Front diff crossmember no longer necessary

[JMortensen]

So I'm sitting here waiting for my Bondo to dry and I'm looking at the underside of the car. I just modified my Ron Tyler diff mount from roostmonkey so that it bolts in to my 70 z, and I'm thinking to myself "I don't have to capture bushings with the crossmember, and I'm not going to have a diff mount hooked to this crossmember, so why am I using the crossmember?"

 

Now I think it's important to have a piece that spans the front bushings for the rear control arm, but that convoluted shape of the stock crossmember doesn't look like the most efficient way to do that. I think it is built primarily with the diff loads in mind rather than the control arm loads.

 

Which brings me to my possibly lighter and simpler solution. I'm thinking just use a simple rectangular .5 x 1 x .065 tube across the bearing retainers, and weld it in place. The downsides that I can see are that I would lose about 3/8" clearance for the exhaust and I wouldn't be able to take the diff mount out anymore.

 

The upside is that it would drop several pounds in weight and might actually be stiffer.

 

Or I could just leave the crossmember that is there.

 

Opinions?

 

 

 

 

[Pop N Wood]

I have considered modifying the crossmember to just be a piece of tubing across the center. Basically just cut the ends off and bridge the gap with a piece of something. That would save weight and give more room for exhaust clearance.

 

I have the 70 R180 cross member and a later R200 crossmember. It is kind of suprising how much beefier the later R200 piece is. Most of it could have been diff loading like you say. But I think there might have been more to it. For example, the flat piece on the underside is considerably more significant than the older piece. That won't do much to brace up and down movement. So it must have been made larger to control twisting and/or side to side movement.

 

I wouldn't leave it open on the underside. Wouldn't want to see that area opening and closing as you go over bumps. Think tension and compression (especially if you get T boned). Plus if your urethane mount ever does give out it could catch the front of the diff from cartwheeling.

[blueovalz]

Would the seatbelt bungs allow a piece to be bolted across them without interfering with the driveshaft?

[JMortensen]

I have the 70 R180 cross member and a later R200 crossmember. It is kind of suprising how much beefier the later R200 piece is. Most of it could have been diff loading like you say. But I think there might have been more to it. For example, the flat piece on the underside is considerably more significant than the older piece. That won't do much to brace up and down movement. So it must have been made larger to control twisting and/or side to side movement.

I wish we had some insight as to why Nissan made it thicker. I think when the diff comes up it also goes back so maybe they were trying to prevent that twist.

 

Terry, there is definitely room on the seat belt bungs to attach something. What did you have in mind?

[jt1]

As the car accelerates, the control arms are pushing the car forward, and the front of the arms are pushing in towards each other, trying to toe in. There's a lot of force there, so the crosspiece needs to be fairly strong IMO. The factory piece had to deal with the diff pulling up also, so it can be lighter than the factory piece, but I don't think the 0.5x1 will do it, maybe 1x2 would work.

 

John

[JMortensen]

1x2 would interfere quite a bit with exhaust routing, so if that were necessary I think I'd leave the stocker in there. I know there is room to do two .5 x 1 bars side by side, so that would be .5 x 2

[Evan Purple240zt]

I have nothing productive to say.

 

Nice stitch welding job!!

 

Evan

[heavy85]

I would want to tie the control arm mounts together directly some way for the most direct load path. It seems the tranny tunnel really weakens the center of the car and the factory crossmember does a decent job of tying the sides together. Maybe just cut out those tubes you added to the crossmember to take the weight out ... or just get another unmodded crossmember as they dont weigh that much. I didn't design the factory part but my oppinion is it serves three basic functions 1) hold the control arm on, 2) hold the front of the diff, 3) add strength to the frame to help handle loads from the control arms.

 

Cameron

[johnc]

Weld the RT mount into the car and forget about a crossmember between the LCA mounts. Just make sure the LCA mounts can handle some strong lateral loads.

[Guest 280ZForce]

Weld the RT mount into the car and forget about a crossmember between the LCA mounts. Just make sure the LCA mounts can handle some string lateral loads.

