On the way to the garage I stopped by at Target and after a while I left with a bathroom scale that goes to 300 pounds. Here she is under the end of my lever/ramp rig.
I started with the left-front wheel (well, okay, I STARTED with putting the soft top and the seats back in place to get the car back into its stock configuration -- I even emptied the trunk of the usual trunk junk!)
These are views of the lever/ramp and the ordinary ramp and the car which is about to drive up on them.
You sight down from above to align the center of the wheel with the appropriate mark on the lever/ramp. I'm expecting about 600 pounds per wheel so I'm using the 3:1 mark. This picture is just to remind me that this is the front/left wheel.
And the magic number is ...
Well, carefull... The scale reads 212.5, but the lever all by itself puts 12 pounds on the scale, and to find the actual weight on the wheel we multiply by 3. So that's 3*(212.5 - 12) = 601.5 pounds.
Get everything set up again for the right/front wheel. Drive up onto the rig. Position the car carefully and...
The raw reading is 209.5 pounds. Again, we subtract 12 then multiply by 3 to get 592.5 pounds
The left/rear wheel.
Decided to take a pic of the view as we position the wheel above the mark on the lever ramp. After getting home and looking at this image I realized that the center of the wheel is about an eighth of an inch out of position. So, what we have here is what mathematicians call a sensitivity analysis. How far off is our estimate of the weight over this wheel given a .125 inch error in positioning?
The raw reading is 180 pounds. That leads us to 3*(180-12) = 504 pounds. Assuming the wheel is actually .125 inches out of position, the factor of 3 should really be 3.015 (which is 72/23.875) so we get 506.5 pounds. I think I can live with an error of 2.5 pounds -- although next time I do this I'll use a carpenter's square to make the positioning a bit more precise.
The last wheel is the rear/right.
The alignment looks dead-on.
Raw reading: 191.5 pounds. Actual: 538.5. Why is the right/rear carrying so much more weight than the left/rear? One possibility is that the battery (I need to start calling that one the "accessory" battery) is positioned back in the right/rear corner
The total weight of the car is then 601.5 + 592.5 + 506.5 + 538.5 = 2239. Wikipedia says the curb weight of a second generation miata is exactly 1000 kg which is 2205 pounds, but that is "without options". Mine has AC and power windows, could that account for 34 pounds? Well, come to think of it, I bet all that AC stuff comes to AT LEAST 34 pounds
Should I be worried that my arrangement has the axle I'm measuring lifted up about 4 inches above the other? Here's a crude diagram to illustrate the issue.
The answer to that depends on where the car's CG lies (in the vertical direction) relative to the centers of the wheels. At a guess, I'd say the CG is about 6 inches above the wheel center -- it couldn't be more than a foot; these cars are low slung. The wheelbase is 89.2 inches so that 4 inch differential height means the car is at a arcsin(4/89.2) angle or 2.57 degrees. Let's go with the worst case possibility that the CG is 12 inches above the wheel centers. The CG will have moved by about .5 inches away from the axle being measured. After a somewhat careful analysis, it would seem that my weights are low by a bit -- they are 98.9% of the true values. Of course that's a worst case estimate -- if the CG is actually 6 inches above the wheel centers we're talking about a .25 inch shift and the errors are halved. Splitting the difference would say my weights are 99.2 percent of the actual values. After correcting for this effect (more or less) I'm estimating the vehicle's curb weight at 2256 pounds.
After all that number crunching I was tired. (Naw, I'm kidding, I love number crunching!) Anyway, some physical work seemed in order so I took the seats back out and finished removing the soft top and loaded that stuff into our station wagon for the trip home.
I remembered that I never did snap a pic of the screws that hold the main hinge plate for the top on, so here it is.
Look at all that space that's available when the rag top's not there! Doesn't that look like a whole mess of batteries could fit in it?
I hope so, 'cause they're gonna have to!