Joined IAM - Too slow?

[StressedDave]

Don said:

OK. Let me clarify what I mean. On the approach to a left hand junction for example. Steam-From-Ears is behind...a little too close...but OK. I want the left junction. So. Mirror - I know where Steam-From-Ears is. Signal. No change in speed. Give SFE time to see the signal. A second or two. Mirror - has he seen the signal? Touch brake pedal. Brake lights go on. NO BRAKING. Mirror - is SFE slowing? Give him time...check again...slowing? SFE is slowing..excellent. Normal brake for junction. Smoothly off brakes, blending new gear into the end of braking whilst a small rev match with the accelerator smooths our the change. Both hands back on steering wheel - and turn in. Accelerate away. At the point where, after pressing the brake pedal, I note that SFE isn't slowing...perhaps I'll abort the manouever if required. SFE doesn't get held up more than necessary. Personally I can judge perfectly well how hard to press the brake pedal - and I'll bet most others can too. To be fair - when we take new Associates out on their early drives they can judge perfectly well what speed to be at for a hazard and they can usually slow down too. The bit they seem to find hard (understandably) is predicting what gear they will need. If you "go-down-the-box" you never need to predict...you just muddle through by being in whatever you've got to. Once the ability to predict what gear is necessary has been learned people seem to have a lot less trouble implementing the system - at least in my experience.

A classic case of YMMV - my experience is that most people tend to have a rising rate on the brakes (i.e. too little at first and then coming in in an unseemly rush when they get a bit closer to the hazard) and thus spend more £'s than they need to looking at the point when they might need to come to rest. If you make them brake later then they tend to get a nice constant pressure throughout and have enough time at the end to get the gear they need, do the final observation checks and flow moothly in and through any stream of traffic.

As for the S-F-E scenario, I'd be doing the same, although I tend to extend the indication for as long as possible to get them vaguely used to the idea that I might actually be changing speed rather than breathing on the brakes.

[vonhosen]

TripleS said:

GreenV8S said: I'm a little surprised that you're talking in terms of understeer on public roads, you'd have to be using far more grip than I would consider prudent before things like understeer and steering on the throttle become an issue. My feelings exactly Peter. I keep getting surprised by these references to instability, anxiety about weight transfer, understeer, oversteer, steering with the throttle etc. in a road driving environment. I must be too restrained with the whole process, as I never feel conscious of understeer/oversteer or any kind of instability. Generally speaking I find the car (not just my own but any car I've yet driven) does what I ask of it without any apparent threat of non-cooperation. Sorry, but that's me mystified again. Best wishes all, Dave.

I'm not talking in terms of getting near to limits of grip at all.
All of our movement on the road is a combination of our inputs & the movement of weight within the vehicle. We should be attempting to do this in a balanced unrushed way, that leads to us being in the right place on the road, travelling at the right speed & in the right gear for the circumstances. Under the right place on the road that will include our lines taken & ideally these will be (where ever we have the vision & it is safe) the lines that offer the greatest degree of balance (least forces acting on the vehicle & smoothest transitions of weight)

>> Edited by vonhosen on Tuesday 9th May 16:40

[Don]

vipers said:

Don said: I find it is best to have finished braking and be back on the gas before turn-in. That way you know what gear you will be in as you negotiate the hazard. IAM Fail if your auto-box changes gear mid-corner or "hunts" is a sequence of bends. The idea being that you have not preselected the suitable "hold" gear to prevent it Interesting, I have been driving an auto for 10 years, (previous to which I drove manual for 40 years), my current car is a 52 plate Volvo S80, what I dont follow from your comments above is this:- 1. You said "That way you know what gear you will be in as you negotiate the hazard", how so, I never know which actual gear I am in, I select "D" and thats it, the display doesn't actually show the gear ie 2,3 or 4. 2. You said "IAM Fail if your auto-box changes gear mid-corner or "hunts" is a sequence of bends. The idea being that you have not preselected the suitable "hold" gear to prevent it", how on earth can I know if it will change mid-corner, and how can I have pre-selected the suitable "hold" gear.