I have thought bout this recently & planned to do this. I'm just waiting for my r/t diff mount. I wanted to give as much clearance possible to run exhaust piping on each side of the diff.

[JMortensen]

I have thought bout this recently & planned to do this. I'm just waiting for my r/t diff mount. I wanted to give as much clearance possible to run exhaust piping on each side of the diff.

I had thought the same thing, but I am really hesitant to remove the link between the two control arms. Would make diff changes a snap...

[heavy85]

Weld the RT mount into the car and forget about a crossmember between the LCA mounts. Just make sure the LCA mounts can handle some strong lateral loads.

 

I'm curious why you say linking the LCA mounts is not needed. Is it due to what Jon has done up top with the diagonals from the cage? The crossmember sure seems like a good way to distribute the LCA loads into the frame. Generally you want to connect the loads in straight lines where possible?

[tube80z]

Weld the RT mount into the car and forget about a crossmember between the LCA mounts. Just make sure the LCA mounts can handle some strong lateral loads.

 

Or if you really think you need it weld a tab along the bottom of the RT mount for a bolt in crossmember. This would be more in line with the lower control arm mounts than a piece further up where the seat belt anchors are.

 

I don't think you need much to strengthen this area given the rest of what you have done.

 

Cary

[johnc]

Is it due to what Jon has done up top with the diagonals from the cage?

 

Yes.

[JMortensen]

Or if you really think you need it weld a tab along the bottom of the RT mount for a bolt in crossmember. This would be more in line with the lower control arm mounts than a piece further up where the seat belt anchors are.

The seatbelt anchor plates are basically in line with the control arm bearing retainers. The bungs are just a little higher in the chassis. The bottom edge of the bung is at the same level as the bottom of the frame rail where the crossmember bolts on. The bottom of the mount is much higher in the chassis than the seat belt anchors are. Maybe you meant further forward...

 

I don't think you need much to strengthen this area given the rest of what you have done.

I never did do the proposed bars from the X to the control arm pivot area, so I really don't have much of anything done to strengthen this area at all, other than stitch welding the frame rail and welding a plate on the inside of the car.

 

I'd love to leave this area open because it would make working on the diff

so freakin easy. Do you really think this can be left alone with 12" slicks on the back of the car?

[JMortensen]

I'm just thinking if I left it as is I could do back to back tests with the crossmember installed and not installed...

[bjhines]

I used the cage to rienforce this area from above.

 

 

I am definitely using the lower differential crossmember along with my RT mount. I modified the X-member to clear dual exhaust.

[tube80z]

I never did do the proposed bars from the X to the control arm pivot area, so I really don't have much of anything done to strengthen this area at all, other than stitch welding the frame rail and welding a plate on the inside of the car.

 

I'd love to leave this area open because it would make working on the diff

so freakin easy. Do you really think this can be left alone with 12" slicks on the back of the car?

 

You have subframe connectors, cage diagonals, etc. all welded in that help keep the floor from moving. Those are all going to help keep this from moving. Take a look at the top of the tunnel, I don't think you need a lot more than that to strengthen the bottom.

 

From personal experience I can tell you that I've broke that crossmember (diff mount issue) and never noticed any handling issues. I even ran one race with just the caps and couldn't tell. I have broken cracked and broken the rear transverse mounts, which I can feel. So I'd say you're safe and I'd spend the effort at the back and leave this area alone.

 

Cary

[JMortensen]

Alrighty then. I'll just leave it as is, weld in the diff mount and see what happens. If I feel there is a problem I can always reinstall the stock piece.

 

Glad I asked! Thanks John and Cary.

[neverenough]

I have the RT mount but not yet installed. My question is what is stopping the nose of the dif. from tearing the rubber mount used with the Rt mount when going in reverse. I know it doesn't see much stress in reverse but if the nos of the diff. goes up when accelerating forward it should go down in reverse. I would like to have something under to keep it supported from below.