Aftermarket Control Arm Failure

[cygnusx1]

Two vertical gussets in a "V" formation. The point of the V should lay on or past the midpoint of the tube section close to the LCA bushing end, and the legs of the "V" should radiate towards the inner bolt holes for the ball joint. They should be welded all the way along, and then the part should be stress relieved. Brittle metals have no place in this piece. This would leave room in between the "V" for the sway bar end link. This is just one way to do it. There are many. I wish I still had Solidworks here.

Edited October 26, 2011 by cygnusx1

[EMWHYR0HEN]

What a terrible design on that arm that broke. Have you contacted them about it?

[ComicArtist]

I personally wouldn't trust that LCA in the first place.... just the fact that the threaded rod is just welded to the side of the control arm is sketchy. Like everyone else is saying, it needs to be boxed in.

[Owen C.]

How thick is the plate? Looks like rather than taking the time to engineer a proper design they just used a thick ass piece of hot rolled flat bar. Id think that 1/8" thick with a few strategically placed bends would not only be stronger but much lighter.

[z-ya]

The guy to made these came by today and has decided to start from scratch and do a complete redesign.

 

Here are some more pics:

 

 

 

 

 

[30 ounce]

Wow!! Kinda the reason I went with TTT control arms. I know they are heavier than stock ones but they are tough. I also dont like the TC rod having just the one bolt. Seems like a pivot point for the control arm under braking. Most of the weight of the vehicle is transmitted to front tires through these! AZC are like that too. Maybe I'm wrong...

 

Glad nobody got hurt.

Edited October 30, 2011 by 30 ounce

[RebekahsZ]

I'm with you 30ounce. All these TC rod kits use a single, tiny little bolt to connect the TC rod to the LCA. It looks scary frail and must see all that stress under braking/potholes. I'm interested in trying to find a good way to install a larger clevis and bolt. I have the zccjdm.com adjustable TC rods and TechnoToys LCAs.

Edited October 30, 2011 by RebekahsZ

[z-ya]

The ZCCJDM TC rods have two bolts connecting it to the control arm, but still rely on one bolt to tie the hockey stick to the rod:

 

 

Same for Arizona Z:

 

 

The TTT TC rods have the hockey stick welded to the rods:

 

 

The only reason to use the hockey stick approach is so that the TC rod can be used with stock control arms. I don't think having one bolt holding it all together is a bad thing, as long as it can handle the load. The Arizona Z parts have been proven for years.

 

The guy that made mine is going to increase the sized of the threaded Y bolt on the end of the TC rod to 5/8". He is also going to a bigger bolt that holds the TC rod to the control arm. He is also most likely going to change the design to one piece.

[z-ya]

Here is what the one that didn't break looked like on the car:

 

[30 ounce]

I'm not saying the TC rod design had anything to do with where yor control arm broke it just amazes me how dainty they can make these parts when you consider the forces that must be applied to these.

[johnc]

Wow!! Kinda the reason I went with TTT control arms. I know they are heavier than stock ones but they are tough. I also dont like the TC rod having just the one bolt. Seems like a pivot point for the control arm under braking. Most of the weight of the vehicle is transmitted to front tires through these! AZC are like that too. Maybe I'm wrong...

 

Glad nobody got hurt.

 

Not true. Vehicle weight and the weight transfer under braking is taken by the springs which is pretty obvious because they compress under braking. The loads the TC rod sees are minor compared to that. The single bolt clevis is fine.

[johnc]

Here is what the one that didn't break looked like on the car:

 

 

So... when was the last inspection done?

[Snailed]

Not true. Vehicle weight and the weight transfer under braking is taken by the springs which is pretty obvious because they compress under braking. The loads the TC rod sees are minor compared to that. The single bolt clevis is fine.

 

What would you say the force is on that rod under hard braking? Say the strut sees 600lbs of compression, what would you estimate the TC rod sees at the same instant? 300lbs?

[Zmanco]

What would you say the force is on that rod under hard braking? Say the strut sees 600lbs of compression, what would you estimate the TC rod sees at the same instant? 300lbs?

Go back to physics basics: f=ma.

 

Assume that z-ya's car can generate 1g under braking. Then the total force across the 4 tire contact patches is ~ the weight of the car.

 

With weight transfer due to braking, the fronts will generate more of that force vs. the rears. There's some further force multiplication (leverage) since the lever arm from the contact patch to the strut tower is longer than the TC rod connection to the strut tower. Seems like the TC rod takes most of the load in the lateral axis - hard to imagine the spring providing much lateral stiffness - hence each TC rod probably absorbs a load close to the half the weight of the car.

 

Does that make sense?

[Lazeum]

We also need to take into consideration the moment induced during braking by the pads. It creates a moment that would be transfered longitudinally. The spring perch & the compression rod are the 2 only paths for stress during braking.

