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tholt

Yet another Rear control arm design

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Okay... trying to get caught up here and my brain is really starting to hurt.

 

So, if I am understanding Dan's sketches and and the output of his spreadsheet, it says that if you have a 1 degree angle between the inner and outer control arm axis the amount of bind that will induce is about .025"...

 

That would be 1 degree of toe per side.... that's a LOT... The most I would ever try and run in the rear is about 1/8" per side, that would be about .3 degrees ( I would really only run about half of that). Using that number in the spreadsheet it says that the induced bind is only about .007" and it only varies by .0015" through 20 degrees of travel. If you loosen your camber plates the .007 will probably self adjust. Chances are the amount of bind we are discussing is way down in noise level or at least well within teh tolerance range. Unless of course your frame is way out of square and you are trying to use the adjustability to compensate...

 

I think I am on board for trying to build something like the arms Jon sketched... the rear suspension is already disassembled for a shock rebuild and the plan was already to build new LCAs. I'll be busting out the CAD soon with an intial design... stay tuned.

 

Tom

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Tholt,

 

Your conclusion is what I came up with. The magnitude of the deflection is less than I assumed (that's why I did the calculations and spreadsheet). What the spreadsheet also says is that if the control arm cannot flex, the top of the strut will try to deflect 0.026" for a 0.3 degree toe in condition (0.3 degrees is equivalent to 1/8" toe in and a 24" tire). In reality, the strut will be much stiffer than the control arm, so a majority of the deflection will occur in the control arm. The ratio of the torsional stiffness of the control arm to strut will determine force applied to by the control arm to the strut. If the control arm has a torsional stiffness of 1000 ft-lb/degree, a significant side load will be applied to the strut. If the control arm has zero torsional stiffness(as in an A-arm toe link), then no side load is applied to the strut.

 

As you observed, much (but not all) of the strut deflection due to splindle pin angle can be mitigated by careful alignment of the strut. By this I mean that after a toe adjustment, the strut must be shimmed forward or aft to minimize the deflection.

 

As an example, if the toe is set at 0.3 degrees, the top of the strut will move backward 0.026" with the control arm horizontal, and will vary from 0.021" to 0.026" to 0.021" as the strut goes through its full range of motion. 0.026" is equivalent to 0.070 degrees of control arm twist. So given 1000 ft-lb/degree, the torque will be 0.070 x 1000 = 70 ft-lbs. The bearings inside the strut are approximately 16" above the connection to the control arm, so the side load on the bearings will be approximately 70 ft-lb*(12in/ft)/16in=52.5 lbs.

 

Given the same conditions as listed above: If the strut is shimmed backward 0.021" after toe alignment, the variation can reduced to 0 to 0.005 to 0 as the strut goes through its range of motion. The maximum twist on the control arm is reduced to 0.0118 degrees and the strut side load will be reduced to 0.0118/0.070 x 52.5 = 8.85 lbs.

 

Now assume the worst case with the toe is set at 0.3 degrees as above. Instead of shimming the control arm to correct direction, assume that the strut has an intial misalignment of 0.125". The twist in the control arm increases to 0.48 degrees. The side load on the strut now increases to 0.480/0.070 x 52.5 = 360 lbs:shock:.

 

If we must use the H-arm strut lower control arm, we need to be really careful to shim the strut forward or back to minimize the control arm deflection and strut side loads. These problems completely go away with the A-arm toe lnk type lower control arm.

 

Dan

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Can you make any use of Terry's example where he was able to get 1/8" deflection in the outer bearings by applying 130 ft/lbs of torque to the spindle pin? Seems to me like this distance can be converted into degrees and then that should be able to be measured in side force on the strut.

 

EDIT--What would really be cool to find out would be how the control arm deflects when you go over bumps. Then you could get real world numbers for side force. The more I think about it and see the examples here the more I think I might just break down and make a new set of control arms. If for no other reason then not wanting to have to disassemble the strut to shim the spindle pin every time I want to make a toe change.

 

Maybe I can use the inner tube from my modified arms, since Ron did some machine work on those parts for me...

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Dan and Tom, what size bearing are you guys going to use for the rear? I'm thinking about using a 3/4" rod end back there, since it carries ALL the load, then using a reducer bushing to 5/8, running a longer spindle pin through the strut to the front and then for convenience sake using a 5/8" turn buckle and rod ends for the toe adjustment up front on a double shear mount to the main part of the control arm. The front part seems overkill, but the spindle pin hole is just too perfectly sized not to use it. Have you guys given this stuff any thought yet? I'm just looking in the Coleman catalog trying to figure out how much this project will set me back...

 

I know Dan is likely going to use chromoly. I'll use mild DOM. Wondering what diameter and thickness to run too. If I used Coleman's off the shelf threaded tube ends that would mean a 1" ID tube, maybe a .095" wall. Looks like they sell .095, .085, and .065. Not sure which would be best, but I tend towards the conservative side when it comes to these things...

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I am planning on increasing the rear track by 100mm overall (50mm per side) by extending the OEM transverse link (LCA). This post is not exactly about that however. What I want to say is that to address the issue of subsequent strut misalignment I will be removing the strut tube entirely from the bearing housing and reboring the housing to an angle which will allow the top of the strut to locate back at its original position and maintain zero camber. I will probably need to insert a thin sleeve to ensure the strut tube is a snug fit before re-welding.

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The more I think about it and see the examples here the more I think I might just break down and make a new set of control arms.

 

Jon - repeat after me:

 

My name is Jon

I'm a fabaholic

My car will someday see the track again ... maybe

(repeat)

 

Step away from the welder and get that thing on the track already!

 

Cameron

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That is pretty far off topic for this post ...

My apologies if the intent of my reply was a bit obscure. All I was trying to say was, if a particular LCA arm design was being limited by a resultant alteration in the location of the strut top (as mentionrd is some posts) then here was a way to move the top of the strut back to the OEM position, i.e by repositioning the strut tube's location on the bearing housing. Whilst my particular misalignment will be caused by an extended LCA the same technique could be applied to misalignment caused by alteration to the toe-in.

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You will not get any binding as long as the spindle pin is parallel to the axis of control arm rotation. So lengthening the track shouldn't be a problem.

 

The issue that might occur is having enough adjustment at the top of the strut to compensate for camber. Adding 50 mm per side to the length of your lower control arms will give you about 6.5 degees of negative camber. I don't think my Ground Control camber plates can compensate for that.

 

This does sound like the topic for a new thread.

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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

LCA2-B-sm_thumb.jpg

lca2-S_thumb.jpg

LCA2-EXPLODED-S_thumb.jpg

LCA3-sm_thumb.jpg

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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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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...

 

Since that's what most people run on the front for a tie-rod setup I don't see why it wouldn't work on the rear.

 

Cary

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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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