Tuesday, October 2

rack update

Originally one idea for the supereasy DIY rack was to pit J-B Weld against PC-7 Epoxy in a no holds barred deathmatch of destruction. The J-B Weld died a quick and painless death. Perhaps PC-7 will do better...

The consistency of PC-7 is very different - more like roofing tar. Pasty, but kinda stringy too, and without the nice surface tension that makes the "fillets" look nice. Definitely lumpier. It also takes way longer than J-B Weld to dry/cure. Overnight at least, and 2 nights would be better.

Stay tuned for the stress testing... Can it hold a 30-pack? On the trail?


Blogger Joel said...

I'm going to predict that the PC-7 will not fair any better. But I don't think it's due to the strength of the epoxy. It's more the mechanical design of the joint. I suggest you stick with PC-7 for this since I have seen and used it like this before. First, take the tube end that butts into the T-joint and cram a piece of rubber or something similar about 1.5" down the end of the tube. Then fill the end of the tube with PC-7 (or -11) epoxy. Let it fully cure. Then drill and tap a hole in the epoxy, your choice of bolt size but make sure to leave enough epoxy in there. Then drill a through hole through the cross piece. If you think the cross piece will collapse under stress, then drill through one side of the tubing and squeeze in a bunch of epoxy to form a plug in the area of the hole. Then after curing, redrill all the way through. The epoxy forms some seriously strong threads when tapped and the bending stress will be transfered completely to the bolt rather than the epoxy. The key here is to play to each component's strength. Epoxy is good in direct tension/compression or even shear, bad in bending moments. A steel bolt while best in the same situation has significantly more strength in bending than a polymer.

Wednesday, 03 October, 2007  

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