Upper Control Arm Options

[RogerRamRod]

68 Dart 2door
The parts:
Boxed lower arms
SPC Gen 1 upper arms from BAC
Hotchkis Fox shocks
Hotchkis front & rear bars
Hotchkis rear springs
RMS strut rods
1.12 Torsion bars

The Question
What difference would I expect between leaving the factory upper arm cams at full in, midway, or full out and making all adjustments on the SPC arms?
Moving the cams all the way in would in effect require a longer control arm. All the way out, obviously, shorter arms (provided I could screw the arms in far enough)

I am “flagging” those that I assume might have good answers about this, though anyone else please chime in
@72bluNblu @BergmanAutoCraft
@autoxcuda

Thanks to all of FABO for your assistance with my build thus far

 

[Demonx2]

Subscribed....

 

[CUDA69]

68 Dart 2door
The parts:
Boxed lower arms
SPC Gen 1 upper arms from BAC
Hotchkis Fox shocks
Hotchkis front & rear bars
Hotchkis rear springs
RMS strut rods
1.12 Torsion bars

The Question
What difference would I expect between leaving the factory upper arm cams at full in, midway, or full out and making all adjustments on the SPC arms?
Moving the cams all the way in would in effect require a longer control arm. All the way out, obviously, shorter arms (provided I could screw the arms in far enough)

I am “flagging” those that I assume might have good answers about this, though anyone else please chime in
@72bluNblu @BergmanAutoCraft
@autoxcuda

Thanks to all of FABO for your assistance with my build thus far

Move them all the way in. With the pivot moved in you gain more camber as the wheel goes up during hard cornering.

 

[autoxcuda]

Negligible geometry change.

But I would move them downwards then inwards some IF running an adjustable arm where you can change the arm lengths. Just for added camber gain like stated above.

 

[RogerRamRod]

Similar to this, thanks

 

[72bluNblu]

So, if you assume that the static alignment is already set, then moving the UCA pivot point in (toward the middle of the car) makes the control arm longer, not shorter, which will decrease camber gain. Just assume that the upper ball joint is fixed in position where your static alignment is where you want it. If you want to increase camber gain, the upper control arm gets shorter, which means the pivot gets closer to the upper ball joint (pivot moves out). If you move the pivot in, the distance between the pivot and the upper ball joint gets longer, so the UCA gets longer, which reduces camber gain. Camber gain is mostly the UCA length vs the LCA length, so if you figure the LCA length is fixed, then making the UCA shorter means the upper ball joint will get pulled in more than the lower as the suspension travels, which angles the spindle in, so, negative camber gain.

Some really basic generalities/theory:

• You want the lower control arm to be as long as possible. The longer the LCA, the shallower the arc the spindle travels as the suspension goes up and down. That arc is where a lot of your alignment changes come from (especially bump steer)
• You want the camber gain to be equal to the lean angle of the car. So, if the car leans 5° in a corner, you want the camber gain at that point to be -5°, so the outer tire (the one with the most load) remains vertical and the contact patch is flat on the ground. This is complicated, because you have to calculate all the lateral forces, get the lean angle, and figure out how much the suspension is compressed when the lean angle is 5°, and of course that’s just one angle so that whole operation is a graph depending on the angle of lean the the suspension travel. Bottom line is there is such a thing as too much camber gain.
• According to Steve Smith anyway (lots of books on suspension design for race cars), you want your upper control arm to lower control arm length ratio to be around .6 to 1 for what he calls "a race car based on passenger car suspension". Your camber gain is mostly based on the length ratio of the upper to lower control arm (although other stuff like spindle height comes in too, but more of that stuff you can't really change)

To really do all that right you need to draw up your suspension, calculate the roll center, figure out your bump steer, graph out your camber gain, etc.

My suggestion would be to put the UCA eccentric bolts in the middle if you're not planning on doing all that. If you put them all the way on the inside, you've effectively lengthened the UCA. That will be less of a problem than something very non-theoretical, and that's the thickness of the frame. The further you put the pivot in, the more likely your UCA will hit the frame. Most of the UCA mounting ears already need to be clearanced for tubular UCA's, and the SPC's fall into that category too.

And of course none of those changes occur in a vacuum, changing the control arm length will change a lot more than just camber gain so it's all a trade off.

So yeah, on my Duster with the SPC Gen I UCA's I put the eccentrics in the middle. At some point I am going to calculate all the suspension geometry for my car, graph out all the changes in camber gain, bump steer, roll center etc and at that point I might make some changes. But until then the eccentrics are in the middle and I adjust the alignment with the adjusters on the SPC UCA's.

 

[RogerRamRod]

Thanks,
Middle is where I put them, but sitting & thinking today (look out) I started wondering what changes would happen by making the arms longer or shorter.

