Swing arm question

Joined
Nov 2, 2021
Messages
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I’m pretty new at welding. I tried to make my swing arm as strong as possible. The original design was to have two shock absorber, one on each side of the arm to balance the reaction force of the shock to avoid twist and bending of the swing arm.

The issue here is that with two shock absorber, there’s almost no suspension at all. So, I removed one and it’s perfect.
Load capacity is 600 lbs each and i can't find a pair with 300 lbs each.

I’ve seen some professionally built system with only one shock and they are far less beefy looking than mine.

Do you guys think having just one shock will off balance the force and twist the swing arm?

My swing arm:
6.jpg
272201298_10159716849292840_8669852926909770416_n.jpg



Professional swing arm:
032__shock_location_is_perfect_75603.jpg
 
That "professional" arm isn't really all that professional, it should have much more bulk around the pivot area and the side plates are far heavier than necessary for most of their length. Nice tidy job though, the side's are likely for the sake of convenience and it gives a clean look but I'd be wary of the lack of bracing between the two sides, at the least I'd expect it to be putting unnecessary extra load on the the torque flats.
Edit: Some of the welds are also undersized, if materials are well specd then fillet size should be over half material thickness, if it's less the weld has less cross sectional area, it's inevitably weaker,

Yours... very respectable for a first attempt! I'd keep a very close eye on it all the same and maybe consider some extra gusseting where the width changes, you may get flexing under breaking/power that can lead to unexpected failure at any time. A kludge that may help, use a very hard paint and it will crack if you're getting the kind of stresses that could lead to failure. I certainly wouldn't attempt to use just one shock and don't see an easy way to add enough bracing for it (and single-sided always comes with a hefty weight penalty).

How far would your budget go for a different set of shocks? You should be able to get a mediocre generic set from a motorcycle/scooter breakers for very little and if you explain what you need they'll likely let you swap them until you find what works. Getting them custom made isn't all that expensive, the last set of Hagons I got where about 300 euro iirc and they where spot-on, just needed the weights, geometry, mounts, intended use, etc.
 
stan.distortion said:
Yours... very respectable for a first attempt! I'd keep a very close eye on it all the same and maybe consider some extra gusseting where the width changes, you may get flexing under breaking/power that can lead to unexpected failure at any time. A kludge that may help, use a very hard paint and it will crack if you're getting the kind of stresses that could lead to failure. I certainly wouldn't attempt to use just one shock and don't see an easy way to add enough bracing for it (and single-sided always comes with a hefty weight penalty).

How far would your budget go for a different set of shocks? You should be able to get a mediocre generic set from a motorcycle/scooter breakers for very little and if you explain what you need they'll likely let you swap them until you find what works. Getting them custom made isn't all that expensive, the last set of Hagons I got where about 300 euro iirc and they where spot-on, just needed the weights, geometry, mounts, intended use, etc.

Thanks for the kind words on my first attempt. Will look into extra gusseting.
The hard paint is definitely a good idea, i did not think about that. Is there a specific kind you recommend? car paint, house paint?

I spent a good amount on my build. I still have $$ left for shocks. Will be trying a pair of 400 lb shocks.
Based on what you said, i will definitely use a pair of shocks instead of just one.

Thank you!
 
kingkongshrimp said:
Based on what you said, i will definitely use a pair of shocks instead of just one.

Good decision.
I would not change a double shock rear end to single shock by simply leaving away one of them,
because it might not only cause problems on the swing arm, but also on the frame,
when the symetrical stress is replaced by an asymetrical one !
 
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