Progressive Rate springs

[EFA]

I need to understand these a little more.

 

I have a problem with body roll on my car. Increasing ARB stiffness makes the car undriveable on the road (the back end gets very lively given the amount of power and there is a complete lack of traction on anything but smooth roads), as does increasing the spring rates beyond 190lbs.

 

150lbs gives the best traction compromise, but as I said, the car rolls too much. Also, with 400lbs of passenger and driver on board (such as when Count is in my car! 😳) the rear tyres do touch the edges of the wings on big bumps!

 

I am using the thin (3/8) ARB on the 3rd from stiffest setting. The ARB uses the over diff mountings as opposed to the under diff solution on newer (96>) cars. This configuration means the ARB links run at a significantly obtuse angle if mounted on either to the two stiffer settings. A 1/2 ARB (next size up) is too stiff and the back end becomes a nightmare. If I lengthen the drop links I can probaby address get the 3/8 ARB stiffer in a mechanically correct manner, but I dont think this is enough.

 

I think the solution may be progressive rate springs.

 

I see these as rated at two values example 90/180

 

I can only assume that the compliance in the 90lbs section is largely accounted for in supporting the sprung weight of the car, so the real springing (with the exception of a few which would aid traction) is supported by the larger if the two values?

 

If someone can give a better explaination of the effect of progressive springs (and also comment on whether they use them - particularly on tracks) I'd be most grateful.

 

Ta

 

Arnie

 

 

 

 

[Mike Bees]

The Caterham springs are progressive over a range rather than being dual-rate, so the more lard you put in the car the stiffer the effective rate gets (since more of the softer end goes coil-bound).

 

Mike

[Petrolhead]

Taken from R300.net curtesy of Simon Lambert

 

Std springs are generally good enough for anything. If you are more track orientated than road, then brave the 250lb front 215lb rear combination. Very bumpy (be warned), but flat. (Your current rates are 150lb front with a std progressive rear). Understeer city if you stiffen the front and stay soft o­n the back. There is the option of a 170lb front and the Caterham 21 stiffer progressive rear - that should be a good compromise. There are zillions of other spring rates o­n the market - you choose.... Thing to remember is, it is cheaper/easier to do the springs and the platforms together.

 

"There are three springs for the current rear:

 

-Standard progressive

-SV progressive

-Race linear

 

Spring Length Rate

 

Standard Rear 345 mm From 0 - 95 mm: 110 lb

Next 87.5 mm: 130 lb

Remainder: 200 lb

 

SV Rear 325 mm From 0 - 92.5 mm: 138 lb

Next 82.5 mm: 200 lb

Remainder: 246 lb

 

Green Front Linear 250 lb

 

Green Rear 280 mm Linear 215 lb

 

The std spring is 345mm o­n-shelf-length.

The SV spring, previously used o­n the uprated 21 suspension, is a stiffer progressive, 325mm.

 

This spring is an excellent compromise for those that want stiffer suspension, but don't want to be bounced into the nearest ditch by the race springs. I used to use them and may go back to them. The race linear (green) is 280mm long." - Simon Lambert

 

Hope this is of some help

 

NE7Club Web Site

R5 no 65 😬

 

Edited by - Petrolhead on 2 Dec 2005 12:08:09

[EFA]

Thx guys,

 

Sounds like SV Rear may be the way to go.

 

325 mm From 0 - 92.5 mm: 138 lb

Next 82.5 mm: 200 lb

Remainder: 246 lb

 

 

[John Howe]

Hi Arnie, I have a set of progressives sitting on my w/shop bench, waiting to be fitted. You are most welcome to try them out in the short term.

 

I am currently running 175 lbs (understood that was what R500 springs were rated as) but the car's rear end jumps all which ways, hence the planned change.

 

Anyhow, if you want to have a play, you are most welcome to try them for a month!

 

 

 

JH

Deliveries by Saffron, the yellow 230bhp Sausage delivery machine

[John Howe]

Arnie, a thought... the car originally had progressives fitted but was always bottoming out!

 

Hence the change...

 

 

 

JH

Deliveries by Saffron, the yellow 230bhp Sausage delivery machine

[Neill Anderson]

Springs & dampers are a real area that is very difficult to get right unless you have some experience, some access to test equipment and a helpful damper supplier or manufacturer.

On a road car the damper has the MOST significance after the tyres themselves and the geometry alignment and corner weights. The damper forces ideally need to be matched to the spring forces but there are ways around this a little but stiff springs need more damping than soft ones ultimately.

