My Rennline strut brace has pulled apart

[450knotOffice]

18 months ago, during my suspension rebuild, I installed a Rennline strut brace. It's worked great since then.

However...today I was out driving on the typically bumpy freeways we have here in SoCal (you know, those rhythmic concrete section lines). The radio was off and I began to notice a subtle rattle coming from the front of the car, almost like something was loose. My first thought was that something in the glove box was jiggling around so I checked in there. Nothing. I poked and prodded all over the cabin looking fo the very subtle rattle. Nothing. I then started to sense that it was coming from my trunk area so I decided I'd check the strut brace to see if it somehow loosened up.

When I got home I checked the brace and sure enough it had loosened up a bit. However, the lock nuts were still tight. That seemed impossible so I thought maybe the strut brace had stretched a little over time (it's aluminum). No problem, I thought. I loosen the lock nuts, give it a little twist, and that'll tighten it right back up. Except that didn't happen. Instead, when I twisted the brace, one of the threaded inserts on the tube started pulling out. The other one stayed put.

So that's it. The bar is useless at this point because I can't put any tension on it without the insert pulling out.

Have any of you guys seen this happen before? I'm trying to figure out what to do about this. Should I call Rennline and ask them to send me another one, or should I simply find some steel tubing that I can thread my existing hardware into and use that instead?

For now, my trunk has more room in it than it's had for quite some time. Sigh.

The brace. Note the inserts at each end. It looks to me like these have simply been pressed and glued in.


The good end. Note that the insert is flush with the aluminum tube.


The end that pulled out. It's out only a little right now, but I was able to pull it out 1/16 of an inch earlier. It's too tight to pull out by hand, but the leverage that twisting it on the threads provided made it easy to pull out.

[scottb]

I'd give Rennline a call. The worst that could happen is that they'll tell you "pound sand."

[klaucke]

Quick TIG and off you go!

[450knotOffice]

I thought about that but it appears that the materials are not similar. They might both be aluminum but the inserts seem to be made of a much stronger alloy than the strut itself.

Further searching of this forum shows that "drums" found this problem too, earlier this year. Seems that I'm not the first one to find this issue. Interestingly, in that thread there was a picture of another brand of brace that used a couple of locking pins to secure the insert in the tube.

This is the thread:
Which strut brace?

[450knotOffice]

Rennline has been emailed. Hopefully they'll get back to me promptly.

[rfuerst911sc]

If Rennline won't fix it could you drive in the one end that's pulled out and then drill and "pin" both inserts with a roll pin? If you think there is enough depth you might be able to use 2 roll pins ( small diameter ) in a x or + cross pattern.Just make sure you offset your drilled holes so they are in a different plane. Good luck.

[gestalt1]

i don't think the roll pin will work - the rod end needs to thread into the tube and therefore the pin can't go all the way through. you could drill and tap a hole in the side and add a set screw however. you could also get a steel tube, cut to length and tap both ends replacing the aluminum part. i have the same strut brace but have not installed it yet. i did not realize they only relied on friction to hold the ends in - lame.

[kevin993]

Scott - I'm not sure which Porsche god you upset, but you've had a tough run with car issues lately. Perhaps your cab is finally letting you know what it thinks about sharing a garage with another Porsche! Hope everything gets fixed and you're back running around soon.

[Wil Ferch]

For my Rennline unit...and for completely different reasons....I replaced the VERY thin-wall tube originally provided.... with a solid piece of round-stock aluminum ( McMaster-Carr).

In my case, putting a compressive or tensile load on the main cross bar had it take a "set".. ( as seen as a curved reflection upon the shiny surface). This was confirmed y taking it out and rolling it on a known, level surface. Sure enough...whump...whump...whump..it was bent.

Trying to "gently" bend it back in my pine-padded vice, the original unit promptly "kinked". Enough of that..I figured..and went with a sold bar. No issues since.

Having said all that, the guys at Rennline have always been upstanding, upfront guys, and if you told them of your pull-out, I'd be willing to bet that they'd make right by you.

- Wil

[berettafan]

that seems like a really bad design. am i missing something here or should the designer of this part be flogged?

[Grady Clay]

It looks like both the Rennline 2-point and 3-point braces
potentially have this problem. Is that part simply pressed
into the tube?
"
"
© 2006 Rennline

"
"
© 2006 Rennline

I can’t imagine that this isn’t a common problem with that design.


The “turss” version appears to have the swaged ends to the
tube that allows the spherical bearing joint to thread directly
into the tube and not into some pressed-in intermediate piece.
"
"
© 2006 Rennline

Depending on the satisfaction you get from Rennline, I would
change to a swaged aluminum tube or have these intermediate
pieces TIG welded in your tube.

Best,
Grady

[Cory M]

Another option to fix it is to bond the two pieces together. There are a lot of industrial and aircraft adhesives out there that have remarkable shear strength.

I'm sure Rennline is very interested in this failure and I'd be surprised if they didn't help you out though.

[berettafan]

Cory would the adhesives you mention be of comperable strength to threaded inserts?

[450knotOffice]

Guys, you bring up some good points. I have serious reservations about simply receiving another bar of the same design from Rennline due to the fact that those inserts are simply pressed and glued in. It just looks like a poor design to me. That design would have been fine for compression loads but seems entirely inadequate for continuous tension loads. It seems Rennline is the only company that does this. All the others I've checked use threaded inserts (or no inserts at all).

[Wil Ferch]

get the solid 3/4" bar from McMaster Carr and drill to fit....pretty cheap.

- Wil

PS-- the stock design by Rennline has these glued-in with industrial strength stuff

[450knotOffice]

A Rennline sales rep contacted me just now and told me that they changed the design of the strut brace and told me to send it back for an exchange. I'm guessing they figured out they had a design flaw and rectified it with a new design, which is good because I wouldn't have accepted the old design in exchange.

I'm happy that Rennline has come through so quickly. That's good customer service.

[Wavey]

Originally posted by Wil Ferch
In my case, putting a compressive or tensile load on the main cross bar had it take a "set".. ( as seen as a curved reflection upon the shiny surface). This was confirmed y taking it out and rolling it on a known, level surface. Sure enough...whump...whump...whump..it was bent.- Wil

My stainless Brey Krause has the same problem! Although not bent very much, it's definitely bent.

[911pcars]

Anytime you have a threaded bushing that is of a different material than the rod end, there'll be this potential situation. The carbon fiber strut bars also use a glue (high-strength epoxy?) to install the rod end bushings.

A better solution with aluminum tubing would be to TIG-weld threaded, aluminum bushings in each end, then use with high strength al. rod ends for an all aluminum solution. However, this might increase the cost of the unit.

Sherwood

[jaydubya]

Originally posted by Wavey
My stainless Brey Krause has the same problem! Although not bent very much, it's definitely bent.

Just curious if you guys preloaded the bar in tension. In my mind this is one of the biggest advantages of the strut "brace" on the 911 - pull those shock towers together and get more negative camber. I get at least an extra -0.5 degrees doing that.

[Briana]

Originally posted by 450knotOffice
I'm happy that Rennline has come through so quickly. That's good customer service.

I'm not surprised. They're a great bunch of guys.