Modified (cut) spring measurements

["Mark Battley" <m.battley@xxxxxxxxxx>]

He's back: the man with the angle grinder and testing machine.... be afraid,
be very afraid!

Ahem, where was I.

Well I attacked my springs with angle grinder and compression testing
machine, starting with (as far as I know) standard front springs (6.75
coils, 16.7mm wire diameter, 60.5mm coil radius) from a 1973 105 series 2000
GT Veloce. I measured the load deflection curve (typically at 6 or more
points) in a standard electro/mechanical testing machine (between flat
platens). I used the rubber pads at each end, mainly to make sure that the
spring didn't slide anywhere (yes I did have other security measures in
place to keep things safe). I don't believe that the rubbers will
significantly affect the rate, and I intend to use them in practice anyhow.

Both springs were within 1% of each other (deflection for given load),
standard and modified.

Brief summary of results: 

Relative to original spring (320mm static length, 75 N/mm, 435 lb/in rate)

                            Static length    Loaded length                
Rate   (N/mm/lb/in)
                                                    (at 2250lb/1023kg)

Less 0.5 coil                -7mm                -8mm                     
+2%   (78/446)

Less 1 coil                  -30mm              -20mm                   +12%
  (85/487)

I stopped at 1 coil: I'll see how the car sits/handles before going any
further. This should be enough for my requirements.

So removing only half a coil makes virtually no difference to the rate, and
only a small difference to the length. This first half is at a very shallow
helix angle and is not very "active". Once you get beyond this things change
more quickly.

Note that the rate is not actually completely linear, it increases slightly
as the spring effective length reduces. Cutting wheel in angle grinder
worked very well: with spring held in a big vice the heating was very
localised.

Enough for now, gotta run...

Mark Battley
Auckland, New Zealand.

1974 Alfa Romeo 2000 GTV

Alfa Romeo 105 series Bertone Coupe home page and register at:
      http://www.geocities.com/MotorCity/1806/105GTV.HTML

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