RE: Auto Weighing

[Roger Stockton <rstockton@delta-q.com>]

Hi David,

The bathroom scale with a lever approach works fine.  I weighed my
Suzuki Forsa (Chevy Sprint) this way to get original weights on each
wheel and (so far) weights on each wheel with electric motor installed
and batteries placed in the proposed locations.

I haven't managed to get any pics of my weighing setup on the web yet,
but here are the important details:

- I used a lever made of 1" mild steel angle iron, bolted together to
  form a rectangle about 3 feet long and 6 inches wide.  Additional
  6" cross-pieces were bolted on at appropriate locations to act as
  the fulcrum points for the various ratios I thought I might need
  (0.25x, 0.33x, and 0.5x if I recall correctly).  This approach allows
  you to get the carefull measurement stuff dispensed with on the
  workbench where it is easy, rather than on the garage floor repeated
  each time you set the scale up under a wheel.

- I built my lever so that the scale position and tire position do not
  vary depending on the lever ratio in use; only the fulcrum position
  needs to be altered to change ratio.  This is easier to work with.

- I built my lever with a little hinged platform for the tire to be
  lowered onto so that I know the weight is applied to the lever
  *exactly* through the hinge pin (the location of which was carefully
  determined relative to the scale end to provide accurate ratios)

- you must provide rollers to prevent sideways motion of the wheel as
  the vehicle is lowered onto the scale from causing its mechanism to
  bind.  I used 4 pieces of brass dowel (about 1/4" diameter and 6"
  long): 2 were laid on top of the scale, about 3" apart and parallel
  to each other, the other 2 were laid across the first 2.  This
  prevents the wheel from imparting any side loading to the scale at
  all.

- the car must be level *while the scale is under one wheel*.  I jacked
  up each corner of the car and placed a short piece of 4x4 under each
  of the first 3 wheels; the scale and lever went under the 4th.

- the lever must be level; I used a couple of thin boards with 2 short
  pieces of 1" angle on top of them for a fulcrum the appropriate height.
  The pieces of angle are laid parallel to each other and face down to
  form a knife edge for the appropriate lever cross piece to bear against.
  (Again, provided the cross pieces were located accurately when the lever
  was bolted together, this minimizes the need for precise positioning of
  the fulcrum blocks each time - just get them under the appropriate cross
  piece and you're set.)

This is where a picture is worth a thousand words, but since I don't have
a picture scanned to share, I'll attempt it in ASCII:

   A    B     C          D                      E
                       _____
    _____________________|_______________________
   |    |     |                                 |
  ----                                        ------  (Crossed rollers)
 [Fulcrum]                                     *  *
 [Blocks ]                                [0-300lb Scale]

(Not shown are the vertical sides of the long sides of the lever;
they also point downwards, like the cross pieces that are shown.)

A, B, C are the locations of the fulcrums for the different ratios,
D is the location of the tire platform pivot axis (hinge pins), and E
is the location of the point that bears against the scale through the
rollers.

If D is located at the midpoint of the lever A-E, then the configuration
shown yields a 0.5 ratio (weight indicated on the scale is 1/2 of that
on the wheel on the platform at D.

The ratio at B is the 1 - [(distance D-E) / (distance B-E)].  For instance,
locate B at 1.5x the distance D-E away from E for a 0.33 ratio.

Locate C at 1.25x the distance D-E away from E for a 0.25 ratio.

The short bits of angle iron on top of the fulcrum blocks are laying face
down, so that their "spine" provides an edge for the lever to bear against,
like so:
  __________
 |__________| <- end view of lever
   /\    /\
 +----------+
 | Block    |

NOTE: my Forsa weighed 1500-1800lbs and had a 60/40 front/rear weight
      distribution, so no wheel exerted more than 600lbs on the lever.
      Very slight flexing of the 1" angle lever was detectable as the
      load was applied to it, so you might want to use 1.5 or 2" angle
      if you expect more than say 800lbs on any wheel and don't want
      to sacrifice your first lever to see if it'll take the weight!

You should be able to get an idea of the CG location using the idea of
weighing the car while it is tilted sideways and then while tilted
to the front or rear, but must first weigh it accurately while level
(since CG will be computed based on how much the weight distribution
changes from its level value).  The accuracy of the measurement will
depend to some extent on how much of a tilt you subject the car to;
you may need to resort to the use of jack stands on the high side
given that the low side will need to have the wheels 4" off the ground
to remain level with the scale under one wheel.

Hope this helps,

Roger.

-----Original Message-----
From:	David Dymaxion [SMTP:david_dymaxion@yahoo.com]
Sent:	Monday, April 30, 2001 1:39 PM
To:	ev@listproc.sjsu.edu
Subject:	Auto Weighing

First right to the question, and then the details: I'd like to weigh
each corner of my car, any suggestions?

I'm about to start wrenching away on my 911. First step is to take
out the motor, figure out what is wrong, and then decide whether to
fix it or electrify it.

Anyway, I'd like to weigh the car, for these fun reasons:

    Weight before and after
    Weight of components as they come off
    Preserve the front/rear balance
    Weight on each tire
    CG before and after

The Porsche has an adjustable suspension, so if the weights on the
wheels are too far mismatched that can be adjusted. Alledgedly this
type of balancing can improve the handling for autocross.

The CG number would be just for bragging rights. The CG should be
lower after conversion to electric if the batteries are kept low. My
idea here is to weigh each corner. Then put the rear wheels on
blocks, and weigh each corner. Then put the front wheels on blocks,
and weight each corner, and then do it side to side. With a little
math, I can calculate the CG.

My cheapest thought to date is something like this:

        tire
==============================================
^                                        scale

You put a strong board on a narrow board or pipe, and hold up the
other end with a bathroom scale. Put the car tire on about the 1/4
point, and the scale will read 1/4 of the weight of that corner of
the car.

I assume "real" scales to do this directly would be expensive, but
I'm open to suggestions. How thick a board would I need? I'd guess
about a 4x8, or is this overkill? I have also thought about using
multiple scales.

The car is not driveable, so no easy drive down to the farmer's
scales. Also, I want to be able to do the weighings while the car is
taken apart.

Thanks for any suggestions.


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