Well - your auto-box may be different - there are quite a range of systems on the market now. All the auto-boxes I've had though have been the same. You have a "D" and then a "3","2", and a "1". (A four speed box)

In "D" the car may choose the full range of gears from "1" to "4" - and it will - with little prompting from the driver! In "3" however the box is restricted to only "3","2" and "1". In "2" it is restricted to "2" and "1" and in "1", clearly, only first gear will be used.

So. Imagine you are approaching a sequence of bends of moderate pace. At the speed you need the car could be in "4" (most auto-boxes change up as quickly as possible) - but if you press the gas pedal the car will need to change down to "3" to accelerate. This was always called kick-down on the cars I had - mine even had a button at the full travel of the accelerator which changed down again - "double kick down" - certainly made it surge forward, that did!

Now - again this may have changed - with the auto-boxes I have used you certainly know when the car changes gear - you can feel it through the seat of your pants. On a poor surface (icy or damp) a gear change whilst cornering can destabilise the car. In a manual you would NEVER change gear mid corner - but in an auto the car could decide that it was going to - say just after the apex as you feed in power. (Most auto-boxes change down when your foot goes on the gas).

So - the IAM will expect you to get on the gas before turn-in as this should ensure that the car stays in the gear its in through the hazard. If there is a possibility that it will not you can manually select "3", say. This means the car will NOT change up to "4" whilst you are turning - once on the straight again you can push the lever back to "D" and the car will change up to "4".

This is what they mean. A "hold" gear (say, "3" simply holds the car in "3" rather than letting it change up to "4" as it would in "D".

Of course - if you own a modern Audi it is quite possible that the car is fitted with sensors that tell the computer that you are cornering. If you are - the car won't change up all by itself with no need to use a "hold" gear.

Or you may have one of the fabulously smooth SMG systems that have a computer controlled clutch/rev matching ability and are so smooth that F1 drivers can change gear mid-corner with no fear whatsoever. These systems are so clever that they all but eliminate any of the downsides of brake gear overlap by ensuring that the driver never need take his hands off the wheel and also that the drivetrain never experiences any shunt/power on-power off instability during gear change.

In any case. Before taking an IAM test with a car equipped with any sort of auto-shift system it is essential to read the manufacturer's instruction manual thoroughly and carefully. If its a non-traditional system (like that in a Toyota Prius, for example) it will be necessary to explain to the examiner how it works and how it must be driven according to the manufacturer's recommendations. The examiner will then take this into account and the test candidate accordingly...sadly I do not yet know of anyone who has taken their test in a F1 Ferrari 360 / 430 although I await the examiner's comments with great interest!

[GreenV8S]

vonhosen said:

I'm not talking in terms of getting near to limits of grip at all. All of our movement on the road is a combination of our inputs & the movement of weight within the vehicle. We should be attempting to do this in a balanced unrushed way, that leads to us being in the right place on the road, travelling at the right speed & in the right gear for the circumstances. Under the right place on the road that will include our lines taken & ideally these will be (where ever we have the vision & it is safe) the lines that offer the greatest degree of balance (least forces acting on the vehicle & smoothest transitions of weight)

Good. I wouldn't have expected you to be getting near to the limits of grip. I absolutely agree that it is desirable to drive smoothly, and that the forces on the car should be applied and removed smoothly. In general pootling it makes it more comfortable for the passengers and minimises wear and tear on the suspension and transmission, and if you are anywhere near the performance limits of the car it means you stay further away from the performance limits for a given performance. My comment was really reflecting surprise at the suggestion that any driver was having to drive round understeer, or was close enough to the limit of grip for the throttle to have any noticeable effect on the line that the car follows through a bend. I'm sure you're well aware how far you have to load a tyre before the slip angles become significant, and the idea that somebody was planning to use this much of the available grip in normal driving was quite alarming.

