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 Post subject: Bent lower control arms
PostPosted: January 28, 2018, 9:18 pm 
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Hey guys. I'd like some input into my issue.

So I finally had a chance to poke at the Seven. My lower control arms are strut-style where you have one control arm going straight outwards, then a strut link bolted to the bottom near the balljoint and to the chassis. See pics to hopefully clarify (new phone/camera and couldn't get good pics). So what bent is the 3/4" solid heim that bolts to the bottom of the control arm. One side is bent ~10° in an upwards direction; the other side less.

I removed the coil-over and moved through the entire range with no bind. Control arm drops 2" lower, and goes up high enough to contact headlamp. Steering turns correctly with no bind at both ends of range. The only contact I can see is the coil-over ends have some nicks where they contact their mounts, but they are on spherical bearings so should rotate out of the way once they contact (would also snap the ears off before bending a heim). I do not believe I am bottoming-out and I do have rubber bumpstops if so. Shocks are set fairly soft.

So.... I'm confused. These are 3/4" heims that bent on the threads. That is a LOT of metal. I have had bad hardware from this supplier before, but even if it was made out of cheese that is a lot to bend. This stuff is sold for use on heavy dirt-track cars. Of all the things to bend, I didn't imagine it could be a 3/4" heim. It probably is the weakest area of the control arm though.

Thanks for any help. At this point I can't think of what else to do but try installing better quality heims and see what happens.


Attachments:
CTRL ARM.jpg
CTRL ARM.jpg [ 83.7 KiB | Viewed 912 times ]
bent strut.jpg
bent strut.jpg [ 308.59 KiB | Viewed 912 times ]

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PostPosted: January 28, 2018, 9:26 pm 
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Did you hit anything, like a huge pothole?

Is it on both sides? If not, that tends to indication doing something bad to one side.

About the only other thing I can think of is if that "trailing link" didn't have sufficient travel in compression, like that stop nut hit something and the suspension kept moving, bending the rod end. For example, dropping it off a jack with something wedged above that stop nut.

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Last edited by KB58 on January 29, 2018, 10:03 am, edited 1 time in total.

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PostPosted: January 28, 2018, 9:45 pm 
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I can't think of any impacts that were abrupt. I have video of hitting a hard dip at autoX and it wasn't enough to hit the bumpstops. Heims on both sides are bent: Left side worse. I should also mention this car is sprung fairly softly.

I wonder if it's braking loads. I see the oval track cars that use these have them in tension, where I have them in compression. Pretty sure I have seen them in compression before though. Still seems unlikely that a 3/4" thread can't handle that.

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PostPosted: January 28, 2018, 9:58 pm 
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In the drawing, you labeled that piece as a "solid heim". Is it NOT a spherical rod end bearing? :?

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Last edited by ngpmike on January 28, 2018, 10:18 pm, edited 1 time in total.

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PostPosted: January 28, 2018, 10:02 pm 
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Nope it is a solid rod-end bolt. It does not rotate. It slips over a nub on the bottom of the ctrl arm and is bolted solid to it. The ctrl arm and rear strut link can be thought of as a solid piece (like an A-arm effectively. All nuts/bolts were tight and still has no bind through entire range of steering and suspension.

Image

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PostPosted: January 28, 2018, 10:12 pm 
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It looks to me like your transverse arm is trying to move rearward under braking. With the offset between the transverse arm and the diagonal you may be getting a lot of twisting along the transverse axis and it's bending your rod-end!

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PostPosted: January 28, 2018, 10:39 pm 
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If it was from brake twisting, would we expect it to be bent in the opposite direction? I'm not doubting the idea: It is probably the most likely cause (and highest loads) but the load-path should be bending it downward? I guess if my rear pivot point was higher it may bend upwards.
I know I made a design mistake by having this rod in compression, but I guessed a 3/4" rod would be able to take it. These have been done in compression before by OE but maybe were more in-plane.

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PostPosted: January 28, 2018, 11:03 pm 
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Your transverse arm would rotate around an axis that runs between the lower ball joint and the bearing on the inner end of the arm. Under braking the "T" arm is trying to move towards the rear of the car, but the Diagonal arm is trying to stop it, but the "D" arm is offset from the "T" arm's center of rotation. This causes the "T" arm to rotate because the anchoring force applied by the "D" arm is holding the mounting spud stationary while the "T" arm is moving. Hope you were able to follow that. I had trouble putting it into words! :roll:

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PostPosted: January 28, 2018, 11:10 pm 
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C10CoryM wrote:
These have been done in compression before by OE but maybe were more in-plane.


That's probably why Jack does kinetic's arm in double shear, mounted in back of the "T" arm in line with the center of rotation.

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PostPosted: January 29, 2018, 12:06 am 
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Disconnect the shock and verify that the suspension isn't binding as it's moved through its full range.

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PostPosted: January 29, 2018, 12:17 am 
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Yeah, it makes more sense that the ctrl arm is rotating over the top of the strut rod. I was thinking backwards.
Easy enough fix I think. I'll add some tabs to the rear of the ctrl arm and bolt the strut rod in-plane to get rid of the twist. I'll keep the stud on the bottom for swaybar mount.

Thanks for the help guys.

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PostPosted: January 29, 2018, 2:00 am 
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I don't think it bent in compression. That would mean it buckled and it would take a lot to do that. Maybe you have been bottoming out your coilovers? If that happens the lower arm will try to pivot on the bearing at the bottom of the shock.

Thanks for the picture, I was looking at those parts in an oval track catalog the other day and was thinking it would work something like that but couldn't tell from the picture because it didn't show the attachment for the lower connection.

I'm surprised to see that, it's pretty robust stuff you have there.

Instead of compression and tension try to think of what it would take to put a bending force on it. If you hit a pot hole while braking the ball joint will push back on the rod end, but it will also try to bend it, not just compress it. The rod end is being used to keep the orientation of the link vertical.

Looks like you figured it out while I was writing this :rofl:

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PostPosted: February 18, 2018, 3:44 am 
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ngpmike wrote:
Your transverse arm would rotate around an axis that runs between the lower ball joint and the bearing on the inner end of the arm.


Yes.


C10CoryM wrote:
I'll add some tabs to the rear of the ctrl arm and bolt the strut rod in-plane to get rid of the twist.



Yes.


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