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.

[Dave Adams]

As you can see his S3 used a Pinto 2.0L and he does not recommend 300lb/in springs, Vern seems to have a background in racing cars and for them300lb might be ok but even then softer will give a better result in Andy's  opinion.

From my personal experience reducing my spring rate and tyre size transformed my handling by almost as much as all the work i did over last winter. My car was a very different car once done, though it wasn't a bad car before i started.

[Big Vern]

Yes Steve your dampers probably are wrong, they were designed for a triumph herald/spitfire.

[Big Vern]

Dave, I have been involved in developing road car suspension for several years and some of the work I've done is on cars millions of people are driving around in every day!

Unless your racing, forget CPM other than to confirm you dont have identical frequencies front and rear - also forget what Allan Staniforth has to say on this as his stuff IS only for racing! and his calulation gives a circular refferance error in my computor program!.

I will calculate both types of front suspension and then get my boss to check it.

Andy S. changed his S4 front suspension to something totally different so its not relevant here as he had different motion ratios.

All your doing is confusing people.

Have you driven an S3 or melos with 300lb/inch springs on the front? I suspect not, so you cant pass judgement on what others have PROOVED to be correct!

Experience has shown a page full of numbers doesn't always give the suspension behavour you're expecting if it did why do so many race teams spend so much time on set up then!

[2zb8cfd8f5ygm]

I agree with Big Vern and it's important to remember where we are.

We have all read the technical books in our attempts to understand how cars work, but these are kit cars, and 'pile it high and sell it cheap' kit cars at that. Don't get too technical. As Big Vern says, there are so many compromises with cars, you can get lost in the technicalities that don't apply to us.

If there was an 'ideal solution' every car would be exactly the same.

[James Doulton]

I think that for each car, for each use, for each loading and for each bump or corner there probably is an ideal answer. The trouble is that some bumps are different and sometimes you have a different load so you have to chose a set-up that is safe and adequate under all circumstances and that is always a compromise. The roads near me used to be pretty good before last winter but now we have pot holes and ridges aplenty and so I would now choose a softer longer travel set-up.

Before I got my Melos I used to have a Smart with low profile tyres that cornered like it was on rails on a smooth surface, it was like driving a go-kart, it just went where you pointed it with almost no body roll. But hit a bump and it would get thrown off line, and you'd have to put your teeth back in! After a long journey your neck would ache from trying to keep your head on. They obviously didn't expect roads like we have in South Gloucestershire.

[Big Vern]

The smart going off line like that is down to the damping not the spring rate, most people make that mistake, they think they need softer springs but its the damping rate that's wrong.

[James Doulton]

Is that because most of the damping is on the rebound? I could see that that would make sense as the damper would stop the car from pushing the wheel back onto the road surface, leading to a reduction in the contact force and a resultant loss of grip. Or have I got that all wrong and the damper operate equally in both directions.

[Big Vern]

The stuff you've posted of Andy S's makes no sense. Firstly the suspension on the early and late cars is different with different leverage ratio's, he has assumed them to be the same at 1.3 whereas they are 1.2 for the triumph based and 1.45 for the escort based. 150lb/inch springs on a late car and it sinks straight to the bump stops. I know as I've tried it.
He says 127cpm is way too high but you have aprox 133cpm. As I've said already 300lb/inch on a late car is almost exactly the same wheel rate as you have (25N/mm for mine 27.7N/mm for yours) its the different leverage ratios that explain it which Andy S has failed to take into account as he's assumed they were the same. Most of his article is just a reprint of the Allan Staniforth stuff which is race car orientated. If I did as Andy S is suggesting my car would be sitting on the bumps tops which is dangerous!
As I said to start with 300lb/inch is correct for later cars and this is based on first hand experience.

[Dave Adams]

to be fair i have always maintained that my front end is too stiff and i have been looking to soften it to somewhere around 180lb/in

[Pete Clayton]

I think its quite a personal thing spring rates. I would go with what is collectively recommended and then work from that to what I liked best, like you are doing Dave. I have no idea behind the science (get an idea from what BV has said) but I know how I like it to drive!

[1midz0wwbl268]

Vern i have the original bike type on mine, koni coilovers with the long  narrower springs , i didnt change the dampers and springs  because they are still servicable, the dampers are on their highest setting,  At some point, i will change these for dampertech units, but will have to stick with what ive got for the present  what  are the likily rating of these springs?  

[Big Vern]

I've measured two sets of these and they both had 300lb/inch springs but they came from duttons with pinto's and yours was a xflo I believe. I'm sure there must be places in your area that could rate one of your springs to see what you have. Use the car with them as they are if you can at least to start with so you can decide whether you like the car with those spring rates.