FE-FC Holden Discussion Forum

i am sorry to bring this up again but i have been reading a chassis suspension book by herb adams who is a respected auto engineer who  is an engineering consultant to goodyear,pontiac,koni and  many others.             he explains the benefits of chopping your springs already on the car, over buying heavy duty springs etc.
so what i am wondering,
 Is it really that bad to chop a spring ?
i know its illegal but why?

This is what I understand to be the mechanics of spring compression.
Force = spring constant by length of spring (F=Kx)
If you shorten (or cut) a spring, you lower the force it is able to withstand.
This is why professionally lowered springs are made of thicker steel because the spring constant is beefed up to counter the shorter spring.
So if you cut your springs you not only end up with a lower car but softer suspension as well.

This is not a good thing - you want lower car & stiffer suspension

Ken

thanks so much ken
thats exactly what i was wondering

not to counter anything anybody has said as i have no technical training, BUT i have lowered hundreds of cars (yes hundreds) by cutting springs and have had no ill effects they may be softer but not that i've really noticed usually put in a stiffer shocks, and when it comes to cornering i'de rather my cut springs than stock height springs, i've had well over a thousand cars mostly early holdens if i lowered them it was by this method i bought springs for 2 cars and both times end up cutting them as i wasn't happy with them

Read the question - most people have chopped springs in the past and beefed up the suspension with heavy duty shocks and sway bars.  Chopped springs improve some driving characteristics and make others worse. Herb Adams' book covers the positives in detail but perhaps doesn't talk as much about the characteristics which are not improved as it should.

The question asks "why is it illegal?"
I have endeavoured to give a short answer to a simple question and it doesn't matter how many thousand cars you have lowered - it still doesn't make it legal :P

Ken

Lets look at this from three fronts legality, engineering and practicality.

In reading all the legislatiion and regulations most which have been harmonised between the states there is nothing that says you cant chop coil springs with an angle grinder.  It says you cant heat or weld on hardened steering and suspension components.  This of course outlaws using an oxy.  Yes many years ago some modifiers just heated up the spring without removing it and waited for it to collapse to the right height.  Definitely form over function (read unsafe).

If you want to stuff your head into books about suspension you will find the formula about spring rate.  The formula includes two specific parts being the length and wire diameter.  The wire diameter is squared and that is why only a small increase can make a big increase in spring rate.  The relationshiip with the spring length though is linear and more importantly shorter springs (less coils) have a higher rate all else bieng the same.  So if you have a spring with 10 coils and chop the one coil off the spring will be 10 per cent stiffer.  On this basis chopping springs increases spring rate and therefore doesnt "weaken" them.
.

Of course you may want to consider a related formula and that is about wheel rate. (find that darn book again).  This is important for holdens as the spring is located about halfway along the bottom wishbone.  Again the formula involves a square sign meaning the wheel rate is not 1/2 the spring rate but a 1/4.  This means a spring rate of 600 lbs/per inch only provides a wheel rate 150 lbs per inch.  (If you had a strut front end the spring rate is the wheel rate).   What this means is with a HR front suspension that you chop one coil off you get about 2.5 per cent increase in wheel rate.

On a practical basis it would be very difficult to achieve an exact ride height by resetting springs.  This is why coil over set ups are so nice as they are infinetly adjustable.  For HR type springs and suspension the only way to get an exact height (without having a blacksmith in the back yard) is to have a spring which is marginally too tall and then trim to height.  This is still quite time consuming as the springs may have to trial fitted a number of times before you get it right.  OH yes little by little as you cant weld a coil back on if you were a bit keen with the angle grinder.

Hope this helps

John   

 

What about taking in to consideration the progressive winding of the spring at the ends which you lose when you cut the coil  ??? ???.......FC427.....

just remembered when i read that, that cutting springs actually makes them stiffer
there's plenty of laws that somebody somewhere who's prolly never driven a car has made up, that could quite possably be why it's illegal
don't want to get into arguments with people that have to do everything by the book  ::) some of us just do things (educated guesses) and it works and if that was never done there'de be no modified cars cause i'm sure it's how it all started

Ratbox

You are right. Sometimes we inadvertently are doing the right modifications even though we dont know why.  Of course any modification taken too far can cause issues.  Eg authorties want us to have 2/3 of the original suspension travel.  For a HR front end this means a miniimum 38mm bumpstop clearance (without making the bumpstop shorter).  In addition they dont like springs that fall out of their seats on full droop. 

