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Subframe fab

Whisky_Richard said:
Also, if you want to stiffen up the back half of the subframe, just weld some strips of steel to the top and/or bottom of the tube. If you plan to reinforce the back half of the subframe, definately add a gusset as you will have a stress concentration at the joint you have drawn.
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Actually the reinforcement strips should be on the sides so that the vertical loads goes through the plane of the material.

Imagine taking a flat metal ruler and laying it flat on a table-top. Hold down near the middle and lift one end, it bends upwards.

Now flip the ruler up so that it's laying on the long edge vertically. Hold it down by the middle and try lifting an end up. It doesn't bend one bit and is much stiffer when handling loads in-plane.
 
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DannoXYZ said:
Actually the reinforcement strips should be on the sides so that the vertical loads goes through the plane of the material.

Imagine taking a flat metal ruler and laying it flat on a table-top. Hold down near the middle and lift one end, it bends upwards.

Now flip the ruler up so that it's laying on the long edge vertically. Hold it down by the middle and try lifting an end up. It doesn't bend one bit and is much stiffer when handling loads in-plane.
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I understand what you're getting at, but you're wrong. What would be strongest would be to turn the ruler on its side and add material to the top and bottom so you end up with an I-beam... which is what I'm effectively suggesting.

I don't want to do this, but I think we need to... :nerd

Deflection = Load (P) * Length (L)^3 / 3 * Modulus (E) * Moment of Area (I)

The most effective way to improve moment of area is to add material to the top and bottom of the bar since the area become more effective as a function of the distance from the centerline cubed.
 
eh, I ended up just painting it and deciding to forgo the gusseting for now. I'll be keeping a close eye on those welds - luckily, I can see them quite well without pulling anything other than the seat.
 
Whisky_Richard said:
I understand what you're getting at, but you're wrong. What would be strongest would be to turn the ruler on its side and add material to the top and bottom so you end up with an I-beam... which is what I'm effectively suggesting.

I don't want to do this, but I think we need to... :nerd

Deflection = Load (P) * Length (L)^3 / 3 * Modulus (E) * Moment of Area (I)

The most effective way to improve moment of area is to add material to the top and bottom of the bar since the area become more effective as a function of the distance from the centerline cubed.
Click to expand...
Your equation is for a simple beam, you'll need to expand on that into three differentials for the two flanges and the web. Also given two I-beams of equal-weight, the one that's stronger in vertical direction has has more material in the middle vertical web section and less in the horizontal top and bottom flanges. The optimum ratio is to have about twice as much material in the web as in the flanges. In the OP's structure it's 50/50. He needs to add more to the vertical webs and elongate it into a vertical rectangle.

Think about how to stiffen a knife-blade for cutting. You don't add a top flange to it, you thicken and widen the blade. Look at how swing-arm cross-sections are optimized for vertical-stiffness and horizontal-flex. It's a rectangular cross-section (I-beam with 2 webs) oriented with most of the material vertically oriented.

Or in frame-design. Extruded aluminium rectangular sections are oriented with most of the material oriented vertically. Same with steel trellis frames like Ducati (the truss is oriented vertically). Force in-plane with the most material in all these cases.

We are actually talking about the same structures, just debating on the ratios of how much to distribute the vertical versus horizontal components. A rectangle is stronger than I-beam of same weight, especially torsionally, such as when his body-weight leans to one side (or the pillion). This is party due to the 2nd web and partly due to to the larger moment. For the weight/amount of material, you'll get significantly higher strength by increasing the moment (diameter). Stiffness goes up by the 4th-power of diameter.
 
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Junkie said:
eh, I ended up just painting it and deciding to forgo the gusseting for now. I'll be keeping a close eye on those welds - luckily, I can see them quite well without pulling anything other than the seat.
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Yes, keep an eye on it. It will fail first at the top-welds, so you'll have to take the seat off to inspect. Might be a good idea to anneal the joint and polish it to improve fatigue resistance.
 
DannoXYZ said:
He needs to add more to the vertical webs and elongate it into a vertical rectangle.
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Agreed. If he wants to cut the bars all up (and basically undo all the work he has done), then making the bar taller would be ideal. However, to roughly keep the same height, the material serves its purpose best added to the top or bottom of the bar.
 
I won't be taking a passenger often, and the rear part will take very minimal loading without one - so I'm not too concerned about it.
 
