RE: Springs and all that (long)

[M.Battley@xxxxxxxxxx]

> > >>>And I recall a comment that if you cut a spring, then the car
> > might raise-
> >
> > The key to it is the shape of the ends of the springs....

> still catching up, was there ever a definitive conclusion to this?  all of
> the above makes *no* sense whatsoever to me.

> cutting a spring will increase the spring rate.  cutting a spring will decrease
> the spring length, under any and all loading.

I'm not 100% sure I'm right, but I've more or less convinced myself 
that it is possible. If anybody in Auckland has got a spare 116 rear 
spring they don't mind having cut up I'm happy to try and prove it 
experimentally.

> if i've got a spring with 10 coils in it, and cut one off either end, the
> resulting spring will be shorter.  if i compress the before and after
> spring, the middle eight coils of the before spring will be the same length
> as the middle eight coils of the after spring,  and the before spring still
> has a coil at either end.  the before spring has to be longer.

BUT, the middle eight will not be the same length as they used to be: 
some of the ends of the eight will be inactive, so the eight coils 
will be slightly longer than they were. Yes, the same applies to the 
original 10 coil spring, however this had the last coil flattened 
off at each end (which was the coil that was removed). The length 
(height) of the inactive region will be greater for the 8 coil spring 
than that of the 10. I believe that because of the flattened ends of 
the orginal spring the length will not reduce as quickly as the rate 
increases.

What actually happens depends on the geometry of the particular 
spring. My (very crude!) calculations indicated that this behaviour 
is only likely for springs that are relatively long with a shallow 
helix angle, such as a 105 or 116 rear springs. In my tests on a 
front 105 spring even with only half a  coil removed the loaded 
length reduced, and with 1 coil off  I got a  20mm reduction in 
loaded length as predicted. I'd like to test a rear spring to see 
what happens.

As a side issue the behaviour at the end of the spring is one of the 
potential problems with cutting springs. The load transfer from the 
car to the spring is not as gradual as it would be originally which 
has the potential to cause local overloading of the spring or car 
spring seats, again depending on the geometry and materials of each. 
Cutting one end rather than both seems wise to me: at least this 
problem (and any instability) will be only at one end.

> however, if i cut a spring and then add a spacer of equal thickness of the
> amount  of the spring that was removed, then the ride height would be
> higher.  perhaps this is what les was alluding to?
You're correct, but it wasn't from what he's told me.

> should i throw out my physics books?
Nah, they're okay. But it's the subtleties of real life engineering 
that catch us out. The laws of physics still apply, however we just 
need to apply them to the geometry of a real spring, not an 
idealised one.

Do my explanations make any sense to anybody?? 
Has anybody else come across this behaviour?

Mark.
Mark Battley
Auckland, New Zealand

1974 2000 GT Veloce

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