 

The single point or double bolt system will differ from the way the stress is going to be sprend. No bending moment would occur with 2 ball joints at the extremities of the rod whereas there's most likely some with stock setup. I would assume it would be very small based on design otherwise Nissan/Datsun engineers would have made the rod much beefier.

 

 

[JMortensen]

If you look at strut rods (T/C rods in Z car lingo) on other race cars, you'll see that a clevis is a pretty popular way to do it. Most clevises are bolted to the arm, but having the tab that the clevis bolts to be an integral part of the arm seems a stronger way to do it to me. Most of those clevises have a 3/8" gap in them, so that's a hefty chunk of metal. Another way to say it would be: What's stronger, a 3/8" piece of metal bolted to the ball joint or a 3/8" piece of metal that is either welded (older AZC suspension) or integral to the control arm?

 

I agree with you guys who think the TC rod takes a lot of force. The weight transfer and spring compression is a side effect of the g forces and cg height during decel. Those g forces are still going into the suspension somewhere, and I think somewhere means strut top, ball joint, LCA inner pivot, and TC rod. I think the force created at the strut top and ball joint transfers to the TC Rod and only a relatively small amout of force is seen at the lower control arm pivot.

 

You really don't want joints at both ends of the TC rod because that would allow the lower control arm to swivel, which would mean that the TC rod would no longer be in line with the rear bracket. I think this is a recipe for TC rod failure. The TC rod should be solid up front.

[johnc]

Braking load on the suspension is a torque load, not a linear load - brake caliper clamping rotor. That torque load is spread through three mounting points (LCA, upper shock, TC rod mount). The TC rod is just one leg of a virtual A arm with the LCA the other leg. The chassis does not push against or lean on the TC rod under braking. All the TC rod does is stabilize the outer end of the LCA. I've driven a car on a race track with a broken TC rod and yes, the car pulled to that side because the rod was broken and that LCA moved rearward but the car still stopped reasonably well and let me get back to the pits. The front suspension didn't collapse, the car didn't explode.

[30 ounce]

I'm sure the anti sway bars would hold the LCA enough to limp back to the pits but I would bet a lot of $$ that without the sway bar and TC rod holding the LCA in place you would have a very difficult time getting back. The LCA would have almost nothing to do with braking torque, the strut and TC rod would. I don't understand how braking force would not be transmitted mostly through the TC rod. I'm not talking torque here but the weight of the vehicle.

[JMortensen]

Braking load on the suspension is a torque load, not a linear load - brake caliper clamping rotor. That torque load is spread through three mounting points (LCA, upper shock, TC rod mount). The TC rod is just one leg of a virtual A arm with the LCA the other leg. The chassis does not push against or lean on the TC rod under braking. All the TC rod does is stabilize the outer end of the LCA. I've driven a car on a race track with a broken TC rod and yes, the car pulled to that side because the rod was broken and that LCA moved rearward but the car still stopped reasonably well and let me get back to the pits. The front suspension didn't collapse, the car didn't explode.

You didn't have a monoball on the front control arm. If you have poly or your kevlar LCA bushings, that would help a lot, along with a poly sway bar bushing. I've seen two pop in real life while racing and both were able to drive back to the pits, both were using poly. I'm quite sure that if mine were to fail with monoballs everywhere, there would be a much bigger problem.

 

From: http://forums.hybridz.org/index.php/topic/22762-scary-tension-rod-failure/page__st__20

 

I have too broken a T/C rod. But I didn't know it broke until I hit the brake. I heard something that sounded like a rock hit the underneath of my car, but I didn't think anything of it. Just prior to that I hit this pot hole, not too large, but large enough to jolt the car. I have 205/50ZR15, so you can feel ever thing on the road. I was cruising at about 80 mph and I'm coming up on the exit ramp I need to get onto. So, I let of the gas and at about 40 mph I hit the brake, my right wheel got shoved into the back end of my wheel well, sending me turning in that direction off the road. The car was okay, I was freaking out. I was forcing the wheel in the other direction the whole time of the slide. It was the T/C rod that snapped. Now I have that performance rod kit with the ball/socket...hasn't broken since. I'd just thought I'd add my story...still have yet to read every one else's insight. Good luck...not a fun experience.

[johnc]

You didn't have a monoball on the front control arm. If you have poly or your kevlar LCA bushings, that would help a lot, along with a poly sway bar bushing. I've seen two pop in real life while racing and both were able to drive back to the pits, both were using poly. I'm quite sure that if mine were to fail with monoballs everywhere, there would be a much bigger problem.

 

Correct. And that does show that the TC rod primarily performs a locating function and doesn't support a large braking load. If an anti-roll bar end link with a class 8.8 10mm bolt can keep the LCA located or poly LCA bushings can keep the LCA relatively in place under braking then then fore/aft loads must be low.