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Engine CG

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Aaron_JHU View Drop Down
Bolt Sorter
Bolt Sorter


Joined: Aug/14/2012
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aaron_JHU Quote  Post ReplyReply Direct Link To This Post Topic: Engine CG
    Posted: Sep/18/2012 at 8:21pm
Hey Guys,

Trying to find the X-Y CG of the engine. I assume its not centered on the bolt holes. 

I have an idea as to how to find it using 4 scales, detailed in the screenshots below.

Top View:

Side View:

Blue: scales
Red: mounting plate at bottom of engine
Green: square tube test rig

As long as I do a good job of centering the green "feet" on the scales, I could calculate the CG position in the X-Y Plane. Thoughts on if this would work? Better ways to find the CG in the X-Y? Or, does anyone already know the CG location?


Aaron Rips
Hopkins Baja - Drivetrain
2010-
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otto View Drop Down
Milling Master
Milling Master


Joined: Oct/06/2011
Location: OR & Colorado
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Post Options Post Options   Thanks (0) Thanks(0)   Quote otto Quote  Post ReplyReply Direct Link To This Post Posted: Sep/18/2012 at 9:02pm
should give you a rough idea

if it were me, i'd replace each scale with a single load cell (4 s-beams shouldnt be too expensive) and tie the load cell into the fixture (just make sure you arent transmitting any moments)

make sure your fixture is nice and stiff

the further from the CG the scales/load cells are, the more sensitive they will be to the CG location
Cornell Baja 2008-2012, suspension stuff
Oregon State 2012+, saw chain & some more baja
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sandres913 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sandres913 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/18/2012 at 9:51pm
If you go with bathroom scales, use digital ones. Or better yet, don't use bathroom scales Thumbs Up 
Just make sure they're all the same kind and calibrated, or at least know the error on them. 

Use material that won't deflect under the weight of the engine, aaaaaand....zero your scales with the fixture (including bolts/mounting hardware) on it. That way you're not imposing a second center of gravity into the picture
Cheers! Beer
Shaun
Purdue University Calumet - PUC Motorsports
2010 - Washington #18
2011 - Peoria #30
2012 - Wisconsin #54
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Aaron_JHU View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aaron_JHU Quote  Post ReplyReply Direct Link To This Post Posted: Sep/27/2012 at 5:05pm
Hey guys, thanks for the advice. Here are the results and some info about our process.

I ended up going with 4 digital bathroom scales. turns out load cells are more expensive than I would have thought.

Process info:
4  digital bathroom scales
.065" welded square tube test frame. symmetric in the x-y to .05". 
weight was zeroed out.
Rig centered on scales to .2" accuracy (approx.)
10 Trials
Note: engine weighed without gas tank

Results:
Avg weight: 52.05 lb.
Total weight stdev: 0.189 lb.
max stdev of 4 scales: 0.356 lb.

Here's a basic drawing of the bottom of the engine with the CG.

If you have your own data, let me know if this makes sense. If you don't, I hope this helps. I am confident that this result is at least close.
Aaron Rips
Hopkins Baja - Drivetrain
2010-
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