Subframe connectors for a driver/cruiser, necessary to weld to floor?
- Thread starter hotrod swinger
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moparmat2000
Well-Known Member
Thanks for the kudos Rusty.
This is an old thread i replied to that seemed to pick up traction again. I used 2x3 box steel 1/8" thick. The lap plates against the torsion bar crossmember are 3/16". They allow a better load transition into the thinner crossmember since they are laying alongside it, and not as some people do by just butting the connector to the torsion bar cross member and welding. Dont forget the drain holes either lol. Mine are 3/4" it's as big as my unibit was.
Re routing the cable to inboard of the connector makes it really clean as well. I also did this same subframe connector method to the 69 cuda coupe as well. I have not been on here as much lately as the messiness known as lifes utter bullshit has most certainly gotten in the way of things.
I have a build thread on here called 67/68/69 cuda dilemma , that has a lot of weld and fab, including a super clean way of modding and installing a 42/46RH trans in an A body along with everything to make it work including stock using the console shifter and using the factory kick down cable, as well as how to make a bullet proof front lower radiator crossmember that looks factory made. I like modified stuff that appears stock
One day I will get back to all this and get further, but for now it sits and mocks me every time I go in my shop for a tool.
Thanks
Matt
mopowers
Well-Known Member
I'm a believer that welding the connector to the floor is better than not. I went that route on both of my Darts. Since the '68 has a back seat, I ended up pushing the 2x2 tubing through the floor, marking it, slicing off the sides where it penetrated into the cab, and rewelding the top back on while welding it to the floor from the top. Now, the carpet lays flat. Lots of ways to skin a cat on subframe connectors.
OK, I'll bite, how do you know?
moparmat2000
Well-Known Member
Your stress is concentrated on the bolt attach points. This is where over time depending how you drive the car you will likely get cracking looseness and failure.
Sure cracking can start at a weld seam, but is less likely because the entire structure is more rigid.
As far as bolts go, case in point, bridges are bolted together and designed to flex so they dont fail. You are bolting together a frame connector to a car chassis with the intent to make it function as 1 solid structure (no flex, or minimum flex).
Will it work? Sure, it will stiffen it up. Will it work as well as a welded in set of connectors ? No over time it will not. It will still allow a bit of flex, and along with that the stress loading and unloading of the bolts.
Over time the clamping force of the bolts will become less as the oem structure deforms behind them, along with cracking from stress loading and unloading.
Here is something for you to think about. Bolts are made for things you want to be able to take apart to repair or replace if needed. Subframe connectors, are not one of those things.
Sure cracking can start at a weld seam, but is less likely because the entire structure is more rigid.
As far as bolts go, case in point, bridges are bolted together and designed to flex so they dont fail. You are bolting together a frame connector to a car chassis with the intent to make it function as 1 solid structure (no flex, or minimum flex).
Will it work? Sure, it will stiffen it up. Will it work as well as a welded in set of connectors ? No over time it will not. It will still allow a bit of flex, and along with that the stress loading and unloading of the bolts.
Over time the clamping force of the bolts will become less as the oem structure deforms behind them, along with cracking from stress loading and unloading.
Here is something for you to think about. Bolts are made for things you want to be able to take apart to repair or replace if needed. Subframe connectors, are not one of those things.
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I have a 1980 Dodge D150 and I'm having trouble finding anyone that makes frame connectors for it .....
Depends how there designed, like I said if designed right they can be just as strong. Doesn't mean that there are ones available that are designed right.
The factory K frame seems to work fine with only 4 bolts lot of stress/force goes through it, steering, braking, engine etc..
MopaR&D
Nerd Member
For them to be "designed right" would make them expensive and enough of a pain to install (read: drilling lots of holes and installing lots of bolts) that it wouldn't be worth it vs welding them in.
The k-frame doesn't experience the same kind of loading as the unibody between the front and rear subframes. That portion of the car was engineered from the beginning to utilize bolted joints so it's not a good example to use for "well they made it work there so it must be able to work elsewhere."
