Round or square subframe connector tubing?

[97 Octane]

Planning to build and install subframe connectors for the Mark. I'm looking to tighten up the performance and handling now that SCT tune has given new life to both my Marks.

I will be using chromoly tubing, either round or square.
What are the benefits for using one over the other?

 

[OchoMarko]

I think square is stronger and stiffer and definitely easier to weld/bolt onto a vehicle.

 

[OchoMarko]

Please take lots of pics and show us your work!

 

[SATURN5]

I used mild 1 by 2 x .090".

These are all the braces I have currently on the 96.

 

[driller]

Square (or rectangular) has a 'stiffer' cross section only in two planes when comparing the depth of the cross section and equal wall thickness. Round has the same stiffness in all directions. Round has better stiffness for the same weight square or rectangular section. Round is also better in torsion.

I used round chrome moly tubing for subframe connectors. I also bought some rectangular tubing for jack rails but discovered I didn't need them when I could jack up the entire side of the car using the subframe connectors as the jacking point.

 

[racecougar]

JP is correct.

That being said, I went with square mild steel tubing as it is easy to work with, plenty suitable for the task, and most importantly, it's what I had on hand.

http://www.lincolnvscadillac.com/fo...racing&p=2037249368&viewfull=1#post2037249368

 

[Markviiiedrea]

I would think you could gain a little more ground clearance with the square tubing as well.

 

[olddavid]

You guys are really changing the entire chassis dynamics. What would be some of the downsides of this improvement? Do the braces get in the way of the alignment guys when they do their work? Now that you've lived with it for a while, what would you change? I cannot think of a more useful handling alteration for a V8 rear drive unibody car, especially if you have the skills to do your own fabrication. When I look at Cougar's pics, I see that coils are his choice of suspending the car. Is modifying a bagged car this way sensible?

 

[racecougar]

You guys are really changing the entire chassis dynamics. What would be some of the downsides of this improvement?

Carrying an extra 33 lbs of weight.

Do the braces get in the way of the alignment guys when they do their work?

Not in the least.

Now that you've lived with it for a while, what would you change?

Not a thing.

I cannot think of a more useful handling alteration for a V8 rear drive unibody car, especially if you have the skills to do your own fabrication.

Bingo. There's a reason why you'll often see the first suggested mod on just about any Mustang (especially the '79-'04's) website is to install full length subframe connectors. Anything you can do to reduce unibody flex is beneficial in a number of aspects. Properly designed SFC's make a very noticeable difference on these cars.

Is modifying a bagged car this way sensible?

You certainly could.

 

[Markviiiedrea]

Carrying an extra 33 lbs of weight.

In comparison, SATURN5’s kit is 50lbs for what is pictured.

 

[racecougar]

In comparison, SATURN5’s kit is 50lbs for what is pictured.

The 33 lbs I mentioned only includes the SFC's and related mounting hardware that I produced/sold. I never weighed the LECB's, V-brace, or additional roof structure, as those were all one-off's for my car, and I can't recall the weight of the RSTB's that I produced/sold at the moment. The whole package would likely be right around 50-60 lbs, if I had to guess.

EDIT: IIRC, the shipping weight for the RSTB's was right around 8 lbs each (including hardware, instructions, padding, and the cardboard box).

 

[driller]

I forget all the numbers, but a good rule of thumb is chrome-moly tubing is about 1/2 the weight of the dimensionally equivalent mild steel and has the same strength. Put another way, if the weight per foot is equal then the chrome-moly has twice the strength.

Most chrome-moly tubing is sold in a thinner-wall (light weight) dimensional equivalent to standard mild steel tubing. I think the round I used was 1.9" OD which is equivalent to 1.9" OD steel tubing more commonly known as 1-1/2" steel pipe.

 

[97 Octane]

JP, Rod and Bob, you guys have made some valid reasons why I should go with one over the other. One thing I did not disclose was concern for the added weight but that is something I was willing to accept. I guess for the added 30-50 lbs sacrifice, it is negligable in terms of the benefits an SFC would add to the car's performance. While I want to keep weight gain to a minimum, I don't plan on putting a heavy ass subwoofer box in the trunk either. A-p yes but Not a sub box.

Cost is the other reason why I wanted to tackle this myself. As every metal fabricator has had a first project under their belt at one time or another....this is mine....sort of. There are plenty of steel distributors competing in my area so my cost will be low and because I don't have a garage full of specialty cutting equipment, this project will be assembled using simple tools like my sawzall, drill press and mig welder. I don't have any budget right now for new tools.


JP, you mentioned round tube has better stiffness and torsional rigidity. I might be swayed to go this route and use mild steel instead as Rod mentioned which would be easier to work with than chromoly steel of equal diameter.

Nevertheless, round tube would have a better resistance to lateral torsional buckling. Even then, the majority of the forces that stress the SFC is the tension and compression excerted between the crossmember and the unibody chassis.

-Ren

 

[driller]

Before you choose, pickup a piece of chrome moly and compare to a like sized section of mild steel. You'll be surprised at the difference.