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Old 04-13-2020, 11:30 AM   #1
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Check my geometry?

I've been using this attachment I built to keep an approximate idea of the weight added as I rebuild the trailer. I think the length from the hub to the scale might affect the reading, but I'm no engineer. Is the 6" significant and causing a lower reading, and by how much. Hub to hub length is about 74" Thanks for your help.
J.L.
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Old 04-13-2020, 11:55 AM   #2
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How is it sitting on the other side? Is the tire installed, sitting on the ground, and trailer level side to side and front to back?
If you set it up like that and move your scale along the fixture so it is out from the hub face a distance equal to where center of tire would be....then no additional math is required.
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Old 04-13-2020, 02:14 PM   #3
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Rich,
Would not the tire center be directly under the brake drum?
The wheel is offset to the inside.
Maybe lock the drum with the brake adjustment wheel, and lower onto the scale. Put a jack stand under the axle mount plate as a safety measure. Tongue jack down and opposite wheel chocked.

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Old 04-13-2020, 04:24 PM   #4
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Rich,
Would not the tire center be directly under the brake drum?
The wheel is offset to the inside.
Maybe lock the drum with the brake adjustment wheel, and lower onto the scale. Put a jack stand under the axle mount plate as a safety measure. Tongue jack down and opposite wheel chocked.

Bob
I would assume it is a zero offset wheel, but I dont know that. In any case, if op slides the scale point close to the drum, and it's an inch off of the "center" of the wheel....AND....he's got the tire on the floor on the other side, it might make a 10 pound difference. That's close enough for floorplan and loading estimates. Now he could put a jack stand under the spring, or axle plate, and put the scale on the opposite plate and accomplish the same thing. Assuming all is level.
Theoretically, center of the tire would be on a plane with the hub face, not center of drum width.
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Old 04-13-2020, 04:28 PM   #5
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Or, OP could make up another hub jig for the other side.
I do like Bob's idea of setting the brake, but I think I would fab a wood piece to the curvature of the drum for the scale to be placed under. Other pieces of wood, stood off but under the drum block, could stop rotation if something went wrong.
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Old 04-13-2020, 05:32 PM   #6
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It's pretty simple, assuming the axle is level when you take the measurement.

If the load at 80 inches (74"+6") is 2000 lbs, then the load on the wheel centerline would be 2000 lbs x 80"/74" = 2160 lbs
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Old 04-13-2020, 05:54 PM   #7
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Centerline of the wheel is about at the hub face, so I think I'm 6" away from where the load is normally carried. Mark's calculations are pretty much what I thought I would have, but I did want another opinion. The trailer is level side to side and fore and aft throughout the weighing. Thanks for the help, weights are lining up where I want them to.
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Old 04-13-2020, 06:13 PM   #8
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The amount of weight (force) each ground touch point sees is directly proportional to the relative distance to the center of gravity of the load. So the six inches will materially affect your estimates. No need to weigh the other touch points unless you want a very accurate reading. You will need a reasonable idea of the center of gravity though.

I presume the other side is on a jack stand just under the brake drum but it might be on the tire as others suggested. Lets say the center of the touch point is under the hub.

You could stipulate it is in the center of the trailer over the axle and in that case the torque lever arms are 37 for the other side and 43 for this side.

So the load on the other side is the weight measured (Wm) times the ratio of levers or 43/37 and the total axle weight is Wm(1+43/37) or roughtly 16% more than is his estimate used by doubling the measured weight so the 6 inches is making a 16% difference in the estimated axle weight.

Now since the true Center of gravity is forward of the axle the error is probably around 12% since the lever arms are a bit longer and the relative difference is less.

To get the total weight of course the OP must also weigh the tongue jack load. I'l presume there is only one axle with ground touch point but if not those will have to be measured also.

The physical principle involved is that the trailer is static so all torque moments must balance each other otherwise the trailer will shift until they are balanced and the sum of all support loads must equal actual total weight.

If this is unclear, ask any questions you have.

edit: posted before I saw mark's response though he should have split the 74 in half.....
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Old 04-13-2020, 07:22 PM   #9
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...edit: posted before I saw mark's response though he should have split the 74 in half.....
OK, I'll buy that.
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Old 04-13-2020, 07:26 PM   #10
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Correction, the other hub weight is 16% more, the total weight is (2.16/2 -1)*100 or 8% more than the estimate, again since the center of gravity is forward of the axle, the overestimate is likely more like 6 or 7% under.
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