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scary tension rod failure


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Well I just had a scary moment, I was racing my car on a winding highway (smooth surface) and just as got on the straight section my tension rod actually snapped in two :shock: . I'm so thankfull that in did not happen in a turn, for at that speed i would have been severely injured, to say the least.

I never pictured my tension rod failing. Has anyone heard or experienced such an event ? The only thing I can think that may have contributed to this scary suspension failure is the polyurethane tension bushings I had, which I guess could have been a little overtorqued (but not by too much, if at all) along with old control arm bushings causing too much load on the tension rod. Does anyone have any insight to what else might have caused this, I don't want it to ever happen again. The car does not have a single spec of rust, straight chasis, good steering rack, new tie rods and is properly aligned (however I did not recheck castor angle after putting in the new tension rod bushings). Should I consider new non-polyurethane (stock) tension rod bushings? what do you guys think? I'm obviously going to get new control arm bushings.

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I've now seen this twice in person, and you are the second person I've heard of but not seen this happen to.

 

The reason is that the poly bushings do not let the TC rod pivot. The rod flexes and flexes, and usually over the course of 5-10 years, it breaks. I would suggest upgrading to an adjustable TC rod with a rod end, or if you don't want to do that, get a G Machine setup and run a stock rubber bushing in the back. On the G Machine setup the front aluminum ball and plastic cup takes all the braking load, so putting a poly bushing in the back is not necessary. You can run the rubber in the back so that you don't flex the TC rod.

 

Good thing there wasn't a Prelude with a family in it going the other direction...

 

Jon

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poly bushings do not let the TC rod pivot. The rod flexes and flexes, and usually over the course of 5-10 years, it breaks

Ah, that makes sense, i hadn't thought of it. Perhaps the lowered suspension by increasing the angle between the tension rod chassis mount and control arm also added to the amount rod flexes against the stiff bushing. (I just bought some bump steer spacer yesterday)

 

Good thing there wasn't a Prelude with a family in it going the other direction...

No worry, I have been driving for 20 years and am done with the many stupid moves I used to make (the good old days when I felt immortal), I also have a wonderful family (wife and two kids) to get back to. I was racing the road (divided winding highway), not the traffic. I'm getting to old for that.

 

I'll think about the G machine set up. thanks for all of your input Jon.

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Would replacing the poly w/ rubber in the rear also soften the "small bump" response a bit? I seem to recall a post some time ago recommending rubber bushings on the T/C rod so as to make the ride a bit more comfortable, less "intense", something I need badly - my suspension setup provides a "vibrating massage" on all but the smoothest road surfaces...

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Guest Carrman

Lifted from the Dime Quarterly site:

 

Tension / Compression Rod Modification

A simple modification may help prevent T/C rod failures

Article and Illustrations by Kelvin Dietz

 

 

One of the most noticeable changes you can make to your car's handling is to replace the old rubber suspension bushings. By now they are well past their useful life and should probably be replaced. This was all covered in a previous DQ article, and one of the recommendations was to replace the stock Tension/Compression (T/C) rod bushings with Energy Suspension polyurethane bushings.

 

For a street driven or an autocross/street car, I think this is the best option. Parts are easy to come by, simple to install and maintenance free. However, I found an unpleasant side effect from these bushings that I thought I would share.

 

On a recent trip across town, I experienced the failure of my passenger-side T/C rod. I hit a small bump, heard a pop and noticed a slight pull to the right, but didn't think much of it as the car still went FAIRLY straight. I drove the car home and took the wheel off to find the problem. I nearly had a heart attack when I noticed the T/C rod had snapped in half at the shoulder behind the bushing. Not a problem at 30 M.P.H. on a city street, but potentially VERY damaging on a tight lefthander at 60 mph.

 

I'm surprised we haven't heard of more failures like this. Even though I've attempted to restore the stock front control arm angle by using bumpsteer spacers, my T/C rods are still angled up in comparison to stock. Between that and the stiffer bushings, the T/C rods experience much more stress than they do at stock ride height.

 

In light of this experience, I suggest modifying the polyurethane T/C bushings. I drilled six 5/16" holes around the perimeter of the center mounting hole. These should allow the T/C rod to pivot more easily and still offer the advantage of the stiffer bushing when braking and/or cornering. I recently replaced both of my T/C rods with a set from a car that had never been lowered. I also had that set Magnifluxed to be sure they were still in good shape. I shudder to think of the consequences of a high-speed failure.

