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Bracing - here's my interpretation of 109's thoughts, I was thinking of welding the tube to a piece of trimmed 1/8th" angle iron accross the two engine/gearbox allen bolts and then weld in a verticle fillet as well as a outer one to the tube and angle iron as I've clumsily highlighted in the photo - what do you think?

2nd question - Tube - it's only going to be some 8"/20cm in length - just how strong should it be?
In the mock up I've been using some 0.625 x 0.038-0.040" seamless stainless tube - considering its length - wouldn't this be strong enough?

I don't think the angle between the braces will do much. It will mean taking the whole brace off if you need to get at one side for whatever reason.
The little triangular braces are on track, but they will be more effective the closer they are to the bolt hole. Where the braces are positioned on the tube is ok, but I'd tilt the vertical brace outwards, and the horizontal brace upwards.
If both braces nearly touched the washer, they'd be positioned best as possible.
When the triangular braces are welded to the tube, make sure they don't pull the 'foot' out of alignment with the tranny case.
Pipe size........one way to find out.

I don't know that anyone is going to be able to answer questions like is it strong enough and will this work, etc. One way to know is to trial and error and see what works. Of course there are experts who have been done these roads before. Trial and error might not show anything for miles and miles until you see where something has pulled the aluminum out or cracked a case or something.

A real cool way to see what works would be to build a finite element model of the frame and to some degree the engine case. I know guys at my work that do this sort of thing for aircraft wings and fuselages. It's easy enough to measure dimensions, thicknesses, and build in material properties. But no model is good until validated. Validation is done by applying a known set of loads to the actual article, then take measurements of displacement and strain. Then you apply the same loads to the model and see where the model outputs match the actual results...if they don't, you have to tweak the model until they do. And the model would never be perfect anyway, but be good enough where you wanted it to be.

One thing we'll likely never know is what are actual inputs and loads/stresses? Anybody willing to put strain gages on their bikes and ride them around, changing various aspects of the frame/bracing and see what the results are?

On the current direction of putting the short brace between the rear downtube and the back of the engine case. We first don't know what the loads are or the relative displacements in this region. If they're substantial, those loads have be borne somewhere...back into bending the rear downtube, deforming the engine case (locally where the bracing attaches), and possible transfer of loads to the lower engine mounts, leading to additional loads into the lower frame rails.

There is typically a zero-sum game. You might stiffen something up, but you're going to be putting loads somewhere that an engineer didn't design for that.

I still think a cool finite element model, even an unvalidated model, would reveal a heck of a lot!!

Kurt in S.A.

Thank You 109 for your responses.
Your advise about the fillets are great but I can't get my head round not tying the two braces together - wouldn't it stop the tubes from trying to rotate around the allen bolts? - should they want to that is!
I was thinking too that welding these fillets would want to pull the joint out of alignment and was wondering if brazing it might be better - I've not the ability to bronze weld.

"A real cool way to see what works would be to build a finite element model of the frame and to some degree the engine case. I know guys at my work that do this"

How well do you know them? - Come on Kurt- get too it

Nourish wrote:"A real cool way to see what works would be to build a finite element model of the frame and to some degree the engine case. I know guys at my work that do this"

How well do you know them? - Come on Kurt- get too it

I'm sure they get tired of my talking motorcycles...none of them ride. But I wish I had the skills and software! It's not a simple undertaking...much more than just a stick figure diagram. Google "finite element model motorcycle frame" and then click on images!

Kurt in S.A.

Kurt in S.A. wrote:

Nourish wrote:"A real cool way to see what works would be to build a finite element model of the frame and to some degree the engine case. I know guys at my work that do this"

How well do you know them? - Come on Kurt- get too it

I'm sure they get tired of my talking motorcycles...none of them ride. But I wish I had the skills and software! It's not a simple undertaking...much more than just a stick figure diagram. Google "finite element model motorcycle frame" and then click on images!

Kurt in S.A.

Here's one low-tech approach:



From here:

http://yamaha-triples.org/forums/thread ... &posts=155

I'm not sure what there is to brace on the Yamaha frame in question--I've got one naked here at the house and bugger, it is ever a heavy thing.

I just noticed - the speedo cable - the brace is in the way

Nourish wrote:[img]

I just noticed - the speedo cable - the brace is in the way

Ah yes, the old speedo cable hole thingy.
Silastic?

No I want to keep the Speedo. I'll have to move the brace inwards onto the cross tube - not as good I know.
Might make it easier to mount for easier removal - I could turn that rear lug around 90deg and bolt it downnwards with a countersunk bolt/screw.