If you're gonna build the frame up in that section, IMHO, just plate the outside of the framerail from the firewall body mount forward. You should have that whole area pretty much flat after you cut/grind off the IFS, and adding metal in the upright Y-axis of the box is bigger benefit there than adding in the X-axis.

With Isuzu's stout channel frame, you're buttressing a pretty strong base.

Randii

To really add strength, the frame needs to be deeper in the Y-direction. I was wondering if the frame was about the same width as a 2x4 tube, and the tube could be sectioned to match the contour of the bottom of the frame. Extra material in the x-direction will actually do relatively little. If you look at the section modulus (resistance to bending) of a 2x2 solid bar, and a 2x2 tube they are not that different (the solid bar is actually not much stronger in bending). Plates on the sides will add some strength, but you won't get the same bang for the buck as you get from more depth. It is up to you, how to do it, but I suggest more depth. Even if you just added a plate along the bottom of the frame, it would add considerable strength. Look at semi-truck flatbeds; the web in those frame rails is often as thin as 3/16", and the flanges are often no thicker than 7/16", or even 3/8", yet you can put a 60,000 lb concentrated load in the center, and they don't fold. It is material away from the horizontal centerline that will add stiffness. A vertical plate on the side of the frame will add stiffness, but not as efficiently as more depth. If the Isuzu frame is sufficiently stout, a plate would probably work really well. But I have always like the motto: When it doubt, make it stout, out of things you know about.

I agree with some of what you say, but IMHO the rails will be stiffer with 3" tall quarter-inch vertical plating than with quarter-inch tall 2.25-inch wide horizontal plating. I understand that generally, flanges are thicker than webs, but that applies better as the web-height to flange-width ratio increases.

Randii

This could actually lead to some very interesting discussion. There are certainly trade-offs. Without doing the math, the biggest bang for the buck comes from more depth. The reason being stems from the equation for the area moment. On a rectangular section, this equation is (base*height^3)/12. As you can see, having the height cubed adds the greatest stiffness. To obtain the section modulus, you then have to divide the area moment by the distance from the center of mass to the point in question. Of course, for irregular sections there is some additional math required, but additional height is the greatest contributor to stiffness. In the case of the Isuzu frame, a plate is probably more than adequate. I suggested the extra depth, only because of the amount of strength you can gain from it. Not that the plate on the outside will not add stiffness, it will add a great deal. But, when it comes to things like frames on off-road rigs, I am a big fan of overkill. If I get some time to day, I will do a few calculations and post the results, I think it will be good info for anybody on the board considering an SAS. Does anyone happen to know the dimensions the frame at the point of the engine mounts, including the thickness of the frame? If no one posts dimensions, I will base the calculations on 2x3x1/8" tube.


Okay, I just ran a few quick calculations. Using the equation for section modulus from the Machinery's Handbook, 25th edition. Z=(bd^3-hk^3)/6d. For a standard 2x3x1/8 tube the section modulus is .978in^3. If I add 1/4" to top and bottom, increasing the height by 1/2" the section modulus increases to 2.35in^3. If I add 1/2" to the sides (basically a 1/4" inch plate on each side of the tube), the section modulus is 1.73in^3. You can see that a 1/4" strap, top and bottom is 1.4 times stiffer than a 1/4" plate on each side. Either way, you have increased the stiffness of the tube, but additional material added away from the horizontal neutral axis of the material gives you more bang for the buck. Notice, with the side plates you have almost doubled the stiffness of the tube, but with straps top and bottom, you get more than double the stiffness.

I'm putting my slide rule down and backing away from this discussion - I'm clearly out-classed! <img src="/forums/images/graemlins/pfft.gif" alt="" /> My seat-of-my-pants 2D planar estimates are no match for the real 3D thang! While reflecting on this, I realized that I was thinking more of discrete planes (well, we're adding plate, right?) and not the assembled box member.

Thanks for posting up a math example of the stretngth ratios! <img src="/forums/images/graemlins/kewl.gif" alt="" /> I'll try to measure the motor mount area tonight when I get a chance, for real numbers.

FWIW, since plating the top of the rail is gonna be tougher than the inside and/or bottom, how do the calc's work if you only plate one side of the box? How about two adjacent sides (definitely better to fight twisting)? There's also racking to be considered...

Randii (who admittedly fabs without the calcs)

If I could only understand what you guys are saying, lol, sounds interesting.