Mark

The usual holden coils are wound flat at the end that fits in the tower and  therefore I would always cut the staggered end. 
I dont think I have seen an early holden spring (FX to WB) that was specifically designed to be progressive. Certainly no standard spring anyway.  However, a progressive spring gets its extra stiffness essentially through "binding" the close set coils at one end which in effect is shortening the coil making it stiffer. 

Please note this is general advice which can always be challenged.  There are heaps of suspension and steering books out there to help.  In regard to legality call you friendly engineer and seek advice from your state transport authority.   

John

The authorities also don't allow shortening of bump stops either ( at least in NSW that is ) so the 2/3rds of original travel is measureed with original height bumpstops.I have seen a NSW RTA spec sheet that a vehicle inspector showed me which lists the front and rear length of original GMH bumpstops for the various models.

twas the night before christmas and no need for petty bickering over nothing ;D ;D

merry christmas and a safe and happy new year to all 8)

thanks for all the input,i definately need it
i have been doing my head in as of late
i was chopping different springs to see how they would handle after getting the right ride height so i can go get some made.
so
 i have gone and bought stock hr springs(hr front end)gonna cut them till i get the right height then go to kmac and ask them for a slightly stiffer spring.i am a metal fabricator not a blacksmith or id do it myself.i really want the best suspension possible so any aftermarket spring advice would be greatly appreciated also
thaks guys
 merry christmas

mate i love the bickering
its hilarious
ho ho ho

For my two cents worth, (and yes I'm guilty of both chopping and heating springs in the past) the price of lowered springs is miniscule compared to how much we spend on these cars yet it makes a huge difference to how the car looks and handles, we think nothing of spending thousands on stereos, engines, chrome, paint, wheels, upholstery etc. yet we balk at spending less than $200 on something so critical and potentially life threatening as springs ???, the last couple of cars I've done I've bought heavy duty 2" lower on the front and had the rears reset 2' lower and another leaf thrown in which has looked great and given me no problems, by all means keep the debate going though it gives us something to read and keeps us amused. ;D ;D
                     Pedro

i totally agree its stupid that there is people out there as ya say that spend 3 grand on a stereo or whatever but are too tight for a suspension upgrade.
i am more so doing experiments with the chopping etc so i can actually get the best possible handling when i get some made                 (or if i can source the right height in an aftermarket spring off the shelf

The problem here is that the stock springs have a different compression rate to that of HD springs, as an example a stock spring cut to lower a car 3" then replicated in a HD spring may only lower the car 2"  :-\

As a moderator of the forum, despite the vast amounts of experience on offer, I can only recommend seeking professional advice or be prepared for a lot of fart-arsing around to get the right combo legally.

can't agree more ;D ;D

i agree my sujestions are only things i've tried or done and worked for me and I'M quite happy with the out comes and am still here to tell the tale
never gunna say they are all legal or by the book and anything i say if you have a go, it's at yor own risk ;D stinky i'm sure you've seen the ratbox rotissary ;) :o

and what a great method it is, done it myself a few years ago  ;D

i understand about the difference with a heavy duty spring ,.i have learnt that the hard way years ago. i just thought if i had a height with the stock spring i could hopefully talk to someone that knew what they were talking about,(a suspension expert)and they could tell me in tech terms what i really need in spring dimension,how many coils etc etc.maybe i am thinking too technically about this .i spoke to a guy at pedders in blacktown a while ago that  understood what was going on,but couldn't answer all my questions.
anyone know an expert?i wanna build a front suspension of my own one day so all this info helps

Some very good reading here.
http://www.eatonsprings.com/
Graham

Look at this page and find the link to calculator, it may be what your after?
http://www.ozrodders.com/board/viewtopic.php?t=33636