Front Spring rate /ARB ratios

[zefarelly]

ok, the scenarios are this

front engined, Rear wheel drive, 750 kg car, 140 BHP, probably ( I'm guessing) about 65/35 weight distribution ( its a cortina ok! )

pretty much everyone, inc me, seems to run single leaf capri 2.8 springs on the rear. 140lb

I run 500lb front springs and a 1" ARB, and the cars understeering slightly testing in the dry at Goodwood. I have 2 options, go to to 450lb springs ( free and easy as I have them) or go 7/8 or 3/4 ARB ?

other set ups I know on Loti Cortina are

600lb and 7/8" ARB
550 & 7/8
700lb ! 3/4 ARB

a few run soft, ie 350lb, but theyre slow !

My general philosophy is change one thing at a time so I think 450lb springs and test to see if its better ?

any one got an opinion or anything constructive to offer ? ( other than buy a proper car !)

[Chris Y]

There's nowt wrong with 'Tinas!

I would go with the 450 lb springs first, then fiddle with the roll bar later if you're still getting understeer. Just don't go too soft on the front, otherwise you may start picking up the rear tyres on heavy cornering. The ARB will help with that, of course.

[Al Weyman]

500lbs and a 1" bar! To me that sounds way over the top, I am only running a 350lb spring on my Black Iroc ( 53/47 weight split but 300lbs of V8 up front) now it is pure coil over and I can tell you that is definitely not understeering, quite the opposite as it happens. This set up was supplied to me by a top American race tuning company. Maybe your rear could be a tad firmer (maybe I should rephrase that). I found on my old Sunbeam Talbot a front spring rate of 190lbs was about right and the final setting I went for, similar car to a Cocotina.

[zefarelly]

I know I'm close, form the LC's which are faster, BUT like you Al, they have a lot more oomph under the bonnet which will induce oversteer even with light carpet slippers on.

the other option is to run 4.50 rear tyres rather than 5.25, the down side of that is I'll shred them as fast as fronts !

[phoenix]

Originally Posted by zefarelly

I know I'm close, form the LC's which are faster, BUT like you Al, they have a lot more oomph under the bonnet which will induce oversteer even with light carpet slippers on.

the other option is to run 4.50 rear tyres rather than 5.25, the down side of that is I'll shred them as fast as fronts !

Reducing tyres will reduce overall grip - so don't go that route! It is possible that the rear could go a little stiffer to dial out the understeer but without losing grip. Maybe a little more bump stifness on the rear dampers and a little less on the front would do the trick?

If not, two coil springs are cheap to buy (and easier to change?) than the rear springs so I would soften the front. 450lbs represents a 10% reduction so a good place to start, but it is difficult to know how much this will reduce the weight transfer without knowing the proportion of roll stiffeness given by the springs and what proportion of roll stiffness is given by the anti roll bar.

But one thing should be remembered when comparing different anti roll bars. All other dimensions being equal, a 1" solid anti roll bar is over 3 times stiffer than a 3/4" solid anti roll bar and 1.7 times stiffer than a 7/8" solid anti roll bar. So maybe softening the springs by 10% will not be a noticeable as you might think. If you have the option of changing the anti roll bar (i.e. you have one!) then maybe try that first?

[zefarelly]

Thanks Phoenix, I'd already calculated the different cross sectional areas of the ARBs, I only have 3/4 and 1" though, and I have 450lb springs, so thats what I'll try first. we had tried firming up rear dampers etc and changed tyre pressures, and it did help, but 1 thing at a time, and note the results, I just need a test day now !

[b1ackcr0w]

I think your motto of "try one thing at a time" is the wise route.

The first thing that springs (no pun intended) to mind is that a change in your suspension should have an effect on tyre wear rates. How long is your typical race?

[maddogf3]

[QUOTE=b1ackcr0w]I think your motto of "try one thing at a time" is the wise route.

Too True, One thing at a time and see what the stop watch says .Just had a look in Allan Staniforth book he has info on Talbot sunbeams with a 5 link rear axle and says that the works team were experimenting with 300 front springs and 350 rears for tarmac rally's That would get me thinking about stiffen rear up .It " can " stop weight transfering to the rear as much and get front doing a bit more .I must admit I find the experimenting the best bit about motor racing . Try it and see what happens dont always go along with what the other are doing . On the track its usualy Stiff is best . I would see if I could scrounge some springs and try it with different bars and springs at a track that you are consistant at . Did you say understeer was when turning in or mid corner ?

[R59]

Zef,

How far off the pace are you?

The rears - my ol' mate's Mk1 Escort was running 200lb progressives on the rear, they helped "make" that car. With the 140lb singles, the understeer was there, with the 200lb, it became much more neutral, but not so easy to drive as it was on a knife edge, but faster by miles.

So, maybe, going stiffer on the rear is the way to dial out your understeer.

You have to think different.....

The obvious is "understeer at the front - reduce the spring rate"

But, in reality, it may be a case of stiffen the rear to compensate.

As you said - a test day is needed - lots of spares to try - fast spanners to make best use of your time - and an analytical head on your shoulders....

Rob.

[maddogf3]

Zef
Thats two people now suggesting you stiffen it up at the rear me quoting what the works did with a rally sunbeam and racing 59 with some info on mk1 escorts If it were me . I would get Stainiforth's book and work out your wheel frequency ( It gives you a good theoretical balance between front and rear) .I think you will find it pretty well spot on . I am no good at maths and got my niece to do a spread sheet for me .
All the paddock rumours about spring rates then become interesting when you have a bit of theory up your sleeve .
" Dont listen to gossip , work it out for yourself "

[phoenix]

Originally Posted by zefarelly

Thanks Phoenix, I'd already calculated the different cross sectional areas of the ARBs, I only have 3/4 and 1" though, and I have 450lb springs, so thats what I'll try first. we had tried firming up rear dampers etc and changed tyre pressures, and it did help, but 1 thing at a time, and note the results, I just need a test day now !

I sent you a PM on the 11th asking for some numbers so I could offer you some properly calculated options. Still haven't received the numbers, so I can't do the calulations!

[breezeblock]

i would try more front camber i had the same prob and it cured it

[zefarelly]

I did reply Phoenix but I just don't know exactly, car weighs circa 750, I will try 450's back on the front, but may then try playing with the rear, I can't do anything with camber as its all fixed.

[phoenix]

I'm trying to get a weight for the back axle from another source - your figure sounded a bit low to me! When I have got a 'second opinion' I'll do the calcs!