It may be, but in this application, I wonder if the standard tube is acceptable with corner bracing where it jogs out and back in?

I usually weld something in and watch it for a while. I tend to go more on the truck side than aircraft so it does not usually break. Hopefully somebody who is more of an engineer will respond.

Does Ferrari make a truck?

grandpaul wrote:At this point, my concern is with a satisfactory solution for the offset upper (shock) leg to clear the drive chain on the Left side. I'd like to use the original section, beefed up, but is oval cross-section really necessary?

The oval section was just suggested to give you additional clearance. Do you know exactly how much room you'll have for the chain on the upper run yet?

By the way,

I've found that old bicycles can be an excellent source of hard to find tubing. I've bought good quality stripped frames in the wrecking yards for a fiver and gooten some nicely engineered tubes out of them, from oval to round stuff.
I usually go to Allen Steel in Redwood City from more common stock but bike stuff makes good brackets, fender braces, and a nice neck brace when I changed the angle on one.

If left as-is, the chain would cut a groove just about the size of itself in cross-section, at the bottom of the swingarm's stroke. Once it was compressed to the top, that interference would be less than half that amount. The problem is, that I would need to cut half way into the pipe to allow for the chain guard to clear as well. I think it would be stronger to re-route that leg with a jog at the top, than to chop out a segment and weld in a piece of angle iron bracing around the chainguard (although that would serve as a nifty mounting point for it)

My Dreer-designed unit just clears the chain by a tiny amount, with no provision at all for a chain guard.

Would it be possible to space the engine tranny package to the right a hair?

I suppose the vernier isolastics could be adjusted a bit, but not nearly enough for 1/2" clearance, more with the chainguard...

I wish I could see it. You can move a rear wheel and sprockets but I can visualize much tolerance with the drive sprocket on the tranny. It's kind of a decision as to how much is practical, you could move the output to the other side but the cost would be silly.

"no thank you" on the 'Silly Cost' idea!

I'm working it out; my oldest brother (mechanical/electrical engineer) is more mechanically inclined than our structural/architectural engineer brother, he's working on it. He's a home-builder of aircraft (Flying Squirrel), so he's studied up on his mechanical structures...

The main load on the that tubing on the upper left side is compression. The stock swingarm tubing is way strong enough to afford a divot with a curved plate welded over it. Or you could just cut that whole section of tubing off and put in a bent piece.

On second thought, probably nix on the divot idea if it's half the diameter of the tubing. Honestly, using the stock swingarm for the upper section is overkill. You could use lighter tubing there and get a piece bent with a slight S to go over the chainguard. I don't think you can use the iso's to move things around because the swingarm's going to move with it.

Most times if it looks right it is, even without engineering.
I would think the original swingarm would be the main structural piece, and the upper section could be of a lighter material.
This upper piece, while having to deal with the loads of the suspension, could have offsets in it to suit the chain clearence problems.
While a bit off the subject, ducati did similar offsets and lighter tubing for canterlever and rising rate. The shock mount at the top is offset to clear the rear carb. They used alloy or square or round chrome molly tube.

IMG]http://i579.photobucket.com/albums/ss232/GRM450/TT31-5-08005.jpg[/IMG]

That didn't work, I'll try again.

My eldest brother replied in the affirmative as to the cut-out notch with a section of angle iron to reinforce that segment. The actual cut-out, in section, is less than 1/3 of the diameter of the tube (at it's deepest point) , and less than 3" long. Indeed it is way over-designed and will be structural overkill.



My main purpose in using a second swingarm was many-fold: 1. I had a spare laying around, 2. It fits in the available space perfectly, 3. It made designing and fabricating the main pivot mount for the rising-rate linkage a "done deal" by simply fabricating a set of bushing blocks to fit the existing swingarm bushings, 4. Fabrication work is cut in half, 5. Integrity of the existing welds is very good, 6. The Dreer-designed unit has some shortcoming that are going to require additional work on my 880 to produce a satisfactory finished system, and 7. The Dreer swingarm also required custom fabrication of a new rear axle, spacer, and chain adjuster blocks.

Net result should be a better system at lower cost. Some may like the "trick look" of the rectangular-section tubing of the Dreer unit, but I think I prefer the "beefy look" of my unit.