RE: Shortening springs - don't heat them!

["Mark Battley" <m.battley@domain.elided>]

>Please don't use this method.  If you want to lower a car, installed
>different springs, or carefully cut yours, but don't heat them to
>collapse them.

I agree  - unless you understand and have the facilities to re-heat treat
them properly afterwards to restore the correct (strength) properties. Which
I have had done by commercial spring manufacturers. But not quite for the
reasons suggested by Ian.

>Springs collapse when heated because you're changing the metals
>fundamental tensile properties, a key value in the spring formula.  And
>you're doing just the opposite of what you want, not only does the car
>now sit lower, but the spring will be softer so you're more likely to
>bottom out.  But more importantly, you've now greatly increased the odds
>of the springs failing and breaking.

No, no, no... you are changing strength properties (which is why it yields
under the compression load and shortens), but you CANNOT significantly
change the stiffness (modulus if you prefer) of the metal by heat treatment.
The shorter spring will have the same stiffness as it used to (until the
springs compress enough to bind). The "key value" in the spring stiffness
formula is the Shear Modulus, often denoted by G.

BUT, the metal will have a lower yield and ultimate strength, so is much
more likely to either continue to yield and get even shorter, or fracture. 

Strength and stiffness are very different material properties (but are often
confused), as a generalisation for metals strength can be greatly changed by
heat treatment, stiffness cannot (or not significantly for this type of
material and application). Composite materials (my little corner of the
engineering world), add even more complication to the issues of stiffness vs
strength.

I have been through the exercise of measuring the stiffness of GTV front
springs, cutting 1/2 then 1 coil off, remeasuring the rate at each stage,
and comparing to predictions. I measured 74, 78, 85, N/mm (419, 446, 487
lb/in) respectively. Note that the springs are not completely linear, since
the active length changes as the ends flatten into their supports (and when
you cut the end off you change the rate at which this happens). The
difficult input parameter to assess when predicting properties is the active
number of coils - std front springs on my car were 6.5 coils, matching the
measured data required inputting active coils of 5.6, 5.3 and 5.2
respectively. For what that is worth to anybody.

Mark Battley (yeah, I'm a mechanical eng too)
Auckland, New Zealand.

1973 Alfa Romeo 2000 GT Veloce
1989 Fiat Uno Turbo (yee-hah)
1999 Fiat Marea Weekend 2.0 20V - new addition to the fleet, naf name, but
lovely sounding and performing 5 cylinder engine, excellent handling,
leather, ABS/airbags/climate air/CD changer etc and it will do until a 156
SW gets cheap enough... We nearly bought a 156 sedan, but apart from being
50% more expensive that we planned, its boot is just a joke for a family
car. Not that the SW is much better (officially they are smaller), but they
have more flexibility with the load space. Shame they are still too new. But
I did get to sit in a 156 GTA, and hear it running.... yummmm!
1989 Fiat Croma (just sold - to American tourists about to cruise around NZ)
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