Who's got the junk yard dog eye? & structural question

When you've got an angle grinder and welding machine along with these raw materials, how can you not want to build an ebike on a beautiful Saturday? Who else can see the ebike in there? 8)


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I've got a structural engineering question. I'm going to use the leaf spring material for much of the frame. Let's Ignore torsional rigidity, which I'll have to address with cross bracing, and talk about rigidity inline with the bike. First, it's seems like putting the springs under pressure would be more rigid. My plan is to create a somewhat oval shape with 2 leafs facing each other like this:
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With the red as a brace there are 3 options.
1. No compression of the springs
2. Compression from the ends which would put the brace under tension
3. Compression of the springs from the top and bottom with would put the brace under compression from the ends.

Which would form a more rigid structure? My gut is telling me put that brace under tension, since I can use a simple thin strip of steel for that brace. I'm going to rip the springs lengthwise to make 2 of these shapes, one for each side, and join the 2 with panels and braces around most of the perimeter. It will get side panels too for something along these lines as a dual suspension high power mid-drive ebike.


Any suggestions regarding where I'm going wrong on the structural ideas?

Now the big question is to get cranking with the angle grinder, or go pick up that plasma cutter I was eyeballing the other day. As long as a cheap one is reasonably durable it could pay for most of itself just in cutting discs on this one project. Leaf spring steel is tough stuff.

John

It will never work John.

Since it won't work, you won't be needing those forks, so just send them my way and your problems will be solved

Yeah, I see your problem!!! Just push those parts a little closer together , and it will weld itself! You already did the hard part.......

NeilP said:

It will never work John.

Since it won't work, you won't be needing those forks, so just send them my way and your problems will be solved

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LOL! Good eye alright, but you can pry my 888's from my cold dead fingers when it doesn't work and it bucks me off in traffic. I found a guy in California who sells refurb Marzocci's. I picked up 2 sets of 888's for under $600 including shipping. He's a surfer/snowboarder/MTBr type with a fluid sense of time, so as long as you have patience he's got the good stuff.

John

hydro-one said:

Yeah, I see your problem!!! Just push those parts a little closer together , and it will weld itself! You already did the hard part.......

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or I could just glue it together with some of that 3M epoxy. Dropouts see more force with a big hubbie than any frame member, so it's gotta work. :lol:

BTW, I've got a friend in Winnipeg who's been sitting on a mountain of Konions and an Xlyte for a year and a half, so he could use a bit of hands on encouragement or someone to take over for a song. He and his GF are hanging with some jet-setters in the orient right now, but real jobs back home. Good people. If you ever call 2 when you get hungry, you're probably getting it from his mom's place.

Neil,

You shoulda asked for the hubmotor, and yes those are 6 phase wires, and the entire motor shell is AL.

Do you have contact details for him?

Ummm ok well send me that as well

Kinda concerned about the weight. Your design could be made from plywood and be more rigid with just a fraction of the weight. Think top and bottom of 3/4" plywood cut on the inside to allow bending, then fiberglassed to add strength. Sides could be 1/4 " plywood. Make the rear drop outs from 1/4" aluminum. Saw them out with a sabre saw. Fab the headstock from steel since you would probably need to weld it up from smaller pieces. Bottom bracket could be done the same way. Make the width of the box to accommodate the length of your cells. If you need to stiffen the sides, you could add 3/4" partitions. Make the top and bottom cords from mahogany and the sides from a nice thin Luan/Lauan or Philippine mahogany plywood. Make little doors to access storage compartments in side. Put some little doors in the sides that flip down with LED fog lamps mounted inside them. You could put red and green running lights and go with the nautical theme The shape keeps reminding me of a fish. It could be really cool with the dropouts for the lower tail fin and a plywood upper tail fin. A dorsal fin could be the seat mount. Paint the whole thing to look like a stylized fish.

I cracked a leaf spring years ago on my dump truck. Welded it, and it cracked again at the weld again... but that was with a tri-axle trailer and a backhoe behind it and 3500 lbs of stone in the bed... I think I violated about every metallurgical requirement as it deals with springs.

bigmoose said:

I cracked a leaf spring years ago on my dump truck. Welded it, and it cracked again at the weld again... but that was with a tri-axle trailer and a backhoe behind it and 3500 lbs of stone in the bed... I think I violated about every metallurgical requirement as it deals with springs.

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At least you got the job done.

I am not here to discourage you by any means John. Took metallurgy 20yr ago, so can't be 100%, but spring steel alloys are not suited for welding. It has more to do w/ the way molecules line up during heat treating. Heat treating springs requires a quenching process, rapid cooling, this creates the hardness and resistance to deformation. It also has to do w/ the non-ferrous alloys contaminating the weld, manganese in particular. Wish you the best in completing the project, but the connection may have to be mechanical and not welded, if you are going to use those leaf springs. Ask yourself this, have you ever seen a commercially produced product w/ a welded spring?
IMO, it is in the wrong shape and will increase the weight much more than expected.