The very low speed area of the damper graph is of the utmost importance and it is this area where high quality components, machining and tolerances pay dividends; none of the cheaper brands can control the repeatability of this low speed area sufficiently for the low forces required by a light car. Any friction here represents a high percentage of the desired damping force and it is to negate friction that you end up paying Nitron and Ohlins prices.

The bumpstop should also be considered an integral part of the springing and damping medium. A proper progressive bumpstop and upper spring collar design will allow relatively soft springs and still support the body mass of two occupants etc. Effectively this gives a rising rate spring but the problems are the hysteresis in the bumpstop and the inability to have effective rising rate (or position dependant) damping. Note that dampers are inherently velocity sensitive, springs are displacement sensitive.

The damper bushes are also important, conversely softer bushes sometimes make the ride worse as they introduce an area of undamped motion until the excess “slop” has been taken up, and on direction reversals this lag is more or less doubled. Often rod ended dampers give both a better ride and better traction, but they can be noisey.

To answer your original question ALL dual rate springs are actually progressive in that there is never a step change from one rate to another. If you simply fit two different coil springs in series (i.e. one on top of another) all that happens is the overall rate will be non linear in a manner that depends on both the rates of the two springs as well as their lengths (or to be pedantic, the amount of travel available between the coils on the two springs). Initially the softer rate spring will compress more than the stiffer one, in direct but inverse proportion to their stiffnesses; the stiffer one will compress though. This continues as load increases and soon the softer spring will reach a point where it goes coilbound and then the overall rate will be that of the stiffer spring.

Progressively wound single springs do the same as a pair but cost less and are not adjustable. Proper progressive springs use tapered wire as then the stress distribution stays uniform as well and thus are lighter, but a pig to make!

The easiest way to experiment is to get some nice long progressive bumpstops and some simple nylon shims (say 5mm thick) that can be slotted to fit over the damper shaft above or below the bumpstop. These can be sued to bring the progression of the bumpstop in earlier by effectively moving it down the shaft. Note that doings this also reduces the ultimate “closed” damper length by shortening the available stroke. As such if the problem was one of bottoming out this may help or hinder the problem, depending whether the problem originally was one of clearance or forces.

Note also that as axle pairs of wheels rarely go up and down in perfect unison the rear anti roll bar can help stop bottoming out. The dampers can nearly prevent bottoming out all on their own, but ultimately they need the springs and ARB to support the final masses. It is almost impossible to ramp up the damper forces quick enough to control all of the bottoming problems without spoiling the low speed traction.

ANY play in damper ends or roll bar links or bushes is fatal, you will never sort the car out.

 

In the absence of a long and painful test program try some of the things others have tried but be prepared to find ot that you and your car react dfferently, driver inputs (being transients, just like the damper forces) have massive influence, as does tyre dynamics.

 

Neill A

 

 

New 7 Owner

1996 VX 2.0

[oldbutnotslow]

Wow ❗

 

Grant

 

😬 183 BHP of Black and 'Stone Chip' excitement. 😬

here

[Fathead]

hmmm sounds complicated (just slightly)

 

Good luck arnie

 

Tom

The most southerly uk blatter

😬Yellow 1.6 Supersport 😬 Photos here

[Petrolhead]

R500 have 150 front and std prog rears

 

NE7Club Web Site

R5 no 65 😬

[EFA]

Thx Neill, You certainly did a "Carmichael" there!

 

John, I'll give you a call soon.

 

Thx

 

 

[Peter Carmichael]

Oi!

 

Worth pointing out that Eibach do flat section springs that are designed to bottom out all at once, giving two effective rates. Sounds like a tricky idea to me, as Neill says: springs are position sensitive/dampers are velocity sensitive. If you have the Eibach setup, then you have four possible situations:

 

Bump: soft springing

Bump: hard springing (flat tender spring bottomed out)

Rebound: soft springing

Rebound: hard springing

 

You'd need to do some clever laplace mathematics to model whether any of that represents an advantage...

[EFA]

Afternoon Peter......

 

I was hoping you'd pop up.

 

I've been given a formula for calculating the effetive rate, but I am not convinced:

 

Effective rate before Tender becomes coil bound = (Tender Spring Rate * Main Spring Rate)/(Tender Spring Rate + Main Spring Rate)

 

Calc that with a 200lb/in tender and a 200lb/in main and the result is 100lb/in

 

Im looking to get an effective pre coil bound rate of about 130lbs, then a main rate of 200lbs. I'm bugger4ed if I can find any rates which allow that.

