The US Cartool subframe connectors require a TON of prep. A TON. Not to mention a crapload of welding. And not super easy welding either I might add, there's a fairly large difference in the thickness of the metals, pretty large gaps even with decent trimming and fitting. And of course, welding upside down if you don't have the luxury of a fully stripped car on a rotisserie. And welding upside down just sucks. They're the most stock looking, that's for sure. But to install them on a car that's anything other than stripped down to the bare chassis and on a rotisserie or a lift is a pain in the butt. I built and installed my own home made tubular subframe connectors on my Duster in probably a 1/4 of the time it took me to install the US cartool connectors on my Dart, maybe less. Not to mention the whole tank of welding gas I used up. The US Cartool subframe connectors are a great product, but they are labor and skill intensive to install. Especially compared to tubular subframe connectors.
The MP bolt in connectors are a little small for cross sectional area. I think they're like 1.5"x1.5"? And the landing plates for the front crossmember and rear rails are a little small. If they're welded in they are definitely better than nothing, but they're not the strongest option out there. Tubular connectors can be plenty strong, but the MP connectors are on the small side. I made the connectors on my Duster out of 1.5"x3"x .120" wall tubing and used pretty large landing plates to gusset them into the existing structure. They work pretty darn good, and with that size you don't have to cut the floor pan, they will sit flush with it across the rear foot boxes. If you're not a welder or fabricator though the MP connectors are not a bad option, they can be installed quickly with little prep and will still be an improvement over nothing at all.
There are quite a few things you can check to see if you've actually added any stiffening. Most are pretty simple and not super scientific, but simple things like less rattles and squeaks after install are an indication that you don't have as much flex anymore. You can jack up of a corner of the car and see if you can open and shut the doors easily, which isn't something you can do on most of these cars without subframe connectors, usually that causes some door sticking (you can check before you have them installed and compare). And of course if you have the tires to really put some loading into the chassis, you can actually feel some of the chassis flex in these cars without any stiffening. Kind of a spooky feeling, you can tell things are moving but it's not body roll. But you have to push pretty hard for that to start to be obvious. Generally the car just feels more crisp afterward, but of course that's a pretty subjective measure of things.
The MP bolt in connectors are a little small for cross sectional area. I think they're like 1.5"x1.5"? And the landing plates for the front crossmember and rear rails are a little small. If they're welded in they are definitely better than nothing, but they're not the strongest option out there. Tubular connectors can be plenty strong, but the MP connectors are on the small side. I made the connectors on my Duster out of 1.5"x3"x .120" wall tubing and used pretty large landing plates to gusset them into the existing structure. They work pretty darn good, and with that size you don't have to cut the floor pan, they will sit flush with it across the rear foot boxes. If you're not a welder or fabricator though the MP connectors are not a bad option, they can be installed quickly with little prep and will still be an improvement over nothing at all.
There are quite a few things you can check to see if you've actually added any stiffening. Most are pretty simple and not super scientific, but simple things like less rattles and squeaks after install are an indication that you don't have as much flex anymore. You can jack up of a corner of the car and see if you can open and shut the doors easily, which isn't something you can do on most of these cars without subframe connectors, usually that causes some door sticking (you can check before you have them installed and compare). And of course if you have the tires to really put some loading into the chassis, you can actually feel some of the chassis flex in these cars without any stiffening. Kind of a spooky feeling, you can tell things are moving but it's not body roll. But you have to push pretty hard for that to start to be obvious. Generally the car just feels more crisp afterward, but of course that's a pretty subjective measure of things.
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