Following Mals request for information on the "other" board I will write a small piece on spring rates.

First off we will consider what was fitted as standard as Mr. Dutton intended.

Most of the front ends are designed around a spitfire coilover-shocker assy - and as standard these had 150lb/" or 180lb/" early and late spitfires respectively
now these give a fairly stiff ride as standard with a x-flow car and are in the firm sporting area for car of the Duttons weight - bear in mind the spitfire was an 800kg car.

Springs are rated on a measurement of there compression for a given weight and in the main its linear so on a 150lb/" spring if you place 300lb on it it compresses 2" - hence why you have open and fitted lengths - 11.5 inch open and 8 inches closed means a weight of 525lb has been applied.

Many cars also run a cortina or escort mk1 axle and cart springs - thes are rated at between 95 and 115lb/"

These are not the rates that the car see's as we have a leverage that needs to be applied and this varies on the cars with a triumph lower arm or an escort as the leverages are different. On the rear of the car the position of the spring along the axle effects the rate - I have seen some later S4 models move the spring further out along the axle compared with the S3 etc - this reduces the leverage and makes the spring "effectively" stiffer

Next consideration is the weight of the car - its distribution front and rear and side to side and also the unsprung component weight. The parts of the car that the springs support are known as "sprung" and all the other bits - wheels brakes axles and half of the weight of the srings/shockers and locating arms are known as "unsprung" As the axle wheels and brakes springs etc at the back of the car are almost as much as the chassis and body work its well worth remebering this and taking it into consideration when doing the calcultions.

So what does it weigh? whats the distribution? and what is the unsprung component?

For example lets say our S4 phaeton weighs in at 770kg all up with driver with its Pinto and that the distribution is approx. 53/47 front/rear now the unsprung component is going to change that to give further bias as far as the springs are concerned to the front. and reduce the weight - so lets say they see 650kg and its 55/45

So we now have some figures to work with 650kg = 1430lb and with our distribution the front corner see's 393lbs and at the back 322lbs - it wont be equal al round in practice but we have to start somewhere.

At the front of our S3 we have a standard Dutton TCA which has a leverage of 1.3 so for equilibruim our spring would need to be 510lb.

So back to our Spitfire spring at 180lb applying a 510lb weight will compress it 2.8inches to approx 8.5 inches.

Now you have a basic understanding of the priciples I will post another with the link to the calculator



Cheers



Andrew

:D

Part two.

Now i must give credit to Mr Alan Staniforth - who's book the "race and rally car source book" has provided the insight into the mystery that is car suspension systems. :D

the spring calculator I wrote as a samll excel program - its nothing flash very basic :blink: .

SpringCalc

Now the fundamental theory behind this is based on the wheel frequencies described in the above book.

Wheel frequency is measured in Cycles Per Minute CPM and its this figure that will decide the nature of the suspension on your car - by years of experience of engineers far more capable than I the following has been established.

For comfortable road cars the CPM should be 60-80
For sports cars the CPM range is 80-100
For racing cars the CPM range is 100-125 and up to 175 for certain applications

As an idea below 50 and suspension movement is so large the vehicle risks bottoming out and at 150 vision would be impared - F1 run 200-500.

Also from experience the rear of the car should be 10-15 higher than the front - If you want to know the full story as to all the whys and wherefores buy Alans book.

So gather the information on your car weights - suspension leverages - and the type of suspension firmness you are looking and feed it into the calculator - you will see what the different fields do by playing.

I ran Seans S2 through it and for a hard fast road/race setting it came up with front springs in the 210lb/" for a CPM of 115 - He had 300lb/" spring originally which gave a CPM of 138. - he chose 225lb/" which gives 120CPM and It is very hard and well into race car levels of stiffness.

As an idea the standard 180lb spitfire springs give - a CPM of 103 on a car of 600kg and a front bias of 53%.

So you want a guide as to rates

Car S3 with a Pinto - 650kg with a front bias of 53%

Front

CPM - 80 = Springs at 120lb/"
CPM - 100 = Springs at 180lb/"
CPM - 120 = Springs at 265lb/"

Back

CPM - 90 = 109lb/"
CPM - 110 = 134lb/"
CPM - 130 = 225 lb/"

And for those fitting 300lb/" springs - CPM = 127 - well into smooth tarmac only territory and not at all good on the road.

As you can see Mr Dutton was quite canny with his selections as the escort leafs and the Truimph coilovers are quite nicely matched to give a good sporting ride :o .

For fast road work less is more as compliance with sensible damping is preferable over rock hard with dampers strugling to hold on to the spring.