If we plate only the outside, you end up with an effective section modulus of 1.35in^3, still considerably more stiff than the original frame. What also needs to be considered are some of the torsional effects from the motor. Any plate added anywhere, will add some torsional rigidity. What is good about the plate on the side, it adds more torsional rigidity than the plates top and bottom. Which brings up another issue, crossmembers. With an SAS we are fighting differential clearance, and this does not make adding crossmembers easy. Personally, I think it would be beneficial to add a crossmember to the frame, once the IFS is cut out. That being said, this crossmember could be rather small. Something as small as a 1.75" OD tube would do quite nicely. A frame plated on the outside only, with a crossmember right in front of the sump, tucked as tightly as possible, would be great. I have not done an SAS, so I can't say what clearances are there, and what are not. For me personally, I am an advocate of low lifts so the center of gravity is as low as possible. I would like to SAS my Trooper, but I only want to run a 33x9.50 tire. I really only want a few inches of lift, so I know that differential clearance will be at a premium on my rig. What crossmembers are being kept and what crossmembers are being discarded on most of the Isuzu SAS's?
By the way, if you can't tell, I get almost giddy when I can apply my vocation to my avocation <img src="/forums/images/graemlins/smile.gif" alt="" />! I am a geek by profession, and it is exciting when I can apply schooling to real world stuff. Not that I don't do that at work, but frankly, I design and build boring stuff at work. If I wasn't such a chicken, I might venture out on my own and design things I like. But I am afraid if I did that I would not be able to adequately provide for my family. There is a lot of security in a benefits package! Dan, sorry for hijacking your post, but I think this can benefit anyone considering an SAS.
That being said, a plate on the side is probably quite adequate. If you want to overkill it a little, add a crossmember, and if you really want to overkill it, add straps to the top and bottom.
One other thing, any plates you add top, bottom, or sides, it is a good idea to taper the ends so you don't have an abrupt change in cross section. Abrupt changes create stress concentrations, which can lead to premature failure.

You went right where I pushed you with the twisting effect of the motor mounts. <img src="/forums/images/graemlins/pfft.gif" alt="" /> I gussetted the <img src="/forums/images/graemlins/ignore.gif" alt="" /> outta my motor mounts and tied them into the shock mounts, yet still I worry a bit about 'em. It is on the list to fab a tower-to-tower shock brace to tie it all together a bit more (a crossmember up top, if you will)... but right now it is handling being parked quite well. <img src="/forums/images/graemlins/rolleyes.gif" alt="" />

Making low lift and SAS work together is a challenge -- you wind up with pitman arm, axle housing, frame member, steering linkage, and other bits all wanting to share space at the most inoportune time. Adding cross-members is pretty ambitions anywhere other than the traditional places -- up front behind the bumper or aft of the firewall. The crossmember that Isuzu uses to tie the lower control arms together goes away in most swaps I have seen... but the suspension loads go away, as well. The motor mounts loads remain, though, as you noted.

FWIW, the frame is about 3" tall, 2.25" wide, and mebbe 3/16" wall thickness... in metric, I guess. <img src="/forums/images/graemlins/pfft.gif" alt="" /> That probably brings the ratios a little more even, but I bet the short sides of the rectangle still win... which really makes me want to rethink some of my past fab. <img src="/forums/images/graemlins/zombie.gif" alt="" /> I'm not worried about it falling apart, but I'd guess some of it could be stronger.

Randii

The motor mounts are always a concern. Years ago, had a buddy with a '63 Ford Unibody shortbox. Being the impetuous teens we were, we could leave nothing alone. A 460 soon found it's way under the hood, needless to say, the frame (originally designed for a straight six with less than 100hp) was not too happy about a 500hp big block. We welded those motor mounts in time after time. Being young and dumb, we never did get it right, but man it was fun! With weight in the back we could almost get the front off the ground! This was an instance where we had totally exceeded the capacity of the existing structure. Knowing what I know now, I would have boxed the whole front, and likely put a brace up and over the top of the engine. Is this necessary in the case of a 2.6, maybe but probably not. But, the motor mounts are definitely an area that deserves some attention.
If you would like, get me a front axle weight, we can WAG the load from the engine on the motor mounts, if someone happens to know the weight of the engine and tranny, that would help (but we can always just throw a conservative estimate at it). I will also need some spring lengths with center pin to shackle distances, and I can calculate some stresses in the frame itself. Using what we know of the stock ZU frame, I could design some stiffeners that would put the frame in a nice low stress region, to give it long life. I would be happy to help anyone who like to put a little engineering into their SAS. Maybe we could even come up with a nice canned kit like All-Pro has for the Yota trucks. <img src="/forums/images/graemlins/notooth.gif" alt="" /> Or at least some plans we could all share.

I have been working on getting alot of the parts I need to rebuild them axles. I have my dana 300 all tore down,washed and painted. I was putting it back togeather when Dan hooked me up with some real cool tricks to do to the case. So I order all my stuff and have to get out there and finish that t-case.

Oh yeah what do you guys think about 350 to 375 stock hp? I can't say alot right now but I am working on getting a new motor which came stock pushing that kind of hp. That should push them 31 inch tires I am going to be running. <img src="/forums/images/graemlins/smile.gif" alt="" />

Later,
Daniel