bonesetter

iTrader: (9)

What changes would be needed to reduce oversteer from these present settings?

Car: '04 JDM
Suspension: Eibach coilovers, lowered 1" all around

Edit: Have 24mm adjustable rear anti-roll bar. Should i be looking at that instead of geo settings?

lookout

Geometry looks fine but I would add a 22mm front ARB. 24mm rear with standard front ARB can make for a tail happy car if rear wheel drive bias like a JDM Sti.

bonesetter

iTrader: (9)

Thanks for your reply lookout.

The Whiteline ARB is in the middle hole at present. I may change that to the front hole ('softest') before considering a front ARB as doesn't this effectively 'reduce' front grip?

Alternatively would a reduction in rear camber help? The 24mm ARB is quite stiff, would the rear wheels tolerate the little less camber as they lose less when loaded in cornering, or am I just off the wall with that?

lookout

Changing to the softest setting on the rear bar will help to reduce wash-out oversteer or traction related oversteer. It won't help roll oversteer.
Reducing rear camber is unlikely to reduce oversteer.
If you run decent tyres, a 22m front bar will increase front AND rear end grip as well as rear traction in most situations. Sometimes this is perceived as increase in understeer, but the reality is that the increased rear traction can now 'push through' the front end grip. The overall grip will be higher and the car will have a more benign balance.

If you go too big on front bar, you will reduce grip, but 22mm is by no means too big if you have decent tyres and road biased spring/damping rates.

dunx

iTrader: (3)

Adding a front ARB will make the front end work harder and reduce the oversteer.

I have a 22mm rear bar on the stiffest setting, adding the front ARB has made the car neutral overall.

HTH

dunx

P.S. I prefer my car now as it has more grip overall and is less "tail-happy", but still manages a slide on track...

bonesetter

iTrader: (9)

Sorry for putting you on the spot, but could you elaborate why you say the JDM Sti is RWD bias? It certainly feels like it is at the moment!

I've been searching all the forums and can't find much JDM specific when it comes to handling set-up.

bonesetter

iTrader: (9)

I'm trying to work through your replies (apologies for slowness) and what the different types of oversteer are you mention. I understand it as initial turn-in is all front end, then there is the squat phase before the power is reapplied at apex or thereabouts for exit.

If I take a bend 'correctly' (smoothly) all's well, any lifting of the throttle or indeed panic braking and the rear feels worryingly light (lift-off O/S right?). What does this correspond to in your descriptions?

bonesetter

iTrader: (9)

I've just measured the FARB and it is ~19mm so I'm presuming that is the std one.

I have changed the fixing to the front hole on the rear ARB today and am going to see how that behaves (Bank Holiday traffic today made it impossible to test drive).

Local roads are rural and need a degree of compliance I'm thinking

Thanks for your reply

lookout

Wash-out type oversteer is when the rear end starts sliding (on steady throttle) without the car rolling too much, due to the rear suspension frequency being too high. Normally this means that the springs are too hard, too much low speed bump damping, too much rebound damping or too much ARB.

Traction related oversteer normally occurs when the torque applied to the driven rear wheels is too high for the amount of grip available from the tyres and the road surface. If this happens excessively on a dry road, with reasonable tyres, it can have various reasons. Spring rate to high, rebound damping to high, low speed damping too high, ARB rate too high, worn dampers etc. or a combination of the above.

Roll oversteer quite often happens after turn-in, when the outside front wheel is loaded, before the weight transfer to the back happens due to throttle application. If the car does not have enough roll resistance at the front for the amount of grip available from the road and the tyres, the car will roll onto the outside front wheel. This results in the diagonally oposed, iniside rear wheel to unload most. When the inside rear wheel has lost most of it's weight, the partly unloaded outside rear wheel can lose grip, resulting in an oversteer situation. If roll oversteer happens a lot, a few reasons can be pinpointed. Not enough front ARB, not enough front low speed bump damping, not enough front spring rate, too much rear rebound damping, rear dampers being too short etc.

In my experience, the roll oversteer situation is normally the most violent and most difficult to control.

Wash out understeer can also happen for a variety of reasons.

-If a rear wheel drive biased car starts pushing on when exiting a corner under power, it normally happens because there is too much traction from the inside rear wheel, allowing it to 'push through' the available grip at the front. Normally, adding a small amount of rear ARB sorts this by creating a small amount of inside rear wheel slip.

-If a rear wheel drive biased car starts to understeer immediately during or just after turn-in (and you haven't tried to bend the laws of physics by going into the corner way too fast) and the car does not roll too much, the front suspension frequency is normally too high for the grip available between the tyres and the road. This means that there is not enough compliance in the car to effectively transfer weight onto it's outside front tyre, before it loses grip. This can have a lot of reasons. Spring rate too high, low speed bump damping to high, front rebound damping way too high, front ARB rate too high, not enough caster or the front of the car being too low (front roll centre too low and not enough front suspension compression travel).

