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Yikes... Movement in moustache bar mounts


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Looked like the bar was bending then springing back, kind of like the original bar was designed to, although it may bend much more with more torque.  The mounting area, the studs, didn't seem to move.

 

You didn't give any details at all about how the mount is made or what material it's made from.

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Looks like it's flexing on what looks like rubber bushings. If that's what it is, and it's flexing more than you'd like, I think stiffer bushings is your solution.

 

Also it looks like you mounted the control arm mounts to the mustache bar. Am I seeing that right? I wouldn't have done that, since now they too are flexing with the mustache bar, instead of being solidly mounted to the body.

Edited by rturbo 930
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 I see flexing of the bar and the bushings.  The back of the diff is pulling down.  Usually the nose pulls up but the rotational force is pushing down on the back at the same time.  So the front is very strong now and the back is the weak link.  

 

Stiffer bushings without a stiffer bar might transmit more twisting to the studs.  Fatigue could be a new problem.

Edited by NewZed
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I originally thought it was flex in the bar but the more I watch it the more I think it's 90% in the end mounts.

 

The mount bushes are polyurethane. The mount is the same as used by Arizona Z with their aluminium moustache bar

 

The suspension is not attached, although the bushes are new OEM rubber and not polyurethane.

 

I can't recall the wall thickness of the 25 x 50 mm bar but it wasn't thin.

 

I'd be interested to see if anyone else has filmed this with 350 or more hp.

Edited by Boy from Oz
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What kind of diff?  The single bolt on the back mount looks unfamiliar.  What's the front mount look like?  The Arizona Z Car mount has brackets on the control arm pivots, yours doesn't seem to have those.  Not really clear what you're working with.  Looks interesting though.  Did you build it or buy it?

 

If you want to confirm that the bar is bending/flexing you could run a tight string across the top of the bar, from mount to mount, as a reference line.  Distortion from the camera lens might be making the bar look like it's bending.  If it's camera the string will look like it's bending also.  All you can really say from the video is that the back of the diff moves away from the car body.

 

But, if you look at how far the back of the diff moves and compare it to how far the bushing flexes, you can see that the back of the diff drops much farther than the bushing flexes.  That's a long lever arm from the end where the mount is to the diff.

 

Also, the hangers for the suspension brackets are tied directly to the body.  So the "dogbone" is a reference point also.  The back of the diff is definitely dropping and it can't do that unless the bar bends.  Even if it twists on the bushings it still has to flex/bend also.

Edited by NewZed
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Almost looks like a S130 or Z31 rear diff hanger setup. IIRC those have rubber in them, and that could be where the movement is coming from. Not sure how you could possibly use those hangers with the long nose R200. You must have some weird combo in there. Give us some more details and maybe we can help more.

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I remember that one.  Good to see you're still back in action.

 

In your video it's hard to see what the front of the diff is doing.  But you have elastic mounts up there also, it's probably twisting up and back, not too solid as I proposed earlier.  And the leverage problem that everyone has who uses a short nose diff with the original holes for the strap mount.  The nose of the diff is pressing upon a fairly long lever arm against the screws in the body.  You might even be flexing those two arms also.  I think that the front is moving allowing the back twist downward.  The whole diff is twisting around the axles.  And the tube you used for the rear bar seems to be flexing.

 

The guys that run the Q45 diff have the same leverage problem on the front.  The suggestions in your other thread about mounting directly above the diff nose probably still are worth considering.

 

Just opinions...

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Edited by NewZed
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In the video from 29-34 It looks like the control arms and hangers that tie the rear lower control arm mounts to the body are moving way to much. They should not move at all it does not look like the control arm bushings are flexing to much. The control arms, hangers and cross members should not move and if they do it will affect handling and tire wear by changing toe in/out. It most likely moves when cornering and hard braking also. 

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I probably didn't make it clear in post #6 but the moustache bar bushes are poly. The inner LCA are rubber.

Oh, I thought there was a bushing between the mustache bar and the rear diff hanger sheet metal thingy. Looking again, apparently not.

