RCA mounts and M-bar doublers

[MONGO510]

Hey fellows, I was installing an RT Diff. mount and got to wondering about any advantages to doubling the mustache bar and the rear control arm support plates.

Doubling the M-bar would possibly aid in U-joint life by reducing twist under extreme load and doubling the plates would help to stiffin the rear suspension under extreme side loading.

 

So what do you all think?

[MONGO510]

Anyone? Beuller?

 

Mongo

Edited March 30, 2010 by MONGO510

[johnc]

Crawl under the car and look again. The mustache bar is designed to control the torque of the diff housing not fore aft position. The transverse link could use some lateral bracing but making the transverse link braces thicker is an ineffective way of strengthening the transverse link against lateral load.

[JMortensen]

As John says, the mustache bar actually sits behind the suspension and doesn't contribute to it's ability to handle lateral loading at all. The lateral loads transfer through the rear control arms to two vertical supports which hang down from the frame rails. While they are connected via the link at the bottom, this looks to be a pretty crappy design and some racers have reported cracking the pieces under heavy g loading. Some of us have tried to stiffen this very weak design of the rear suspension. Terry went the furthest, replacing the vertical supports and integrating the mustache bar into a solid aluminum plate. I braced and gusseted the supports. I know others like bjhines ran a brace from the mustache bar bolt to the bottom of the support. There are probably a number of other ways to get that done, but that should get your brain moving in the right direction.

 

I think they're all shown here:

http://forums.hybrid..._toe%20adjuster

Edited March 30, 2010 by JMortensen

[MONGO510]

Thanks for replies. I am aware of what the M-bar does, I was wondering if there is a benefit to the Axle joints if the bar stiffness was doubled. I recall seeing a twisted up M-bar on this site somewhere. I thought that under load there would be some distortion of the bar, possibly causing some axle joint phasing errors and vibrations. Doubling would pretty much eliminate these potentials. Doubling can be accomplished without any welding by just stacking the 2 bars. The second bar would have the eyes cut off and be attached with clamps at the ends like a leaf spring is held together.

 

I also saw some pics of cracked and bent verticle supports that I would like to strengthen. If I recall the cracks werte at the root of the bend in the support. I thought doubling may be a easy way to help reinforce the plates. I have got the plates and a spare M-bar to use if I did this. I also have it torn down for the install of a RT Diff. mount at this time.

 

Thoughts?

 

Mongo

[JMortensen]

Thanks for replies. I am aware of what the M-bar does, I was wondering if there is a benefit to the Axle joints if the bar stiffness was doubled. I recall seeing a twisted up M-bar on this site somewhere. I thought that under load there would be some distortion of the bar, possibly causing some axle joint phasing errors and vibrations. Doubling would pretty much eliminate these potentials. Doubling can be accomplished without any welding by just stacking the 2 bars. The second bar would have the eyes cut off and be attached with clamps at the ends like a leaf spring is held together.

 

I haven't seen a lot of problems with phasing issues and vibrations caused by the diff moving around. Have I just missed them? Seems to me the vast majority of the issues have to do with the driveshaft angles between the diff and transmission.

 

The mustache bar is pretty heavy. Doubling it would make it stiffer, but it would be like adding a leaf to a leaf spring, and wouldn't make it rigid. It's still a spring, and still designed to let the front of the diff rise, as is the front diff mount. If you eliminate the ability of the front of the diff to rise, then you no longer need the spring in the mustache bar, which is what would allow the axles to move around as the bar twists. Then you could really make a rear diff mount that was rigid and wasn't designed to be a spring at all. If you eliminated the flexibility in the diff mounting you'd still have bushing flex in the suspension. I've been down the road to cure that problem. It's not exactly an afternoon's work, but it can be done. If you rigidly mount the differential and use all rod ends and/or monoballs in the suspension then you can get rid of that twisting action. If not, you can't. You can lessen these problems by doubling the mustache bar, but eliminating them entirely is going to take a lot of work.

 

I also saw some pics of cracked and bent verticle supports that I would like to strengthen. If I recall the cracks werte at the root of the bend in the support. I thought doubling may be a easy way to help reinforce the plates. I have got the plates and a spare M-bar to use if I did this. I also have it torn down for the install of a RT Diff. mount at this time.

I don't think doubling works as well here. I think the problem is that the supports have too small a footprint and the leverage from the control arm at the bottom is too great, so I bridged the gap between the supports with a piece of angle, and then gusseted the corners to stiffen. To clarify, if you were to make the uprights out of much thicker steel, 1/2" for example, then you still have the problem of leverage, and I think that you would find that the frame rail would be the next thing in line to go. I think spreading the load is what helps here.

[johnc]

What I've seen more then a few times is a tubular brace the runs from the mustache bar mount on the chassis to where the transverse link and the transverse link brace meet. I've also seen an "X" brace between the upper transverse link brace mounts in the chassis to where the transverse link braces meet the transverse link.

 

On my old racing 240Z I did the first mod mentioned above and was thinking about doing the second.

[MONGO510]

Thanks for the input fella's it is appreciated!

 

Mongo(John)