Reaction to skids in an ESP-equipped car

[WS]

Hi All,

I would like to ask about your opinion on the best technique for dealing with skids in a car equipped with the ESP. Basically, the question I would like to ask is, should the driver try to help the system or should they not do anything and just let the ESP do the job?

Specifically, when the car understeers:
- will you decrease steering angle for a moment to help the front wheels regain grip and try to turn in again (as you would normally do in a car without the ESP), or will you maintain steering wheel lock and wait for the ESP to intervene?
- will you brake or not?

When the car oversteers:
- will you apply opposite steering lock (as you would in a car without the ESP) or maintain the steering angle as it was?
- will you brake?
- pressing the accelerator (which might make sense sometimes in a front-wheel driven car) will not have any effect during the skid because the ESP reduces engine torque (or makes the engine ignore the fact that the gas pedal is being pressed); however, perhaps it is better to keep foot off the gas to avoid a sudden surge of power once the skid has ended and the ESP stops working.

Theoretically, trying to help the car recover from a skid, if done correctly, should make things easier for the ESP. But at the same time the ESP uses the steering angle to determine where the driver intends to go, so changing the steering wheel lock during a skid misleads the system.

Also, will your actions differ depending on whether the car if front-, rear- or all-wheel driven? Perhaps they will they differ depending on some other factors?

Thanks for your opinions.

[Astraist]

For understeer I would keep the steering more or less steady, but I would brake the car slightly. Stability control works on assymetric braking and, in the event of understeer, it brakes the inside-rear wheel to yaw the car into the bend. However, since it is the least loaded wheel in this condition, the system often struggles with mild understeer and some braking helps it.

Some drivers apply more steering into the bend during understeer, which I find less effective. This increases the skid but than the ESP applies a stronger correction. Also, the steering input increases drag, effectivelly slowing the car as to eventually remedy understeer.

Understeer eventually diminishes on it's own, since it increases the (originally too tight) radius. In that sense, turning the wheel more would magnify the understeer but would thereby make it die faster. In cars with passive rear steering, it might be induced by the additional lock and contribute to reducing the understeer, too.

As for oversteer. I have learned over time that letting go of the pedals and pointing the car towards the desired direction, which might entail some countersteering. The power is usually diconnected and will interfere with the braking that the system applies. Braking would make the sliding rear tyres even lighter.

[jonquirk]

Two things I learned on a skid control course were: feet off everything; look at where you want to go. I think these tenets would apply with or without ESP.

I don't think understeer and oversteer are appropriate terms in a skid. We should be talking about front wheel or rear wheel skids.

ESP cannot tell where the road goes so steering wheel position is only relevant in terms of the difference between where the wheels are pointing and actual yaw angle achieved. Also relevant is the relative speeds of the wheels. ESP will reduce engine output and brake individual wheels in an effort to equalise these inputs; only the driver can decide which direction they want to travel so look at where you want to go and steer appropriately; leave the pedals alone until ESP has restored control.

[Tosh]

ESP has been developed with the average motorist in mind, one with no concept of handling or skid control. In order for ESP to kick in and do its job all you have to do is point the car in the direction you need to go. Hitting the brakes or accelerator will have no effect to the stability as the ESP will take over and maintain the car on the steering course you want. Using conventional skid control techniques of reducing steering angle in an understeer slide or opposite lock to correct a rear wheel slide will actually prevent the ESP from activating as it will not detect a slide.

If you watch Mark Kendrick's Bespoke Driving DVD, High Performance Roadcraft, he has a section detailing this information.

[Astraist]

Hitting the brakes or accelerator will have no effect to the stability as the ESP will take over and maintain the car on the steering course you want.

True, but for mild understeer some braking might be necessary. I remember this well in training with a Sirion at a wet 180 degree bend (simuated on the training grounds). Without braking the ESP actually could not keep the car in the desired path over a certain speed!

Using conventional skid control techniques of reducing steering angle in an understeer slide or opposite lock to correct a rear wheel slide will actually prevent the ESP from activating as it will not detect a slide.

In either case, it's not the steering that controls the skid. Reducing the steering (in understeer) allows to lose the excess speed faster, and countersteering in oversteer (which isn't always needed!) is done to keep the car pointing to the desired direction of travel, so some of it might be need with ESP.

[125isfine.]

Tosh is correct on this as I have experienced myself in a buttock clenching moment a few years ago.
I had a Smart Four Two and I was out in the countryside late one night in the depths of winter, C class road with a ninety degree bend approaching, water sloshing in the wheel arches and then just as I got to the bend...silence, oh bother I said !. Reaction time was non existent, steering went featherlight and I didn't even have time to lift off the throttle. I turned sharp left and braced myself for the impact, airbag and embarrassment, I heard and felt a rapid pulsating noise and next thing I new I was stationary, around the bend, I was mighty impressed as it had reacted many times faster then I could have done.
However, if your car has ESP please find a snowy off road area and test it, they don't all work as well as the manufacturers claim. I had a latest model Kia Picanto with ESP and that didn't work at all!. It would allow horrendous wheelspin on pullaway, it did not shut power off or apply brakes and I could get straight on terminal under steer on full lock in the snow, all I could see was a flashing orange light. Dealers said nothing wrong.