Will be glad to help in any metal fabrication questions you may have.

bigmoose said:

I cracked a leaf spring years ago on my dump truck. Welded it, and it cracked again at the weld again... but that was with a tri-axle trailer and a backhoe behind it and 3500 lbs of stone in the bed... I think I violated about every metallurgical requirement as it deals with springs.

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First question is what rod you used to weld on the spring? Unless you use a furnace and then re-temper the spring, you end up with a narrow brittle band along either side of the weld.

Using spring material for John's build is a poor choice of material, probably made because it is cheap and handy. Better to start with good tubing when you are going to invest considerable time and effort into building anything. John will go ahead with the springs no matter what, so hopefully he will make a little oven out of bricks or just rocks, heat the welded area to dull red and let it slowly cool. The result wont be as strong as the rest of the spring, but it wont crack on you either. Just bend if it is too weak.

bØb said:

Kinda concerned about the weight. Your design could be made from plywood and be more rigid with just a fraction of the weight. Think top and bottom of 3/4" plywood cut on the inside to allow bending, then fiberglassed to add strength. Sides could be 1/4 " plywood. Make the rear drop outs from 1/4" aluminum. Saw them out with a sabre saw. Fab the headstock from steel since you would probably need to weld it up from smaller pieces. Bottom bracket could be done the same way. Make the width of the box to accommodate the length of your cells. If you need to stiffen the sides, you could add 3/4" partitions. Make the top and bottom cords from mahogany and the sides from a nice thin Luan/Lauan or Philippine mahogany plywood. Make little doors to access storage compartments in side. Put some little doors in the sides that flip down with LED fog lamps mounted inside them. You could put red and green running lights and go with the nautical theme The shape keeps reminding me of a fish. It could be really cool with the dropouts for the lower tail fin and a plywood upper tail fin. A dorsal fin could be the seat mount. Paint the whole thing to look like a stylized fish.

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bØb,

At 20hp or more I don't think wood is the right material. I do want to do a composite work/glass frame, just not this one.

Mat and Gordo,

I don't want to use it as a spring, so isn't it a better steel than the alternative, mild steel? Making a brick oven and heating up the weld areas sounds like a good idea to make sure it's not brittle.

John in CR said:

bØb said:

Kinda concerned about the weight. Your design could be made from plywood and be more rigid with just a fraction of the weight. Think top and bottom of 3/4" plywood cut on the inside to allow bending, then fiberglassed to add strength. Sides could be 1/4 " plywood. Make the rear drop outs from 1/4" aluminum. Saw them out with a sabre saw. Fab the headstock from steel since you would probably need to weld it up from smaller pieces. Bottom bracket could be done the same way. Make the width of the box to accommodate the length of your cells. If you need to stiffen the sides, you could add 3/4" partitions. Make the top and bottom cords from mahogany and the sides from a nice thin Luan/Lauan or Philippine mahogany plywood. Make little doors to access storage compartments in side. Put some little doors in the sides that flip down with LED fog lamps mounted inside them. You could put red and green running lights and go with the nautical theme The shape keeps reminding me of a fish. It could be really cool with the dropouts for the lower tail fin and a plywood upper tail fin. A dorsal fin could be the seat mount. Paint the whole thing to look like a stylized fish.

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bØb,

At 20hp or more I don't think wood is the right material. I do want to do a composite work/glass frame, just not this one.

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Don't underestimate wood. There are a lot of wooden airplanes out there. Might I even mention HH's Spruce Goose.

bØb

Gordo said:

First question is what rod you used to weld on the spring? Unless you use a furnace and then re-temper the spring, you end up with a narrow brittle band along either side of the weld.

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The era I did it was before I had the TIG, so I am sure it was simple 6011 or 6013 on a rectified AC buzz box. I'll bet I used 6011 as it cut through the rust and dirt! Heat treat/anneal afterwards, don't think I had time to do that. I had loads to haul and things to do!

mat h physics said:

Wish you the best in completing the project, but the connection may have to be mechanical and not welded, if you are going to use those leaf springs.

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Steel springs can be brazed. It cooks off the temper in the heat-affected zone, but it can be done.

Not strictly relevant to this discussion, but interesting: In my former life as a high tech industry machinist and prototypist, I saw a lot of things welded from alloys that are not generally regarded as weldable. Those usually take a very special welder (the human, not the machine), and a lot of shop time.

There was one reinforced lid for a containment vessel (made of 21-6-9 stainless if I remember correctly) that took two guys two years to complete. It was still cheaper than having the part machined or forged.

As a material it would be on the high end for strength and toughness. The problem is the solid shape, which reduces the modules of inertia. This is what gives I-beams and hollow tubes an advantage, the structural shape. This is compounded by how to connect other components to it. If you could find a rigid means of connecting a weldable grade of steel, you may be able to overcome that aspect. Even drilling holes in it may be a problem, if you can't cut it very well with a file don't attempt to use cutting tools like a drill bit (files are very hard, that is why the break so easy). See if you can soften the metal by getting it red hot then cooling, again use a file for a basic test. Maybe a local has a hydraulic press w/ a die set to punch holes. You can grind most anything, but if the metal is too soft (like alum) it will not shear the dull grinding agent off and eventually clog. I think if you do some experimental welding, you will find the weld to be the weak link. Do a small section, get some leverage, and see how much force it can hold. Annealing can be done with a common gas torch, it just takes awhile. The key is slow cooling, maybe a heat insulator wrapped in foil to reflect the radiant heat (how most heat is transferred).
The material will work, it's just going to complicate doing it right.

