Mohd Azad Jasmi

By: Azad Jasmi

[Recommend this Fotopage] | [Share this Fotopage]
View complete fotopage

Saturday, 14-Oct-2006 04:02 Email | Share | Bookmark
Oversteer - Part IV

Let's try to conclude this topic. It's one of my favs. Oversteer is hard to describe if you've never experienced it before. However, it's more exciting, dangerous and fun than understeer so we'll cover it first. From above, picture a car approaching a sharp right-hand corner. When oversteer occurs, rather than following the path of the corner, the rear wheels lose grip and follow a tangent from the current position in the corner. In extreme cases this means that the car will leave the road backwards as the rear wheels head towards the outside (left hand side) of the corner.

Oversteer can occur in three ways; a. Through excessive power or wheelspin,excessive corner entry speed or lift-off oversteer. Each of these causes are examined below.

Excessive Power Oversteer through excessive power only exists in rear wheel drive cars. It can occur when accelerating away from a sharp corner, typically when leaving a T-junction. Applying too much power and generating wheelspin causes the rear wheels to lose grip and stop following the path of the corner. In these situations the front wheels remain on-course and the car pivots around the front wheels. If deliberately provoked the car can be made to turn 360-degrees around the front wheels - known as a "doughnut". Correcting excessive power oversteer at low speeds is best achieved by reducing power to a modest level (but not closing the throttle completely). At the same time, and as is common to all forms of oversteer, you should also apply corrective steering by turning the front wheels such that they point in the opposite direction. Corrective steering is commonly referred to as opposite lock, although this is misleading as it implies a maximum amount of corrective steering.

Exactly how much corrective steering to apply depends on the angle of oversteer and is a matter of practice makes perfect. If in doubt, apply a little corrective steering and add more until the front wheels and rear wheels are travelling sideways at the same rate. When the right level of corrective steering is applied the car will be travelling sideways in a four wheel drift. When in a four wheel drift, grip will be restored when enough speed has been lost. In the oversteer through excessive power situation, this will happen immediately because stopping the wheelspin (by reducing power) will restore grip.

Once grip is restored it is important to undo your corrective steering. If you don't "unclock" soon enough it can result in the car snapping into oversteer in the opposite direction - known as "fishtailing". Once a car starts fishtailing it can be extremely difficult to recover as momentum increases and the amount of oversteer in each "flap" increases until a spin occurs. Learning how much corrective-steering to apply (and how quickly to unwind) is best done on an airfield day. Here you can spin, fishtail and four wheel drift to your heart's content with loads of space and nothing to hit. I would strongly suggest attending an airfield day or two before going to a motor racing circuit track day where there are gravel traps and armco barrier to hit if it all goes horribly wrong.I always had "fish tailing" during my tenure in the Elise. It's pretty hard to drift in the Elise when I started 2-3 years back.

Once in a four wheel drift, if excessive power is sustained then you can perform a power slide so long as the correct balance between throttle and steering is maintained. More throttle will increase the angle of oversteer therefore requiring more corrective steering. Every car has a critical angle of oversteer beyond which it cannot be recovered. The Elise, with it's relatively short wheelbase and rear-biased weight distribution, has a low critical angle of oversteer and can't cope with more than about 30-40 degrees before a spin is inevitable. I haven't had enough experience on the 4WD on tarmac as most of my 4WD "sessions" were on gravel. I had a WONDERFUL esperience praticing Subaru STi rally prepared car with Mr. Stewart Reid in Australia. I learnt a lot about driving a 4WD drive cars there.

Another useful lesson learnt at the open parking space where the critical angle of oversteer is reached and how best to spin neatly using the least amount of room. Holding a terminal oversteer at opposite lock is fruitless if you know you will spin. Once you realise you have a case of terminal oversteer, the best course of action is to hit the brakes (and dip the clutch to protect the engine) which will result in a tighter spin, requiring less room to come to a halt. There is a saying, "in a spin, both feet in", which is to remind you to dip the clutch. Spinning means that you will travel backwards and if you're in a forward gear that can mean bad news for the engine and transmission unless you dip the clutch. When spinning, remember that the steering wheel will spin violently when the car points in the direction of travel. For this reason, do not put your thumbs inside the rim of the steering wheel - they are easily broken.

