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From the Editor

12th March 2002

by Julian Edgar

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I recently attended a night session at the Mount Cotton Training Centre, located just south of Brisbane. A private session where the facility was hired by a small group of about ten, the evening comprised extensive use of the centre's figure-8 skidpan and a shorter burst on their country road training circuit.

The skidpan is a touch unusual - like many, it's capable of being wet by a sprinkler system. But once the surface is wet, someone walks around liberally sprinkling diesel fuel on it from a watering can! This results in a surface that is slipperier than anything you'd find on a road - perhaps black-ice excepted. Since I live in a country where driving on ice is almost completely unknown, it was certainly the slipperiest surface I've ever driven on.

Sent out just two cars at a time - with about half of the track separating them - and with the skidpan tackled at very low speeds, it's both a safe environment and also one that gives very little tyre wear. That encourages lurid slides - in fact the skilful who were driving RWD cars could hold the tail out right around the track, flicking it from one side to the other with each change in direction.

Me? I am not good enough to do that - especially in my LS400 Lexus, which was by far the biggest car in our group.

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And while it can be argued that the skidpan has little relevance to on-road driving, in terms of practicing in extremis car control, it's very, very good. It also demonstrates in a way that cannot be mistaken what understeer and oversteer really feel like.

This was brought home to me when my fianc?, Georgina, was driving my car. Despite having driven - often quite quickly - all my cars from the six-second FWD turbo Daihatsu Mira (power understeer) through to the R32 Skyline GT-R (in standard form, power oversteer) to the 1995 Audi S4 (constant four wheel drive understeer) and then her own '91 Lexus LS400 (understeer in and oversteer out), I always found it frustrating when discussing handling with her. Why? Because while she could correct slides in own car that happened when breasting a crest at 100 km/h, she could never describe with clarity whether it was the back-end or the front-end that was sliding!

But on the skidpan being able to - literally - wind on two turns of lock and have the car plough on straight ahead, and conversely have full opposite lock on as the rear of the car tried to overtake the front, made the terms 'understeer' and 'oversteer' very clear indeed!

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For this reason I think that the diesel'd skidpan must make an awesome teaching tool for beginning drivers - a world away from dry textbook theory.

And of course, in a group that included a Porsche Boxster, Impreza WRX, Mazda MX5, Porsche GT2, Porsche 993 and Integra Type R, it was simply a ball of fun to watch the other drivers and then personally take part in the parade of spins.

But for me one of the most interesting aspects of the evening was driving my car on the super slick surface with the stability control and traction control activated. The '98 Lexus uses traction control to minimise rear wheel spin (and thus power oversteer) and in addition, has a Vehicle Stability Control (VSC) system that has the ability to correct understeer and oversteer. When either system is operating, a dashboard light flashes, while when the VSC comes into operation, a beeper goes off. It's therefore easy to separate the actions of the two systems. Pushing the VSC button switches off both the traction control and VSC systems.

Driven quickly on the greasy surface with the VSC button on, the car was immensely stable. It was slipping and sliding - but only a little, the traction control dashboard light flashing constantly. However, to my surprise, the VSC beeper was initially conspicuously quiet. It took a deliberate effort of throwing the car around to make the VSC system work - but then it wasn't as impressive as I had expected.

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It took me a while to work out what was going on. Unlike some other cars with stability control systems, at the onset of understeer the Lexus brakes both rear wheels. This is designed to slow the car and also transfer weight forwards, so that the front tyres bite into the road better. (Many stability control systems brake only the inside rear wheel, so that the car's nose is forced to pivot around. This effect is particularly noticeable on the FWD Mercedes A-Class.) But on the very slippery surface, the braking of both rear wheels didn't really have dramatic affect on the LS400's understeer - the grip levels were so low that the weight transfer didn't make much difference.

However, when controlling oversteer, the VSC - working on the front wheels - does use a single-wheel-braking approach. And that part of the system worked very effectively.

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So, given that - admittedly in conditions that would not ever be normally experienced - the car had bad understeer with the VSC working, it made me think of suspension modifications that would reduce it. And that brings me to a philosophical question, one that is very important in cars with stability control - but one I've never seen considered.

