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Yet another Rear control arm design


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...I hear a fat lady singing....   http://www.modern-motorsports.com/catalog/product_info.php?products_id=68&PHPSESSID=fb548f2abfea0ebac795ee81c9ae5a99

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I fail to see the Subaru's relevance.

 

No effort to control strut side loading.

 

Replace all the bushings with bearings and then we could talk about binding issues.

 

I'm game, lets talk. If all points where replaced with spherical's, what binding would there be?

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Seems to me that as the suspension compresses the longitudinal link will cause the suspension to swing in an arc. That would very quickly cause bind in the lateral links, if everything were solid bearings.

 

EDIT--Actually, maybe that would just cause toe change. Still not what I'd be looking for.

 

The RCVD book discusses this suspension on p642 as well. Binding is not mentioned as an issue with this setup, although toe change is and it's suggested as a good suspension design for compliance and ride comfort.

 

RE-EDIT- The reason you don't have bind with this one (as much, still has some like any strut) is that the upright can pivot where the longitudinal link attaches and the lateral links allow the suspension to move fore and aft a bit. With the Z neither motion is allowed by the H arm, so any force that wants to move the suspension in those directions acts directly on the strut housing and causes the side loading on the strut.

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Seems to me that as the suspension compresses the longitudinal link will cause the suspension to swing in an arc.

 

Could be used as anti-squat?

 

That would very quickly cause bind in the lateral links, if everything were solid bearings.

 

Got it, and I agree. I had assumed your were referring to spherical bearing's. I don't believe there would be any binding with spherical's.

 

EDIT--Actually, maybe that would just cause toe change. Still not what I'd be looking for.

 

Subtle use of toe changes are often desirable. I probably would'nt go far out of my way to have dynamic toe, but I'm not opposed to it if 'comes with the package' and the curve is reasonable.

 

The RCVD book discusses this suspension on p642 as well. Binding is not mentioned as an issue with this setup, although toe change is and it's suggested as a good suspension design for compliance and ride comfort.

 

It does say something to that effect, but its related to bushing orientation/durometer, which wouldn't be applicable here.

 

I'm 'arguing' simply becuase I see it as a reasonable alternative and based on similar packaging. Unfortunately, I'm not conviced of the strut side load's being a factor that can be reasoanbly reduced with control arm design, but its outside my ability to either argue or prove.

 

Am I bugging you yet? :-)

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It still seems so obvious to me as to what mechanism is causing the side load that I'm a bit surprised that Dan and I still appear to be alone in our opinion. That said, you're not bugging me at all. I'm here to test my theories and learn. I had to think pretty hard to analyze what makes the difference in the H arm and the tri-link strut design you posted, and I like that type of thing so thank you.

 

In the end I think the exercise was instructive. In the tri-link suspension the links allow the strut to move fore or aft or rotate slightly without binding the links. In the H arm strut setup any of those motions would be resisted by the control arm. Failure of the control arm to control the force would then transfer it to the strut housing which has no freedom of movement in those directions. The strut is then forced to control the motion, which loads it like a beam. But it isn't a beam, it's a strut and so the relatively small connection between the upper and lower halves of the strut is the bushing inside the strut, which gets loaded and there's your stiction.

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Interesting string of contributions here. I will not attempt to justify any design, but pass on my experience with the OEM arms. Yes, they flex, and relatively easily. Prior to the heim joint conversion on the outer end of the arms, I rigidly attached the inner pivot point, and then added a twisting force (representative of acceleration, and braking) to gain some insight on the arms strength. At about 75 lb/ft of torque, I could deflect the outer ends of the arms by about 1/8" (front outer bushing up, and the rear outer bushing down by 1/16" each). Now obviously, that amount of movement is going to be stopped by the strut rod/tube before that happens in a real application.

Once I had completed my modification to the outer bearings, I tested the arm again, and found that the rigidity significantly improved (same deflection occurred at about 130 lb/ft of torque on the end of the arm). This was a fairly simple conversion. It would be interesting to see what seam-welding the entire OEM 280 arm (not the lighter 240 arm) to overlap the OEM spot welds would do to improve on that.

 

It has been recognized already (and any racecar designer sees this ALL the time) that improving one part simply moves the problem on to the next weakest part (and on, and on). And an IDEAL lower arm (no arm or pivot deflection) is going to "pass" this problem onto the unibody. This string was a nice study in the Z suspension, but that's as far as I can go with it.

 

IMHO, improving upon the OEM design is easy, but a perfect solution will eventually have the result of a completely different car. I'm a firm believer of the 80/20 rule, and when I start seeing only 20% more improvement, with 80% more work, time, and expense lying before me, that's when I step back and say "good enough, job done!"

 

BTW: Just watched "Cloverfield"...It rocked!!!

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Dan sent me this and asked me to check it for accuracy... :lol: Well I'll leave that up to you guys, but along with the pictures is a spreadsheet that more accurately quantifies all of this info. I tried to upload it here, but apparently you can't upload Excel spreadsheets to the Hybrid Z server. So it's hosted at my business site.

 

It might help to show graphically how the misalignment of the strut shaft takes place. The force that the strut shaft absorbs resisting this misalignment is side force on the strut.

 

rearstrutcalc11.jpg

 

rearstrutcalc21.jpg

 

http://www.thepetdoorstore.com/rearstrut.xls

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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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