Is that becuase they don't allow any flex whatsoever?

 

Pretty much.


Actually, poly is not completely rigid. But it's stiff enough that in order to force it to allow the necessary suspension movement it stresses the strut rod itself excessively. Bends it right where the strut rod is the weakest - right through the threads. It won't break right away, but fatigue failures of any sort are generally just a matter of time.


Norm

 

I cycled my 67 suspension; adjustable strut rod moves and does not appear to bend. I plan to change it back to rubber none the less.

My 64.5 has non adjustable strut rod and strut rod bushing design appears to put less stress on strut rod. I may just keep the poly bushings in 64.5.

 

I just bought these...

http://www.autozone.com/autozone/par...er=201426_0_0_

...from autozone for 16.99...I'm pretty sure they're rubber...but it must be a heavy duty rubber because they're pretty stiff. Ad doesnt mention what they are though...anyone use these?

 

You probably won't get a visible amount of bending in the rod no matter how good your eyes are, but the bending stresses will still be there. Maximum bending stress will be through the first unsupported thread at the end of the nut.

In fatigue stress terms, this is an alternating stress, since the suspension moves in both directions from the 'at rest' position. The threads create a stress intensification, and it is normal practice in my day job to treat threaded piping connections as having a "stress intensification factor" of 2.3. Fatigue life is not linear with respect to stress, either. Double the stress and fatigue life may drop to as low as 3% in terms of the number of cycles to failure. No better than 10% unless the stresses were way less than the endurance limit to begin with.

If you are determined to keep the poly, you may be able to reduce the bending stress by a little creative drilling of holes in the poly. The trick is to drill them in such a manner that you "throw away" a bigger proportion of the bending resistance than you sacrifice from the straight axial stiffness against tension/compression by doing so. A slightly different version of this trick makes a clearly noticeable difference in poly bushings as used in rear lower control arms (where it is commonly understood that "poly = bind").


Norm

 

That appears to be the same SINGLE picture that everyone throws around the internet whenever poly strut rod bushings are brought up. But how many people use them(myself included) and how many have failed? Also consider that there's NO WAY the companies would be able to sell a product that was KNOWN to cause failure(even for use in race situations) and stay in business without being sued, or even charged for criminal liability.

And if you look at that picture, there's no evidence of a stress failure due to bending forces(which doesn't necessarily mean that wasn't the case either), and without an actual failure analysis it's impossible to tell. What's FAR more likely is that they were over torqued, since people in general have a tendency to vastly over torque ALL things. Combine over torquing the strut rod with a less compressible bushing, and it's more likely that the rod was stressed excessively under tensile load and failed as a result, or excessive tensile load combined with normal operating forces.

It could have been as simple as it was weakened from rust, or had badly cut threads from the factory. Bad threads can cause a world of hurt, ask Norm about that and he can explain how you do NOT want threads cut to a sharp point near the root, but with a radius, otherwise they're virtually guaranteed to crack and fail.

Now, that poly is much more rigid is a valid assertion, and as such it should be treated differently. With poly you should FLIP THE REAR WASHER. The reason for this, is the stock config like in the picture above....as the strut rod moves and tries to pivot through the bracket it WILL try to compress the bushing in the areas where that washer edge moves closer to the bracket. And since poly has limited compressibility, that can potentially lead to strut rod flexion, and the farther the suspension articulates, the worse it becomes. By flipping the washer so the "inside of the cup" of the rear washer points to the rear of the car, you remove the washer's contact area against the bushing around the outside, preventing it from attempting to compress the bushing at the outer edge where it would normally close distance with the bracket. It basically turns the rear washer FROM a cup contacting the entire bushing, INTO a small heavy washer that simply holds the bushing in place against the nut.

That's how mine have been set up for years without issue(and many others). I can even jack the car up so the front wheels are off the ground and the entire strut rod pivots through the bracket with 0 visible flexion. And that's with the wheels off the ground. It's also pliable enough that even with the wheels off the ground and the shocks bottomed, I can unbolt and reinstall the strut rods onto the lower arm, BY HAND. With the poly bushings torqued I can pull the rod down and line the bolt holes up, and hold it with 1 hand and start the nuts with the other. I'm not exactly a prime example of male upper body strength either. I also ONLY TIGHTEN THEM BY HAND. Do NOT use a torque wrench or cheater bars or anything like that. 2 combination wrenches only, and tighten them by hand, enough so they won't come lose. That prevents the bushing from being excessively pre-loaded and removing any compliance it has.

I need to jack it up sometime this week to do other things anyway, I can take pictures if you want of it on the ground at rest and up in the air.

Edit: Also if you guys look at the poly bushing in the pic above, you'll notice that bushing is bulging out around the sides with a rounded appearance....in other words, that bushing has been SUBSTANTIALLY compressed due to excessive preload, removing any compliance it had to begin with. That and the washer is in stock form causing a need for large amounts of compression(which is not available due to excessive preload) to be needed.

 

64.5 bushing appears to present less resistance to bending than 67 design.

 

Absolutely correct about bushing (lack of) compressibility and overtorquing them. But it's still a bending problem.

The tighter you clamp a split bushing, the stiffer you make the connection at that end of the threaded rod to the car structure against bending rotation, which in turn drives the end moments in the rod itself up. The same thing can and does happen (on a smaller and less critical scale) with polyurethane-bushed endlinks if you overtighten those - the long bolt eventually fails in the same place - under the sleeve just outside the nut that's on the control arm side of the bar.

A tensile problem would have the strut rod breaking on the bushing side of the nut, with the excessive tensile load being carried only between the nuts and out of sight within the bushing and washers. In that case, you'd have the nut still on the rod. The two bushing pieces, the rest of the rod, and the other nut may or may not still be on the car since they would no longer be captive.

I'll have to revisit the picture later since it doesn't make it through the filtering. But I won't be surprised at anything except at myself for not looking for the bulging before.


Norm

 

That's true. Which is why I just hand tighten mine, enough that the nut won't come loose and the strut rod locates the lca, but keeping enough compliance in the bushing to allow proper movement. That and my suspension is stiff, so there's not much articulation.

 

Was planning to change strut rod bushings from poly to rubber but changed my mind.

The broken strut rod picture shows an incorrect design or overtightened install which doesn't appear to allow strut rod to move. My 67 has a metal inner bushing which prevents over tightening. Appears broken strut does not have metal bushing???

I disconnected my 67 strut rod from LCA and can move it w/o too much effort. The bushing is cone shaped near washer which allows it to move around; unlike the one pictured w/ broken strut rod. I have been running poly on 67 for about 15-20 years.

My 64.5 poly bushing is different design but is cone shaped for freedom of movement. I have been running poly on 64.5 for about 15-20 years.

I wish more people would post pictures of their broken strut rods and poly bushings.

I am running rubber LCA bushings which may put less stress on the strut rods.