 

[72bluNblu]

Thanks,
Middle is where I put them, but sitting & thinking today (look out) I started wondering what changes would happen by making the arms longer or shorter.

If you're just working within the adjustment range of the eccentrics the changes are going to be pretty small. And if you're not going to do a full geometry analysis on the suspension of your car, accounting for ride height, weight, CG etc then I'd just leave them in the middle.

Hotchkis did get into this with their B/E body UCA's though, they relocated the front UCA mounting point. B/E body suspension had more anti-dive built in, which also changes camber gain. Theory on this was the giant B-body station wagons needed more anti-dive under braking because they were massive and had big blocks, and since all the B/E body models shared the same suspension they all got it built in. But, if you're building a corner carver you're increasing the wheel rates substantially, so, you don't need nearly as much anti-dive because the big torsion bars take care of it. So Hotchkis did two things with the forward mounting point on the B/E UCA's, they moved it down, to lower the anti-dive, and they moved it out, to shorten the front leg of the UCA. You can see this on my Challenger with the Hotchkis UCA's. Hotchkis claims (and I haven't calculated the differences) that this improved camber gain substantially.

My '72 Challenger

Hotchkis didn't do anything with the mounting points on the A-body UCA's though. Supposedly this was because there was less anti-dive built in, although looking at the angles it wasn't much different. But the way the A-body UCA's are mounted definitely doesn't lend itself into an easy, bolt in bracket to change the UCA location like the B/E body mounts do, which may also have had something to do with it...


My '74 Duster

 

[autoxcuda]

If you're just working within the adjustment range of the eccentrics the changes are going to be pretty small. And if you're not going to do a full geometry analysis on the suspension of your car, accounting for ride height, weight, CG etc then I'd just leave them in the middle.

Hotchkis did get into this with their B/E body UCA's though, they relocated the front UCA mounting point. B/E body suspension had more anti-dive built in, which also changes camber gain. Theory on this was the giant B-body station wagons needed more anti-dive under braking because they were massive and had big blocks, and since all the B/E body models shared the same suspension they all got it built in. But, if you're building a corner carver you're increasing the wheel rates substantially, so, you don't need nearly as much anti-dive because the big torsion bars take care of it. So Hotchkis did two things with the forward mounting point on the B/E UCA's, they moved it down, to lower the anti-dive, and they moved it out, to shorten the front leg of the UCA. You can see this on my Challenger with the Hotchkis UCA's. Hotchkis claims (and I haven't calculated the differences) that this improved camber gain substantially.

My '72 Challenger
View attachment 1716002782

Hotchkis didn't do anything with the mounting points on the A-body UCA's though. Supposedly this was because there was less anti-dive built in, although looking at the angles it wasn't much different. But the way the A-body UCA's are mounted definitely doesn't lend itself into an easy, bolt in bracket to change the UCA location like the B/E body mounts do, which may also have had something to do with it...


My '74 Duster
View attachment 1716002795

If you want to remove some anti dive, you would put the front adjuster to the lowest point and out the rear adjuster to the highest point.

 

[RogerRamRod]

Thanks for all your replies so far.
BluNblu, 3 questions,
what is that covering the top half of your shock? a cut tube from a fatter shock? I was worried about the exposed shaft on mine. I was thinking of the old accordion boots from my old dirt bike days.
What upper Ball Joint is that? I changed from Moog to ProForged, but I have to remove the zerk for the socket to fit.
Do you remember the part # of those brake hoses? The ones I have seem a bit tight at full turn. (rear hung 2.75" calipers from Diplomat)

 

[autoxcuda]

My Hotchkis RCD shock have large dust covers like those

Ball Joint https://howeracing.com/collections/howe-precision-ball-joints

 

[72bluNblu]

If you want to remove some anti dive, you would put the front adjuster to the lowest point and out the rear adjuster to the highest point.

Now that's thinking! Wouldn't be a ton but a little can go a long way.

At this point, for me anyway, I think I need to actually analyze the geometry of my suspension. Little things like that can definitely be optimized and tuned, but in order to use fine tune adjustments like that you have to know exactly where you are with your geometry. What's good in theory isn't always best for a particular suspension set up, and there are trade offs for everything suspension geometry related.

Thanks for all your replies so far.
BluNblu, 3 questions,
what is that covering the top half of your shock? a cut tube from a fatter shock? I was worried about the exposed shaft on mine. I was thinking of the old accordion boots from my old dirt bike days.
What upper Ball Joint is that? I changed from Moog to ProForged, but I have to remove the zerk for the socket to fit.
Do you remember the part # of those brake hoses? The ones I have seem a bit tight at full turn. (rear hung 2.75" calipers from Diplomat)

1- It's a shock boot like autoXcuda said. Both the Hotchkis and the RCD's have double seals on the ends so a boot shouldn't be necessary, but because I drive year round and occasionally on gravel roads I installed boots on the Duster. I think at one point, back when Summit was selling the RCD shocks, you could buy extra boots for them. Pretty sure that's what it is. Or maybe I borrowed them from a set of RCD shocks, I have a couple sets and not all the cars are on the road so I may have swapped some things around to have them on the Duster, which is the daily. Summit does sell accordion boots for off road shocks, that would work too!

