suspension and springs

[Dave Adams]

This is copied from Andy Smiths web site and i am sure he wont mind it being copied onto here (i have spoken to him in the past about it) it covers S1/2's, B+'s and S3/4's he was (still is) in to racing his cars and has used his S4 on the track. it had a Rover V8 in it.

Dutton Suspension

Dutton suspension – well nothing spectacular or anything wrong with it for that matter – If live axles and triumph derived suspension was good enough for Colin Chapman then why not a cheap and cheerful sports car like the Dutton.

First of all the front end of the car.

Series 1 (Phaeton Series 2, B-type, Malaga etc.) has the full triumph set up and may even use the triumph coilover as well, Mr Dutton just says that the spring should be cut down to a free length of 11 inches and that the locating lugs on top of the triumph unit are cut off. Series 3 and 4 has a Dutton Ford set-up using a Dutton top wishbone and the Ford Escort lower track control arm/roll bar assy. – the McPherson strut is cut down and a couple of mounting lugs are welded to the side to accept a Maxi ball joint in the top. – The spring shocker can be any number of variants from an aftermarket Spax or AVO coilover to a Triumph Spitfire coilover.

Now the Back end.

Well what can we say, all the cars have pretty much the same set-up no matter what the Series. the only differences - which are very important - is in the methods of location. Series 1 ( B-type, Malaga)Has the same Ford live axles from the Escort MK1/2, Capri MK1, Cortina MK1/2 and Corsair - on these models the location is by 3 or 4 trailing arms and a panhard rod with suspension being taken care of by aftermarket coilover shockers or a coilover from the Hillman Imp. Series 2/3/4 These use the same axles as the other series cars but the location is by the donor vehicles leaf springs and telescopic shock absorbers.

Lets 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 MK2 or Escort MK1 axle and cart springs - these are rated at between 95 and 115lb/"

These are not the rates that the car sees 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 un-sprung 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 springs/shockers and locating arms are known as "un-sprung" 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 remembering this and taking it into consideration when doing the calculations.

So what does it weigh? What’s - the distribution? And what is the un-sprung 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 un-sprung 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 sees 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 equilibrium 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 principles I will post another with the link to the calculator

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

the spring calculator I wrote as a small Excel program - its nothing flash very basic.

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 impaired - 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 Alan’s 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 Sean’s S2 through it - this is a very light car - no frills and a nice Ford "Kent" crossflow - for a hard fast road/race setting it came up with front springs in the 210lb/" for a CPM of 115 - He had 300lb/" springs 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 = 160lb/"

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 Triumph coilovers are quite nicely matched to give a good sporting ride.

For fast roadwork less is more, as compliance with sensible damping is preferable over rock hard with dampers struggling to hold on to the spring.

Some final notes

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 - 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 off a coil or two - or even a half - 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 could it be simpler?

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 in a little more detail.

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%.

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 Sean’s 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 - 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 - however on smooth tarmac the front is stuck down the back end is another story and due a five link very soon.

[1midz0wwbl268]

Thanks, thats what i intend to do, actually even though my car started off with a 1300 crossflow, it at some stage did have a pinto fitted, according to wayne, a 1600 when he had it, that got sold on  , and it ended up with a 2lt , that i sold , when i decided that a 1300 would be a more cost effective machine , so theres a very real chance that they will be 300ibs 

[Dave Adams]

I have made my own spring rate tester and it now lives in my garage. its not 100% accurate but is certainly with in 5% or so. i could test a spring for you.

[Dave Taylor]

My Phaeton S3 as Escort cart springs on the rear, std escort shock, front imp coil overs, ford TCR spitfire top wishbone. no mod's, it goes in a straight line and corners very well under power, the ride is comfy.

Go on ask me spring rates, I'll save you all time... I DON'T KNOW, am I bothered NO!!!!

She goes like stink, holds her own from the lights with most things and puts the biggest daft smile on my face everytime I drive her.

[James Doulton]

Oh Dave, I feel sorry for you as you just don't understand the pleasure that an Excel spreadsheet can give you! Or is it just me? Am I a geek?

[Dave Taylor]

James I love a good Excel spreadsheet especially if it shows the commission I have earned this month. But driving my Phaeton and not looking at it sitting in bits in my back garden is what it's about. On saying that it's sitting in the back garden in bits at the moment.

[Pete Clayton]

I like it Dave, dilligaf!

[Andy Smith]

 Thought I had better offer a response in defence

Couple of points - the Triumph leverage ratio and the "original" Dutton escort TCA leverage were very similar - about 1.23 for the Triumph and 1.3 for the Escort IIRC - I dont think that aywhere above do I state they are the same so any assumptions are that of the reader.

Unfortunately - what has happened over time with TCA repairs is that the welders carrying out the work have not had any idea of how important the position of the bracket is and they have moved further and further inboard - The bracket should hold the shocker almost in contact with the upright - many do actually rub

As regard 300# springs - maybe, maybe not - its not a one size fits all application as the Ford non Dutton TCA's all have different leverage ratio's - may suit a car with a 1.6 ratio but not one with a 1.3 as my car was. Dutton did not want to keep loads of different springs and such on the shelf when easy available Triumph springs worked.

Regards the "Dangerous" statement - well working with suspension you should know that is an unfair statement as all it means is more preload required. - Would be softly sprung if your LR is 1.6 but not dangerous otherwise millions of cars on the road would have dangerously soft suspension.

The piece is not a all encompassing design guide its what it says it is an introduction and to make people think and hopefully appreciate that its not exactly a simple science 

As for Mr Staniforth MHRIP- well he works for me and many others outdated or not. 

In the photo Gallery there are two Melos in restoration take a peak at the TCA and the location of the Shocker pivot 300# suit both ? I doubt it - they would be very different rides and a 225 would certainly not be correct for one without some serious pre-load being needed.

Its not a one size fits all - which is what I was trying to get across in the article - you need to measure and decide what you want to use the car for.

Andrew