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Spring rate/ length 
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Post Re: Spring rate/ length
If I remember correctly, :? unsprung weight is 2/3 of the weight of the suspension arms, steering arms, driveshaft and brake lines and cables, plus the entire weight of the wheel, tyre, hub and brake assembly etc

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Tue Jun 03, 2008 3:19 pm
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Post Re: Spring rate/ length
Oh my gawd.

Dismember, weigh, re-assemble, set the suspension, tracking, all before Newark :(

Oh my gawd

Bob S.

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Tue Jun 03, 2008 4:06 pm
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Post Re: Spring rate/ length
Quote:
Oh my gawd.

Dismember, weigh, re-assemble, set the suspension, tracking, all before Newark


Nopey :D

Jack up the car, remove wheel, wind spring compressor onto spring until wheel flops up and down.

Put wheel back on.

Put wheel on bathroom scales = unsprung weight - or near as dammit.

The 1/2 and 2/3 numbers are for theoretical calculations. The above method gives you the actual.

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Tue Jun 03, 2008 4:44 pm
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Post Re: Spring rate/ length
Well that sounds a lot easier :D

Bob S

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Wed Jun 04, 2008 8:26 am
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Post Re: Spring rate/ length
OK.

This is what you need to calculate the required spring rates and lengths.


Data Needed
Total weight including driver.
Front / Rear weight distribution including driver.
To calculate total weight at each corner.

Un-sprung weight at each corner.
To calculate sprung weight at each corner.

Front Lever Ratio. (Wheel movement/Spring movement).
Rear Lever Ratio. (Wheel movement/Spring movement).

Front fitted spring length with the car at the correct ride height.
Rear fitted spring length with the car at the correct ride height.

Chosen front wheel frequency.
Chosen rear wheel frequency.

The natural wheel frequency (cycles /min) is your choice and determines the overall 'feel' of the suspension. As a guide it should be around 60 – 80 for a comfortable road car, 80 – 100 for a sportier car and 100 – 125 for a circuit racer on smooth tracks. (Alan Staniforth). The front and rear wheel frequencies cannot be the same or the car will oscillate in pitch (backwards and forwards). It is normal to set the front wheel frequency 10 -20 c.p.m lower than the rear.


You could possibly use the GTM quoted figures for total weight and front/rear distribution as long as you make an allowance for the weight of the driver (and passenger if you wish).

Unsprung weight at each corner you will need to determine as described elsewhere in this topic.

To find the lever ratio at each end you need to measure how much the spring moves for every inch of wheel movement. Popping a spring off will give you a longer range of movement and it's important to get this right. In the formula this number is squared so small errors make a big difference to the result.

Fitted spring length is the ideal length you want the spring to be when the car is sitting at your ideal ride height.

Wheel frequency is your own personal choice.


These are the formulae used.


Formulae
Wheel Rate = Sprung Corner Weight x (Wheel Frequency/187.8 )2 (squared)

Coil Rate = Wheel Rate x (Lever Ratio)2 (squared)

Static Spring Crush = (Sprung Corner Weight x Lever Ratio)/Coil Rate

Spring Free Length = Required Fitted Length + Static Spring Crush


As I said, if you can collect the data we'll spend a happy hour at Newark doing the sums.

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Wed Jun 04, 2008 9:18 am
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The Terrible Tims
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Post Re: Spring rate/ length
If only my maths GCSE O level was half as interesting as the above :lol: :lol: 8)

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Wed Jun 04, 2008 9:55 am
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Post Re: Spring rate/ length
Quote:
If only my maths GCSE O level was half as interesting as the above


Hardly, the above is second year automotive engineering degree level - I know, i've been through it. :?

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Wed Jun 04, 2008 11:30 am
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Post Re: Spring rate/ length
Bob have you got anti roll bars on your libra?

if you havnt might explain the high spring rates, if you fit softer springs you may find it of benefit to fit a antiroll bar to fine tune the weight transfer, just looking in to weight transfer myself and looks like i may benefit from fitting an anti roll bar to my coupe.

WEST

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Wed Jun 04, 2008 12:59 pm
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Post Re: Spring rate/ length
No, I haven't got anti-roll bars, my chassis no is 002, they had'nt thought of them in the beginning. :roll:
Phil Cope has 10in/300lb [rear] with anti-roll bars.
Paul Curtis has 12in/300lb[rear] with out bars.

I wish I'd done a maths O level :lol:

Bob S.