[waremark]

I had a latest model Kia Picanto with ESP and that didn't work at all!. It would allow horrendous wheelspin on pullaway, it did not shut power off or apply brakes and I could get straight on terminal under steer on full lock in the snow, all I could see was a flashing orange light. Dealers said nothing wrong.

ESP needs a reasonable level of grip to function. Pulsing the brakes cannot help unless the tyres have some purchase. Possibly in this case there was not enough.

[michael769]

Hi All,

I would like to ask about your opinion on the best technique for dealing with skids in a car equipped with the ESP. Basically, the question I would like to ask is, should the driver try to help the system or should they not do anything and just let the ESP do the job?

The recommeded approach is to steer the car in the direction you want it to go and otherwise allow the system to sort things out for you. Lifting off the pedals can indeed be helpful as it reduces the chances of immediately provoking another skid once the car is stabilised.

ESP does have limitations and, while it can recover from skids far more readily than a human in most cases, it does not always succeed.

[martine]

...there's a whole lot of nasty maths revolving around the need for a decent amount of forward speed for the enhanced kalman filter to be able to work out the other important control parameters...

I blame the reversed polarity of the warp engines myself...

[Kimosabe]

...there's a whole lot of nasty maths revolving around the need for a decent amount of forward speed for the enhanced kalman filter to be able to work out the other important control parameters...

I blame the reversed polarity of the warp engines myself...

Okay. That got my attention. Understeer/Oversteer, it's all so very 20th century.

I don't know. It seems to me they could have simply modulated shield harmonics while reversing the polarity of the quantum flux capacitor, all the while channeling power away from life support through the plasma conduits into the deflector array, emitting a neutrino-enhanced tacheon beam that's modulated at 1.12 Jiggahwatts.

Apollo capsules? Do go on...

(i'm going for little lie down now)

[triquet]

Just don't take the corner at 88mph or you may meet yourself coming the other way ...

[TripleS]

I had a latest model Kia Picanto with ESP and that didn't work at all!. It would allow horrendous wheelspin on pullaway, it did not shut power off or apply brakes and I could get straight on terminal under steer on full lock in the snow, all I could see was a flashing orange light. Dealers said nothing wrong.

ESP needs a reasonable level of grip to function. Pulsing the brakes cannot help unless the tyres have some purchase. Possibly in this case there was not enough.

ESP doesn't generally include traction control per se. It can tell the engine management to shut power off if that will be better to maintain stability but doesn't control wheelspin. It's also worth remembering that the system has limited command authority. It can't change the laws of physics, hence terminal understeer if there's insufficient grip. It also tends to have a speed cutoff (5-10 mph on the earlier systems, god knows what these days) so at slow speed it can't work in any case (there's a whole lot of nasty maths revolving around the need for a decent amount of forward speed for the enhanced kalman filter to be able to work out the other important control parameters - there aren't enough sensors in the system to measure all that needs to be measured so it estimates).

That's what I do, estimate: and that's with a distinctly limited set of sensors/senses.

Aside from the frivolity though, this topic relates (in a way) to my travels this morning.

We'd had a touch of frost round here overnight, and although the main roads (B1416 and A171 to Scarborough) were OK and suitable for the use of a normal speed range, when we turned off at The Falcon Inn, heading for Hayburn Wike, it was a different story. Being a nervous and wary sort of cove, I looked carefully at the surface on the minor roads, and decided it looked frosty/icy. Hell, it was like a bleedin' skating rink! Problem was, there had been ice overnight, but the early morning sun had done just enough to leave a film of water on top of the ice, which, as far as I'm aware, is about the lowest grip level we are likely to find.

Anyhow we completed our travels safely, and old CUBby is still the right shape, so all was well.

I do wonder though what would have happened if I'd been less observant as to the road surface, but I had had the benefit of ESP instead. My guess is that I would have had a bent car.

No doubt modern ESP systems are very good, and I'm not seeking to dismiss them, but quite honestly I'd prefer drivers to be more aware and prudent in their driving, and not start thinking that these systems will always keep them shunt-free, because they won't.

BTW, can you let us know what you think about the technical matter I referred to in the 'Brakes' thread?

Best wishes all,
Dave.

[akirk]

It is also worth noting that the system varies from car to car...
on my Kuga the Haldex 4x4 system acts as a quasi-stability system in that losing grip in place can lead to it transferring grip to the other wheels...
on my z3 it has DSC which is a system incorporating stability control / traction control / making the coffee / etc. it measures lots of things from acceleration to g-force etc. to make its decision as to when to interfere...

there are also times when the car has far better grip with it off (ice / gravel / snow / etc.) - mainly times when it should be in the garage anyway!

Alasdair

[triquet]

Indeed. I think that as the coefficient of friction approaches zero all the bets are off and the most sophisticated of algorithms are not going to help.

[TheInsanity1234]

Indeed. I think that as the coefficient of friction approaches zero all the bets are off and the most sophisticated of algorithms are not going to help.

Well, that much is obvious!