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Yet another Rear control arm design


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On to Phase 2:

Okay... At first pass I'm not liking the forward toe link too much. The basic problem is the toe link really gets in the way of the sway bar link. I haven't figured out a good way to make that work yet other than ditch the sway bar or put the toe link at a bad angle. I did two iterations of this design. The simplest is shown below as LCA2-B (purple). It works the best of the forward link designs and mounts in the stock inner locations. It might be advisable to add one more bar from the inner bar to the toelink joint... but again no swaybar

 

I tried another version using heims on the inner mount (LCA2)(gray), but it really didn't work. There was not enough room to fit two 5/8" heims end to end for the toe link. I do kind of like the inner mount concept though.... see the exploded view. This would use 3/4" bolts and heims with a section of tap tube tying it all together front to rear. I would have to make some aluminum bushings to support the bolts in the stock mount locations.

 

Next I looked at moving the toe link to the rear (LCA3)(yellow). I know this has some drawbacks based on our discussions, but it sure does work better from a design simplicity standpoint. It also lets me use the inner heim design from above. I need to evaluate just how big the drawbacks are to having the front outer mount fixed. It might be a little more tedious on the alignment rack, but probably not by much. I've already thought of adding spherical bearings on the inner bushings and using a fixed tube on the innner mount rather than the heims and that hasn't been ruled out, but it would be about 2X the cost.

 

Go ahead... tell me what you don't like! I can take it. :)

 

Tom

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I like the first example because it is simpler and uses fewer rod ends. It is exactly what I envision. As far as the anti-sway bar goes, I don't use a rear sway bar. If I did, I would rather attach upward to the strut housing rather than down to the control arm.

 

I assume that the connection between the toe link and A-arm is a spherical bearing and that it is free to rotate.

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Tom, have you seen how Terry Oxandale and I got our rear monoball pivots installed? Details in this thread: http://forums.hybridz.org/showthread.php?t=106457 I used a cheap circle track monoball housing and welded it to the frame and the uprights. Terry made an aluminum piece that held the monoball in the stock position. I think this would simplify/strengthen the arm itself, although it's not strictly necessary and entails it's own set of challenges.

 

I don't like the 3rd design, but it sounds like you expected to hear that.

 

Did you think of moving the inner end of the toe link further forward on the arm? That might be another option for gaining more sway bar clearance. I haven't really looked yet, but I suppose at some point that there is an issue with tire clearance.

 

In general, attaching the sway bar to the strut is the way to go if you can. The sway bar will unload the inside rear tire, so it may be advantageous to get rid of it entirely if you can get some stiffer springs to control the roll instead.

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Can I assume these designs are based upon the idea of modifying the strut so that middle link attaches under the tube (intersecting the centerline of the strut tube) as was discussed previously?

 

If so, then these are interesting. The single thing that concerns me is that halving the length between the two bushing (and this could be mitigated somewhat by lengthing the pin on one side) doubles the forces on the bearings during acceleration and braking. Let's continue...

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The strut tube is behind the axle shaft. So without modifying the strut, the rear rod end is below (but inboard of) the strut tube. I like having the fixed point on the A-arm below the strut tube so that adjustment to toe will have a minimal affect on camber. So, the spacing between the rod ends has not changed.

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As far as the anti-sway bar goes, I don't use a rear sway bar. If I did, I would rather attach upward to the strut housing rather than down to the control arm.

 

I thought this too because you can get better motion ratio / longer end links / more range of motion / more consistent through travel / etc .... HOWEVER wouldn't attaching to the strut also add more sideload to the strut ... the thing trying to be avoided here?

 

Cameron

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The angle between the strut housing and the end link for the sway bar would be less than 20 degrees. If the sway bar were applying 200 lbs of force at its end, the force applied perpendicular to the strut (bending force) will be equal to 200 lb x sin (20 degrees) = 68 lbs.

 

This torque would be proportional to the amount of roll and would only be present in turns. Mounting sway bar end linnk will cause a bending moment in the strut.

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The strut tube is behind the axle shaft. So without modifying the strut, the rear rod end is below (but inboard of) the strut tube. I like having the fixed point on the A-arm below the strut tube so that adjustment to toe will have a minimal affect on camber. So, the spacing between the rod ends has not changed.

I just looked to verify for myself, and if anything the spindle pin housing is maybe a hair too long, but the rod end is damn near centered on the strut tube in the rear. This picture from Arizona Z cars website shows it perfectly:

RBILLET51.jpg

 

I thought the rod end would have to be spaced back. Now I'm thinking the 5/8" rod end and single shear might be enough...

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The rod ends that I'm using in my current rear control arms are Aurora XAM10-T which have a 3/4" shank and a 5/8" ball. For axial loads they are rated at ~40,000 lb. Aurora suggests that the rod ends are good for 10% of that in the orientation we use them.

 

I'm toying with the idea of using a ball joint on the A-arm portion. To do so I would have to modify the rear strut. We'll see...

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I'm toying with the idea of using a ball joint on the A-arm portion. To do so I would have to modify the rear strut. We'll see...

 

A spherical on the end of the arm could attach to a double eared bracket on the strut. You could do that in such a way that you could use stock or the new option with the same struts.

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To improve the roll center for severely lowered suspensions, one could consider an application already shown quite a while back in which a separate piece (I don't recall the fabricator) was fabricated that bolted solidly onto the strut pin housing, and then the lower control arm bolted onto this additional piece. It in effect lowered the roll center by lowering the attachment point by about 1", and this piece could be designed to perfectly center the outer bearing in a double-shear arrangement, and extend the front bearing to offer more resolution in toe adjustment.

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  • 3 months later...

Attached are some pics or our interpretation of the new rear control arm. These were built out pf parts recycled from the previous control arm thread that Jon started where rod ends replaced the outer rubber bushings.

 

Two things were immediately apparent when putting these on. It was much easier to get them bolted up and the strut aligned with the camber plate with no fuss. It was one of those why didn't we do this a long time ago moments. And I should also mention that aligning for rear toe is now very simple.

 

These are mounted on the back of Dave Kipperman's LS2 powered autox car. We gave them a bunch of abuse this past weekend with no issues attributed to the arms. We had a record heat wave move through our area so any plans for a back to back comparison were thrown out and at this point we're probably not motivated enough to revisit.

 

Cary

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NOW you've hit the mark! My biggest concern in the current aftermarket designs (and even my own work) is the issue of maintaining an EXACT spacing between fixed heim joints. This design eliminates preloading on a spherical bearing that may be only a few thousanths off the dimension of the spindle pin boss. Kills me that such a simple concept evaded me.

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