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Wed Jun 04, 2008 4:07 pm
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Post Re: Spring rate/ length
There's a formula for working out the stiffness of roll bars too! :D

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Wed Jun 04, 2008 4:17 pm
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Post Re: Spring rate/ length
John,
I bet you thought you had retired from teaching :!: :!: :lol: :lol:

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Wed Jun 04, 2008 7:34 pm
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Post Re: Spring rate/ length
I might as well do the rest while I'm on a roll.

Image

For a simple solid anti-roll bar of this shape the 3 dimensions shown are all you need to work out its stiffness.


Stiffness = 112822 x (Diameter)4 / Bar length x (Lever length)2

The result will be in lbs per inch so it can be compared directly with the normal suspension springs.


* the diameter of the bar is used to the fourth power (Diameter x Diameter x Diameter x Diameter) in the formula so you have to measure it really accurately. Small errors get magnified a lot.

* the lever length is squared in the formula so small changes in lever length can make a big difference to the stiffness of the bar.

* if you use a tubular bar then you can still use the formula but instead of (Diameter)4 you have to substitute (Outside Diameter)4 - (Inside Diameter)4


The big advantage to an anti-roll bar is that it only works in roll or single wheel deflection. When both wheels are deflected the bar has no effect. This enables you to run softer springs for a more compliant ride without compromising the amount of body roll produced when cornering.

A simple way to make the bar adjustable is to vary the effective lever length by having alternative mounting points for the links which connect the bar to the suspension.

Image

You can't go too extreme as the links need to be roughly at right angles to the bar during operation.
The links can be as long as you like as long as they are rigid. Rubber mounts or flex in the links will obviously reduce the effectiveness of the anti-roll bar.

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Thu Jun 05, 2008 9:27 am
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Post Re: Spring rate/ length
Data collected so far;-

Total weight [nearly full tank] 1791lb [814kg] with driver 1982lb [901kg]

Distribution front 44.4% rear 55.6%
front 785lb [357kg] rear 1007lb [458kg]
with driver front 880lb [400kg] rear 1102lb [501kg]

Corner weights NSF 385lb [175kg] OSF 400lb [182kg] NSR 497lb [226kg] OSR 510lb [232kg]
with driver NSF 414lb [188kg] OSF 466lb [212kg] NSR 522lb [237kg] OSR 580lb [264kg]

Unsprung weight [as per Pantera] front 140lb [64kg] rear 146lb [66.5kg]
per corner front 70.2lb [32kg] rear 73.2lb [33.2kg]

Sprung weight front 645lb [293kg] rear 861lb [391kg]
per corner front 322.5lb average [actual nsf 316lb osf 329lb]
rear 430.5lb average [actual nsf 409lb osr 452lb]

Sprung weight with driver, front 740lb [336kg] rear 956lb [435kg]
per corner front 370lb average [actual nsf 363lb rear377lb]
rear 478lb average [actual nsr 454lb osr 502lb]

Lever Ratio Front 1.5" - 1" [1.5" wheel movement = 1" spring movement]
Rear ditto

So conclusion;-

Front fitted spring length upper limit 8" [my choice], lower limit 8".
Rear fitted spring length upper limit 11" [my choice], lower limit 12".

Front wheel frequency [got to be sporty].
Upper limit 90cpm = 200lb/in spring. [Which is what I'm going for].
lower limit 70cpm =120lb/in spring. [Don't go this low, I started with 140lb/in back in 1999, tooo soft].

Rear wheel frequency [got to be sporty].
Upper limit 100cpm = 310/320lb/in spring. [Which is what I'm going for. I've driven two other Libra's with 300lb/in on the rear, one had 12" springs the other 10". It feels right].
Lower limit 90cpm = 200lb/in spring. [Don't go this low, I started with 190lb/in back in 1999, and the offside kept collapsing on left hand corners, 200lb/in were not much better].

Ok, when I've got the springs and fitted them, I'll give you an update.

And many thanks to gtmdriver for his help .

Bob S.

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Wed Jun 11, 2008 4:58 pm
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Post Re: Spring rate/ length
How are you doing with the rest of the data Bob? Is it worth bringing the calculator to Newark. Looking at the weather forecast for the weekend we won't have a lot else to do?

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Thu Jun 19, 2008 8:51 am
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Post Re: Spring rate/ length
I think I've got all the data, need to speak to you to see if I've got it right first?

I'll be bringing my calculator, so you can't get out of it.

As you say, looks like we'll be sheltering in the tent.
Just hope we can get it up before the rain.

Bob S.

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Thu Jun 19, 2008 9:57 am
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