If you are making suspension changes to a car with VSC, do you optimise handling with the VSC operating, or with it switched off?

It's a vital question, because the answer will completely change the approach that is taken. For example, on the skidpan with the VSC switched off, the Lexus was fairly well balanced for such a large and heavy car. The weight transfer through sharp S-bends was a bit abrupt - and that tended to throw the progressiveness out - but all in all, it was pretty good. But switch on the VSC, and as I have said, the car just understeered everywhere.

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Now I am not suggesting for one moment that a car's suspension should be modified solely on the basis of its skidpan performance - on the skidpan the steady-state lateral accelerations are tiny, there aren't any potholes, there is plenty of room, etc. However, thinking about the handling balance does give rise to some issues.

For the sort of modifications that would reduce the VSC-on understeer could also completely unbalance the car when the electronic aids were not working.

For example, to reduce understeer, traditionally the modifications would involve a stiffer rear sway bar, and perhaps stiffer rear springs. (You could also soften the front bar, but the body roll of the Lexus is already fairly large so that wouldn't be a preferred approach.) So, on the road (forget the skidpan now) the car would understeer less, while the greater propensity for power-oversteer and trailing-throttle turn-in oversteer resulting from those mods would be quelled by the VSC.

In fact, for argument's sake, lets go to the extreme - a really big rear bar and very stiff rear springs. That'd make the car turn-in like a go-kart (well, not really but you get the idea) while the car's 210kW could still be put down through the rear wheels with impunity as the VSC controlled all that end of things.

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But what would then happen if, mid-corner, the VSC decided to die? Scenery, here I come!

Obviously Lexus engineers considered this question, and took the approach where with the VSC turned off, the car is fairly well balanced. In fact, except in that its roll linearity is poorer than Georgina's '91 model (something I'll discuss another time), the '98 model handles in VSC-off mode pretty much like the older model, which lacks any form of traction or stability control.

And that's fine if you have the VSC switched off. But I think that the VSC system is brilliant, and (except for deliberate testing) I'd no sooner switch it off when driving on the road than I would pull the ABS fuse.

(Certainly, I'd also not let anyone less than a professional racecar driver pedal my car hard on the road with the VSC switched off. After all, on the skidpan an instructor - who could fling his Mazda MX5 right around the figure-8 in a superb constant tail-out attitude - spun the Lexus almost immediately he tried to show me the best approach!)

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So for me, any suspension modifications would have to take a middle ground of working with the VSC, but at the same time not unbalancing the car to an undue degree if the VSC failed. For example, perhaps just a slightly stiffer rear sway-bar would do it.

It all reminds me of a car I used to own - the R32 Skyline GT-R. That car used a 2+2 form of four wheel drive - the fronts only started pulling when the rears were spinning, and the system was designed to actually reduce the torque which would have otherwise gone to the front wheels when the car was cornering. The result was an unacceptable amount of power oversteer.

As has been covered in one of our articles ["Godzilla Tamed"] my father and I designed and built an electronic controller that increased the speed and amount of cornering torque heading frontwards. The result was a car that handled very well indeed - I usually set the adjustable control knob so that corners were exited with just a trace of power oversteer. In fact, that car behaved very much like the next model R33 GT-R - so Nissan also agreed that the first model was too power over-steery.

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But the relevance to this story is that in the modified GT-R, both my father and I were very conscious of the fact that if his electronic controller died halfway through (say) a second gear corner being taken under power, the car would immediately spin off the road. In that car, the electronic 'handling aid' was indispensable - happily his design and construction were both done very carefully and to a very high standard, and the system always worked perfectly.

However, the scenario that it's the electronics that's keeping you on the road - rather than simple mechanical components like bolts and nuts and steel springs and swaybars - is a little bit disconcerting. But if you have a car with traction control or Vehicle Stability Control, and you modify the suspension without giving a lot of thought to the characteristics of the end result, that's a scenario that might well apply to your car...

Contact:

Mount Cotton Training Centre 07 3822 0033

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