Possibly, you guy's take pointing out a fact as the same as an endorsement, never said it was generally better or the way to go.
You're assuming the factory did what was best, and that's rarely true. They typically do the cheapest thing they can get away with, which is probably why the rails weren't continued for the entire length of the car to begin with. The roof structure being present in that location, along with a lack of suspension points in that area, meant the cabin area was strong enough to meet the factory standards without them.
Welding the floor along the entire length of the subframe connector with two dissimilar metal thicknesses like the US Cartool design does is also very different than having a spot welded flange like the factory frame rails do. It might look more factory when finished, but from a stress/load perspective it's very different. If you want it to look more factory, awesome, go for it. But don't mix that up with "what's best".
I have welded in 2 x 3 tubing for frame connectors for years in lots of Mopars and a few Fords. I have never tried to do the thru floors deal as I have to lay on my old back and do it all. I too have no idea what is the strongest way, but what I do sure makes it stout!!! Makes a REAL difference. But then I have heard some "arm chair engineers" say the car needs to flew some!
Don't know!!
There is a limit to how stiff you can make that section of chassis as well, especially using two frame rails in the locations where they get added. You could weld in a set of railroad tracks if you wanted, but the benefit to the chassis stiffness might not be significantly better than a 2x3" box tube. The weight, on the other hand, would be drastically different.
There is a level of improvement that benefits the performance of the chassis, and after that everything is diminishing returns. Personally my somewhat educated opinion is that regardless of which kind is stronger, the difference in chassis strength between the two basic types of frame connectors on these cars is small enough that the vast majority of people driving these cars won't be able to tell anyway.
Yeah, that's not at all true. Certainly not in theory. There's a reason why bolted connections are used for most engineered structures outside of light duty automotive.
Now, that doesn't mean that the MP bolt in connectors are as strong. Their 2 lag bolts on each end of the connector are definitely not as strong as a welded joint, and definitely not over time. They would need larger landing pads, more hardware and captured nuts in the frame rails to have a chance to be as strong. So for the MP connectors I agree, bolting those in isn't as strong. But that doesn't mean they couldn't be with an improved design.
Making the structure more rigid can actually increase the chances of cracking at the joints/seams. You have to look at how much the metal flexes over the length of the chassis under the loads it will experience. Too much flex can cause cracking from overwork, but too little flex can cause cracking by concentrating more load than the base metal can withstand. Two very different processes, but the result will still just look like a crack to the naked eye. The amount of flex and load has to be matched to the capability of the material used to build the structure.
This is false reasoning. Bolts are used in structural engineering because of their load bearing properties, not because anyone is going to disassemble a bridge or skyscraper. That stuff gets taken apart by explosive demolition, so why isn't it welded? Because of the structural properties needed for the joints, that's why.
Yeah sure, in the automotive world stuff gets bolted together so it can be taken apart and serviced, but that is definitely not the only reason. The entire chassis on one of these unibody cars could be riveted together, and very likely with little to no difference in strength if done right. But they're not, and being able to take it apart has nothing to do with that decision. The factory does things for a lot of reasons besides what's best, what's strongest, etc. and that usually comes down to cost.
I most certainly am NOT. I simply commented on doing it as the factory did. You are inferring incorrectly, because you like to argue.
SO allow me to spell it out. When I said "best", I simply meant that the best way to add frame connectors IS to tie them into the floor system. I was not talking about which way of doing THAT was BEST. Are we clear now?
moparmat2000
Well-Known Member
Fords fullsize pickups are aluminum bodied, and glued and riveted together, but that doesnt make them better either.
No one is arguing against welding them in, you took something I said a couple of years ago that was a reply to god knows who and disagreed, fine, then you had to do an even more elaborate disagreement so I replied with a fact, not an endorsement. Again no one is not saying to bolt them in instead of welding but if done right you could.
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The rear end of that connector would bolt up not too bad but that front bolt connection is definitely terrible. If it cupped around the frame with 6 or so bolts would be better.
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