Edited to add:

Incidentally, I have no objection to people who wish to change gear on a bend, if they pick a time that fits in with the driver workload and do it in a way that doesn't disturb the car. Given that very few roads are absolutely straight, it would be a nonesense in any case to claim that it was always wrong to change gear on a bend.

>> Edited by GreenV8S on Tuesday 9th May 17:36

[7db]

GreenV8S said:

I'm sure you're well aware how far you have to load a tyre before the slip angles become significant, and the idea that somebody was planning to use this much of the available grip in normal driving was quite alarming.

Not much at all. I can feel the difference between cornering a balanced car with the throttle killing off understeer and wallowing round a corner out of the gas at just 20 mph in urban driving.

[Philbes]

TripleS said:

GreenV8S said: I'm a little surprised that you're talking in terms of understeer on public roads, you'd have to be using far more grip than I would consider prudent before things like understeer and steering on the throttle become an issue. My feelings exactly Peter. I keep getting surprised by these references to instability, anxiety about weight transfer, understeer, oversteer, steering with the throttle etc. in a road driving environment. I must be too restrained with the whole process, as I never feel conscious of understeer/oversteer or any kind of instability. Generally speaking I find the car (not just my own but any car I've yet driven) does what I ask of it without any apparent threat of non-cooperation. Sorry, but that's me mystified again. Best wishes all, Dave.

Now that sums up what I thought (as a beginner to advanced driving). In a modern car, at normal road speeds, understeer and oversteer just don't appear - even in my 'grandad's 147bhp Honda Accord'! I can lift completely off the throttle in the middle of a corner taken at 50mph and the car just continues round on the same course with no steering correction needed (I've just been out and tried it). Mind you this is in a FWD car not an enthusiast's high-powered RWD.
Understeer/oversteer will definitely by absent at the low speeds that my observer is currently suggesting are correct for corners.

[vonhosen]

GreenV8S said:

Incidentally, I have no objection to people who wish to change gear on a bend, if they pick a time that fits in with the driver workload and do it in a way that doesn't disturb the car. Given that very few roads are absolutely straight, it would be a nonesense in any case to claim that it was always wrong to change gear on a bend.

Never said that changing gear in a curved path could always be avoided.
Firstly I'll take gears where the vehicle is travelling on a straight path where ever reasonably possible. If it needs taking in a curved path I'll just ensure that I am not "actively" steering whilst doing the gear change (ie the steering is held set) & that changing at that time won't de-stabilise the vehicle adversely.

[Don]

GreenV8S said:

Incidentally, I have no objection to people who wish to change gear on a bend, if they pick a time that fits in with the driver workload and do it in a way that doesn't disturb the car. Given that very few roads are absolutely straight, it would be a nonesense in any case to claim that it was always wrong to change gear on a bend.

I agree. Your caveats about driver workload and disturbing the car are exactly correct IMO.

On a long bend accelerating away from a junction for example it would be perfectly reasonable to change up...as long as it's done such that the change does not compromise steering or unsettle the car as you say.

There's even one of those awful mnemonics for it: "Constant Steer - ok to change gear."

The business about the automatic boxes and changing in bends is, I expect, a reaction to candidates who only ever shove it in D and "hope" rather than taking control. Having said that with how good some of the modern systems are getting it really will be best practice pretty soon - again - IMO.

[Don]

Philbes said:

I can lift completely off the throttle in the middle of a corner taken at 50mph and the car just continues round on the same course with no steering correction needed (I've just been out and tried it). Mind you this is in a FWD car not an enthusiast's high-powered RWD.

Try that in my TVR and - in the dry - you would get away with it no problem too. Be insensitive once its wet and its a different story....

Philbes said:

Understeer/oversteer will definitely by absent at the low speeds that my observer is currently suggesting are correct for corners.

I should hope so!

All the advice given is about ensuring that you never get into a situation where the car doesn't do what you want it to!

