Custom Rear Subframe Mounting

[JohnW]

So at this point I have 3x1.5x1/8" box tubing to build frame rails out of coming back from my front subframe. Like this:

https://static1.pt-content.com/images/noimg.gif

https://static1.pt-content.com/images/pt/2012/09/turbopintoproject0022-1.jpg

It's going in my Pinto, and I'm planning a 3-link torque arm rear end later on. This is a rough sketch:

https://static1.pt-content.com/images/pt/2012/09/aTQPS-1.jpg

But at this point, all I can do is run the rails down the floor. I have until Sunday to get this done and won't have a chance to do much else for a while due to a move. I want the main part of the rails in now so the floors will be pretty much done and I won't have to worry about them. This won't interfere with anything else in the car and I'll be able to drive it until I'm ready for the next step. What I'm concerned about is how I'll be attaching the rear later. I want to weld a gusseted plate in as a bolting point before the rails go in, as I won't be able to get a welder in to add anything later without hacking up the floor. The plate will be lined up with the slope of the rear floor behind the back seat.

This is in a Ford Pinto that currently weighs 2400 pounds. I'm expecting it to weigh around 2600 once I'm done between a lighter motor and some structural reinforcement.

Now my questions are:
1. Is making a bracket to bolt the rear end on like in my drawing a mechanically sound idea? I think it should be if I take precautions to prevent the box tubing from crushing when bolted on and use the proper bolts. The control arms will be mounted to these front rails and the subframe I'm planning to make and bolt on later will be taking the forces from the coilovers and panhard bar or Watt's link.
2. If it is a decent idea, what size and thread pitch bolt should I use? I'm planning on using 4 bolts per side. I was thinking 5/8" would be good but I'm not sure, and I'm under the assumption that fine thread is better here right? I have to know what I'm going with before welding this all together. I'm planning on welding something along the lines of this (http://www.fastenal.com/web/products/detail.ex?sku=37892) in, instead of threading the plate because there wouldn't be much thread contact.
3. If this is a bad idea, what would be a better way to do it?

[19,69camaro]

I would weld it and then run at least a 1/8" plate over it. Make sure you plug weld the plate in several places. That should be strong enough. Steel is very strong in tension.
Bolting it up is half-assing it IMO and it will add weight.

[feeble]

This is the clamp-on bracket I designed for my Mustang II 3 link, but I am contemplating switching to a torque arm similar to what you have sketched and then using the front leaf spring hanger instead of building a whole new frame. The axle bracket would be the same either way.


https://static1.pt-content.com/images/noimg.gif

https://static1.pt-content.com/images/noimg.gif

How big is your rear end tube OD?

EDIT: Sorry, I misunderstood, I thought you were talking about the axle bracket. Why not use the existing leaf spring pocket though for the lower control arm front attachment?

[JohnW]

I would weld it and then run at least a 1/8" plate over it. Make sure you plug weld the plate in several places. That should be strong enough. Steel is very strong in tension.
Bolting it up is half-assing it IMO and it will add weight.

Well the thing is, I can't do the rear yet and if I weld the rail in now, then I can't get in to do the top bead later. I was also contemplating bolting it and then welding gussets on the sides after. I would like to do it the proper way though, so I'll probably have to wait until I'm ready to take everything on.


This is the clamp-on bracket I designed for my Mustang II 3 link, but I am contemplating switching to a torque arm similar to what you have sketched and then using the front leaf spring hanger instead of building a whole new frame. The axle bracket would be the same either way.

How big is your rear end tube OD?

EDIT: Sorry, I misunderstood, I thought you were talking about the axle bracket. Why not use the existing leaf spring pocket though?

I had a similar idea for the axle bracket, but I like yours better. It's a little more polished than what I'd been thinking about using. I believe the 8.8 is 3". I also really like how you made it bolt on. That way I wouldn't have to worry about setting the pinion angle perfectly and nothing will have to be welded to the rear end.

Using the leaf pockets was my original idea until I found the rust around them. It's spot welded to the car and trapped crap in there that rusted the floors a little. It'd be as much work to make new brackets for the control arms as it would be to fix this and connect the stock leaf pocket to subframe connectors. This is also part of why I want to run the rails all the way back and cut out the stock ones. They're solid for now, but the rust has started in places that I can't reach without cutting them out. Not to mention I need to cut out the entire "trunk" floor to make room for the fuel cell, Panhard or Watt's, and put in framing for the fuel cell and coilover supports. It just seems easier to start with two new rails coming back.

I figured I'd keep them at the same width as the rails from the front subframe since it'd be pretty easy to run back like that - and I would be building a little inbound of the current rear subframe. That way I'd still have the support so the car wouldn't sag while doing the framing, then I could cut the stock subframe out at a later time and make new bumper mounts. I'm not sacrificing too much doing it this way, am I? I'm also a little wary about the stiffness of the 3x1.5 box tubing I have as it seems too narrow. It should be alright for running along the floor as it'll have additional support (and may brace it to the rockers later), but I think I might be better with 3x2 where it kicks up.

And I'd like to put the control arm mount a bit further forwards of where the frame will kick up behind the rear seats - otherwise I'd be limited to a 7" arm which is a bad idea. Yours looks to be around 18" and that mount would interfere with the seats. My plan is to make the trunk floor flat where the spare tire well is now and keep the rear seats in the car. That limits my options with the frame somewhat.

I take it you used SolidWorks? Once I finalize my design a bit more I'm going to have to get back into using it.