Broken Swingarm!

[Blaner]

Well guys, I must admit I under estimated the punishment that my bike has to endure...

A few years ago, after breaking my standard swingarm, I built my own swinger. Thinking it was rock solid, it too finally gave way...

Turns out the leverage and twisting force applied to the swingarm transmitted through the wheels is pretty enormous! Strong enough to cause metal fatique and shear right through a 50x25x2mm steel.

Its back to the drawing board!

BEFORE:


AFTER:

 

[OConnor Racing]

We had a similar problem with one of ours too. We built one for Awk to test. It lasted a couple of years but Awk finally won. We stopped making them after that. Your swingarm looks just like a TROCK. Nice job. I think it's the braces we use in the corners making the arm too rigid.

 

[Blaner]

We had a similar problem with one of ours too. We built one for Awk to test. It lasted a couple of years but Awk finally won. We stopped making them after that. Your swingarm looks just like a TROCK. Nice job. I think it's the braces we use in the corners making the arm too rigid.

Yes Ken, credit goes to you for the design, I sort of eyeballed your TROCK's and made my own. What thickness of steel tubing did you use? I am not sure what the next step is, I was thinking a new one but with 2.5mm tubing, but now im not sure!

 

[Blaner]

Obviously weight is a concern. I can understand that the sharp edges and geometrical design can reduce the tensile strength and create weak points where fatigue can manipulate, so perhaps we need to go with a more radius/filleted joins somehow...

 

[OConnor Racing]

I used 1/8 tube steel. That's just over 3mm. I would lose the corner braces and go with a different type of shock mount and I bet it will last. Ours didn't snap but it was full of cracks in the same place yours broke. I built the first one over 5 years ago and it's still fine. The only difference is the lack of corner braces and I ride like the old man that I am. Maybe MR Awk can dig up a picture of the first one I made and posted on here years ago.

 

[OConnor Racing]

Here's an old post of the first one I made. To the best of my knowledge, it's still holding up.

http://www.blasterforum.com/threads/swinger-home-made.24533/

 

[Blaner]

Surely the lack of corner braces would reduce the tensile strength overall? By adding them, the weakest point is isolated picked on, without them, the weak point could be anywhere, I would imagine more reinforcing would be the solution? or perhaps getting the load to be more evenly distributed through the arm somehow.

 

[79 bronco]

You may be making the swingarm to stiff and rigid. You wand your swingarms to flex a little and not to break! And don't weld weld it all at one time so you can keep the heat down.

 

[Blaaster]

The problem lies with the zone of thermal disturbance where the parent metal closest to the weld becomes crystallised.

It is the weakest point of a weld.

It is not the weld that usually breaks, it is the metal in the heat affected area!

Any slight imperfection of the weld like undercut/overfill can cause a hairline crack and riding the bike will finish the failure process.

Lots of folk think that pouring in the filler metal will make the weld stronger, unfortunately the heat affected zone becomes larger and compounds the breakage problem.

 

[speedyslyder]

all i know is stick/arc weld is stronger then mig/wire weld.......
and a million dollars that factory's spend to make and test there parts lol
i have more faith in a bolt sometimes then i do a weld....i forget the width/method/angle i learned in school ect but summin about a 1 inch weld can hold 1000lbs

and yes heat is a factor..seeing how that is how we manipulate metal sometimes

 

[Blasturd]

Cracks follow straight lines. i would use round tube for the cross brace up front and for the shock mount. Dont weld straight down the 1x2 like you did its just asking for cracks.

I would be using a .083" or .120" wall tube, if all you used is 1.6mm (.063") tube that didnt help against the carnage that's very very thin material.

1"x2"x.065" weighs 1.269 lbs per foot
1"x2"x.083" weighs 1.6 lbs per foot
1"x2"x.120" weighs 2.25 lbs per foot

If your length of 1"x2" for the swingarm is lets say 18" per side that would be a total of 3 ft in the swingarm the weights would be as follows;
.065" 3.8lbs
.083" 4.8lbs
.120" 6.75lbs

the difference between .065" and .120" would be about 3 lbs, thats very minimal imo.

Also if weights an issue i believe i seen in another thread that you notched the tube and your plates on the back that the carrier bolts to span through the tube which is dead weight its not really helping with the strength of the swingarm. Instead notch out the plate.

If your using 1/4" plate it looks like from the pictures it goes up the 1x2 around 4"

1ft of 1/4x2 weighs 1.7lbs so 8" of 1/4x2 plate weighs 1.36 lbs that would take a decent dent out of the extra weight of the thicker tube.

 

[Blaaster]

Cracks will follow the lines of thermal disturbance, the most brittle area directly surrounding the weld deposits.

 

[Herpderplol]

would heating the metal up help?

i know when welding cast iron you heat the whole piece of metal to prevent cracks from sudden intense heat

 

[Blasturd]

Cracks will follow the lines of thermal disturbance, the most brittle area directly surrounding the weld deposits.

Yes agreed but 90° welds are most prone to cracking that's why the curvature of the round tube would help prevent cracks as-well as increases the weld surface area.

 

[Herpderplol]

Yes agreed but 90° welds are most prone to cracking that's why the curvature of the round tube would help prevent cracks as-well as increases the weld surface area.

the ones from the swingarm shop on egay have a round tube for the shock mount and i dont think anyone on here has had problems
http://www.ebay.com/itm/Yamaha-BLAS...-12-/370619952412?hash=item564aabad1c&vxp=mtr

 

[Blaner]

Thanks for the input guys. Yes I am leaning towards round tubing, for the reason that sharp angles are prone to cracking. With regards to weight, the swingarm weighs in at around 7kg (15.4lbs). So that is why I went with thinner walled material, thinking the braces would be more important. This was a first attempt and like all 1st attempts, there usually is a 2nd, so its a case of live and learn.

If I go with round tubing, I believe I could get away with minimal to no gusseting, this would then save a huge amount of weight and allow me to use a thicker walled rectangular tube.

We used a DC arc welder and although Blasturd's material theoretical mass is useful, it doesn't factor in the weight of the front tubes which I had machined. the 6mm angle iron gussets, the laser cut 6mm swingarm mounting ears and all the welding. KOC used the same design with 3mm steel, an his failed in the same place apparently, so its clearly a design flaw rather than a material flaw. Metal fatigue is a cumulative process and it builds until it cracks, all swingarms will do it from the flexing and so you just have to find out to prolong it. Thats why I feel the idea of allowing the swingarm to flex is a problem, you can only flex metal so many times before it snaps.

 

[OConnor Racing]

Blaner just called me KOC. LMAO

 

[Blaner]

Blaner just called me KOC. LMAO

Can you believe it! Apologies, Im sure you know I meant KOR!!! HAHAHA, one day when I eventually send you a motor for you to do for me, you can engrave "KOC" on the cylinder for me.

 

[Bluesman]

There's a reason that all the big boys (Lonestar, Stellar, etc.) build swingarms out of chromoly. As far as Swingarm shop, yes they are famous for cracking out around the round tube shock mount. They're a cheap swingarm for recreational riders who don't do serious air jumps.

 

[Blaner]

Ive investigated Chromoly, but its not available in SA, I would have to have it imported at a cost of 3 arms and a kidney. Funny, since we are one of the worlds leading steel producers...we sell it and then buy it back...go figure. I was wondering about Stainless steel? It has a specific gravity similar to carbon steel, but has a higher tensile strength. I am concerned about brittleness though, any ideas?