Whilst a race driver may be concerned about not flying off the track due to under/over-steer the same advice and principles apply when it comes to maximising the margin of safety out on the road. A race driver tries to ensure the tyres are being asked to do one thing at a time - in order to use the grip they afford up completely in that one thing - whether it be acclerating, turning or braking. A race driver blends these things with constant regard for grip. An Advanced driver should also be aware of grip - always ensuring that there is a solid margin for error available to deal with the unexpected - but the principles of knowing how grip is used for turning and slowing and accelerating are the same.

[GreenV8S]

vonhosen said:

Never said that changing gear in a curved path could always be avoided.

Sorry, didn't mean to imply that you had said that although I can see how my comment could be interpreted that way. Some of the other posts about locking out transmission and avoiding gear changes even if the 'box wanted to, seemed to be heading in this direction. I just wanted to get my oar in first!

[GreenV8S]

7db said:

I can feel the difference between cornering a balanced car with the throttle killing off understeer and wallowing round a corner out of the gas at just 20 mph in urban driving.

All I can say is that in my experience you have to be pushing the tyres quite hard before slip angles are high enough to be noticeable, and I wouldn't expect drivers to be using that much of the available grip in normal driving on public roads. I guess either you're much more perceptive than I am, or your car handles very differently to the cars I have experience of.

I do agree that it is desirable in general to accelerate gently through and away from a corner, just not that it is normally necessary in order to prevent understeer.

[TripleS]

GreenV8S said:

....I have no objection to people who wish to change gear on a bend, if they pick a time that fits in with the driver workload and do it in a way that doesn't disturb the car. Given that very few roads are absolutely straight, it would be a nonsense in any case to claim that it was always wrong to change gear on a bend.

Best wishes all,
Dave.

[TripleS]

vonhosen said:

GreenV8S said: Incidentally, I have no objection to people who wish to change gear on a bend, if they pick a time that fits in with the driver workload and do it in a way that doesn't disturb the car. Given that very few roads are absolutely straight, it would be a nonesense in any case to claim that it was always wrong to change gear on a bend. Never said that changing gear in a curved path could always be avoided. Firstly I'll take gears where the vehicle is travelling on a straight path where ever reasonably possible. If it needs taking in a curved path I'll just ensure that I am not "actively" steering whilst doing the gear change (ie the steering is held set) & that changing at that time won't de-stabilise the vehicle adversely.

That's all fair enough, but for my own purposes I'm quite happy to do a bit of steering and change gear (usually at a large roundabout) at the same time - if I feel like it and the circumstances are right. I certainly don't need to have the steering 'set' at that time

Do you remember the old trick of rubbing your tum in a circular motion with one hand while your other hand pats your head? Well I can't do that, but I can change gear and steer at the same time without detrimental effects - which seems more useful!

Best wishes all,
Dave.

[vonhosen]

TripleS said:

vonhosen said: GreenV8S said: Incidentally, I have no objection to people who wish to change gear on a bend, if they pick a time that fits in with the driver workload and do it in a way that doesn't disturb the car. Given that very few roads are absolutely straight, it would be a nonesense in any case to claim that it was always wrong to change gear on a bend. Never said that changing gear in a curved path could always be avoided. Firstly I'll take gears where the vehicle is travelling on a straight path where ever reasonably possible. If it needs taking in a curved path I'll just ensure that I am not "actively" steering whilst doing the gear change (ie the steering is held set) & that changing at that time won't de-stabilise the vehicle adversely. That's all fair enough, but for my own purposes I'm quite happy to do a bit of steering and change gear (usually at a large roundabout) at the same time - if I feel like it and the circumstances are right. I certainly don't need to have the steering 'set' at that time Do you remember the old trick of rubbing your tum in a circular motion with one hand while your other hand pats your head? Well I can't do that, but I can change gear and steer at the same time without detrimental effects - which seems more useful! Best wishes all, Dave.

I'll have the gear at roundabouts as soon as I have a gap, never need to take it will steering actively personally.

