Springs and all that (long)

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

Following up on the spring thread:

>>>And I recall a comment that if you cut a spring, then the car might raise-
>>>this is NOT true.  Yes, the spring rate increases linearly with the number
>>>of coils lost, but the rest height goes down the same amount...

>>Been there, done that. Don't agree, sorry.
>>Car - 2 litre Giulietta (Nuova). Rear springs cut too conservatively (owner
>>was cautious). Ride height went up - measured before and after! My
>>interpretation was spring became stiffer and harder to compress for the
>>same load and therefore the deflection was insufficient to lower the car
>>even allowing for the lost length. The springs were new.

>I don't believe there's any way you can cut metal out of a spring, put
>it back in the car, and have the car sit higher, assuming all the other
>parts went back in the same place.  When measuring the effect of ride
>height changes however, it's best to disconnect the shocks.

I was also skeptical initially, but if Les says it happened I'll believe
him! I think that I know why, and even got keen enough to do the maths to
(more or less) support my hypothesis.

The key to it is the shape of the ends of the springs. The last coil or so
is flattened (has much shallower helix pitch) than the rest of the springs,
so typically the free end is not very far away from the next coil. However
(as I found when testing my spring) most of the last coil is still active,
even when compressed against the spring seat (the spring works in torsion,
not bending, and can continue twisting even when in contact with the seat).

So if you cut only the end coil off you increase the rate by whatever
proportion that was of the total number of (active) coils, BUT the initial
length only reduces by sometimes not much more than the diameter of the
coil. So it depends on whether the reduction in initial length is smaller or
greater than the reduced deflection due to the increase in rate.  Once you
get past the first coil and into the uniform region of the spring the
undeformed length of the spring will reduce much more quickly. The actual
result depends very much on the proportions of the spring.

I don't have any data for 116 springs as in Les' case, but doing (very)
approximate calculations for a 105 GTV rear spring (a lot of coils, quite
flat at the ends), gave the result that cutting one coil off would INCREASE
the deformed length... okay, only by 0.2mm for that case, but the car would
definitely not be lower. 

105 front springs are very different to the rears: shorter, much stiffer,
less coils, greater helix angle. In this case I calculate a reduction in the
deformed length of about 20mm for removal of one end coil. 

However as has been pointed out by a few people, this is only part of the
story: the spring is well inboard of the wheel so there at least a 3:1 ratio
between the distance from the lower pivot to the centreline of the spring
and the line of tyre contact. 

So you might think that 20mm at the spring would equate to 60+mm at the
wheel?? BUT how come my car (which appeared to be stock, apart from 25mm
spacers at the spring pan) was only 25-30mm lower than stock??? I suspect
that because of its position, angle, and the geometry of the lower wishbone
as it rotates the line of action of the spring is not actually at its
centreline...??

The test loads for the front springs are 800-1000kg depending on the model,
whereas the wheel load is presumably more like 300kg (assuming 60% of 1000kg
car weight at front, half on each wheel). Fits with the 3x ratio.

Whatever, I guess that I'll find out when I do it. What I want to do by
cutting the springs is achieve the same height as the present spacers (which
is 25mm at the spring, 25-30 at the wheel), with an increase in rate. I'll
then see what effect removing the spacer under the spring itself has on the
ride height. At least I'm in the position to easily measure the deformed
spring length without having to put it in and out of the car. Hopefully I'll
get time to do some spring cutting and testing today. I'll update the digest
with the results.

One other related point from John H.:
<That does not address any concerns about differences in stiffness between
<his fronts and the previously modified by PO rears, but is he sure the
rears had
<"been lowered by some PO by cutting the springs"? In the USA the
<conventional wisdom is that 105/115 springs invariably sagged on their own
over the
<years. I have heard much about "improving" rear ride heights on
GTV-6s/Milanos by
<boogering springs, but hadn't thought that was a "problem" with 105 rears.

Yes they have been cut: there are less coils than there should be, the cut
end can be seen, and at full droop on the straps the coils are loose.
Subjectively the rear also seems stiff compared to other GTVs.  I have seen
mention of using cut down Berlina springs as a modification: as to whether
it is an improvement... or did Alfa know best...  or am I allowed to tune my
car for its particular usage.... I'll leave that thorny issue alone!

Phew...

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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End of alfa-digest V7 #329
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