Roll understeer happens normally just after turn-in when the car 'rolls off its grip' at the front due to having not enough suspension frequency for the grip available between the road and the tyres, resulting in there being too much roll (and again, if you haven't tried to bend the laws of physics by going into the corner way too fast). The front end of the car rolls so much that the outide front camber angle becomes positive and the inside front wheel loses too much weight. The reasons for this can include not enough front ARB, spring rates too low, not enough caster, not enough low speed bump damping, way too much front rebound damping or the roll centre being too low.

Of course there are many other reasons for a car to mis-behave, but I feel that my post takes up way too much space already :-). But all the above behaviours are a result of the grip available between the road and the tyres. This changes due to having very sticky tyres, a slippery road, cheapy tyres, grippy tarmac, rain etc. For a road car, the secret is finding a compromise that works well most of the time with the driving style of the pilot. Adjustable ARBs are a very easy way to get this compromise working well, quickly.

bonesetter

iTrader: (9)

Thanks very much for your reply. I now know what phases of oversteer to talk about and how they're created

I've been reading about front and rear roll centres, positive & negative rake, and ARB tuning - good stuff, but sooo many opinions What I need to do now is take a step back, to try and simplify what I need to reduce this car's oversteer further... What I am aiming for is neutral handling.

Where I'm at now:
I have changed the RARB (24mm) fixing hole to the front (softest) position. This has helped quite alot.

The car presently has negative rake (see pics). Forumite's are in two camps as to weather this is good or bad. It certainly makes the car squat on hard acceleration, especially coming out of a bend.

While highering the rear may not remedy the oversteer, might it give more confidence to the handling?

Is 24mm just too much for the rear ARB? Adding a front 22mm may well balance the rear but will this combo be too much for the road, especially as I live in a rural area with bad roads?

Has anyone wheel arch to wheel centres ride height suggestions?

I know a standard Sti has F386/R376. My current measurements are F345/R325

Thanks again

lookout

Although your car might be a bit too low at the back, I wouldn't set a lot of rake into the car. A few reasons for this, but the main ones are that you will be reducing rear droop as well as reducing effective front caster angle (sort of). I suppose raising the back by 10mm will be good thing, but your rear toe will probably need to be adjusted to suit.

I do think that adding an adjustable 22mm front ARB will more or less sort all your oversteer problems as well as create extra grip, without making the car more nervous or uncomfortable on uneven roads.

Just for good order, make sure that the springs have been fitted correctly. Harder at the front, softer at the rear. I suspect that they will have a number on them like 60-220-50. Where 60 is ID in mm, 220 is length in mm and 50 is N/mm or Kg/cm.

lookout

Mind you, at your front ride height, the front roll centre will be almost below the ground and might have some sideways migration. Normally this does create some strange handling characteristics as well as the distance between the COG and RC being a bit on the high side, increasing the tendency to roll at the front and therefore a possibilty to hang onto the bumpstop on the outside front during corners.
I suspect that the car will try to grip at the front and then revert to snap oversteer in some situations.

At those ride heights a roll centre correction kit is a must to address the dynamic geometry problems.

bonesetter

iTrader: (9)

....... .... so, I presume just setting 'stock height' will not work either?

jasonius

iTrader: (1)

Bloody hell lookout, you seem to know a bit about geometry , lol can't believe I haven't seen any of your posts before..

What's your background/how did you learn all this very informative stuff..?

lookout

That will do it as well, but you lose the low centre of gravity. Are your dampers long enough to retain a decent amount of droop? I suppose that with the 'off the shelf' available roll centre correction kits, the car will drive quite nice at 350-355mm front and 340-345mm at the back.

lookout

My work requires me to know a little bit about chassis dynamics

jasonius

iTrader: (1)

I've had some major power oversteer moments recently after rotating tyres f-r. Before this the car was unshakable, even in the wet. Tyres are GY asymmetrics around 50% worn, so I think I need to soften the rear ARB a little in order to handle the lower grip.. Oh and use less right boot..

Interestingly, the tread didn't seem much different f/r (4mm/5mm), but the shoulders of the fronts were not as sharp, as you'd expect.

bonesetter

iTrader: (9)

Just got back from a test drive (blast) out, the first since tweaking the rear ARB fixing position to the front setting.

Very pleased with the improvement. Conditions were warm and dry on the roads, with just the start of tree/leaf debri beginning to form in places. Car behaved very well indeed, and even when deliberately provoked mid corner by lifting-off and flicking more turn, was quite forgiving. A friend came with me and was amazed at the performance and said it's very near [being right].

There were two occasions when the rear slipped out, very slightly and nothing worrying, and recovered almost imediately with just lifting off and a hint of opposite lock. I would say this was traction oversteer. The front was NEVER compromised, never a hint of slipping and if the front hadn't been so planted, I would have said the rear slipping was normal.

I think that in the wet the picture would be one of having to go quite a bit more carefully, with regard to the rear. But all in all nothing like before. I would say in the dry, the way the handling is now, it is livable with, but I know with the deterioration in conditions with oncoming seasonal change, I am keen to get things a little better, if not completely right.

