Spring rates for circuit racing

[gmsclassics]

When I first set up my Sprint I followed local recommendations to use lower spring rates than in UK due to bumpier circuit conditions in NZ. Using 350 lb front and 200/250 lb variable rear, coupled with special bilstein shocks, seemed to work well with the then standard (no LSD) diff. Over time simply by adjusting caster, camber and tyre pressures I managed to dial out most of the understeer. Subsequently stiffer front and rear sway bars have been fitted - approximately double the cross sectional area - and of course an LSD.

On the recently opened Hampton Downs circuit there are two very tight infield hairpins that have now brought the issue of understeer back to the fore. With my close ratio gearbox they are first gear, 5000+rpm. I now find I can only get round by taking the widest line possible, yet Porsche, Alfa and even MG can pop down on the inside taking a tighter line. I am lifting the inside front wheel with the softer rear suspension and am having significant front tyre wear. With the Dunlop DZ03G 195.60R14 tyres I run 22psi rear but to avoid tyre roll on the front these are up to 28psi. After the re-shelling, we set camber to -1.7degrees and caster to 4.5 degrees. Toe is zero. On fast flowing corners the car is stable, quite neutral handling and as fast, if not faster than most others.

I am wondering whether now I have the LSD, I should be stiffening up the springs significantly on the rear to try to keep the car flatter through the corner, increase front tyre contact with the tarmac and hopefully decrease understeer.

Would appreciate some thoughts and recommendations from others with circuit race cars.

Geoff

[SPRINTPARTS]

Hi Geoff,

I am no expert on suspension set up (wish I was) but understeer has never been a real problem with my own car. For a long while I struggled with oversteer until a major rethink sorted it out. It can be very difficult to set the car up just for 1 or 2 corners, ie how does the car feel in the other corners? If you get rid of the understeer in the hairpins, will the car still behave the way you want it to in the other corners?

Type of tyre and driving style has a lot to do with feel and behaviour of the car. Usually a locked diff will create understeer in slow corners, Is the understeer on corner entry, exit or the whole lot?

I have always found that the rear spring rates need to be around 1/2 of the front. I run a very small rear sway bar, for a long while I never used a rear bar.

Using 205/60x13" Yokohama tyres the current set up is 600lb front, 280lb rear spring. Front sway is approx 25mm tubular (adjustable) and rear is 18mm solid (adjustable). I don't use the sway bars for set up, just tweaking the "feel" of the car. Neg camber is 4.0 deg. I have always used more tyre pressure than most people and tend to use 32 psi front and 30 psi rear cold).

For a long while I used 400lb front and 200lb rear, but decide to up the rates. I like the way it behaves, generally neutral, with power on exit oversteer. If you do rush into tight corners too fast it can have some understeer (push for the Nascar people), but generally it is easy to get out of by applying some right foot.

Good luck and let us know how you go.

Mark

[DavePoth]

Are we talking about the corner right next to the start finish line, and the one just before that? I can see how that could be a problem if you're losing lots of time in that section.

I'd be inclined to go for a slightly stiffer front, but I reckon the first thing to try is A/B'ing the ARBs you've got. If it's lifting a wheel in the corner you are losing half of your front grip, and a softer front ARB will help that. What it'll do on the faster bits of the track is open for discussion though.

[gmsclassics]

Yes Dave, that is the worst corner although the one before isn't much better. The club circuit part has yet to be built so these corners are by far the slowest.

I have always had a problem that even with full throttle I cannot induce power oversteer - the back just digs in and sticks. Maybe the LSD is set too tight so it acts like a locked diff most of the time? Maybe I should just play with raising rear tyre pressures and see what that does.

Problem is that in fast corners it behaves very well indeed - stable and very fast. Suspension set up has always been a mystery to me - more like a black art! Thanks both for you comments - any other welcome. Will let you know how I get on eventually.

Geoff

This is a recent picture of the car in this tightest corner - you can see the fresh air under the inside tyre.

DSC05519B.JPG (210.57 KiB) Viewed 7445 times

[DF]

Looks to me that your front shocks are at the limit of there travel (extension not compression ) too soon as the
car doesn,t seem to be rolling much and lifting the wheel ???
Is the ride height correct for the travel of the shocks ?

[Dolly racer 33]

It is very difficult to advise on spring rates because there are so many variables. As a rule of thumb the rears need to be about 33% of the front rate but below 150lbs the rear is too soft, so if you start at 600f/200r & work from that. Tyres will make a difference, diff settings, roll bar, tyre pressures etc Rear tyres should be around 2lbs less than fronts, roll bar 1",rear roll bar in the bin.Diff set at about 65ft/lbs.

Remember you may never get your car to handle well on all bends without an adjustable front bar. Every cricuit usually has a 'throw away corner', if so always make this the slowest corner.

All these are only base settings, you will have to fine tune the car to suit you, the tyres & the circuit.

[trackerjack]

Ken this is valuable stuff, thanks as it helps lots of other owners too, I would imagine.
I have never considered chucking the rear A/B away!

[Carl]

I have never considered chucking the rear A/B away!

IIRC the Broadspeed rear ARB was little more than a piece of wire. The ARB had to be retained under the rules but it didn't actually have to do anything

[gmsclassics]

Changed the front springs from 350 to 500lb and raised front ride height by about 7mm.

Best times at Hampton yesterday was 1:24.503, an improvement of about 1.2 seconds (although weather was cooler). What an improvement in turn in at those tight corners - I was amazed - tight turn in plus controllable oversteercoming out meant for the first time ever I could take an inside line and actually overtake rather than be overtaken. If anything the car felt a bit more stable in the fast corners although I don't think it affected speed.

We normally run one scratch race and a couple of handicaps in our series. Started scratch on grid position 21 and made it to 10th by the flag. Great days racing, made even better as for two hours in the middle of the day they ran a special stage on the circuit of Rally NZ - and it decided to rain heavily just at that moment. Never seen so many offs in such a short time!

Thanks everyone for your advice - much appreciated.

Geoff

[SPRINTPARTS]

Geoff that is good news, now can you please tell me how to find 1.2sec a lap at any circuit and I will be VERY happy. I think that I need to adjusting the "nut " holding the steering wheel, but it is very hard to teach an old dogs new tricks.

Mark

[baz]

Looking to renew my front springs for stiffer ones . Where do you guys buy your uprated springs from?? A rough guide on $$$$?
Any help appreciated.
Baz

[gmsclassics]

If you are talking about fitting stronger springs to the standard shocks, then I can only suggest people like Rimmers who specialise in these cars - otherwise you'll run into all sorts of difficulties, especially ride height. With fixed platforms the springs need to get shorter as the spring rate goes up, otherwise the car sits higher and higher!

Ten years ago I went to a set of Bilstein shocks that were set up specifically for the car and incorporate a form of adjustable platform height. These take a standard size spring (narrower than std Sprint) that are readily available in all sorts of spring rates. These would be available through any good suspension shop that deals with race cars, proabably around the $100 mark each. However the full set up with shocks will cost around the two grand mark.

Geoff

[baz]

Car currently has uprated/lower/stiffer springs(about 15 years old) just looking to get new ones that are stiffer. I suppose around 500lbs.
Shocks are height adjustable Spax. Can Rimmers do springs this stiff?

[Mad Mart]

I got my shocks/springs from AVO. 400 fronts & 200 rears. Obviously these guys only sell 2¼" & 2½ I/D springs so you would have to buy a set of coilovers to suit. That's why I went the coilover route as there are plenty of spring rates/heights to choose from.

[Carl]

Where can I get a 1" anti-roll bar ?