 

DQ

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Thanks for all the replies. I guess this serious problem is not so uncommon. Comming back from work in the minivan today :( I was thinking about how to fully correct this problem before I take the car to the track:

1) Obviously a pivoting point as Jon mentioned makes perfect sense. I imagine the MSA kit should be good for that. Any input from people using the kit?

2)Next issue is to use poly or stock bushing on the inner side of the chassis mount. I don't like the idea of the play in the stock bushing, but off course that's better than snapping a T/C rod during a high speed turn and dying. Carman's post brings up a nice idea with either enlarging the whole or soften the area on the poly bushing around the T/C rod by drilling holes to reduce the torque it apllies on the rod. More input on this would be nice.

3) Lastly I was thinking about the increase angle between the chassis and the control arm from lowering the car. This increases the angle for the T/C rod, increasing the amount of torque caused by the bushing on the rod which then causes failure. Well how about making a spacer (Hint for JSK) to fit between the T/C end and the control arm. This would be similar to what the bump steer spacer does to correct the tie rod end to the steering column. I think this would solve the problem with lowering the car, and would be extreemly easy to make and install (I guess it would also require new stronger bolts, since the spacer would increase the torque on the two bolts holding the T/C rod to the control arm). What do you guys think?

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I used the MSA or G Machine kit for a couple of years with no problems. About once a year I disassembled it and regreased it and checked for wear. It did not wear out before I went to adjustable TC rods.

 

There would be no play from using a rubber mount on the other side of the kit. The rubber is just softer, and lets the TC rod move more freely, but there is really no forward pull on this joint. Only push to the rear when cornering and braking. Any play would come from wear of the aluminum/plastic joint. Taking a poly bushing and making it softer is really just spending a lot of time making a rubber bushing. It is already soft and requires no work.

 

I wouldn't put a spacer between the control arm and the TC rod. The rod takes a pounding when you are braking. Any lengthening of the bolts would be asking for a failure, IMO. Not to mention the rod mounts to the top of the control arm, so if you put a spacer in, it would make the angle worse, unless you put a really big spacer underneath and mounted the TC rod below...

 

Jon

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Guest Carrman
Thanks for all the replies. I guess this serious problem is not so uncommon. Comming back from work in the minivan today :( I was thinking about how to fully correct this problem before I take the car to the track:

1) Obviously a pivoting point as Jon mentioned makes perfect sense. I imagine the MSA kit should be good for that. Any input from people using the kit?

2)Next issue is to use poly or stock bushing on the inner side of the chassis mount. I don't like the idea of the play in the stock bushing' date=' but off course that's better than snapping a T/C rod during a high speed turn and dying. Carman's post brings up a nice idea with either enlarging the whole or soften the area on the poly bushing around the T/C rod by drilling holes to reduce the torque it apllies on the rod. More input on this would be nice.

quote']

 

Well, the 510 guys who have done this have not suffred any broken T/C rods after doing the mod.

 

Dave

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Jon has an excellent point with the spacer idea. The thrust vector will still be from the control arm to the unibody mounting boss regardless of the thickness of the spacer. What this spacer does is move the thrust or compression force off, and away, from the centerline of the rod, which introduces stresses that this rod was never designed for.

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Terry, the spacers at the forward end where the T/C rod bolts to the control arm puts the T/C in a combination bending/compression load, but it also puts a larger bending load on the bolts that connect the two. Not advisable.

 

The issue seems to be that the urethane bushings are stiffer and cause a larger bending moment at the area of the step in diameter just forward of them. That step is sharp, and it's a stress concentration for bending loads. That's why a rod end would be a good solution. You could put the cushion between the bracket for the rod end and the T/C bracket on the frame, if you wanted some isolation.

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Not to mention the rod mounts to the top of the control arm

I have a 280ZXT, the TC rod mounts below the control arm. Also I looked under the car and clearly the bushing is compressing on the rod because of the change in angle from lowering the car. So a spacer by correcting the angle will center the TC rod to the mounting point on the chassis and decrease the torque applied by the bushing, which was responsible for the rod failure. It should not increase stress on the rod. However, i agree with jon's concern about bolt failure from the increased stress caused by a spacer. No point in preventing the rod from snapping in two to only come off the control arm. Unfortunately for me, the MSA/G machine kit does not fit the ZX (damn I hate that) and I have not seen any adjustable TC rods with pivot rod ends as opposed to bushings for my car.