Excessive Corner Entry Speed. Oversteer through excessive corner speed is unusual. Most cars suspension and tyre sizes are set up such that should you enter a corner too fast the front tyres will lose grip before the rears. This results in understeer through excessive corner entry speed which is easier to deal with for drivers not used to skidding! To a certain extent the Elise is set up to behave in the same way, although mild in its execution. However, given a slight change to suspension geometry (easily done), use of wider front tyres or worn rear tyres can easily result in oversteer through excessive corner entry speed.I had my my first experience in an Elise in 1999 when I drove the car at "the late" Batu 3 Race Track Shah Alam at the Rothmans corner. At that time, I had very little man hour in the Elise and I did the trial braking and eased off the brake a little too late and had the oversteer with my ex office mate at USPD (now known as Proton Edar. He is Mr. Azam).

Correcting oversteer through excessive corner entry speed requires the right amount of corrective steering and a good deal of room. The faster your speed, the greater the angle of oversteer and the more room required to lose enough speed for grip to be restored. There is of course a critical corner entry speed, above which the angle of oversteer will be unrecoverable. However, in real world situations on public roads you will have slid off the road long before losing enough speed to regain grip. In these situations you have a choice; hit the brakes and leave the road backwards or apply corrective steering and leave the road sideways. The safest choice is probably to leave the road backwards which reduces the chances of turning the car over and puts the engine between you and the scenery. However, unless you have experience of this (or tremendous presence of mind) you are likely to attempt correction - it's a bad situation in any event.

Unless your corner entry speed is only a little over that required to induce oversteer, you effectively have a case of terminal oversteer because there simply is not enough room on the public road to regain grip. That said, there is more room available on left hand bends so long as there is no oncoming traffic. The maxim "slow in, fast out" is particularly apt in these circumstances. The Elise, unless you are particularly talented, is not a car you can simply chuck into a corner and sort it out if it goes wrong. The inherent balance of the car lends itself to a "slow in, fast out" approach because power can be applied early on in the corner and then balanced on the throttle through to the exit.

Lift-Off Oversteer. Lift-off is a result of weight transfer and is particularly relevant to the Elise. Imagine taking a corner a little too fast. The front tyres start to lose grip and your natural reaction is to step off the accelerator so that you slow down and restore grip. What happens in this scenario is that weight is transferred to the front tyres (in the same way as the front suspension dives during heavy braking). This produces the desired effect of restoring grip to the front tyres but reduces the weight over the rear tyres causing them to lose grip suddenly. This is the reason why backing off the throttle when in oversteer has dramatic effects - it increases the oversteer by lightening the rear-end and more often than not results in terminal oversteer.

The Elise is particularly prone to lift-off oversteer firstly because its suspension is set up for neutrality (so it's as likely to oversteer as understeer) and secondly because its weight distribution is biased towards the rear. Reducing weight at the rear has a dramatic effect because the suspension is set up to expect the majority of the weight of the car to be there. For this reason lift-off oversteer is rare in front engined cars because the suspension already expects the majority of the weight to be at the front. That is not to say that front engined cars do not suffer from lift-off oversteer. Most every car will do it but the provocation required is extreme. I just had a wonderful experience with my instructor, Tg Azizan of Proton in an Elise last week, He taught me the wonderful experience of the controlled-drift in an Elise. It took me quite sometime to absorb the drifting in an Elise. Alhamdulillah, I got it. Thanks Sifu!

Lift-off oversteer, depending on the extremity, can be recovered by restoring power and applying corrective steering. This is, however, normally only possible with very mild cases of lift-off oversteer. But there are situations where lift-off oversteer is desirable and can be induced. One way of curing understeer is to jab the brakes, transferring weight to the front wheels and reducing grip on the rears. This technique should have the effect of pointing the car in the desired direction but is very easy to get wrong and should be used with extreme caution.

AZAD


View complete fotopage


© Pidgin Technologies Ltd. 2016

ns4008464.ip-198-27-69.net