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TC rod damage


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There are a couple different clevis designs too. I looked at two and got the chunkier one. The one that you had is definitely the lighter and sexier looking part, but thought this might be stronger. Maybe it just puts more stress on the threads...

 

Ignore the part it's bolted to. Had to trash that and copy Dan's design.

 

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I might suggest making a new version with a wide clevis to sandwich the control arm, most hotrod/offroad shops these days will have some sizes on hand, if you're lucky you can find one a tiny bit bigger and use washers to take up the space.

 

Otherwise make the new plate from 4140HT, I love that stuff...

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My guess is that the clevis shaft bent slightly under the threshold braking which worsened the situation. Already bent clevis got bent more easily. Finally under the hard braking or the corner carving the LCA rotation was stopped by the ball joint motion limit, then both the clevis and dog-leg bent simultaneously.

 

 

Mine were took out today and found the right side dog-leg was slightly bent. Clevis threaded shaft was straight. I think this has happened when I hit the right front rim on the road side rock. The damages of the rim and the shock rod were worsen and trashed. 

The clevises in the plastic bags are HAL-CL6-10 from Summit. Identical shape, carbon steel, $5.97 each.

If I were in your shoes, I'd fabricate CrMo plates and check the straightness frequently and/or change the clevis(es) like socks. Also I'd use the longer tubes for more thread engagement.

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What would happen if I bolted the plate to the BOTTOM of the ball joint mounting web?  Instead of the top?  Will that stop the bending moment?  I was all ready to buy TTT TC rods, but I just realized (and it was mentioned above) that without the clevis, I would lose the ability to adjust caster (which I really want to be able to do-allowing me to run LOTS of caster for landspeed and a little caster for autocross.

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There is a major problem with the design of the TTT TC rods:

 

The end where the TTT TC rod bolts to the LCA is welded.  That means that the angle between the LCA and the TC rod is fixed.  The TC rod, LCA, and the frame between the connections of the LCA and TC rod form a triangle and each of the three legs of the triangle is rigid.  The angles between the three legs of this triangle is governed by the Law of Cosines that states the following:

                                                              A^2 + B^2 -2AB cos(theta) = C^2

 

where A is the length of the TC rod from the frame to the LCA connection point, B is the length of the LCA between the inner pivot and the TC rod connection point, C is the distance between the TC rod connection to the frame and the LCA connection to the cross member, and theta is the angle between the LCA and the TC rod.

 

Rearranging the equation given above, cos(theta) =( A^2 + B^2 - C^2) / 2AB.

 

In other words,  as the TC rod length changes, the angle between the LCA and the TC rod also changes.  If the connection is rigid (as in the TTT TC rod) then either the bolted connection between the LCA and TC rod must have enough slop to accommodate the angle change, or the rod itself must bend.  Either of these options is unacceptable.

 

It should also be clear at this point that the only successful design for an adjustable front LCA / TC rod assembly allows for the deformation of the triangle.  The connection between the LCA and TC is a pin joint that allows the angle between them to change but also keeps them in the same plane.  The connection between the frame and the TC rod and the cross member and the LCA must either be very compliant (rubber) or a spherical bearing.  Rigid bushings at either the TC rod or in the end of the LCA will cause binding and prevent proper adjustment.

Edited by 74_5.0L_Z
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I guess they assume you're using their lower control arms that have a heim joint at the inner pivot. That gives 2 points of flex out of the 3 corners of a triangle. My lca also has a 2nd set of holes to bolt the tc rod to about an inch more towards the center of the car. I think that's in case you get wheel rubbing when using a lot of caster.

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Rob, do you know how much caster your alignment has in it? I talked to Gabe via email and he told me that he has some spacers one can add to his TC rods to achieve 2-3 degrees of caster. I'm running 5 degrees now (limited by tire clearance); I want to run a lot more (maybe as much as 10 degrees) for my 1-mile runs in June. The purpose for me is to achieve a car that is so stable that I could release the wheel at 200 mph. Before this broke, this was gonna be a winter project, but this little breakage moved it up on the list. I wonder if flexing in these dogleg/clevis portion of the TC rod is why I had such scary "bump steer" under braking at the Ohio Mile. Imagine if this little parts failure had happened then! I feel pretty lucky.

Edited by RebekahsZ
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The heim at the frame connection of the TC rod is not enough.

 

Here is a simple experiment to prove the point:

 

With the strut assembly removed and the car properly supported,

 

1. Install the LCA and TC rod on the car and make the bolted connection between the two.  There will be a certain length for the TC rod that will allow you to install the two bolts connecting the TC rod to the LCA.

 

2.  Lengthen the TC rod one inch.  As you do you will start to feel resistance as the TC rod connection to the LCA tries to rotate (but can't).

 

3.  Now remove one of the two bolts that connect the TC rod to the LCA.  If you are able to remove this bolt at all without damage then try to reinsert it (the holes will not be properly aligned).

 

I am running 6.5 - 7 degrees of caster on my set-up with no problems.

