Cutting springs (long!)

[graham.hilder@domain.elided (Graham Hilder)]

In Digest no.740, David Ziglin asked about lowering cars by cutting the =
springs, and asked for comment on some advice he'd received from a friend =
on this topic.  I was looking forward to reading some comments from =
others, on this intriguing and usually contentious topic, but surprisingly =
enough I haven't seen any so far, so why don't I throw my two bob's worth =
into the ring and see if we can set the cat amongst the pigeons eh?   I =
must say firstly, however, that I am no more than an untrained amateur,  =
with very limited knowlwedge and racing experience, and what follows is =
only my opinion, although some of it is based on actually having done it =
in practice. I'd be pleased to hear what others think of what I say...... =
I may be talking nonsense, but if I am, then I'd like to have my ideas put =
right.

David's friend started off with:
"Many people tried to lower their motorcycle and sidecar by "snipping" off =
the springs thinking that the rig would handle better.  I know that young =
people have lowered their street cars this way for many many years to make =
them look better.
But, it is not advised in any serious situation.  It almost always makes =
things worse.  Here is why".

Then followed a description of how a coil spring is basically a torsion =
bar in a different shape...... nothing contentious there.=20

Then he wrote "If you cut off the spring, you actually shorten the length =
of the "bar" and you reduce the surface area of the spring (and) your =
spring rate goes up like a rocket ship".=20

My comment:  I'm not so sure about that. There's a formula for the spring =
rate of a coil spring which indicates that if nothing else is altered, the =
rate increases as the inverse of the number of coils.  E.g. if you cut say =
two coils off a 10-turn spring (and this is a fairly substantial cut!), =
your new rate is 25 percent above the old rate...... I wouldn't exactly =
say that that was going up like a rocket.

David's friend continued:
"A spring that is nice and soft at full length gets has hard as a rock if =
you cut 2 inches off.  That is two inches of length of the spring, not =
height of the coil". =20

This seems contentious!  Two inches of length of the spring might be, what =
shall we say, a quarter of a coil at most?  If your original spring is say =
5 turns, the new rate with 4.75 turns would be only about 5 percent =
higher, by my arithmetic. Can't really say this is "hard as a rock", I =
submit.

Next from David's friend:
"If you want to lower the height of the coil 1 inch, like you said
you wanted to do, you might have to cut 8 inches of spring off, depending =
on the diameter of the coil.  You get a vehicle that rides like a lumber =
wagon".

More contention....!!!!   According to the spring rate formula, if  I =
understand it, the rate's affected by the number of coils, not the wire =
length as such. So I go back to my example earlier, in which cutting say =
two coils off a 10-turn spring increased the rate only 25 percent....... =
not really lumber wagon territory?

As I said, I'm no expert, and I may be all at sea in the above. BUT in =
reality I have cut springs by a couple of coils on 2 or 3 Alfas and I can =
say from this limited experience that the practical result is that the =
change in stiffness is only slightly detectable by the Mark One backside...=
... that indicates to me that the formula appears correct in practice and =
the stiffness hasn't increased more than the order of  20 percent or =
thereabouts.

David's friend also noted:
"You could wind up with something that handles so bad, it won't corner at =
all.  And.....once you cut off the spring.......you must also find shocks =
that are exactly the same amount shorter.  And, the rebound and compression=
 dampening rates of the new shocks have to take into consideration the =
change in the spring rate."

Well this is no doubt true if you want the ultimate, but if you just want =
a bit of an improvement over what you currently have, you don't necessarily=
 have to go to all that trouble.  If your Alfa is getting old, then the =
original springs and shocks will have worn and weakened and will no longer =
be Anywhere Near the perfect match for each other which the designers =
intended........ chances are that minor mods such as a bit of spring =
cutting are only going to improve matters, I submit. That's always been my =
experience in the Alfas I've lowered anyway....... they all have handled =
SO much better. Sure, a proper professionally-matched package of springs, =
anti-roll bars, shocks, bushes, tyres and re-alignment would no doubt give =
even more improvement.......  but we're not made of money here I'm afraid, =
and bang-for-the-buck has to be the driving factor. =20

Thanks for reading this, anyone who's made it to the end! =20
Let the comments and contention roll in please!

Regards,
Graham Hilder,
New Zaland.
(GTV, Alfetta GT, Alfasud TI & Sprint).

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