[7db]

GreenV8S said:

7db said: I can feel the difference between cornering a balanced car with the throttle killing off understeer and wallowing round a corner out of the gas at just 20 mph in urban driving. All I can say is that in my experience you have to be pushing the tyres quite hard before slip angles are high enough to be noticeable, and I wouldn't expect drivers to be using that much of the available grip in normal driving on public roads. I guess either you're much more perceptive than I am, or your car handles very differently to the cars I have experience of. I do agree that it is desirable in general to accelerate gently through and away from a corner, just not that it is normally necessary in order to prevent understeer.

It might be more obvious in my car, but you can certainly feel it in most cars I have driven. Have a go next long corner you get the chance on -- you can feel the difference in track as you vary the throttle.

Should you have a cheekly little roundabout near you with no traffic on it, you can set up a constant deflection, and watch the understeer* vary as you come in and out of the throttle. Feel the car bite in as you squeeze the power on.

Beware, though. After a few laps you'll just want to throw up.



* notice we are talking about normal understeer here, ie car following a wider track than might be expected by the deflection, with full grip in place, rather than terminal understeer where front wheels lose traction and a skid develops. This may be the source of confusion.

[GreenV8S]

And you're saying that under gentle cornering it turns in (takes a tighter line) as you apply gentle power and runs wider when you ease off again?

From the purely theoretical point of view I can see an effect which would cause this, due to the (small) rearward component of force that the front wheels generate due to the skew and rolling resistance of the tyre. By applying just enough drive to overcome this the slip at the tyre could be reduced. But the slip angle is really very small indeed under these conditions isn't it, so the angle between the net thrust from the tyre and the instantaneous turning center would also be small, and the scope to reduce the slip angle must be very small indeed? It's certainly not an effect that I've noticed. Maybe I've been subconsciously and automatically adjusting my steering to compensate for it, all these years?

Just for completeness, I'm familiar with this effect at high slip angles from grass track racing, and of course we all know how power on understeer occurs at much higher cornering loads with much more power applied. I'm just mystified to find that you are noticing this effect under gentle cornering on grippy surfaces. I'll certainly keep an eye out for it.

[Major Bloodnok]

An interesting thread, this one.

To speak the original topic: I always make a point of telling associates at the start of the course that what we're going to do is to take their driving style apart and put it back together in a different fashion. This will necessitate intially taking things rather slowly until they're used to the system and can start to compress it all again. Later on in the course, I'll be spending a fair bit of time trying to get them to speed up and to convince them that there are corners that they can take at 30mph in third - they don't have to take every corner in second.

As for the understeer/oversteer - I drove a Cavalier for 10 years until some toe-rag decided to take it for a joy ride. After that, I decided that I was fed up with boring cars and replaced it with a BMW 318i SE. I don't know whether it was the change from front wheel drive to rear wheel, or simply better grip from the lower profile tyres, but I spent the first couple of months in the Beemer steering too tightly into fast corners. The Cavalier certainly understeered quite dramatically compared to the BMW (which itself seems quite sluggish compared to my current pride and joy: a MkIII MR2).

[7db]

Yes, that's the explanation of the effect (that there is a component of induced drag from the turned wheel) but it's not the slip angle that you experience as the driver of the car. Understeer is not an effect of the the tyre having a slip angle (ie differential heading and direction of travel) but is an effect of the net yaw rate of the vehicle differing from the neutral yaw rate:- a result of different rotational moments (about the z-axis) from the front and rear tyres.

This effect can be much larger than the individual slip angles at any of the tyres, and is noticeable in the cockpit.

This can be taken to an extreme where a full skid develops and the car will undergo quite large devations from the neutral yaw rate at modest (in absolute terms) slip angles.

[vipers]

Don,

Just to say thank you for the clarification on my question re auto box's. No doubt about it, we all learn something every day. Cheers.

[Don]

vipers said:

Don, Just to say thank you for the clarification on my question re auto box's. No doubt about it, we all learn something every day. Cheers.

No problem. Always glad to help. Always glad to debate with the highly knowledgeable people on here. I've learned a great deal from the discussions in this forum.

I have also found new ways to explain various principles to Associates at our IAM Group - I have found that very useful.