 

If I factor the effective rate x2 (which makes the 200lb/in scenario a little more likley) I get better numbers.

 

Mr Staniforths book does not help here! Help!!!

 

 

PS.... You should crouch a little in the TG studio. You were not hard to spot!

 

 

[Neill Anderson]

It is correct.

As the load (in lbs) is the same for each of the two springs then if they are both equal rates of 200 lb/in and we load them with say 500 lbs total then the deflection of EACH spring will be 2.5 inches (2.5 inches x 200 lb/in rate gives 500 lb load) and so the total deflection of the two springs together in series (i.e. one on top of the other) is therefore 5 inches and so the combined effective rate is 500 lbs load divided by 5 inches deflection which is 100 lb/in.

For an effective rate of 130 before the main spring becomes the sole support (i.e. once the tender bottoms out) then you need a 200 main spring and a 372 tender, and very careful analysis of the coilbound lengths and spring internal stresses. The Eibach range do square section true tender springs that are designed to go coilbound and still live. The flat thin section helper springs are more to keep the collars from rattling loose as the springs decouple at fully open damper lengths.

I am sure the Eibach website used to have a spreadsheet or calculator function that showed you examples, and cleverly only allowed combinations of lengths and rates that they made as stock springs. I don’t know the web address however, possibly a German site I suspect.

 

Neill A

 

 

New 7 Owner

1996 VX 2.0

[EFA]

Hi Neill,

 

Am I right in assuming the calculation above assumes the springs are equal in length?

 

If this is the case and I have a 200 main spring and a 372 tender, surely the 200 will become coil bound before the 372?

 

 

How would the rule apply if I have a 2" helper and 9" main

 

or a 4" helper and a 7" main?

 

I am, at the moment, completely baffled by the presence of any logic in this theory!!! *confused*

 

 

I went to the Eibach site but the tool they have ERS Wizard here but it is not available at present.

 

Edited by - EFA on 5 Dec 2005 20:32:10

[Peter Carmichael]

The springs go coil bound when they run out of travel - this is not related to spring rate. Eibach quote a load for tender springs at which they go coilbound. You need this to be appropriate for your static loading.

 

From the Sevens list archives (Jan 1999):

At the Autosport show I got a private lecture on spring rates on the Eibach

stand. It seems they had some involvement in Caterham's current springing

policy in between sorting out various F1 teams. My questions were:

 

Q: Why a progressive rear end?

 

A: By the time you match the ride frequencies for front and rear, you get

far too much travel at the back unless you have a rising rate.

 

Q: Ride frequencies. What's all that about?

 

A: The human body likes things happening to it at or around 1hz. The

preference of most British drivers is for the ride frequencies of their cars

is for 1hz to 1.2hz. 1.3hz is a very sporty ride. (lots of anecdotal

evidence of people just going wild on the ride frequency. Turbo F1 running

20hz. Current F1 around 5hz to 8hz). The rear ride frequency should be

higher, so that the rear catches up with the front having hit a bump (bump

activates the suspension at the front first.

 

Q: Why the soft springing at the front?

 

A: Having designed the progressive rear end, you need to match the front

end. The easiest way is to whack on soft springs and then bump up the

anti-roll bar size until it starts understeering like a pig. Back off a

bit, calculate up the front roll stiffness and then switch to having more of

the roll stiffness carried by the springs. Caterham only got part way

through this process.

 

Q: So does that means the front ride frequencies are too low?

 

A; They will be too low and there is too much reliance on the front

anti-roll bar for roll stiffness.

 

There was a lot more to this lecture that started looking at designing

progressive spring rates. This will form chapter two, later in the week

when I get the time.

 

PART2:

In this post I will answer the question: 'Why a variable rate spring at the

back of a Caterham?' - according to information gained from Eibach at the

Autosport show.

 

A fundamental of suspension design is the ride frequency. Choosing a ride

frequency decides how sporty a chassis setup is, which in turn decides how

rapidly input must be applied to keep the car on the straight and narrow and

also how rapidly the chassis can respond to input.

 

The rising rate springs on the rear of a Caterham affect this in two ways.

 

1. When the car is carrying a light driver, or a payload of heavy driver,

heavy passenger, heavy luggage and full fuel tank, the ride ferquency can be

maintained at approximately the same design value. Albeit with an

underdamped effect developing.

 

2. Because a Caterham has a low ride level, but is (in most applications) a

road-going sportscar. The first part of this statement suggests a ride

frequency in the region of 1.3hz. Choosing a spring rate to give this ride

frequency would give rise to too much spring travel and grounding-out. A

rising rate ensures that a target ride frequency is set but large suspension

excursions are controlled.