Also does anyone know of a custom TC rod for the 280ZXT. We can't all be just engine donors :wink:

I can certainly use more opinions/experiences, ideas.....thanks

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Is the TC rod in front of the control arm, as in a 510? If so, they can use the G Machine setup. I don't know if there is a different G Machine bushing setup for the 510, but I've got friends using them on 3 or 4 510's. They just get mounted backwards, so that the cup and cone are on the front side of the TC rod bucket. If it is behind like on a Z, I have to wonder what makes them different from the Z setup.

 

Jon

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An alternative to using the stock tension rods or modifying them to incorporate a free pivoting arrangement like a rod end is to find heavier stock adjustable rods from another car. For example, in Oz the strut suspension model Ford Falcons have such (adjustable) rods whch can be modified to fit. They are quite a bit heavier in section than the stock Z ones.

 

Of course no matter what rods are used, their ability to pivot freely throughout the suspension movement range should always be checked prior to final installation.

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OK, I agree with Jon, Pete and Terry the spacer is a bad idea and hence dead.

Is the TC rod in front of the control arm, as in a 510?

Yes it is. I don't know why the kit is not listed for the 280zx. the Z, ZX and 510 use the exacte same TC rod bushings. The ZX and 510 have very similar set ups, same rear control arms bushings and same bump steer spacers...

Does anyone know anything more about this? I will try looking into some more since it would be a nice and simple solution. If memory serves me correctly Jeff P had made custom TC with rod ends for his ZXT, I guess I will try emailing him.

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By itself, lowering the Z doesn't induce additional loads into the TC rod beyond what it was deisgned to handle. The increase in deflection force that poly bushings require push things over the design limit for the TC rod and if there are any stress risers, the failure will start there. The one poly bushing suggestion is a good preventative measure because it resuces the deflection force increase by about 50%. You might still be exceeding the deisgn limit but failure should occur later in the fatigue cycle.

 

FYI... I both poly bushings race 240Z but disassemble and check the front suspension annually.

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Hi JC, I was looking forward to some input from you. In the parallel post you said:

The failures we are discussing are the result of improper modifications and/or a lack of inspection

Well I’m certainly aware now that putting on poly TC rod bushings on a lowered car is an improper modification. Unfortunately I was unaware of it and never heard of any warnings prior to this. As far as inspection, the TC rods were inspected 10 months ago when I installed the new bushings, the car gets frequent alignment checks and is within specs (except for rear toe being slightly off, I will get adjustable bushings for that). I'm very cautious with my car and always use good quality new components. This failure made me nervous because I thought I was being careful with my mods and frequent inspections and still never saw it coming.

 

The one poly bushing suggestion is a good preventative measure because it reduces the deflection force increase by about 50%. You might still be exceeding the design limit but failure should occur later in the fatigue cycle

What I'm trying to figure out now is how to fully fix this, while maintaining a "tight suspension". I don't like addressing the same problem twice and I definitely don't want to risk the same critical failure twice. I drive the car on mountain twisties all the time.

 

I both poly bushings race 240Z but disassemble and check the front suspension annually
Again the suspension was just inspected so I don't know how to prevent this. I don't know if a lowered 280zx places more deflective force from the bushing on the rod as opposed to Z(?different angle). Also my car has relatively low mileage and was bone stock until two years ago when I bought from a non-aggressive slow driver. So there wasn't much chance for long term suspension abuse and the poly bushings were on for only 10 months.

Thanks again for everyone’s useful comments and advice.

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The failures we are discussing are the result of improper modifications and/or a lack of inspection

 

The above statement by me was wrong... improper modification was not the right term. I should have just said modification. And I didn't know the car was a 280ZX, so there may be differences on how the TC rod is stressed compared to a 240/260/280.

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How much can you lower the front of a car before you start getting the extreme T.C. rod flexion? I was thinking of about 2 inches on my 280Z. Also, I have a T.C. rod kit from (somewhere), do you really need the cupped spacer & teflon thing or can you just drill the bushing?

 

Where do you get the G machine stuff?

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