Why would you want such a heavy frame? There are far better ways to build a frame. You have a great fork, a sweet motor, plenty of batteries, a good rear wheel, why ruin it with a lame 80lb frame, steel tubing isn't that expensive! 15-20 lb frame thats stronger and easier to build, like Luke's! You have built some awesome bikes, that haul butt, keep it up!

Chalo said:

Not strictly relevant to this discussion, but interesting: In my former life as a high tech industry machinist and prototypist, I saw a lot of things welded from alloys that are not generally regarded as weldable. Those usually take a very special welder (the human, not the machine), and a lot of shop time.

There was one reinforced lid for a containment vessel (made of 21-6-9 stainless if I remember correctly) that took two guys two years to complete. It was still cheaper than having the part machined or forged.

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Strange; 21-6-9 is similar to 316 and can be welded or machined using identical techniques. A buddy of mine often TIG welds 2" SS pressure vessels, use in paper mills. How big was this lid and how thick?

Where were you guys telling me I couldn't weld on leaf spring steel when I started doing it 2 years ago? I've been making clamping dropouts and shock mounts with the stuff and it's worked really well, much better than mild steel. Maybe I've just been lucky, and the pieces are small enough that combined with my over-welding that I end up with a decent heat treatment avoiding being brittle in the HAZ. I'm sure it helps that I no longer use it as a spring.

Oh well, curves would have been nice, but I'll just have to make an angle iron frame look decent like I originally planned. That will almost certainly be heavier than what I had in mind for the spring steel build, but I have some ideas to save weight there too.

For those talking about tubing, I'm sorry but I have to disagree. It may be an easy route to rigidity, but we don't build bicycles around here. Space is the biggest premium on our ebikes, and tubing of any kind wastes what could be better used as battery space. Minimizing the outer dimensions without sacrificing battery space has to be a primary goal even at the expense of added weight. As long as I don't come close to the weight of a motorcycle then I'm ahead of the game.

John

That would have made for a beautiful bike. With cutting torch skillz you could even cut out some cool patterns without sacrificing strength, and follow up with some grinding to get looking tight. Maybe elongated diamonds or perhaps bananas?

Anyways. I think welding would definitely be complicated. I think the fire would be a good start. Some sort of strip that you could heat and bend/press/and hammer (as last resort) to get the curves to match up closely with the curves of the spring. Maybe some grinding to remove hammer defects. It would need a fair amount of close contact and decent surface area. A c-clamp you don't mind submitting to the fire gods. Heat the whole clamp strip spring apparatus up, get some of your heavy duty weld on the end and let it treat in the fire. Begin removing it from the fire, and as it cools to that special glow feed some brazing material capillary like into the space between the strip and the spring. But don't let it ignite. Did I forget to mention flux? Heck throw a couple of well placed button head bolts through where the strip might move from the braze. Rinse and repeat.

I like funky materials, and leaf spring is right up there on the challenge scale. I would also bury some clams in sand near the fire, some beers. Young ladies chanting and dancing (gotta keep the fire gods happy). Definitely could make for an interesting build.

Passing on what I learned about metallurgy. Welds are rated in pressure per length of bead. Make a test weld of say a cm, find out how much pressure it will take. If your weld bead is long enough, it may work just fine. Being you are in motorcycle power range study some MC frame designs. Rotate the proposed oval 90deg, and you will start to see similarities to superbike boxframe designs (ie the side view becomes the top view). But if you feel confident in the weld, don't let my opinion hold you back.
Welds are annealed to remove internal stresses, hardened materials are tempered back from being super hard to remove higher internal stresses(that create cracks).
I'm not say you can't do anything, just trying to give an understanding of potential problems. Would hate to loose one of the better builders on this forum.

I did some poking around on welding and blacksmithing forums and it seems that a combination of factors by blind luck resulted in my successful use of leaf spring steel. Armed with a bit more knowledge now I'm going to give one of 4 main pieces a go and see if I can break it and make it suitably rigid with a small amount of added material and weight.

Since I wasn't fixing a broken spring, and my welding tools and skills are lacking, my multiple shallow and overlapping welds to get to my comfort level of overkill resulted in a much less sharp and well defined HAZ, so I didn't get the immediate cracking that a pro would get welding the stuff. 5160, the most common for leaf springs, tempers at pretty low temps, 150-250°C, so it's quite likely that my grinding and cutting inadvertently tempered the pieces. The grinding probably created some surface treatment too, though I am going to add some hand peening to my process any time I use it in the future.

It's great that you guys pointed out the weakness in the plan. I will secure the ends with bolts, and I'll mechanically attach the seat spring instead of welding it. First, let's see if I can make it rigid, because doubling the weight to make it stop acting like spaghetti would be pointless. I need to make those arcs quite rigid. Then I can address torsional rigidity.

John