 

The consequences of going to a linear spring rate at the back of the car

are:

 

1. The ride frequency will vary between light load and heavy load. This is

particularly bad in the Seven design because of the loading variations being

concentrated at the back of the car.

 

2. If set up for a reasonably sporty ride frequency, there will always be

the risk of developing large suspension excursions and grounding when

running at the design load over bumpy roads.

 

3. If set up for a light load, the risk of grounding will be particularly

severe when carrying a passenger.

 

4. If set up for a heavy load, the ride frequency will be uncomfortable

when lightly loaded, but will still be susceptible to grounding when running

with a heavy load.

 

5. The alternative of choosing a spring rate which ensures that excessive

suspension excursions do not take place will give rise to an uncomfortable

ride frequency under all load conditions. A ride frequency of 2hz is

probably excessive except for track use.

 

6. Use the car on smooth tracks only, having set the desired ride

frequency. A lower ride frequency is more forgiving, while a high ride

frequency allows more driver iinvlovement and flickability.

 

Part three on this topic will concentrate on matching a suitable front end

to the rear, once I have completed the calulations.

 

 

PART3:

As many of you have observed, the double wishbone front end of a Caterham

has a geometry that guarantees a falling spring rate. This is exactly the

opposite of what you want. This is the area that is most critcal with

regard to ride stiffness because of sump/ground proximity. The standard

Caterham setup makes heavy reliance on the bumpstops and the latest

widetrack, 13" wheel ACB10 setup is not recommended for road use because the

bump stops don't come into play until your sump is ploughing a tarmac

furrow. It seems odd that the standard front end has such soft springs and

the Supersport suspension tries to achieve its aims using a fat anti-roll

bar.

 

All pretty unsatisfactory in my book.

 

For all the same reasons that part 2 applied to the rear of the car, a

rising rate would be ideal on the front of the car. Because of the

constraints of class regulations, most of what I describe will be applicable

to owners who mix recreational track driving with road driving rather than

to racers, but check the regs just in case. My lecture from Eibach got as

far as defining a dual rate suspension, but not a genuine rising rate. I

have done my own calculations looking at the components available in

Eibach's spring catalogue.

 

Eibach's twin spring solution (main and tender spring) allows an initial

soft rate followed by a stiff rate. The setup can be arranged so that the

car rides at the interface between the spring rates (i.e. where the flat

tender spring has just closed), or normally in either of the soft or stiff

regions depending on the application.

 

For example:

 

On rear drive track cars, it is normal to run a very stiff rear end with

high ride frequency. When the car is unweighted over a crest a linear

suspension rate would mean that the tyres would lose contact with the ground

and the car would lose traction. Running a twin rate set up, normally in

the stiff region would give the desired control over smooth sections with

maintained traction over crests. You would choose a tender spring with a

closure force just less than that exerted by the car at rest (this is where

corner weights become important).

 

This would work on a track Seven, but for roadgoing use you want a different

setup. You want the rising rate to prevent large suspension excursions

without inflicting a devastating ride frequency on the passenger and driver.

You could run a twin spring system, normally riding in the soft portion, but

this is not markedly different to planning to run on the bump stops. The

best solution is achieved with a progressive rising rate spring system which

gets softer as the suspension extends, stiffer as it compresses.

 

Eibach manufacture progressive tender springs which gradually increase

spring stiffness as they are compressed. For my analysis I have come up

with a solution using the 60mm spring range. To get a genuine rising rate

it is important that the tender spring never closes throughout its travel,

which is a different presumption to that used when designing a dual rate.

To achieve this, I could have chosen either of the two progressive tender

springs in the 60mm diameter range, because the closure force is more than

twice the static force exerted by the car's weight. One of these is the

60N/mm to 150N/mm spring. In familiar units this is 342lb/in to 856lb/in.

 

The next step is to choose a partnering main spring which moderates the

spring rate into the desired range and keeps the overall spring stack length

under 8 inches (suitable for use on the Bilstein dampers); this spring must

also not close under the expected force. It is also important that the ride

height can be set in the correct position, which may require a flat helper

spring (more stack length) unless the initial spring rate is soft enough.

It turns out that there is a range of springs which are suitable and allow

fine adjustment of the ride frequency. They are in the 80mm long range and

stiffnesses from 120N/mm (680lb/in) up to 180N/mm (1026lb/in) achieving the

desired results.