2- The ball joints are from Howe Racing. Rebuildable and the ball joint tension can be adjusted. Also pretty expensive. 1974 DODGE DART Howe Racing 22320S Howe Ball Joints | Summit Racing

3-The brake hoses are from DoctorDiff and came with my 13" cobra style brake kit

 

[RogerRamRod]

Thank you all for the help. I’m not to the point yet of understanding the subtle changes the mounting points would make to anti dive and all. Once I get it on the road, I’ll be more inclined to study further and learn about the finer tweaks etc. I am trying to get the right parts in place so I don’t have to be buying replacement parts very soon.
Thanks again

 

[BergmanAutoCraft]

If you want to remove some anti dive, you would put the front adjuster to the lowest point and out the rear adjuster to the highest point.

Why would you want to remove anti dive on an already nose heavy car?

 

[BergmanAutoCraft]

68 Dart 2door
The parts:
Boxed lower arms
SPC Gen 1 upper arms from BAC
Hotchkis Fox shocks
Hotchkis front & rear bars
Hotchkis rear springs
RMS strut rods
1.12 Torsion bars

The Question
What difference would I expect between leaving the factory upper arm cams at full in, midway, or full out and making all adjustments on the SPC arms?
Moving the cams all the way in would in effect require a longer control arm. All the way out, obviously, shorter arms (provided I could screw the arms in far enough)

I am “flagging” those that I assume might have good answers about this, though anyone else please chime in
@72bluNblu @BergmanAutoCraft
@autoxcuda

Thanks to all of FABO for your assistance with my build thus far

I suggest pushing the front eccentric out all the way and agree with others to put the rear in the middle. One has to judge how much thread engagement into the sleeves is needed, based on the desired caster. The higher caster improves stability, but induces lift on the inside tire in turns. It also reduces the camber change through the travel. There are a lot of factors involved depending on the desired result.

 

[autoxcuda]

Why would you want to remove anti dive on an already nose heavy car?

It was just brought up what Hotchkis did to the E-bodies. If you have a fully adjustable arm like a SPC, you could do something similar to an A-body manipulating the eccentrics.

Whether you should or shouldn’t, I’ve not really explored. I thought one of the reasons hochkis said they did it was bumpsteer on the E-body.

I’ve done bumpsteer on a A-body it it was pretty good even without adjustment.

 

[BergmanAutoCraft]

It was just brought up what Hotchkis did to the E-bodies. If you have a fully adjustable arm like a SPC, you could do something similar to an A-body manipulating the eccentrics.

Whether you should or shouldn’t, I’ve not really explored. I thought one of the reasons hochkis said they did it was bumpsteer on the E-body.

I’ve done bumpsteer on a A-body it it was pretty good even without adjustment.

I hear you. The only time I would do this is for drag racing where weight transfer is more important. Mopar tried this and decided the anti dive was more beneficial.

 

[72bluNblu]

I'm sure Hotchkis looked at all the numbers when they modified the mounting points for the E-bodies. I can understand the thought process though, the amount of anti-dive you'd want on a B-body station wagon with 100 lb/in torsion bars and the amount you'd want on my Challenger with 270 lb/in torsion bars probably wouldn't be the same. I never had any issues with the car under heavy braking, so they must have done something right.

The T/A cars were modified like that, the anti-dive causes caster changes (and effects camber gain as well). But they only did one car and at least according to Bob Tarozzi they removed ALL of the anti-dive. The result was that the car would "dart around" under heavy braking, so even though the drivers didn't like the caster changes they put the anti-dive back in. He talks about it in this article under "Suspension Snafus"
‘70 Dodge & Plymouth Trans Am Season: Crazy Insider Story!

But using the T/A cars as a guide can be a bit misleading, they did stuff because of class rules and what was available to them at the time. Like raising the back end of the cars for tire and exhaust clearance, that's goes completely against what you typically want to do to improve handling. And yeah, that alone would shift the balance forward and make the anti-dive part more important. Lowering the rear compared to stock would shift the balance back and make the anti-dive less important, so it really depends on the individual cars set up.

On my Duster I run the front and rear eccentrics in the middle of the adjustment range. The SPC's have plenty of thread engagement, but that makes sense because running negative camber and positive caster means shortening up the legs of the UCA's vs the stock arrangement. So running -1° camber, +6.5° caster and 1/8" toe in leaves plenty of engagement on the threads. Heck you can see in the picture above I posted that the rear arm of the SPC upper on my Duster has very little adjustment left to get shorter.