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Corner-Weight
Going to a race shop for corner-weighting or renting scales is inconvenient and expensive. I followed some tips from other people and made my own low-cost version of flat-floor equipment. 
 
It is made using old fashioned mechanical bathroom scales (at $10 each). Wich some pieces of lumber the leverage doubled the capacity of each scale to 600 pounds, sufficient for a Seven.
The 2 front scales are supported by 1/2" metal tubes (as rollers) to prevent horizontal forces on the scale (June 2004).
 
 

cornerscales.jpg

 
 
 
 
 
Before I made the corner-weight scales I tried to weigh the whole car with simple means. Here is my recipe for a pretty precise measurement:
 
1. Load the car on a single axle trailer. If you have a 2-axle trailer you need to raise one axle with some lumber to have the other one hanging free.
2. Position the car so that the tong is only slightly loaded (10-20 pounds on the hitch).
3. Block the trailer wheels and the pull the car's emergency brake.
4. Move the tow car away and leave the trailer supported by the trailer jack. It is advisable to also support the it (loosely) in the back to prevent it from tipping backward.
5. Position the bathroom scale (precision digital preferred) under the trailer hitch coupling and put some spacers on it up to the coupling (paint cans etc.). Last put a wedge on top of the spacer cans (aluminum angle or similar) and mark the contact point of the wedge edge with the coupling. See followong picture.

tongscale.jpg

6. Weigh how much the tong is loaded. For a digital scale you will need to jack it up, turn on the scale (tare) and lower again until the jack is hanging free. Then read the scale. On a mechanical scale you can read it right away.
7. Attach a pointer (bamboo skewer) to the side of the car  with masking tape so that it almost touches the trailer bed. Position and tape a ruler (at least 300mm long) or tape measure on the trailer bed besides the pointer. Note the position of the pointer (see following picture).

cornerscaledetail.jpg

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pointer.jpg
8. Now move the car forward on the trailer (about 200mm) and weigh again. If the scale is maxed out move it back a little. If the scale still has a lot of reserve you may want to move it further forward. Don't move it so far that the paint cans are crushed. Set the brake.
9. Read the tong load again (with lifting/tare/lowering for the digital scale) and note the exact new position of the pointer on the ruler.
10. Calculate the difference of the pointer positions "DP" in mm and calculate the difference of the two weight readings "DW".
11. Measure the exact distance in mm "DIST" from the center of the trailer axle (or center line of wheel bearings ) to the wedge edge mark on the coupling (see #5 above).
12. The car's weight is: DW times DIST divided by DP. It is pound or kilo whatever the unit of the scale is.
 
With a little careful measuring and a good scale this should be precise to 5-10 pounds. I found it easier to read precisely from the ruler and to do the difference in mm (old habits). But it is of course also possible to do all that in fractional inches.