Diff crossmember structural rigidity questions

[Zetsaz]

Hey everyone, I feel like it's the first time in a while I've made a new thread about anything but I'm curious if anyone here has experience. 

I'm running an RT Diff mount with the GM top mount. I know of at least one person (can't find the thread anywhere) who cut his stock crossmember to improve space for exhaust routing. Instead of being a crossmember for the diff, it's now just brackets to hold up the front of the control arms. Z Car Depot now offers billet pieces that do the same thing as pictured.

 

The big question is whether or not this would worsen lateral rigidity. I can't imagine the RT diff mount adds nothing but it's not exactly the same as the crossmember adding a brace straight across. 

 

Would love to hear people's opinions/thoughts.

Edited March 23, 2022 by Zetsaz

[NewZed]

Pretty pretty pretty sure that @JMortensen looked at that in the past.

[JMortensen]

1 hour ago, NewZed said:

Pretty pretty pretty sure that @JMortensen looked at that in the past.

Yeah, I asked that question in the past. I don't think I got a firm answer from anyone else about it. I ended up running monoballs instead of bushings, and welding the monoball holders into the chassis, so I didn't need the crossmember. What I ended up doing was using a piece of 1" square tube across the span, just to tie the chassis together. It's lighter and straight so probably more rigid. If you wanted more clearance for exhaust, could make a short little crossmember that bolted in at the strap mounts, on top of the RT mount, and get it a little further out of the way of the exhaust pipes, not sure if how much that gains, but I think the stock crossmember is probably 1.25" or 1.5" tall and those bolts are higher than that, so might be worth it.

If you're just street cruising, I think you can live without the crossmember and use your bolt on caps. The RT mount won't add much if any rigidity, that's not what it's meant for. 

Edited March 23, 2022 by JMortensen

[jhm]

I did exactly what Jon did on his, and it hasn’t been a problem in three years of track and street use.  Those billet brackets look nice, but I just used an extra set of the OEM rear u-brackets on the front LCA bushed mounting points....they’re the same shape and size front & rear.

 

I have seen failures at those LCA forward mounting points; but only on mega-HP/tq  setups with drag slicks.

[Dat73z]

I was researching this a while back due to AL driveshaft coming close to the crossmember on a short nose diff setup. 

 

I've seen pictures of where people just run another set of rear caps up front and no bracing at all but imo if you're going to cut it I'd at least weld in some bracing across.

[AydinZ71]

Man I have torn this area up multiple times now, both from inside the car and under the car. 
 

Purely a guess, but I would agree with Jon & John as long as you are not doing something extreme like doubling your OEM torque or cornering hard. The RT mount stiffens the tunnel walls above your diff, but does not brace straight across and therefore does not effectively resist the strain on your front LCA mounts. 

 

What concerned me on the Series one is the lack of stiffening across the tunnel in general. The little “shelf” right behind your seats acts as a pretty effective brace Right where your diff mounts to the side walls with the RT, but other than that there is no brace across the tunnel besides the trans mount and front LCA mount. 
 

in my case, I removed the shelf for over 7lbs in weight savings, but that is even more reason to keep the front LCA crossmember.

 

an example of how the Chassis would strain without the crossmember is over rough terrain or uneven surface at speed. The ocilating/uneven forces on either side will want to “yank” the front LCA mounts away from one another, and “push” the rear LCA mounts closer to one another. There is a moment about the strut caused by the accel/decel tire/road forces, and the LCA & mount is resisting that moment. In an extreme case, you could see toe-out in the rear at high speeds. You could keep an eye out on your rear tire wear and see if you spot anything unusual, if you are really worried. 

Edited March 24, 2022 by AydinZ71

[JMortensen]

We're getting off topic, but what I did to strengthen this area was to run an X from the strut towers to the bottom of the main hoop like so. In retrospect if I had to do my cage over I'd put a horizontal bar across the top of the trans tunnel as well, I think that would give a lot of strength in a side impact. The main diag and the X would have to move up. Might have to make huge plinths to make this compliant so the cage attached to the "floor." 

[AydinZ71]

@JMortensen Thanks Jon! Apologies to the original post. 

Super timely advice! What I have to be careful of: I am allowed to remove structural material that does not subsequently require additional reinforcement (amongst other restrictions of course). So I am trying to think of ways to reinforce this area without making it painfully obvious that the new material is performing the same role as the 7lb shelf. 

 

I did leave some material there as gussets and flanges. I was planning on putting a 3/4" thin-wall hoop over that sharp edge which will stiffen the flange quite a bit and eliminate the sharp edge. hopefully that will fly. 

