Rear beam main mounts - captive nut reinforcement

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Rear beam main mounts - captive nut reinforcement

Post by DPDISXR4Ti » Thu Aug 26, 2010 11:19 pm

Luke brought to our attention the weak design of the on-body mounts for the rear suspension beam. Ideally, these should be re-welded, if not replaced entirely.

But if you're not a welder, or maybe you have a truck bed in the back of your car which prevents welder access, you come up with an alternate solution.

My prototype design is simply a 1.25" I.D. pipe of 1" length, which neatly fits around the captive nut, once the floor under the rear seats is cut a bit (see below) to provide access.

A large washer is put under and over the pipe and then a nut is screwed down to provide a secondary clamp load. A replacement bolt 15mm longer than stock is required - MC2Racing sells a pair of Allen head bolts that are 10mm longer - that's what I'm using now, but 15mm longer would be ideal (dunno if that even exists, now that I think about it)

The pics below don't show things very well given my lack of overhead access, but it's all I've got. The last pic shows the secondary nut installed (just barely) and a spare 1.25" pipe. All pics are taken of the passenger side...

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Brad

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Post by Ray » Fri Aug 27, 2010 7:17 am

So, basically, you're putting another nut on there with that piece of pipe as a spacer?
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Post by DPDISXR4Ti » Fri Aug 27, 2010 10:51 am

demonfire wrote:So, basically, you're putting another nut on there with that piece of pipe as a spacer?
Plus washers at either end. At the bottom, it's a big washer bored out to 1.25" I.D. The top washer is actually two stacked washers - a 5/8" washer so it covers the top of the pipe, and then a 12mm washer. Keep in mind, I imagineered and threw this together with what I had at hand in a day.

On the "production" part, I'd weld the top and bottom washers in place and/or use a thicker pipe. Or, just machine the whole thing on a lathe from some 1.5" steel rod.

One additional thought... Use 120mm length bolts and bolt in a length of angle iron across the two points. Not sure if it would clear the hump in the middle - clearance it there and then weld it in so it might actually do something. :P
Brad

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Post by RatFink » Fri Aug 27, 2010 11:16 am

Interesting fix. I missed what exactly the original problem here is. Can you provide a link or something?
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Post by DPDISXR4Ti » Fri Aug 27, 2010 12:25 pm

RatFink wrote:Interesting fix. I missed what exactly the original problem here is. Can you provide a link or something?
Luke posted on MS about this a couple months ago. I'll re-post his findings here....


Hey guys, haven't been on here in a little while. This week was the 1st time I've gotten a few hours to work on my Merkur in several months, and I thought I'd share the results of my project.

Anyway, the rear beam is located in the unibody structure by the tubes that run up through the bushings. The tubes then locate into a thin walled nut/locating tube that appears to be merely pressed into the body. In 2005, the right hand one in my Merkur physically pulled out of the unibody at 70 miles per hour, which caused my entire rear end to cock at an angle and made the rear end kick out to the right. Quick counter steering got me safely to the shoulder. I bandaid fixed it, and it held fine for the next couple years I was daily driving it. Next time I removed the beam from the car I realized that the left side had begun to have the same problem, only it had not yet pulled through. Basically the locator/nut comes unpressed from the unibody, and that allows it to rock back and forth as you drive. After thousands upon thousands of miles of rocking back and forth it actually enlarges the hole enough that the locator/nut pulls completely out! Here's my fix!

I started with a short section of pipe, and tack welded a flange nut to the top of it. The pipe accepts the post from the rear beam, while the nut on top gives the bolt something to screw into. I then welded the pipe to a circular sheet of steel, and drilled holes to allow me to affix it to the unibody with spot welds. After dropping it in I found that I also had clearance to weld it around the perimeter of the flange. I decided to use the spot welds as well, as less is not more in this situation. BTW, I had to cut metal away under the back seat to get to this, but once I have it primed to prevent rust I will weld the floor back up and I should never have to access it again.

The replacement unit:


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Welded in place, with the beam bolted in as well.


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For those of you who asked for more info:

Here's the factory setup. It's a tube/nut thingy that's pressed into a plate, the plate is then spot welded into the body.


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Mine separated from the plates on both sides and began to rock, wearing a hole in the plate and unibody large enough for it to pull right through.


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.095" wall.... too thin and weak for such a critical piece, IMO!!!


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Location of all this. This photo is taken under the back seat on the driver's side. I was standing in outside the driver's door when I took the pic.


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Post by Rotisserie » Fri Aug 27, 2010 4:24 pm

funny. thats what happened to me this past couple of years.

took some angry welds to keep it from popping out. i'll be watching your progress.

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Post by DPDISXR4Ti » Sun Aug 29, 2010 9:34 am

Rotisserie wrote: i'll be watching your progress.
Well, you won't see any in the near future on this particular project. More than anything, I just wanted to get the sheet metal cut before the car got painted.

