suggestion for spring rate

[Dom]

Having ditched the rusty old xr2 I am now planning preparing my shiny 2001 Fiesta ZetecS for racing. However i'm a little unsure on what spring rates I should be using and don't fancy buying several different sets to test.

I used to use 280lb front and 800lb rear on the XR2 - is this a sensible rate on a slightly heavier modern fiesta?

[Alex Hodgkinson]

Any half decent suspension unit manufacturer should be able to tell you what spring rates to run. If they can't tell you, don't deal with them.

[terence]

Interesting front to rear rates,800 on the rear?

[Tim Falce]

The fronts may be too soft, on our old XR2 we use 300lb on the front. Although you do want it to be oversteery 800lb rears may be a tad stiff. Try looking at Zetecinside.com for some useful tips.

[Casper]

I was talking to a suspension bloke the other day and he said it is not all about spring rates, the bump stops have to be perfectly matched to the spring rate for it all to work properly. It is an area I knew about as everyone knows what happens when the bump stops take over but did not know that it was possible to match the two to give good progression. You live and learn.

[Notso Swift]

That is fine if you plan on running it on the bump stops, which is common in sedans because of the compromises they have, but it is not a goal in itself.
Koni sell a range, and it is not uncommon for people to mod them as well.

As for the first question, my gut would say the front is a little on the low side but since we do not have your front/rear weights for each car it is a bit academic. I presume you have struts all around on both cars. Still, contrary to fashion I always work to have as little spring as I can get away with.
Why don't you try and borrow some springs for testing, then commit once you know what you want.
What I did ion the past was select from a range of "cheap" springs and them bought some quality ones once I narrowed it down, you can make the range quite wide, 15% steps is fine and then plump for something close. Remember springs do not go off and come in handy later.


EDIT when I say they do not go off, I mean when they are not in the car, they do when you use them!

[tristancliffe]

Springs shouldn't go off noticably when you use them either, as long as you don't crush them too far. If you overstress them then, like all springs, they will lose some rate, but keep them within their design limits and they should last nearly forever.

[Dom]

done some searching and found the weight to be around 600kg front and 300kg rear. Would spring rates of 300lb front and 400lb rear sound sensible?

[JohnD]

Allan Staniforth (81 yesterday! Happy Birthday, Allan!) sets out in his "Sourcebook" how to calculate spring rates, using as the target the wheel frequency you want, depending on the application. He uses only the suspension dimensions and the sprung weight as the inputs, but it would be unfair to his method to try and precis it here. Buy yourself a copy of one of his books!

John

[tristancliffe]

Dom- What are your suspension ratios, and what wheel rates/frequencies are you aiming at?

[terence]

Originally Posted by Dom

done some searching and found the weight to be around 600kg front and 300kg rear. Would spring rates of 300lb front and 400lb rear sound sensible?

Sounds about right,time for testing then.

[Notso Swift]

tristan if it is struts not enough to worry about 0.95 and the like. Also note I said cheap springs, I find the steel fatigues a lot quicker than the quality nickel chrome alloys.

300/400 seams like a reasonable place to start. Beg/borrow/steal a few options, but you will be fine with that (locked diff up front?) Get an ARB or two while you are at it.

[Dom]

thanks, the car will be racing in a standard class and the only thing that can be changed from oem is springs/damper to coilovers. It is macpherson struts allround with front rollbar but rear beam with no rollbar.

[VT-R]

Originally Posted by Dom

It is macpherson struts allround with front rollbar but rear beam with no rollbar.

Its not Mac strut all round if its got a rear beam.

[Splatz the Cow]

If you have "Driving Ambition" by Doug Nye, about the McLaren F1, it is in there pg 192. One of Gordon Murray's note pages is shown in detail, and it is so easy to follow, I taught myself, by that one page. And hey, the McLaren turned ok..

[Y.N.M]

From my understanding, what tristancliff said is right, your better of knowing your wheel rates and deciding the spring rates from that.

Also I've heard that on a front wheel drive car like yours it can be a good idea to run a stiff rear so the rear tires can do more of the cornering work and leave the fronts to use more off their availible grip for putting the power down, (correct me if I'm wrong) so the rates you suggested look like you were heading in the right direction balance wise.

I'm an ameture in all this so forgive me if i'm wrong guys

[phoenix]

Originally Posted by Dom

thanks, the car will be racing in a standard class and the only thing that can be changed from oem is springs/damper to coilovers. It is macpherson struts allround with front rollbar but rear beam with no rollbar.

Are you allowed a rear ARB?

If so I would soften the rear springs to get a decent wheel rate (e.g. if the rear wheels carry half the weight of the front wheels, halve the front spring rate) and add a very stiff rear ARB. If you can't fit a rear ARB, throw away the front ARB and adjust the balance with the spring rates alone. The front ARB is only fitted to make the car 'safe' on the way to the shops..... With a front wheel drive car you want the front wheels on the track all the time - but it doesn't matter a jot if you lift a rear wheel.