 

That made me watch the video again and you can see the mustache bar tube flexing and the poly at the ends flexing. There is a reason the stock one is about 10mm thick and spring steel, it's supposed to twist. Maybe this is just what you're going to get with this combo.

 

If you want it stiffer you could go to a thicker wall tubing on the bar itself and solid aluminum or stiffer (Delrin?) bushings on the end of the bar. If you stiffen things up considerably then you should do the same to the front bushings, swap them out for stiffer or solid too. If it ain't broke though...

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^ What JM said. Agree 100%

 

Poly can be fairly flexible, especially when you have a larger diameter bushing with a thick wall. Poly comes in different durameters and you obviously need stiffer material.

 

BTW,  the Poly bushings that come in most aftermarket suspension kits are generally junk. Way too soft for suspension applications.

 

Back to your diff mounting. Delrin would be a good choice as mentioned by JM, or even solid Aluminum bushings. Delrin is much stiffer than Poly. Stiiffen up the front bushings as well. They are probably a soft poly as well. Replace those with Delrin, Del-A-Lum or Solid aluminum.  You'll get more road noise, but you can't have your cake and eat it too...

 

Del-A-Lum bushings are a very good alternative. They use an aluminum outer casing, a thin liner of Delrin and then a steel inner collar. They are far superior to Poly in every respect. They will also transmit a bit less noise than solid aluminum bushings. Commonly used in suspension Control Arms of Pony cars and they work very well. You would have to get a custom set made, but any competent machine shop can easily do this.

 

https://www.google.ca/search?q=Del-A-Lum+bushings+pictures&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwj6vIaaoe7NAhVL8WMKHYF3An8QsAQIHA&biw=1232&bih=615

Edited by Chickenman
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I'd get a video of the front mount and diff nose, just to know all of the facts.  If the axle axes are static (which they seem to be in the video) and the back of the diff is dropping, then the front of the diff has to be rising.  Just saying, that's the way it has to be.  Might just be taking up the elasticity of the front urethane or there's more flexing happening.  

 

This is different from what I first proposed.  The short nose diff has a lot of leverage on those two arms used to make the front mount.  

Edited by NewZed
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I'd get a video of the front mount and diff nose, just to know all of the facts.  If the axle axes are static (which they seem to be in the video) and the back of the diff is dropping, then the front of the diff has to be rising.  Just saying, that's the way it has to be.  Might just be taking up the elasticity of the front urethane or there's more flexing happening.  

 

This is different from what I first proposed.  The short nose diff has a lot of leverage on those two arms used to make the front mount.  

Yes, that's why you have to make sure that if you stiffen the back bushings you also have to stiffen the front. You can't have solid aluminum bushing in the back and poly or rubber bushings in the front.

Edited by Chickenman
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Nissan also made a big mistake with the rear suspension LCA and diff support design. The Mustache bar should have been solely used to support the diff. The two suspension supports for the rear LCA should never been attached to the Mustache bar. They should have had there own separate and solid mounting directly to the frame. That way any movement of the Diff does not affect the position of the rear LCA pivots. Look at how much your LCA pivots move when the Mustache bar mounts flex. Not good... That' sending your suspension geometry all over the place.

The suspension doesn't bolt to the mustache bar. The uprights go right next to it but they aren't connected. IMO the big mistakes were the flimsy uprights and the stupid stock front diff mount.

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The suspension doesn't bolt to the mustache bar. The uprights go right next to it but they aren't connected. IMO the big mistakes were the flimsy uprights and the stupid stock front diff mount.

Yes, thank you. You are of course correct. When I was looking at the OP video, he has custom made MB to mount the GM Holden differential.  Initially it looked like the LCA uprights were attached to the MB as they were flexing at the same time the MB was flexing. That's what threw me.

 

After looking closer it appears that it's just suspension loading that is bending the uprights. They need to be reinforced with a gusset of some sort.

 

I've edited my previous post to correct my confusion...

Edited by Chickenman
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