But thinking it through some more now, I think the best way to approach this would be with some 2" steel rod 1.25" in length, machined as follows... Bore out the center 12mm all the way through, then re-bore 1.25" except the last .25". Then reduce the O.D. down to 1.5" except for the last .25" on the other end. It would look sorta like a top-hat. The part could be used as-is or welded in place.
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Post by RatFink » Sun Aug 29, 2010 9:47 am

DPDISXR4Ti wrote:But thinking it through some more now, I think the best way to approach this would be with some 2" steel rod 1.25" in length, machined as follows... Bore out the center 12mm all the way through, then re-bore 1.25" except the last .25". Then reduce the O.D. down to 1.5" except for the last .25" on the other end. It would look sorta like a top-hat. The part could be used as-is or welded in place.
I was thinking the same thing...
Daniel Llewellyn

"You can never turn a pig into a race horse, but with enough development you can have a pretty damn fast pig".
Randy Pobst

I just love this guy:
hEaT wrote:Bit more work, but saved me $100 and I got to learn things, cut things, burn things and hammer things. All in all, a solid move.

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Post by DPDISXR4Ti » Sun Aug 29, 2010 10:03 am

RatFink wrote:
DPDISXR4Ti wrote:But thinking it through some more now, I think the best way to approach this would be with some 2" steel rod 1.25" in length, machined as follows... Bore out the center 12mm all the way through, then re-bore 1.25" except the last .25". Then reduce the O.D. down to 1.5" except for the last .25" on the other end. It would look sorta like a top-hat. The part could be used as-is or welded in place.
I was thinking the same thing...
Feel free to build it and sell it - I'll hold no grudge. :D
Brad

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Post by RatFink » Sun Aug 29, 2010 10:20 pm

Dayum! I'm beginning to wonder if this is a right coast problem. I went out to my newly acquired wreck today to simply pop one out and couldn't get the damn thing loose at all. Beat, pulled, beat some more, made a jig to pull it out, only to break the threaded portion out. I'll eventually get it loose, but the one in this car was definitely not going anywhere soon.
Daniel Llewellyn

"You can never turn a pig into a race horse, but with enough development you can have a pretty damn fast pig".
Randy Pobst

I just love this guy:
hEaT wrote:Bit more work, but saved me $100 and I got to learn things, cut things, burn things and hammer things. All in all, a solid move.

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Post by DPDISXR4Ti » Sun Aug 29, 2010 11:15 pm

RatFink wrote:Dayum! I'm beginning to wonder if this is a right coast problem.
Oddly enough, the couple people I've heard of that have had this failure claim the cars were rust-free. I'm thinking it might be that random parts just didn't get pressed in right and/or things loosened up and allowed a rocking motion that eventually broke it free.
Brad

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Post by MerkXRTurbo » Fri Sep 17, 2010 4:44 pm

Has nothing to do with rust. My car is an Arizona car, 1st of all. Second, you can see that the parts that came out of my car (the post of mine that Brad quoted above) are all in good condition.

Basically I don't think this should ever be a problem as long as the beam mounting bolt stays tight, and has ALWAYS been tight. If at any time it has come loose in your car's life span, then the metal is already damaged to some extent. When it loosens, the movement of the rear beam then pushes the bolt around rocking the nut with it. It would seem to me that harder bushings, or solid mounting, would further compound the complications of a loose bolt, as you don't have a soft bushing that's absorbing some of the force of the movement of the beam.

Ideally the captive nut needs to be WELL attached to the sheet metal so that a loose bolt cannot rock it. Maybe even some red or green thread locker on the bolt to make sure that it can't come loose. Brad, if I were in your situation, I would do similar to what I have already done, only I would cut the surrounding sheet metal out the bottom of the car, modify it accordingly, then weld it back in from the bottom, if that makes sense.
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Post by DPDISXR4Ti » Sat Sep 18, 2010 11:32 am

MerkXRTurbo wrote:Brad, if I were in your situation, I would do similar to what I have already done, only I would cut the surrounding sheet metal out the bottom of the car, modify it accordingly, then weld it back in from the bottom, if that makes sense.
I considered that, but after verifying that my nuts are solidly in place (no comments! :lol:), I think I'll be fine with my approach.

Relative to your suggestion to lock-tite the bolts, I don't think that's really addressing the problem. I think what happens, especially when I think about the failure on your Arizona car, is that the stock bushings dry out and shrink, allowing more wobble forces on the fitting.

Thinking it through some more, the Powerflex bushing is probably helpful in avoiding this condition, as it puts a compressed load around the whole area rather than the wacky stock approach which leaves an air gap.
Brad

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Re: Rear beam main mounts - captive nut reinforcement

Post by DPDISXR4Ti » Tue Jul 03, 2012 9:11 am

I know Joe is putting solid beam mounts in his car. I'm not sure if that makes reinforcement of the captive nuts more or less of a requirement - thoughts???
Brad

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Re: Rear beam main mounts - captive nut reinforcement

Post by Ray » Tue Jul 03, 2012 8:05 pm

Brad, i haven't done the reinforcements on my car for that mount - i solid mounted last year. Although, i'm tempted to cut my floor up just to inspect.
-Ray
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1985 Merkur XR4Ti -#141 CP "Miss Daisy"
2005 Subaru LGT
http://www.cartct.com

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