One of the mounts for our sidecar broke. Found it today. From the looks of it none too soon.
Please look at the photos whose links I have enclosed.
http://picasaweb.google.com/106794675346045217557/BrokenHackMount?feat=directlink
The mount that failed was used when I got it and is of unknown origin.
It was constructed of 1 ½ OD pipe of some sort, with a weld seam on the inside.
The wall thickness is .125
It cracked on the bottom where it was stressed the most and worked it’s way around. It failed along the edge of the weld. The crack following the weld most of the way around. When it got to the top, the crack turned out into the tubing away from the weld. The starting point could have been in the center of the bottom side of the weld. Whoever welded it started at the corner and welded back to the center then stopped.
Then went to the far corner and welded from there to the center again. Where the two welds met there was not full penetration. Aside from that the penetration looks to be 90 to 100%.
I have some DOM 1 ½ inch tubing .125 wall on hand. We are considering making another duplicate mount with gussets this time. I can't increase the OD as it fits in a Dutch clamp on the sidecar. It could be made from thicker walled tubing, I just don’t have any thicker material on hand.
To get thicker tubing involves a 6 hr trip to purchase it. I would rather do that than have a catastrophic failure of course. If this had failed completely it would have sucked..
I’m concerned that the .125 tubing might be too thin. I can make the corner substantially stronger with proper side gussets, if it is really too thin, it may fatigue from torsional movements and crack where the gussets end.
The sidecar weighs 200lbs and the bike 900. We live in a really rough washboard dirt road. Occasionally we have drop the car wheel into some fair size potholes, off shoulders, etc. It takes a beating.
Does the metal breaking along the side of the weld mean that the tubing is too thin?
Or is it the use of the wrong type of tubing and imperfect welding that brought on the failure? Or is more likely that the this mount was fatigue already when I
mounted it 7000 miles ago?
Is DOM substantially stronger than this structural tubing in this type of application and there for will the same thickness in DOM be adequate? Or should I use a .187 thick DOM tubing instead of the.125?
Thanks for you’re help. I’m leaning towards using the 1.5 x .125 DOM . Then weld gusset plates on both sides of the connection. Probably 1/8" thick and 2.5 to 3 inch length on the shortest sides.
Any input would be welcomed. Thought I would run it by everyone
and get some opinions.
Thanks for your time.
Joe
😮
I think your failure was due tostress on the ungusseted 90 degree butt joint. The weld has held up but the weakness lies in the heat affected zone on either side of the weldment. This is where failure normally occurs. Proper gusseting will overcome this.
Lonnie
The wall thickness was not the problem. The use of .120 wall DOM tubing is time proven and very common.
Thanks,
We will make another out of DOM with proper gussets.
Wanted to be sure we were using the right material.
Joe
When you weld up the new mount, rig up some way to purge the air from the inside of the tube with the shield gas. This prevents carbon precipitation along the weld which can lead to fatigue cracking along the weld.
