how to figure center of gravity height ?

[working on it]

partially copied from my post in "foamies" in another thread;perhaps someone else may be asking this question too I, for one , am using Andrew's weight balance calculator for designing and moving weight aound for best effect. The calculator shows the center of gravity, LCG, horizontally, in relationship to the rearmost edge of the trailer. But, in view of wind effects and overall trailer handling on the road, I was wondering whether or not someone (angib, perhaps?) has a corollary formulation to locate the vertical component of the center of gravity? In rough form of course, since no height measurements are called for in the basic balance spreadsheet, much less the squat imparted to the tires and suspension with load.

[Corwin C]

I noticed that you haven't received an answer, so I'll give it a go...

The vertical CG is calculated in exactly the same way as the horizontal ... you can use the same spreadsheet, just pick a reference point (I'd use the top, or an imaginary point above the top if you ever plan on a roof rack) and enter your measurements (vertical) and weights the same way as before. Suspension travel would be very hard to predict without test data with your actual axle/spring/tire combination, but you can load it and measure it.

Interestingly enough, because they are 90° out of phase with each other, one doesn't affect the other. You can also do this left/right as well if you really want to.

I hope that this was helpful.

[angib]

The thing is, once you've worked out the vertical centre of gravity, what are you going to do with it? I can't see the number helps you much - you can do all sorts of calculations with it, but what do they achieve?

As far as I can see, some commercial trailers have awfully high CoGs, but they seem to get away with them. A foamie should have a lower CoG, though it will have more side area than its weight might suggest. But I don't see any way of using the data to tell you if the result will be acceptable.

[working on it]

I was mainly interested in seeing the "vertical" location of the cg as an adjunct to the "horizontal" cg from Andrew's balance spreadsheet. I 'd like to see if I can manipulate weights and locations of the added weights to bring both cg's to a location just forward and slightly above the axle centerline. Having the combined CG in that region would verify that the trailer was dynamically balanced, in the way we scale and move weights around in building a race car. I'm probably not going to be able to pinpoint the exact location of the combined CG, or will be able to redesign what I've built, so far, to be able to reach an optimal location; but as a mental exercise or as a future basis for another build, I'd just like to see the results.

[bestclimb]

get a datum like the ground.

wight of axle times distance from ground to center of axle=moment arm of axle
10 inches X 75 lbs=750 inch#

weight of trailer frame times distance from ground (vertical center)=moment arm of frame
15 inches X 200 lbs=3000 inch#

weight of whole trailer minus axle and frame times distance from ground to vertical center of trailer body.=~moment arm of body
38 inches X 500 lbs=19000

add all those moments
22750
add all the weights
775

devide total moment, by weight = CG of ~29 inches.

anything installed into the trailer gets it's moment and weight added to adjust the CG.

or tip it on it's side and weigh the top...

[working on it]

I've figured about 28", but I don't know if that's comparable to any other TTT that people have built ( similar size and design). I'm familiar with cars and changing their CG for better traction on launch (dragracing), but the trailer ride and tracking ideals are new to me. Theoretical changes may not actually translate into real-life improvements. Just have to finish the TTT and actually test it to really know.

[Corwin C]

get a datum like the ground.

I would avoid using the ground as a datum because it will change all of your distances according to how heavy your trailer is loaded. If you use, for instance, the top of the roof rack, then the only distance that changes would be for the unsprung portion of your suspension (axle, tires).