Edited by 74_5.0L_Z
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74_, I'm pretty sure you and I are on the same sheet of music. And fortunately, I understand the issue with rigid connection of the LCA to the TC rod intuitively, cause I sucked at trig (11th grade). If you had tried geometry(10th grade), you might not have lost me. Remember, I was raised in Arkansas and I now live in Alabama. We know how many stars are on the Confederate flag cause we can count 'em on our toes! Rob is from Louisiana, so he probably understood you loud and clear, although I need subtitles when he talks. From what I understood in your earlier texts, you kept the same basic design, but made it heavier duty: stronger dogleg with more meat around that rear bolt hole, and 3/4" clevis where I have 5/8, and larger diameter tubes. Am I missing something else? I wonder if there is a point relative to the ball joint, any point, at which the dogleg is not in flexion/bending? This may be part of why Cobra Matt went non-S30 on his latest front suspension make-over (?) although I'm pretty sure it had cost him a year or two of racing the Z. I was surfing the net for swaged tubes, clevises and inner tie rod ends. Not a huge job to go bigger (the only issue is non published specs on the inner tie rod ends). But I'm thinking about putting a dogleg on the underside of the ball joint (too) to put that clevis in double shear. Whatcha think? I'm off to church.

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Im running just over 4, my limiting factor is the outer tie rod ends. When I adjust them to over 4, it starts to take camber out, so when I start to lengthen the lower control arm to get some more negative camber, I run out of threads on the outer tie rods. I need to get the longer heims he has to correct this.  A question I have directly related to this, is the way TTT control arms inner pivot on the heims mount. He has a heim with spacers, which allow you to move the inner pivot point a little forward or back. The way it works, it allows the  control arm to angle towards the front of the car, be square to the car, or angle back in the car, when looking straight down from above. What is the best way to set the control arm? I have mine pretty much square, but not sure if angled one way or the other would help, hurt, or not make a difference.

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As you stated, with the control arms mounted in the stock position, the arm angles forward as you add caster.  The angle of the arm away from perpendicular (as seen from above) creates a virtually shorter control arm.  The virtual shortening of the LCA reduces the amount of static camber that you can achieve with camber plates.  You can correct that to some degree by shimming the inboard side of the LCA as far forward as the stock cross member will allow.  Even with the spherical bearings on the inboard side of the LCA and the pivot shimmed as far forward as possible, you will start to get forward angles on the LCA after about 5 degrees of caster.  I fought that problem for a while and decided to do something about it.  I made a custom front cross member that moves the mounting point of the LCAs forward but keeps the fore/aft position of the rack constant.

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I think I just popped my LCAs in with the short side of the spacer forward, then added washers on the back side until the assembly was snug in the crossmember. From the wear pattern in my race tires I could afford to have a little less static camber. Shame race tracks have so many straight-aways! I'd opt for less straight and more curvy (funny hearing that from a drag racer!). I was thinking about the TC rod issue during the sermon this morning. My mind wandered back to my days in helicopter and I recalled the "Jesus nut" on a Huey. The Jesus nut was a simple, but imperfect solution to the problem of keeping the rotor attached to the helicopter. Its a situation where a single part or fastener separates one from a horrible flaming, painful death. My Datsun has a couple of Jesus nuts. One is the inner tie rod, another is the outer tie rod. The single nut that holds the rear stub axle in is yet another. Seems this TC rod attachment (dogleg/clevis/moment arm/associated fasteners) is also a Jesus nut of its own.

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I have 4.1 degrees caster with no binding and the bolts came out with no jamming. I  just took them out about a month ago when I swapped springs from front to back. As I add more caster, I also have to add more length to the lca to keep camber, so that may be whats keeping mine from jamming? I'm not trying to says its a great design or a bad design, just saying it seems to work with my setup at this point without issue. Is that why these other arms he makes have forward heims also?

https://technotoytuning.com/nissan/240z/gtx2-front-lower-control-arms-and-tc-rods-datsun-240z-260z-and-280z

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74- what does the frame end of your TC rod mount look like? Heim or inner tie rod? I think Im gonna just make a heavier dog leg out of chrome moly (or whatever the best alloy I can get is) for now since I think the failure initiated at the rear bolt hole (picture later). As I see the ball joint swinging like a pendulum from the camber plate, to eliminate ANY flex moment would require mounting the the dogleg approximately 3-feet below the ball joint and would assume no bending of the strut tube: a totally idealized situation. I think some flex/rotation at this point must be accepted as a reality. Maybe this winter I can step up to heavier clevises (3/4") too.

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Rob, there may just be enough slop in the bolt holes to allow the theoretical misalignment to be neglected in reality. As we bench race, we assume zero manufacturing tolerances, which isn't really true. You are giving me good data to consider. If I misunderstood Gabe, he may be saying that from your 4-degrees, perhaps I could get 3more for a total of 7, which is no slouch. I haven't totally taken TTT rods off the table. If I can, I want to make my doglegs wider at the rear bolt as 74_ has done. I think that's where the actual bend started due to weakening from the loss of material after the hole was drilled. TTT rods have about as much material in that area as mine did. I think the bending of the clevis saved my ass. So I want to be careful not to throw out the baby with the bath water. The car has been squirrelie on braking for a while-the dogleg may have been flexing for as much as a year before it finally said "enough."

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It's been a while since I really was considering all the ins and outs of TC/LCA angles and all that, but as I recall I was much more concerned with getting the inner pivots in line than I was worried about the LCA pointing slightly forward. I don't see how that has much of an effect on anything so long as you can adjust the camber where you want it and you don't have a solid TC rod end like on the TTT parts. If the inner pivots of the LCA and TC rod are out of line though, then IIRC  you get suspension which will try to level under braking loads. I don't think the effect is huge, and I ran some TC rods that had a pretty big clevis and then rod end so they were well out of line and the car still performed reasonably well. Still, I thought it was worhwhile to redo the entire TC rod bucket and use rectangular tube and rod ends to get everything back in line.

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