 

With these main springs, the spring rates are:

 

Main spring ----------Unloaded spring rate-------------Fully compressed

spring rate

 

120N/mm => 40N/mm (228 lb/in) ------------ 67N/mm (380lb/in)

180N/mm => 45N/mm (256 lb/in) ------------ 82N/mm (466lb/in)

 

Using the softer rising rate tender spring with a stiffer main spring gives

a more abrupt rising rate and a shorter spring stack and quite possibly the

ideal solution, but I cannot remember the rates off hand to list them.

 

Taking into account the Juno calculated wheel rate:spring rate ratio of 2.67

for the widetrack suspension and assuming a linear rise in spring stiffness

from the tender spring I have calculated the effective wheel rates,

displacement from full sag and displacement at 1g rebound acceleration. I

will post this once I have transcribed them, although I think a downloadable

spreadsheet may be a preferred route. Does anybody have a website for which

this could be a valuable addition? Ride frequencies depend on wheel load

and damping characteristics, so I have been unable to presume and calculate

values.

 

The biggest downside is undoubtedly the expense of the rising rate tender

springs ~53UKP per spring. But once purchased the particular

characteristics may be modified for individual purpose using cheap single

rate main springs.

 

Interestingly, Eibach used to distribute Bilstein products and they claim

that the black bodied dampers are rebuildable for a modest fee. I may well

investigate this once I have found out the SI units for damping constants

and I have done a bit more research.

 

Edited by - Peter Carmichael on 6 Dec 2005 00:45:02

[John Howe]

Thank you for that Peter but I wish I hadn't started to read it before bedtime.

 

Never mind I 'll read it again with the morning coffee. However, you seem to be suggesting heavier springs at the back than at the front, which, unless my memory is all to shot, is a the opposite of Caterham's usual set-ups.

 

JH

Deliveries by Saffron, the yellow 230bhp Sausage delivery machine

[Peter Carmichael]

Wheel rate vs. spring rate. The mechanical ratio of the double wishbone setup squared is responsible.

[captain chaos]

One thing that confuses me is whether i should be looking at adjusting the compression setting of my damper (or having it re-valved) or playing with spring rates and/or bumpstops.

I find my car Ok for the road and Ok for the track, HOWEVER, at the bottom of paddock hill for instance, particularly 2 up, she bottoms out and something grinds on the tarmac, probably the sump. In which direction would you go to prevent this? or should I acccept that I'm in that area of compromise?

 

Comments appreciated

[Jason Plato]

Arnie , are you still using your xtra wide track front wishbones without an anti roll bar ?

 

This will be affecting the chassis I would have thought

[EFA]

Dave,

 

I am, but this is not the problem. The issue is a compromise between the rear end bottoming out (on the wings, not the bumpstops!) and maintaining enough compliance that the suspension absorbs smaller bumps instead of the whole rear of the car being deflected on uneven surfaces - this only happens under power, but is very undesirable.

 

The front of the car is sprung medium hard and works very well. Remember many of the SLR racers used to normally run with no front ARB!

 

Figuring out a main and tender spring rate for the rear is the issue here.

 

I went to bed last night in the belief I was just starting to understand Neills logic. This morning I awoke to find Peter's latest submission. I think I'll be joing JH for coffee at this rate. Pass the Nurofen, I'm beginning to wish I hadn't started

 

 

 

Edited by - EFA on 6 Dec 2005 09:47:46

[EFA]

Right, I'm chosing to ignore Peters post for now.....

 

What I want to now understand is how to determine the static compresion of each spring in the tender/main setup. i.e. how do IU determine the length of each spring to be certain the tender spring has a limited amount of travel before becoming coil-bound and that the tender spring becomes coil bound before the main spring.

 

Where I am now confused is: Will the tender spring not be the spring with the lower lb/in rating?

 

Sticking with Neills values to give me an intial rate of 130lb/in, imagine the folowing:

 

Prereqs:

The overall spring length on my dampers is 11"

 

I use a 374lb tender spring 4" long, and a 7" 200lb/in main spring. This gives the 130lb initial rate. The car has a corner weight of 160kgs on each of the rear corners with driver installed. With this corner weight applied, how much travel will remain at the initial rate, before bias shifts to the main rate of 200lb/in? I'm after about 20mm.

 

As I said above, I cannot figure out how the 374lb spring will become coil bound before the 200lb spring.

 

Help!

 

 

 

 

 

 

 

[Jason Plato]

*tongue*or fit rear wheels with the correct offset 😬

[EFA]

You're a bit vicious today Wonky.

 

 

[seymore butts]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Go to Freestyle!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

🙆🏻