swingarm extensions broke

[mg82]

got these extensions made from a fella who runs 4 cylinder nitro hillclimbers.
just straight line drag racing on the ice. my chains tension held it together. thank god it chain was tight or else it could of got messy.

[mg82]

i think the box said " 80 horse and under "
the way i look at it ..... i got through about 6 gallons of vp so we must be pushing the envelope...

[alan]

Looks like the wrong alloy to me!

[stewart]

the old saying weakest link in the chian  could be applied here,,,,wow  ,,1 k5 thats right eqauls nitro 4 cylinder 4 poke$$$$$$$$$

[BDI]

I'm sure the aluminum started out 6061-t6 but after welding it would be something far less then that. Judgeing by the home made look of the extensions they were not reheat treated after welding not to mention you are asking a lot of that little bar of billet. Thank god you were not hurt.

[mg82]

ya i would say i am lucky.
that would not feel good to have that ice tire rip off the back at 90mph

[js]

you might be getting twice the traction with your ice setup over the hillclimber so your putting alot more stress on it than the hillclimber. show us a pic of the other side when you get it off. maybe he undercut the weld and started the crack. BDI's right they didnt heat treat the 6061 after welding it. It has to be heat treated twice. first heated to 900+ then quenched to put it back to T-0 then second baked at 400 to get it to T-6.

[Jeeks]

That's called "the notch effect".

Perpendicular bracing would most likely eliminate it.  You'll need to address the whole pice otherwise it will break here, just past the weld.  The weld acts like a gusset. 

[mg82]

ya thats exactly where it broke. right along the edge of the weld. i'll get a pic of it when i get home this weekend. i just emailed the guy we'll see if he responds.

[k5abuser]

you must be living right . the mojo good with you . could have been worse . could have broke and a honda could have won . lol. a friend of mine is an engineer and has a program you type in the metal , shape and the streesssss it will take . it shows you where it will bend and or brake at what force's. also it will show you where to add or take away . handy thing to know  ahead of time .

[Hillclimb#42]

Look up Thomas Racing Service. I think its thomasracingservice.net or .something. He is making swingarms for multi-cylinder nitro-burning bikes for National Pro's, Amatuers and some International Customers. I personally use him for everything from plating cylinders to powder coating. He Hillclimbs everywhere, builds bikes for many competitors, and still has reasonable pricing with quick turn arounds. You don't have to take my word for it,like I am trying to sell you something. Check out his page. He can make swingarms no problem.

[js]

FEA software can't calc welding. and don't bet your life on the results. its good at telling you how it bends and breaks but the loads are almost never right.

[mg82]

just talked to the the guy who made them. super nice guy that is really apologetic. says that these were a prototype that he tried for the first time. he's machining me a new set on him. this time there will be no welds. all out of one piece.

i'll still look into thomas racing service tho.

thanks for all the input

[Jeeks]

There's a book, I call it my Bible, called Welding Secrets that explains how common welding failures can happen.  For instance on your swingarm, say you V'ed out the crack and welded it back together, the piece will crack again just past the new weld because the weld is still the "notch". You also have the points where the two pieces overlap eachother, those a re notches too. The "notch effect" can happen on pieces that arent welded too.  That's why you'll want to incorporate gusssets to help spread out the stresses.  It happens all the time on various things.  Say you have a bolt stuck and it breaks, where does it break?  Flush with the surface it was threaded into.

Gussets, and welds and joints that aren't perpendicular to the ends of the piece can eliminate fractures.  You'll see some examples on your swingarm and frame.  If that was one solid piece of the same design, it will still break at those two points.

Good Luck.

[don46]

I think part of the issue is the bolt up design, the bolt doesn't hold the material solid enough and will flex. If it was me I'd get a swingarm off Ebay, cut off the existing chain adjustment area, and machine the extensions to slide into the swingarm about 4-6" then you could bolt it or weld it and they'd be solid. I've built numerous sets this way with no problems, one set is on a ZX-10 bike, no issues what so ever. I was pressed for time and had a set built (KX 250), the first trip out they tore up a new sprocket and my extensions, home they went and were modified, no more problems, I still don't like them, so I bought a swingarm and am going to make another permanent set up