Frame Strength - Mountain Buzz
 



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Old 02-01-2019   #1
 
Fort Collins, Colorado
Paddling Since: 1999
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Frame Strength

I've been tinkering with the idea of making my own cataraft frame for hiking into runs. (I'm getting some cutthroat tubes).

I've been reading lots of threads regarding pipe/tube sizing and materials.

I'm hoping for some input about what pressures/forces have caused frame failures.

For example - I can think of two forces on cross bars. One is the weight of people sitting on the bars and various gear that is hung off of them. To me it seems that if my cataraft is only 2' wide between tubes, versus the more typical 40" or greater - that my my tube can have a wall thickness could be less than typical. (For convenience I'm going to stay with standard ODs. Moment is directly related length between supports and deflection is a cube of the length between supports. The other force would be axial - my thought being that if the boat is moving sideways and rams into something one tube is stopped and the frame is taking the force between the tubes.

Forces on the crossbars? I'm also really curious about what forces are there on the siderails?

Any help would be appreciated!

Thanks

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Old 02-01-2019   #2
 
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Kalispell, Montana
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You're correct, engineer.

You are also over-thinking it.


If you want to shave weight, the frame is a good place...but the tubes and oars are probably better. JPW makes great boats, but PVC is heavy.


Look at Edge oars, they're 1/2 the weight of other oars.
NRS atomic aluminum oarlocks.


NRS and Hollaender/speedrail fittings are great for adjustability, but are heavy. Go for a welded frame.
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Old 02-01-2019   #3
 
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Kalispell, Montana
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Now, if you're really serious about shaving weight on the frame, don't try to over-engineer it. Don't plan on others sitting on the framerails. Consider lightweight (thinwall) everywhere except for your seat bar. Mount your footrests off of the yokes instead of a crossbar.


Consider double-butting your tubing like a custom road bike frame. Add that 2nd wall thickness where you need it at the bend/stress/weld points and not in the middle of the bar.

Quote:
The other force would be axial - my thought being that if the boat is moving sideways and rams into something one tube is stopped and the frame is taking the force between the tubes.

Water and rubber should "give" enough that this axial force shouldn't be a big concern. I would think your largest concern for a catastrophic axial load would be in a wrap or center-punching a crossbar with a rock.




Oh, and for a welded frame, you might heat treat it after welding to relieve stress, rather than having too large a frame to compensate for the HAZ.
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Old 02-01-2019   #4
 
Fort Collins, Colorado
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Great advice. Yes - over thinking it. I might be making up for not having river trips to plan right now.

I do have one question regarding tubes . . . I've spent a lot of time looking at tube weights.

Since I'm going for a smaller diameter tube it's saving weight beyond what I could get in other materials - since other manufacturers only sell large tubes. Is there a manufacturer who does smaller tubes?

Ideals on doing a mostly welded frame, but being able to pack it in?
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Old 02-01-2019   #5
 
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I'd actually recommend larger tubes with thinner walls rather than smaller diameter tubes with thicker walls.


Obviously there's a tradeoff between stiffness/weight/toughness. A small diameter (relatively) thick-walled tube that weighs the same as a large diameter thin-walled tube will have more ding-resistance and toughness, but less stiffness.



...and I wouldn't vary radically from existing designs.

NRS tubes are 1 1/4" SCH40, or 1.380 ID with a or 0.140 in wall. OD is about 1.66"

Speedrail uses 1 1/2" SCH40, which is 1.61ID, 0.145" wall, and 1.9" OD.

You could consider 1" Sch20, which is 1.097ID/1.315OD/0.109 in wall...I wouldn't go much thinner/smaller than that to start with.




Or go a lot smaller, break it, then start stepping up! haha


Quote:
since other manufacturers only sell large tubes. Is there a manufacturer who does smaller tubes?

Are you building this yourself or having it fabricated for you?


Search around here, there are several custom frame builders whose names frequently pop up. Recretec, Mad Cow, Eddyline Welding, Neff's Whitewater Customs and Riverboat Works are several of them. Neff's and RBW are Buzz sponsors. If you're having one built, pick a vendor, and work with them to source smaller tubing and a lighter frame. I'm guessing they've done it before.
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Old 02-01-2019   #6
 
Fort Collins, Colorado
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Oops - Yes, I agree larger tubes. Thinner wall.

I meant that only JPW seems to make cataraft tubes that are 16 or 19". Everyone else makes 22" and up. So even though it's PVC, it's less weight because of the smaller diameter. I was asking if there is a cataraft tube manufacturer that makes smaller tubes.
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Old 02-01-2019   #7
 
Fort Collins, Colorado
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I have definitely been poking around the Buzz to see what people think about different manufacturers. Have 100% decided what route I'll go.
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Old 02-01-2019   #8
 
Jenks, Oklahoma
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I purchased a super cutthroat plus their frame from JPW, I think sometime in 2013.

The frame was a smaller diameter tubing.

The frame and tubes have been bomber big time and I have used the rig quite a bit and it still looks good.

Check with JPW on the tubing they use on cutthroats
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Old 02-01-2019   #9
 
Join Date: Jul 2018
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Steel or aluminum tubing?

If the frame is fully welded, you could add laser cut gussets to greatly reinforce tube junction.

Do you have access to solidworks?
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Old 02-01-2019   #10
 
Fort Collins, Colorado
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No solidworks but I do have acad 3d
Interesting idea. Makes sense.
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