reynome

Member
Sep 1, 1999
143
0
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

Member
Dec 3, 2000
66
0
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
 

reynome

Member
Sep 1, 1999
143
0
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

Member
Dec 3, 2000
66
0
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
 

reynome

Member
Sep 1, 1999
143
0
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.
 

reynome

Member
Sep 1, 1999
143
0
I didnt ask what you would do, xrsforever.

I want educated opinions from fellow engineers who are familiar with metals.
 

placelast

Member
Apr 11, 2001
1,298
1
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

Member
Dec 3, 2000
66
0
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
 

osheen

Member
Feb 27, 2000
202
0
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..........
 

reynome

Member
Sep 1, 1999
143
0
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
 

Kaw_Boy_5

Member
Apr 18, 2001
253
0
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.
 

dodgedogjb

Member
Jul 25, 2001
39
0
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.
 

reynome

Member
Sep 1, 1999
143
0
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.
 

JohninKY

Member
Nov 18, 2000
131
0
A small longitudinal weld should not be a problem for your swingarm with or without post weld heat treat. I'm not real familiar with PWHT requirements for aluminum, but for carbon steel it would not be required here. A weld perpendicular to the length of the swingarm would not be a good idea. The problem with drilling holes is not with a sudden catastrophic failure, but with crack propagation over time. Spacing between the holes is also an issue,but as long as they are small holes a few inches apart that shouldn't be a problem.

Either way you go you'll probably be okay. If something goes horribly wrong send us an update from the IC ward.:silly:
 

Max Factor

Sponsoring Member
Oct 18, 2000
155
0
My KTM 520 exc is a street legal version as sold in Australia. It has (had) a chain gaurd fitted. The 'factory" supplies it with 2 x 4mm taped holes in the side of the swing arm to mount the gaurd. My bike is a 2000 model and no drama's at all to date. IMO you're right to go ahead. (definitely preferable to welding as well)
 

mopowa

Member
Dec 6, 2000
75
1
The wrap around holder sounds like a good idea. Something like a hose clamp,or the duct tape someone mentioned.
If you cant figure all the intracasies of the engineering then just ride it with no chain guide.
I wouldnt recomend that for eveyone, just you.

That is a highly educated opinion
 

snaggleXR4

Member
Aug 5, 2001
309
0
Hey,

I thought about the bracket install also. The type they use to put a kickstand on 2-stroke bikes. Unfortunately, it would not look so great, hold very well, and would probably mar up your alum. swingarm.

I think you would be just fine to drill a couple holes in your swingarm. Just make sure you use very sharp tools. I would coat the drill/tap with grease to try to catch all the chips, instead of letting them fall into the swingarm. You could also install Heli-coils in the holes to ensure that the steel bolts will have steel threads to screw into and not soft aluminum ones.

Aluminum is a rather soft material, it can crack, especially around welds. But, it tends to bend and distort a good bit first. Just do it. Good luck.
 

pyromaniac

Member
Jun 25, 2000
378
0
If you done make too big holes i dont think it matters. When i polished my swing i removed probably 2-4mm around the while swing and it still working.
 

Jaybird

Apprentice Goon
LIFETIME SPONSOR
Mar 16, 2001
6,452
0
Charlestown, IN
Unless you heat up the hole real bad during drilling, you should see no stress related problems. You are, after all, replacing the metal drilled out by the metal in the bolt, albeit not the excact surface area you took out, it's darn close. The bolt will support the structure. 6m-8m maybe?

You would probably need software like Ansys to be able to figure stress factor changes from what you are going to do. I wouldn't sweat it.

Thought you couldn't hurt hondas anyway? :think
 

reynome

Member
Sep 1, 1999
143
0
It wasnt my bikes fault, there was this yzf on the trail you see. He just wouldnt get out of our way so, just like a bull dog to cats my Honda was out of controll. Later that night when he finally came home I saw his chane-guide broken with bits of blue plastic wedged in the guide. I spanked him and put him to bed early. :silly:

Thanks for all the input, Ill give a update on how everything turned out.
 

will pattison

Sponsoring Member
Jul 24, 2000
439
0
i've welded plenty of stuff, plenty of ways, including aluminum. i wouldn't hesitate to make the repair you are discussing on my own swingarm. i concur with the engineering theory stated so far, and the screws probably wouldn't be a problem for reasons already mentioned, but why take a chance? i have seen some bizarre metal failures, and in a high-stress application i would always rather have continuous metal. a really good fab shop that works with aluminum will know how to pre-heat, properly select the filler rod, and then possibly heat treat after welding. if not, a lot of aluminum guys will tell you that 8 hours in a 350 degree oven will get you right up to a t-6 condition with no problems. just make sure not to tell the missus.
;)
 

reynome

Member
Sep 1, 1999
143
0
Well the holes are drilled, tapped and the chain guide is bolted on. I rode all weekend and a day at the track too with it this way without any sign of cracking or bending. Probably 13 hrs on it at this point, very hard ones too. The enduro I rode this weekend was nothing but rocks that battered the chain guide and no probs.

thanks again for all the input
 


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