K-Member / Rack & Pinion Steering Suggestions
Actually, the blue print and assembly instructions are easy, you can just download the Factory Service Manual right here for your '68 Barracuda https://www.mymopar.com/downloads/servicemanuals/1968_Plymouth_Service_Manual.zip Shows you how to do everything. And the aftermarket parts included come with their own instructions where they differ from the FSM.
And that list I posted makes for a full on autoX monster with the right tuning. With a /6 you do not need all of that stuff, I'd leave off the QA1 K member right off the bat. And although its always a good idea, you don't need to do anything to the factory K member as long as it's straight and nothing is damaged.
As for the chassis loading, it's one of the biggest differences with the torsion bar suspension. The torsion bars twist, so, the vertical suspension movement is translated into radial loads. Those loads are carried by the K frame and the torsion bar crossmember. So, all of the heavy chassis components that carry suspension loads are down low, keeping the CG down. And unlike a strut, coil, or coilover suspension there is no need for a heavily reinforced suspension tower out in front of the firewall to mount the suspension to. Those suspensions all take the vertical suspension movement and just translate it into vertical suspension loads- so you need a big old suspension tower to carry it. On a Mopar, because the torsion bars put all the loads into the K frame and crossmember the shock tower is not built to carry the weight of the car. And, the frame rails themselves are not triangulated very well with the firewall. Suspension loads are low, not high. That said, the downside to the Mopar chassis is that it is not reinforced well up high because it wasn't carrying suspension load there, and you get things like "cowl shake". Basically there's flex between the rails and the firewall/cowl.
The coil-over conversions load the chassis vertically, which is where the Mopar chassis is weak. Even with torsion bar suspensions the chassis benefits from triangulation. Look at US Cartools inner fender stiffeners Mopar A Body Inner Fender Shock Tower Brace Kit . From their website
There are also more obvious and obtrusive ways to do this, like the J-bars on my Duster which go the whole way, triangulated the front of the frame rail, tying it to the shock mount, and then the firewall
RMS deals with this by mounting their coil-overs to a tower built into their K-member. But as Denny pointed out, that limits the length of the coil-over. The HDK uses the Mopar shock mount, but adds those shock tower hoops to tie the shock tower back to the frame. Both take care of the issue of mounting the coil-over, but neither really address the frame rail to cowl/firewall issue of putting a suspension that creates vertical loads on a chassis that was designed to carry the loads radially in the cross members. Which is where the inner fender braces, J-bars, etc come from.
Seems spot on to me, I got 31 lbs for manual to manual but most of my parts were weighed with a bathroom scale. So a few pounds one way or another wouldn't surprise me at all.
Ok, so most of that weight savings comes from the rear suspension swap not the front then. I've got no issues with the rear conversions, leaf springs are pretty darn basic and not all that easy to "tune". Triangulated 4-links have their own particular pros/cons, but all suspension does. You can still handle well with leaf springs though.
The MotorTrend article is a big nothing burger, it just says what we've already covered. Header room, rack and pinion. That's it, and here's why-
I have a T-56 Magnum in my Duster, I used the ToddRon crossmember which was designed to retain the upper hoop of the crossmember and keep the torsion bars. And I have a Milodon road race oil pan. And long tube headers. And 13" brakes. So, I have all the stuff on my car they say they went to coil-overs to address.
Actually, the blue print and assembly instructions are easy, you can just download the Factory Service Manual right here for your '68 Barracuda https://www.mymopar.com/downloads/servicemanuals/1968_Plymouth_Service_Manual.zip Shows you how to do everything. And the aftermarket parts included come with their own instructions where they differ from the FSM.
And that list I posted makes for a full on autoX monster with the right tuning. With a /6 you do not need all of that stuff, I'd leave off the QA1 K member right off the bat. And although its always a good idea, you don't need to do anything to the factory K member as long as it's straight and nothing is damaged.
As for the chassis loading, it's one of the biggest differences with the torsion bar suspension. The torsion bars twist, so, the vertical suspension movement is translated into radial loads. Those loads are carried by the K frame and the torsion bar crossmember. So, all of the heavy chassis components that carry suspension loads are down low, keeping the CG down. And unlike a strut, coil, or coilover suspension there is no need for a heavily reinforced suspension tower out in front of the firewall to mount the suspension to. Those suspensions all take the vertical suspension movement and just translate it into vertical suspension loads- so you need a big old suspension tower to carry it. On a Mopar, because the torsion bars put all the loads into the K frame and crossmember the shock tower is not built to carry the weight of the car. And, the frame rails themselves are not triangulated very well with the firewall. Suspension loads are low, not high. That said, the downside to the Mopar chassis is that it is not reinforced well up high because it wasn't carrying suspension load there, and you get things like "cowl shake". Basically there's flex between the rails and the firewall/cowl.
The coil-over conversions load the chassis vertically, which is where the Mopar chassis is weak. Even with torsion bar suspensions the chassis benefits from triangulation. Look at US Cartools inner fender stiffeners Mopar A Body Inner Fender Shock Tower Brace Kit . From their website
There are also more obvious and obtrusive ways to do this, like the J-bars on my Duster which go the whole way, triangulated the front of the frame rail, tying it to the shock mount, and then the firewall
RMS deals with this by mounting their coil-overs to a tower built into their K-member. But as Denny pointed out, that limits the length of the coil-over. The HDK uses the Mopar shock mount, but adds those shock tower hoops to tie the shock tower back to the frame. Both take care of the issue of mounting the coil-over, but neither really address the frame rail to cowl/firewall issue of putting a suspension that creates vertical loads on a chassis that was designed to carry the loads radially in the cross members. Which is where the inner fender braces, J-bars, etc come from.
Seems spot on to me, I got 31 lbs for manual to manual but most of my parts were weighed with a bathroom scale. So a few pounds one way or another wouldn't surprise me at all.
Ok, so most of that weight savings comes from the rear suspension swap not the front then. I've got no issues with the rear conversions, leaf springs are pretty darn basic and not all that easy to "tune". Triangulated 4-links have their own particular pros/cons, but all suspension does. You can still handle well with leaf springs though.
The MotorTrend article is a big nothing burger, it just says what we've already covered. Header room, rack and pinion. That's it, and here's why-
I have a T-56 Magnum in my Duster, I used the ToddRon crossmember which was designed to retain the upper hoop of the crossmember and keep the torsion bars. And I have a Milodon road race oil pan. And long tube headers. And 13" brakes. So, I have all the stuff on my car they say they went to coil-overs to address.