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Antony
03-12-07, 11:01 PM
Spring rate calculations I have been measuring up my front and rear suspension arms and calculating the Wheel rate eg if I move the wheel 1" then measure between the shock eyes for the change in movement there, with the shock out of course!

front suspension arms measure from inner point to outer balljoint 13.66" and inner point to shock mount 9.84" the outer joint is 3.81" further out than shock mount. The new rear arms measure 14.76" and 12.59" and 2.16" respectively.

We fitted 500Ib front springs, 300Ib rear springs and Bilsteins valved to suit, found the front didnt move with me standing on car so went down to 400Ib fronts and it feels nice. I guess this is really trial and error at the track and harder is better with slicks and limits suspension travel and inbuilt problems dont show as much, but I have to be practical and have it so there is some compliance for the road.

My main question is with these type of spring calculators,
Pro-Formance Shocks, Inc - Spring Rate Caculator for Indepndent Suspension (http://www.proshocks.com/calcs/imotion.htm)

I input the above information and end up with a spring rate of 2.08" I am not sure of the desired balljoint rate but that should be similar to wheel rate and I entered 1" in that box, so I am wondering with my limited mathematical brain what 2.08 is (208Ibs?) and if it is that would be to light.....!

mylesdw
04-12-07, 12:23 AM
Ant: the spring rates are in lbs/inch so the springs you mention will compress 1" if you put 500 lbs on them and so on. The spring rate fields in the calculator show these numbers so you enter the desired rate at the outer balljoint and it will tell you the spring you need. My Ram with an iron Chevy has 350 lb springs at the front so your 400 lb springs will give a pretty rigid ride considering you car should end up quite a bit lighter.

Running the calculation 'backwards' shows that the ball joint rate on mine is 165 lbs. My car weighs 2550 lbs roughly equally distributed so each corner weighs 638 lbs. So at a standstill the spring will compress 3.8" (638/165) assuming no preload and pulling 1g vertically will compress the spring another 3.8" and cause the oil filter to hit the ground (in my case!!). This figures seem to match what I have seen in practice.

Your 400 lb front springs give a ball joint rate of 190lbs so a good starting set of rears would be 260lb springs (giving the same balljoint rate front and rear)

wilf
04-12-07, 08:59 AM
Has anyone ever played with/tried variable rate springs to prevent grounding? Are they even available except as dedicated to OE applications?

Antony
04-12-07, 10:44 AM
Myles,

What is the Desired outer ball joint rate?
I think I am over complicating this or missing something!

A chap once quoted to me springs only hold the car in different conditions including braking etc, the suspension designed properly with good geometry is the key to good handling, and softer or harder springs is more for drivers preference. I think that is fine but heavy springs obviously limit body roll and with good tyres the suspension travel is limited and can result in lower lap times........!

mylesdw
04-12-07, 07:02 PM
Has anyone ever played with/tried variable rate springs to prevent grounding? Are they even available except as dedicated to OE applications?

Wilf: I believe that rising rate springs (non linear) are just seen as 'too complicated' for aftermarket applications; I'm not sure if manufacturing them is difficult but I guess just about every application would be a custom wind because you couldn't just use seat preload to adjust the ride height and corner weights because that in turn would alter the spring rate for that corner.

Progressive bumpstops (rubber) are probably the best solution to grounding. I think the likes of Merlin used to sell them. Rubber is very non-linear in compression so it is ideal for the purpose.

mylesdw
04-12-07, 07:25 PM
Myles,

What is the Desired outer ball joint rate?
I think I am over complicating this or missing something!

A chap once quoted to me springs only hold the car in different conditions including braking etc, the suspension designed properly with good geometry is the key to good handling, and softer or harder springs is more for drivers preference. I think that is fine but heavy springs obviously limit body roll and with good tyres the suspension travel is limited and can result in lower lap times........!

I don't agree with your chap. The harder the spring the closer the car becomes to all unsprung weight! Imagine how badly the car would handle with solid suspension; every lump in the road would cause the car to bounce about and the tyres would spend a lot of their time unweighted and useless. The same goes for excessively stiff dampers. The softer the spring the better the tyre will follow the road but the roll and the suspension travel increases and the car feels soft and vague. So I reckon springs need to be as soft as possible to limit suspension travel within sensible limits and damping needs to be as soft as possible to keep the tyres in contact and stop the car bouncing. Extra roll stiffness can then be added with the ARBs. US chassis tuner Herb Adams uses this philosophy very successfully.

So to your desired BJ rate; here's my fag-packet calculation for getting a start point:

1. Set the car at desired ride height.
2. Measure the amount of suspension travel to fully compressed; let's say 2" for your car.
3. Weigh the car; lets say 1000 kg for yours.
4. Cobras a roughly 50/50 weight split so assume the corner weight is 1000/4 = 250kg
5. Let's say we are happy for the suspension to hit the bump stops when the car experiences 1g downforce. At 1g the car appears to weigh twice as much as its static weight so the corner sees an additional 250kg. So we are happy for the wheel to move 2" up from design ride height when 250 kg is added to the corner weight. So the desired BJ rate is 125 kg (approx 275 lbs).
6. Run the 275lbs though the calculator to see what springs will give you this desired BJ rate.

Antony
04-12-07, 07:39 PM
Works out at 573Ib springs, that will be solid!

That brings wheel rates to mind with moving the front wheel 1" up, the shock eye to eye moved from 310mm to 285mm = 25mm, the rear moved 18mm, so that would give a very different percentage?

mylesdw
04-12-07, 07:58 PM
Works out at 573Ib springs, that will be solid!

Sounds too much; perhaps that figure of 1g for full compression is not realistic!

Just checked: 1g IS considered typical for normal road irregularities, so the factor that suggests stiff springs is the 2" travel not the 1g load.

mylesdw
04-12-07, 08:17 PM
The geometry is quite different at the back, so feed in those dimensions and the desired BJ rate (275lbs) to find the required rear spring rates.

Antony
04-12-07, 09:34 PM
The rear works out at 375Ib springs!

Rear 14.76" length arm with shock out from inner pickup 12.59"
Wheel rate could be out a further 5" on the rear as the front is 4" out approx, I think this is a bit like how long a piece of string is!

Front arm length 13.6", shock out at 9.84"

I know with 500Ib front springs and the nose of the shock on bump was stiff, there was no movement in the front and that is with me standing in the engine bay but without radiator/oil and basically just the engine/trans.

Might see you will be in Wed Lunch.

Antony
07-12-07, 11:21 PM
I had another go with the calculator and it is questionable if you enter decimal points, so entered data in millimetres and came out with,

front 529# (from 573#)
rear 377#

That is going to be hard in the front with this car being a radial tyred car, the back is quite soft for weight transfer, so will stick with the 400/300 springs and go up if necessary and try and strike a compromise between road and track.

Some people have two sets of shocks/springs which is ideal but I am sure I can get it a bit hard on radials and it will be okay on slicks........!