 

With my new seat rails and the continuation of the floor pan stiffening rails, I have a few quite stiff locations to place another crossmember across the bottom of the tunnel. I was thinking of welding one on that was positioned in such a way to still allow my driveshaft to pull in-and-out. Or I could fabricate a crossmember that can be bolted on. This tunnel is certainly the most flimsy part of the series one when the dash and OEM seat rails are removed. You can drum on it, and it sounds like a 36" wide kickdrum. 

[JMortensen]

Yeah, I think all of this hit home when Cameron (heavy85) was building his car for Pike's Peak. When you have to consider rolling off of a cliff, then suddenly your cage looks like it protects from top impacts but does almost nothing for bottom ones. I think really a full tube chassis is the way to go if you want bottom impact protection. If you're just looking to stiffen the chassis though, a butterfly brace on the bottom would probably do a lot. Would love to see this tested in FEA.

My plans include eventually doing a flat floor, and if I ever get there I'd like to incorporate a simple bolt in brace between the subframe connectors. Maybe just a big X across the bottom with a front and back straight member. There isn't a lot of room on my car and it's really low so was thinking about something like a .5" x 1" rectangle tube, and then just welding some angle iron feet to the corners that would overlap the SFCs and bolt on, then attach the flat floor to the bottom of the brace.

I also had the thought that I would use sheet aluminum between the frame rails (non-flammable) and that alone would probably stiffen things up quite a bit. Again, some testing would make this less guesswork, but I have zero cad drawing experience and not much desire to figure it out, so will probably just do what should work and see what happens, just like the rest of the car...

This is all cribbed from the Flying Miata braces. https://www.flyinmiata.com/default/flyin-miata-butterfly-brace-v2-0.html

 

[JMortensen]

Shit. Now you got my brain rolling on this. Thanks a lot.

I don't think there is room for this front brace, I think the oil pan and bell housing would be in the way, but I'm thinking the rear one would definitely work. Maybe if there is too much crap in the way up front I could make a single horizontal bolt in xmember possibly bent to go under the oil pan, and then I could do the sheet AL on the bottom. 

[AydinZ71]

These are all great ideas Jon! I think I’m going to do an X brace where my new seat rails meet the tunnel. I made mine from 2.5” SS 16-gauge square stock, cut in half. You can see them in this photo. They welded to the floor pan stiffening rail mid-way, so tying the passenger and drivers sides will go a long way.  
 

only blooper is they are not aligned. I left room behind the passenger seat for the fire suppression bottle, and the drivers seat needs to be as far back as it can be now that it is much lower. The tilton pedals also have a long throw (intentional, no booster). 
 

excited to see what you end up with on yours 

 

 

 

[JMortensen]

Gonna be really low on my priority list. Maybe next winter. Was supposed to do the flat floor two winters ago...

[Michael]

 

Another case of removing this structure….

 

By one reckoning, the “tool box” is the upper part of a lateral beam, of which the lower part would be the OEM crossmember featured in this thread.  But I take umbrage: this “upper part” connects to the wheel wells… a flimsy structure unrelated to any suspension pickup points.  Indeed, I am confounded, as to what structural role this “tool box” plays at all.

 

Instead, some race sanctioning bodies call for a diagonal connection between the main roll hoop the frame rails.  This is what’s attempted in the photograph.  The “tool box” is removed, a plate is welded the floor, and a diagonal is welded between said plate and the main hoop.  Ideally the frame rails would extend to “near” the LCA front pickup point, so these diagonals to some extant react to prevent the LCA pickup points from moving away or toward each other.  Still, I would be inclined to retain some facsimile of the OEM crossmember, perhaps duly notched to accept twin exhaust.

 

In the photo below, we're looking just behind the passenger seat (folded forward); the passenger-side wheel well is to the left, and the transmission tunnel to the right.  The "tool box" would, if still remaining, have begun just behind the plate to which the diagonal is welded.

 

 

[JMortensen]

The wheel wells are pretty stout. Not the wells, but the walls that the connect to. There are 3 or 4 layers of sheet metal on the inside of the dog leg area. I know because I picked the seam sealer out to stitch weld in there. Huge PITA. Should have skipped it. I've seen some people lay a piece of sheet over the gap and then stitch there, that would have been a million times easier.

Those NHRA rules probably make sense on a ladder frame car, but that bar that you're showing doesn't make a whole lot of sense on a Z. Same with the main hoop supports connecting to the floor.

[Michael]

The main hoop connects to a triangulation between the rocker panels (beams?), forward edge of the wheel wells, and floor.  I don't presently have a good photo of the subject area, but here's another shot of the removed "tool box" (shelf?) area:

 

[AydinZ71]

Speaking of rockers, I did quite a bit of reinforcing when I replaced mine. I agree Jon, it became much easier when I just rebuilt it without the complex overlapping sheets. Mine was too far rusted.