Drilling holes in swing arm

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#1
I was thinking of drilling 2 small holes in my swingarm to bolt my chain guide directly. I broke off at the welds on the last enduro I was on.

Will this weaken the swing arm significantly? I am not sure how to make a judgement.

Without more info on forces and strengths of the square tubbing I cant do any calculations.

My feeling is that there are allready holes drilled for the rear brake line and for the chain rub pad that is attatched to the swing arm close to the frame, therefore I am probably safe if I keep the holes small.

If I had it welded I think I also run the risk of someone damaging the swingarm just the same. I know that the box tubing is only about 1/8 " thick.

Opinions from those familiar with metals is appriciated.
 

EK

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#2
I am not sure of the specifics of where you plan to put the holes. From an Engineering perspective here is what I would look at:

1) The smaller the hole you drill is better to a point. Very small holes can actually cause stress risers. How small is very small? probably less than 1/16"

2) Drill holes in the lowest stress areas. Generally the highest stress areas are where the highest loads are. On a swingarm, the highest loaded areas are near the shock linkage attach points, followed by the frame pivot attach area.
The bending load in the swingarm progressively reduces linearly from the shock linkage toward the rear wheel. Notice how the swingarm section often reduces as you move toward the rear wheel - because there is less bending load as you move toward the rear wheel.

3) There are a number of loads that also put stress on the swingarm to consider such as twisting and shear. However, these for the most part are often much less than the stress due to bending.

The Engineering can go much deeper. You are probably wondering where does all this lead? For the swingarm, the center of the side of the box structure near the rear wheel (but not into the axle attach area) is likely to be the lowest stress place and probably your best candidate for the least risk of stress cracks or distortion.

Eric K
'01 GasGas 300XC
 
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#3
Good info EK,
I will used a 12/24 tap and bit with bolts to match. It will be about 3/4 of the way back on the swing arm (towards the wheel) on the bottom and in the middle of the beam. What do you think?
 

EK

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#4
reynome,

Holes in the side would be better due to lower stress, but probably not your preferred location. Could you attach a bracket to the side of the swingarm?

Eric K
'01 GasGas 300XC
 
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#5
No, it has to be on the bottom so that it lines up with the chain and sprocket. If I have to jump thru a bunch of hoops to put in on the side I might as well have it welded. Since the alum is so thin I was a little sceptical on having someone just weld it up. A burn hole would ruin it and cost me ton to replace the swingarm. I personally dont think it will hurt it, if it starts to crack out I could have a alum plate welded over the holes.
 
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#7
I didnt ask what you would do, xrsforever.

I want educated opinions from fellow engineers who are familiar with metals.
 
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#8
Perhaps you can have some tabs welded on instead of drilling. Oh wait - why not ask for IM12RSPCT'ss opinion? We haven't seen a lively exchage in a few days...:eek:
 

EK

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#9
OK folks let's just take a deep breath...

No need to get too excited about this, after all it is a swingarm on a motorcycle. Reynome can do what he wants with his machine. The value of this forum is the diversity of experiences and perspectives.

That said, I have a few additional thoughts to offer from an Engineering perspective.

1) Welding can add much more local stress than drilling a hole and can significantly reduce the material strength of aluminum unless it is properly heat treated after welding.

2) It would be better to fabricate/purchase a bracket similar to the brackets used to attach aftermarket kickstands on motocrossers swingarms. That way you do not have any drilling or welding and you can effectively secure the chain guide to the swingarm.

Eric K
'01 GasGas 300XC
 
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#10
I'm appalled at the thought of using inch size fasteners on a metric bike. Drill all the holes you want but buy a metric tap and die set and use metric bolts like the rest of the bike.

Just my 2 cents..........
 
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#11
I was thinking about the wrap around style holder, like the kickstand. I am just not sure how well I could fabricate it.

I chose use the 12/24 tap because it was readilly available at Lowes, they didnt have metric taps.

Thanks for the input EK.

ps: placelast, your not FINGRPOKR's brother are you? :p
 
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#13
I have had brake disk guards welded on swingarms several times. If you get any decent welder he will have no problems fixing you up. I think in the end it would be a lot easier.
 
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#14
Why can't you just use Duct Tape? Not only would you not weaken the swing arm you would look really cool...maybe even start a new trend? ;)

Seriously though...drilling a small hole or two shouldn't cause you to many problems. It is true though that a hole drilled in the center on the side will cause the least reduction in strength from a vertical load. In that position the bending situation from a vertical load would create forces by the hole that are almost zero, theoretically, but I can't find my book so don't quote me on that.

With that said I doubt the reduction in strength will be a problem if you drill on the top or bottom either. I mean has anyone bent a swing arm during normal...or even abnormal riding? A bad crash could probably do it but then it would likely be bent sideways...in which case a hole on the top or bottom wouldn't matter because it would be like the case I described above and in the ideal spot. My guess is that they are over engineered anyway and minimal reduction of strength isn't going to be noticed.

Welding would probably be okay as well. If I am correct on where you are talking about, these are pretty small welds with only slight penetration and the heat affected zone should be pretty small...especially in aluminum because it conducts heat so well. Also...you would be welding in an area that has already been welded so the manufacture probably put the original welds in a spot that doesn't see a ton of stress anyway. While its true that after the welds are done at the factory the entire unit is likely heat treated but I would guess that is for the benifit of the big welds. Also the aluminum is heat treated to be as soft as possible before manufacturing to aid in cutting and forming and then heat treated when finished for the ideal strength properties.

Since this is likely a matter of cost I would say just drill, tap and be done. Welding could cost some money if you find a good welder that is equiped to properly weld aluminum.

That wrap around bracket sounds like a good option as well. Depends on your ability to fabricate of course. The good thing about the bracket is that if you break it again you can just make another.

Good Luck.
 
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#15
dodgedogjb,

thanks for the detailed reply.

The heat treating is something that I didnt count on. Sounds like if it is re welded it could change the properties around that area if someone didnt know what they are doing and got it too hot. I think that I will drill and tap. If it breaks off again I will just leave it that way, I rode my cr500 for several years without a chain guide. I just kept the chan adjusted correctly all the time and never had a prob.