 

[rcadd1ct]

How far over is the ball joint on the GW arm vs the OEM?

Look at the second picture

Looks like an inch or so, but I didn’t measure

 

[rcadd1ct]

So I figured out a way to measure

Looks like in moves back 7/8”. I am using the center of the spring perch as my center reference which lines up with the center of the pivot shaft.

 

[Bentworker]

So I figured out a way to measure

Looks like in moves back 7/8”. I am using the center of the spring perch as my center reference which lines up with the center of the pivot shaft.

Just those control arms alone should give you 3-4 degrees positive caster added over the the stock symmetrical geometry.

 

[rcadd1ct]

Just those control arms alone should give you 3-4 degrees positive caster added over the the stock symmetrical geometry.

Yes, 3 degrees in the A arms.

my main point was I don’t think you can shim the bolts enough to move the ball joint back 7/8”

I may try to jig something up to see how many shims it would take to move it back almost an inch.

 

[basstrix]

So I figured out a way to measure

Looks like in moves back 7/8”. I am using the center of the spring perch as my center reference which lines up with the center of the pivot shaft.

Thanks. I'll be modeling this and will be interesting to compare what GW says is needed for 3deg vs what the cad says. I'm thinking I may mock it up on my ranchero parts car...the front end is already stripped and all the parts available. I'll post up the results.

 

[basstrix]

Here's a plot of caster angle and rearward shift of the Upper Ball Joint vs shim thickness under the forward bolt on the UCA mount point. The measurements were done with tape measures, steel scales, bubble levels etc....in other words, the accuracy isn't stellar. A kinematic CAD model was created to mimic the 4-bar linkage and adjustments as designed for our cars. It should be noted that the chassis mount points were measured from a 1971 ranchero. I believe they are the same as 71-73 mustang, but don't know that for fact.

The model starts with camber set at near zero , no shim, and adds shim in 1/32" increments. The LCA is neutral (at 90 deg to roll axis). The model predicts 3 degrees of caster at 1/4" of shim. This resulted in a camber change of +0.6 deg. A camber correction was done (not on the plot) to bring camber back to zero and required 0.11" outward movement on LCA. The resulting change to caster was minimal (0.005 deg).

Interesting takeaways: The model is somewhat aligned to Hemi's info....the model predicts 0.36 deg for 1/32" of shim). A 7/8" rearward offset of UBJ gives 4.8 deg of caster. Without validation, I can't say whether or not the model is accurate.

This should not be taken to indicate the amount of shim that is acceptable to these front suspensions....for all I know, they cannot be shimmed at all. Use at your own risk!

Edit: Just realized that I wrote down negative camber change...it actually went positive per the way camber is measured.