K9VYN

Quick question for those in the know. Can stiffening the rear of the car too much generate or increase understeering characterstics?

My setup is: Eibach springs (-25mm), Whiteline anti-lift kit, MRT front and rear (adjustable) anti-roll bars and uprated drop links/bushes. Powerstation set up the geometry, but this was some time ago. My tyres are pooh at the moment. The rear bar is now adjusted to its maximum and (although I probably don't corner fast enough) I don't appear to get any oversteer.

My tyre pressures were 35f/33r PSI and I suffered quite a bit of understeer in the wet. I have now lowered them to 30f/30r PSI which appears to have reduced understeer (certainly better in the wet now). I suspect 30 PSI is a tad low. So my questions are:

1) Should I increase the tyre pressures and or should they be even or higher at the front to improve steering response - say 32f/30r (from a unwanted understeer perspective)?
2) Should I adjust the anti-roll bar back to its midpoint (less stiff)?

I'm basically after as neutral handling as possible and a little more confidence in the AWD system on wet roads. Its been a while since my geometry was checked/aligned and this will be on the cards, but I'd like to see what I can do myself first.

Cheers,

Kevin

[Edited by K9VYN - 11/14/2003 8:21:43 AM]

RichiB

Kevin:

Re-fit the OE front roll bar and give it a try.

Rich

7 Foot

Agree

TopBanana

Set the rear anti-roll bar to its softest setting first though. You could also look at your geometry

Leslie

Did Powerstation do the bumpsteer removal mod Kevin or was it before they had developed it. If so I would advise getting that done and you will not believe the difference.

Les

ozzy

Stiffening the rear too much will reduce mechanical grip to the front, so if you go too far you may induce more understeer. What size is your front ARB?

Also, lower tyre pressures will give a slighty larger contact patch and help get some grip back through the tyres, dialling out some of the understeer. If you increase the pressures, you may take a step back - depends on the tyre really. My Toyo's seem to like a higher pressure than my previous Goodyears (+4-5psi of a difference). Increasing the pressure should open up the tread pattern, so with some tyres it would help with wet weather performance. Bit of a juggling act as you can probably tell by now.

Just how "poo" are your tyres? The ARB's will make the tyres work much harder, so it could be them that's letting you down.

My RB5 has Eibach Pro-Kit springs, Whiteline adjustable ARB, Whiteline front & rear drop links and a Cusco front strut brace and handles very neutrally. I can get it too understeer, but this is usually on tight, wet corners when my Toyo's let me down.

I can happily get the car into a 4WD through most corners and it's a joy screetching around a roundabout

It maybe worth re-fitting the front OE ARB and setting the rear to the middle position.

Stefan

superstring

Posted by ozzy:

AFAIK stiffening the rear promotes oversteer and INCREASES front grip.

I agree with the idea of fitting the original front bar and backing the rear off to the middle position until you get used to the new setup. Before you do that though, are you sure you have the bar set to its' stiffest position now, not its' softest?

Also, you might find this thread interesting:

http://www.scoobynet.co.uk/bbs/thread.asp?ThreadID=221456

Good luck, John

Lars

I read a lot of wrong statements and try to cler up
Stiffening the rear ARB, move load from rear outside wheel to rear inside wheel, In total this will decrease rear grip and promote oversteer.
If you are running street legal tyres ( Not R or Racing tyres) higher tyre pressure gives more grip. Do not exceed max hot pressure and never go below manufactor recommended pressure.

I know all the stories about high pressure deform the tyre, and you will only have contact in the center of the tyre. This was true 40 years ago. To day we all use Radial tyres and it's no longer true.

ozzy

Hi folks,

Sorry I didn't explain myself correctly. I agree 100% that stiffening the rear should increase front end grip, but there is a limit to the tyres grip if you put too much extra force through the front suspension.

Thinking about it, the rear tyres are still much more likely to lose grip first and give violent oversteer, but the charactaristics would depend on the manouvre - corner turn-in, mid corner or exiting on power.

I'd be interested to hear an explanation from Lars as to why higher pressures increase grip. I've found pressures vary between tyre makes/compounds. My Toyo's seem to like high pressures, but my Pirelli's and Goodyears like a good 4-6 psi lower pressure to work well.

Logic would argue that lower pressure = greater contact area, but I guess this isn't the be-all and end-all.

Stefan

superstring

Hi ozzy

Well I'm not Lars, but here's my 2 cents...er, pence, worth anyway.

As far as I understand it, increasing tyre pressure creates a stiffer sidewall and therefore less deflection in the sidewall when a side (cornering) load is applied. Tyre slip angle is directly related to tyre deflection and therefore, for a given cornering force, slip angle is reduced which, in turn, translates into more grip. Or, in other words, the vehicle can handle higher cornering forces before the tyres reach their maximum slip angles and run out of grip.

John

JIM THEO

Read carefullythis thread and you'll understand how it works!
JIM

K9VYN

Guys,

thanks for the replies... stupidly i hadn't checked the email notification box and so have only picked up your responses today. all very useful pointers, cheers. i did have the TSL 4 wheel alignment/geometry setup yesterday as well as fitting a front strut brace just before.

i have noticed the wet road understeer tendency has been reduced by some way despite some technical setup issues with my car which meant they couldn't get it spot on. i am currently running 34/32 pressures in the Technics tyres. i'm sure a better quality tyre will improve things further still but i have loads of tread to see out on these first.

today i may try some fine tuning - i.e. backing off ARB to midpoint and playing with the tyre pressures, but i won't go too far as things have improved greatly. thanks again for all the advice/pointers.

Kev

911

I hill climb my STi V3 so it gets push to the limit and almost beyond...
The cars suspension is well modified and with quiet radical(Austrailian) geometry.
I used to run Whiteline arb's front/rear, rear set stiffest setting, Toyo's at 35psi all round and alk on the front.
This seemed to give the best all-round set-up wet and dry.
Graham.