DannoXYZ said:
Yes, keep an eye on it. It will fail first at the top-welds, so you'll have to take the seat off to inspect. Might be a good idea to anneal the joint and polish it to improve fatigue resistance.
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The best thing to do, design-wise, is to elongate the lower brace so it attaches further back than it does now. Then brace & gussett from there toward the front of the bike. & shorten the two front mounts to accommodate the drop in the back. This will give more space behind the engine for stuff too.

And then you can make a lattice frame under the seat to the tank mount.

But hey, this is revision 1.0A, as long as it provides those three major requirements; which was to hold up his seat & mount the electrics and not break; then it's good to go. Right? :laughing
 
Ironbutt said:
The best thing to do, design-wise, is to elongate the lower brace so it attaches further back than it does now. Then brace & gussett from there toward the front of the bike. & shorten the two front mounts to accommodate the drop in the back. This will give more space behind the engine for stuff too.

And then you can make a lattice frame under the seat to the tank mount.

But hey, this is revision 1.0A, as long as it provides those three major requirements; which was to hold up his seat & mount the electrics and not break; then it's good to go. Right? :laughing
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Yeah, you're right on. Basic design idea using trusses is to connect the nodes first with one big triangle. Then sub-divide into smaller triangles as needed for additional strength.

You guys did a great job. I think it's overbuilt to at least 100% extra on strength. Fatigue is an unknown variable, but that's just time. Be it 20-years or 40-years depending upon usage, still well over the anticipated time-frame needed.

I have a server with SolidworksPlus and linear-FEA function if you want to use it.
 
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Danno,

imaging the subframe is solid. the two test cases still exist, weld plates onto the top & bottom and compare to the same original beam w/ plates welded to the sides. which bar is stiffer in y... the taller one, the one w/ the plates top & bottom, obvious.

remove the center section to make both bars hollow. the amount of stiffness removed is the same for both bars so the taller one is still stiffer. therefore, for a load in the y-direction (vertically), plates top and bottom will make the original beam stiffer.

DannoXYZ said:
Your equation is for a simple beam, you'll need to expand on that into three differentials for the two flanges and the web. Also given two I-beams of equal-weight, the one that's stronger in vertical direction has has more material in the middle vertical web section and less in the horizontal top and bottom flanges. The optimum ratio is to have about twice as much material in the web as in the flanges. In the OP's structure it's 50/50. He needs to add more to the vertical webs and elongate it into a vertical rectangle.
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im sure u werent being vague on purpose, but its not correct to say "material" of the web. the thickness of the web does not get the cube-term, the height does. so, whats in bold is not always correct.

my turn to :nerd :laughing. not the example i posted above, just the calculation for only the additional plates. ---> Ix = bh^3/12 + Ay^2, assume existing subframe bar has outer dimensions 1x1 for simplicity & wall thickness = t

increase in Ix from addition of plates on the side: t/6
increase in Ix from addition of plates top & bottom: (t^3)/6 + 2t[(t+1)/2]^2

2t[(t+1)/2]^2 = (t^3)/2+ t^2 + t/2 is always larger than t/6
 
hmm, I suppose I could build it in Solidworks pretty easily. problem is I don't know what the loading is.
 
Junkie said:
hmm, I suppose I could build it in Solidworks pretty easily. problem is I don't know what the loading is.
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I like to use 4-5G for dynamic loading to test ultimate-strength of material. In your test with standing on the rear of sub-frame, you can hop up and down to generate about 2G of loading. Add sudden bumps like potholes and you can be up to 3G.

You'll have 3 moments to calculate. The 1st one is the unsupported cantilever from the end of rear to 4-way weld joint. Then 2nd & 3rd moment from end to mounting points.
 
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Whisky_Richard & stangmx13, interesting discussions. Let's work up some FEA models to test and do some real-world loading and see how they compare. You guys familiar with SolidworksPlus?
 
sadly, i havent had a license for SolidWorks in about 4yrs. if u model it, id def be interested to see your results. then we can argue about constraints, ha. i have a feeling that all the results will tell us is that the existing subframe is overbuilt :laughing.
 
auntiebling said:
god i love watching a good nerd fight, as opposed to being part of it. keep it up!
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Bah, I believe ME degree qualifies me for geek status!
 
Heh, throw up some dimensions and we can figure some stuff out. I am currently running autodesk cause my solidworks got all messed up when I moved some files around.
 
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