I hope this has been of use - It only scratches the surface with suspension and many more factors need to be taken into account when sorting out suspension rates ride heights and damping rates. On Seans S2 I still say the springs are too hard - be he likes it that way - I hope to show him with my S4 that more compliance is actually faster :ph43r: - We changed the spring rates and moved the top mounting so that everything lined up and worked as it should at the ride height that he wanted - the result is that the suspension is where it should be the chassis height is where he wanted it and the suspension is so hard it hurts :angry: - however on smooth tarmac the front is stuck down :ph43r: the back end is another story <_< and due a five link very soon.

Please leave comments if this or any technical post is of use is does give us authors the incentive to carry on posting.

Regards


Andrew

I like reading technical articles to try and understand some of the things that make kits cars handle/perform in certain ways. I don't think I'll have a hope of putting some of the stuff into practice as I can usually muck up an oil change :)
Keep posting your wisdom Andy. Maybe others will follow your lead. I'm sure there's people out there who find it useful.

A good informative guide keep up the good work. When i put my S4 on a weigh bridge a few years ago (1600 XF 4 speed box) including the driver it came in at 770kg with a pinto i would expect to be 800kg+. I have yet to weigh the Legerra but will let everyone know when i have.

With the article printed in the DOC mag and a couple of comments added by Piers I better complete this topic.

Piers raised a couple of good points.

1) - The angle of the shocker

Well to all intents and purposes unless the shocker is well over 30 degrees the angle makes very little difference - from 45 degrees over what starts to happen is the spring rate effectively reduces due to changes in the leverages - so you have reducing rate suspension - not at all ideal - however for Dutton purposes we can ignore it. - why i did.

2) - Cutting coils off increases the coil rate - not sure if I have commented on this in the past - there is plenty of mis-information on this and it all gets a bit confusing.

So I hope to clarify it here.

Firstly if you cut a coil off a spring or reduce its height by cutting you DO increase the effective rate.

An example - if you have a 150lb spring with ten coils and you cut a coil off the spring rate goes up to 167lb - the calculation is multiply the rate by the number of original coils then divide by the new number of coils.

150 x 10 / 9

However - you have also reduced the height of the spring from say 12.5 inches to 11.25 inches - as the shocker you have taken the coil off has an open length of 10 inches you have lost some preload - lets look at this

150lb spring - preload 2.5 inches = 2.5x 150 = 375lb

167lb spring - preload 1.25 inch = 1.25x167 = 208lb

This means that when installed on the same car with our leverage funtion of 510lb for equilibrium we get.

510 - 375 = 135 135/150 = 0.9" of drop (1.17" at the wheel) - shocker fitted length - 9.1"

510 - 208 = 302 302/167 = 1.8" of drop - Shocker fitted length 8.2"

So we have cut 1.25 inches from the coil and the car has droped another 1.1 inch on the shocker (total 1.9" at the shock and 2.5" at the wheel) and we have increased the rate by 11%.


Hope this helps


Andrew

Having done a bit more digging into the subject of shocker inclination on suspension systems the above article has a few errors in it :unsure:

Firstly any angle from vertical has an effect on the rate of the spring reducing its effective rate - this is quite a small effect at 10 degress but increases quite dramatically so its well worth investigation if your shocker is going to be mounted any where over say 15 degrees from vertical and you are seriously looking at acheiving a specific CPM

In rough terms if your shocker is inclined at 30 - 40 degrees you should look to increase the rate of the spring by arond 40% to get a similar average effective rate as you would with a vertical shocker.

If anyone is serious enough about the suspension design - I will send an ammended program that takes into account the suspension unit inclination. However its use requires far more inputs and information about your suspension system.

Cheers

Andrew

Andy
did you manage to find the "springcalc" to allow opening in Msoft Works ?
Regards
Steve

It was one of the first I wrote using Excel and when I tried to convert it it fell over - I spoke to Rob and he said he had sorted you out with Excel so I didnt give it anymore thought.

Drop me a personal mail and I will see what we can sort out

Cheers

Andrew

:)

Bit more tech info on the suspension front.

Imp front units are rated at 195lbs - used on the back of B+ Malaga and S1.

Cut down Triumph units as fitted to the above and the S2 are usually 180 but with a coil cut off these uprate to 200 lb.

Now this is what I would expect if the distribution was 45F/55R and the car had independant back end with lots of sprung weight but things just dont add up and it could explain the cars tendancy to throttle off oversteer.

My S1 runs much lower rated rear springs the same as a live axle SE Westie at only 90lb and even with battery and fuel pumps these cope adequately although with the damper wound off its a little its bit soft and probaly due to the position of the shocker being more inboard on the axle reducing the actual wheel rate.

Later S3/4 types used the Escort leaks and these are of a much lower rate than the Imp fronts for heavier type of car - the fronts remain pretty much the same rating.

Whats a pity is that the information on the rates that really work on these cars seems to be secreted away with people - well whatever I discover will be posted here and kept on my website.


Andrew