Frame sag? - Mountain Buzz

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Old 06-23-2016   #1
Fort Collins, Colorado
Paddling Since: 2008
Join Date: May 2008
Posts: 121
Frame sag?

I've got a double rail frame built with 1.9" OD aluminum pipe- I am told it is 1.5 IPS ASTM B429 (6061-T6XX extruded structural) with an 1/8" wall. It's thinner/lighter than Sch. 40 1.5"/1.9" OD pipe. My frame builder set the inner frame rail width to be the same width as the inside of the tubes, so the inner frame rail does not rest on the tubes.

In playing some more games with frames, I noticed that my inner frame rail is sagging about 1/2" or so (see red rails on attached image). I carry a couple coolers and a drybox so it's got some load on it, but I was surprised nonetheless. Is this amount of sag worth worrying about?

If yes, I've thought of a couple potential solutions cooked up:
1) swap out the existing rails and replace with the thicker wall sch. 40 aluminum pipe, or
2) change the configuration to have a partial double rail frame (see example frame in picture on right), so it's got more cross rails resting on and supported by the tubes.

Any thoughts on which way I should go?

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Old 06-23-2016   #2's Avatar
lafayette or Grand Lake, Depends on mood, Colorado
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Time is like a river. You cannot touch the water twice, because the flow that has passed will never pass again. Enjoy every moment of life on or off the river. is offline   Reply With Quote
Old 06-23-2016   #3
Great Falls, Montana
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Old 06-23-2016   #4
Fort Collins, Colorado
Paddling Since: 2008
Join Date: May 2008
Posts: 121
Alright, so I got new sections of pipe (that will either be for this or another raft project I am working on) to use as my level. I placed it along the top of fittings that should all be the same elevation above the top of the pipe.

I've got a 1/4" gap at the ends (no sag) and 1/2" gap in the middle- looks like it's only actually sagging 1/4". You can see there is the corresponding 1/4" gap between the straight pipe and the fitting where it should be touching in the middle photo.
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Old 06-23-2016   #5
Alpine, Wyoming
Paddling Since: 2010
Join Date: May 2014
Posts: 35
I sympathaize

I have a custom built frame for my Cat that is over 10 years old. It is double rail--both rails rest on tubes. It is the cross members that were beginning to sag for me, over 3/4 of an inch--in particular the front and rear ones. I think this is the case because they get stepped on a lot getting into and out of boat; I attach my winch to them when trailering; and the front one carries part of the cooler load. Anyway, I just replaced both. I'm using 1&3/8in galvanized top rail used for chain link fences. Long story short, the frame will last forever, as I can just replace parts and even modify as needed; the frame will way outlast the tubes. If the frame is sagging from get go, though, I'd make it a double rail.
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