So, what about this plan: rather than fit too many trick bits all at once, what about going for one thing at a time? This way, I can see the change each thing makes, one change may 'fix' enough to make the difference and improvement I'll be happy with, and be cheaper, while being more of a slightly longer term fettle.

Question is then, should I go with this plan and probably fit the front ARB first and then the roll correction kit later, or is my plan a bad one, and should I just go for both straight away?

Cheers

lookout

At the car's ride height or slightly higher, I would fit a Roll Centre Correction kit first, to allow the geometry to work better. This will replace your bottom ball joints as well as the track rod ends. So any slight wear that is present on the standard items will be taken car of as well :-)

After resetting the geometry on the alignment machine, then drive the car again and see what you think. You will probably end up fitting a 22mm front ARB and have the rear ARB on the middle setting to get even more grip after you have explored the limit a few times.

bonesetter

iTrader: (9)

Took a gamble yesterday and ordered the arb before reading your post

No worries as I do intend to follow your advice and will order the RC Kit and fit that first, along with going a little higher and a geo

Quite excited now to see to the results

lookout

You're now on the slippery slope of trying to extract more and more grip and a perfect balance :-) A slave to chassis performance.....

Caster adjustable top mounts, subframe bracing, lock bolts, geometry experimentation, Micheline PS2 Cups etc. all waiting to be done :-)

bonesetter

iTrader: (9)

Oh dear, I suddenly don't feel as happy anymore

dunx

iTrader: (3)

Congested wallet, buy some R888's !

LOL

dunx

bonesetter

iTrader: (9)

What are those Dunx - tyres?

dunx

iTrader: (3)

Google Toyo R888.... soft compound road legal (currently !) cut slick tyre.

Only last about 6000 miles but by god do they grip. Not bad in the wet, but they HATE standing water.
I did a whole trackday, where others were shredding new road tyres, and I only removed about 1.5 mm, down to 1 mm now

HTH

dunx

P.S. Very expensive in larger sizes....

bonesetter

iTrader: (9)

Went out yesterday and ended up totting 150 miles of the local shire's best tight, twisty, open, bumpy, fast, flat, diving, yumping and cattle griding roads...

The JDM's overall performance was fabulous. The experience began to remind me of my Lotus Elise in the 'being in the centre of the car feel', and the sense of being connected to the car's activity around you. Tuning into any particular part became easy. You want to feel each coilover, you can. Rear arb and links, no problem. Cross country is devastatingly quick. Roads I've traveled many times seemed shorter and to pass more quickly. Speed could easily be kept with the combination of huge power and twin scroll.

So, fab points:
The Eibach coilovers felt very active, fluid and confidence inspiring.

Being smooth on the bends and in the corners the car was well balanced, with good front and rear grip levels. When the limit was reached although it was the rear which went out more, it was a close call as the the balance was almost equal.

The not so fab:
The Eibachs like to be up to speed to work well. Small bump compliance feels quite firm.

The car is still too low to be fully practical. The front splitter has taken some more knocks.

There is a rural lane which I commute many times a week, which is usually taken at moderate speed and has quite an undulating and yumpy nature with repeated small bumps. It is a problem for most performance cars, and seems to suit standard family cars with soft suspension best, where a higher speed than a performance car can be comfortably used. Not unexpectedly, the Impreza didn't fair too well here, worse than most cars in fact. The whole car lifted and was tossed around at the mercy of what came before it.

The rear is too easily unsettled. A hint of wallowing crept in occasionally, especially at higher speed, if say the road dropped away mid bend.

I feel the car capable of quite a bit more grip. There is an awareness of lack of synergy in the whole set-up. Not quite as fluid as it could be, some parts being pulled a little too much, others not active enough.

This week should see the next change in roll centre correction kit & geo....

lookout

One of the main reasons that your car won't like undulations or bumps is the ride height. The car will not have enough bump travel before it hits the bump stops. When you fit the Whiteline RCC kit, I would set the ride height to 350-355mm front and 340-345mm at the back at the same time. This compromise will keep it of the bump stops a bit more and allow more speed over undulations.

bonesetter

iTrader: (9)

The front Whiteline arb arrived today and I am now waiting on the RC kit which should be here Wednesday, for fitting then or Thursday.

Are there any changes which need/could be tweaked into the above settings when the alignment is done post RC kit fitment?

The geo does not need re-alignment post front arb fitting I'm presuming?

Shaun

bonesetter,
The 24mm on the rear is fine (on the softest setting for the road), with a standard ARB on the front. I run this on my Spec C and it is absolutely fine, even with -2.5degs front and -2.0degs rear camber. I even open the diff up on the road (2nd position from fully open) and it's great fun!

lookout

You could increase front camber a touch. A couple of minutes extra toe-in front and rear will probably suit you as well.....

Change of ARB does not require the geometry to be re-set.