Shortening front springs

[rotoflex]

JohnD wrote:

    Quoted from JohnD
    The spring seat should be 82mm above the centre of the bush eye.
    That's 3 1/4" - on your phot I think your's is maybe more.
    Even if it's only a little  more, the geometry exagerates it when it comes to ride height.

    Long discussion at : http://www.tssc.org.uk/index.p.....&catid=195#70944

    John

I am not familiar at all with the Vitesse, but I am as a compulsive wonderer wondering if the height of the spring seat was different on any of the dampers fitted during GT6 manufacture:
208022 Mk1
215398 Mk2/+
217591 Mk3
It only comes to mind because I have always wondered about 217804 (Mk3, Heavy Duty).

[AndyV]

Cutting springs shorter will not stiffen them, the ruler analogy is false as the ruler does not get stiffer – that is impossible as you are not changing the properties. But by cutting some off you need to put more energy in to bend to the same deformation as the long ruler. As you are not making the car heavier then the energy going in is unchanged.  If cutting it shorter makes it stiffer was true then cutting off 90% of the coils would give very stiff spring which is clearly not true.

Cutting the spring will not change its rate, it will just get shorter. and by bieng shorter it will have less travel so the coils will bind (touch) sooner.

I say cut the springs and give them a go, you can always buy new ones if you don’t like it. I did it long ago and ground a flat onto the top surface of the end of the cut coils to soften the toe a bit.  Worked just fine.

Andy

[JohnD]

Not quite true, Andy.

Stiffness of a coil spring is given by K = W^4 G/8ND^3

Where:
K = rate lbs/ins
W = thickness of steel wire (ins)
G = Torsion constant - 12 Million!
N = Number of coils (Add 1/2 a coil to the num ber that can move)
D = Diameter of coil to center of wire (ins)

As you can see, fewer coils make a spring stiffer.
But say, a 180lbs/in spring, with ?12 coils (a standard Spitfire spring)
Cut two coils off and its stiffer by 12/10, or 1.2, so 216lbs.ins.
Stiffer, but not by very much.
AND,
the car is now two coils (an inch and half?) closer to the ground, and will hit bumps that more often.

John

[uksnatcher]

Nice one John

Andy, the way i got my head round it was more hands on, actually cutting coils off a spring and compairing the force required to compress it, it gets harder to compress as more coils are removed.

Also the std springs are pre-loaded when installed to the strut by quite a compression... 10% ??, how does that effect the rating? As the uprated springs are loose/uncompressed when fitted to the strut as they are shorter in length.

One thing to note is by adding a toe by cutting the end coil off, there will be more force on one spot of the base/top plate and i assume more localised stress on the now new end coil forcing the spring to curve on compression, possibly rubbing on the shocker as the surface area or footprint of the spring has been dramaticaly reduced.  

[piman]

Hello UKsnatcher,

"One thing to note is by adding a toe by cutting the end coil off, there will be more force on one spot of the base/top plate and i assume more localised stress on the now new end coil forcing the spring to curve on compression, possibly rubbing on the shocker as the surface area or footprint of the spring has been dramaticaly reduced. "

Exactly, cutting coils without closing the end coils and flattening them as original is a poor way of doing it.
Compressing the spring to fit the damper does not alter the rate at all, basically it is taking some of the pressure before the car's weight is put onto the spring.

Alec

[JohnD]

The other way of thinking about it is to imagine that the coil is unwound, into a straight bar of steel.
One end fixed. you lean on the other to bend it.
A long bar would bend fairly easily, but start cutting it shorter and it would soon feel stiffer and stiffer.

Of course in a coil spring, the bar isn't bent, it's twisted, but the same imaginary model would hold true, if you keep the same short lever on the free end to twist it with.

John

[Freebird]

Right, today Joe and I braved the sub zero temperatures and fitted to Spax front shocks and apparently standard springs that good old Bodders1 provided for a very resonable amount of beer tokens. I say "apparantly" as they are a good 1" shorter than the ones fitted originally

The spring seats of the Spax are 75mm from the lower eye whereas the old shocks were 90mm - see pic below. This coupled with the shorter springs has brought the front down to sit at just the right height. Happy days. :) :) :)

Now I need to sort out the rear which seems to list excessively with a driver in the seat.
The threat of hyperthermia stopped play today though, so that will have to wait. Any suggestions as the cause much appreciated though.

Glen.

[Clive]

Good rear spring here  ;)

[Freebird]

cliftyhanger wrote:

Good rear spring here  ;)

I may well alleviate you of that  :)

[starman]

JohnD wrote:

Glen,before you chop your springs, have a close look at the shock absorber and the spring seat.    A lot of shockers, sold as "For Triumph model X" are nothin gof the sort=, and have the seat too high.  Your's look nicley new, unless you've painted them, and the seat looks a bit high to me,  so I fear you have been misled.

The correct height of the seat above the lower eye is well known, except I can't remember what it is just now!  Please do a search here - I'm sure that others and even I have quoted it in the past.

John

Agreed.  Here's a photo of a damper with the spring seat in the correct place (white damper on the left) and the incorrect place (black one on the right):

[starman]

Where your ride height ends up depends on spring rate, spring free length, geometry of the front suspension elements and sprung weight on the front suspension.  The suspension springs are preloaded at rest.  The relationship is not linear:



For the small-chassis Triumph lower A-arms, d1+d2 = 10.125 inches, d2 = 8.5 inches, d1 = 1.625 inches.  For the "lowered" geometry, theta is about 53 degrees.  I constructed a spreadsheet to calculate what free length of coil spring you need for any given rate spring to achieve the target "lowered" front ride height wherein the installed length of the coil is 7 inches, downloadable here:

http://auskellian.com/paul/links_files/spring%20free%20length%20calculator_v7.xls

(note: this is a spring free length and rate calculator for achieving a given target "lowered" ride height and is not a wheel rate calculator)

You can see that 10 inch, 250 lb/in springs and 9 inch, 350 lb/in springs are whole-inch free length solutions for later Spitfires with non-adjustable spring seat dampers (compare with TT4302 330 lb/in springs).  If you use springs stiffer than 350 lb/in, then either go with 8 inch free length ones and use shims to adjust height (preferably under the spring on the bottom spring seat rather than above the spring on top of the spring seat in order to retain full damper travel) or use 9 inch free length ones and live with a little less net lowering.  Of course, with adjustable spring seat dampers, you have lots of flexibility, in which case it's good to go with 9 inch free length springs for 350 lb/in and stiffer and then adjust the spring seat to dial-in the desired ride height.