RE: IAM issues cornering tip
Discussion
Don said:
To say 'limit point getting nearer means slow down' is misleading and wrong.
Whoa there. I agree with everything up to that point. If your range of vision decreases you must slow down (if necessary) to ensure you can always stop within the distance you can see to be clear... I'm being pedantic...but when we "teach" limit point analysis we do need pithy phrases. Like: "Limit point rushing towards you, slow down, limit point rushing away speed up." and "Limit point rushing to the right or left, road is turning so slow, limit moving to the centre road is straightening speed up." etc etc
Limit point could be getting nearer with no turn in the road...e.g. a crest. Still means you need to slow down.
Whoa there yourself! The 'limit point technique' is much more than just evaluating 'the distance to the point where the road surface goes out of sight'. The crucial thing about this technique is that it tells you about the *shape* of the road way out at the limit of vision, way earlier than you can judge just by a casual glance. I find it frustrating that so many people miss the point, I'm sure you don't but you seem to be encouraging others to.
As well as that, there are two main issues with the over-simplified explanation. Firstly, the limit point does not define the distance you must aim to be able to stop in. There are many situations where you can see that the road is clear some distance beyond the limit point, and others where you may need to stop earlier than that. To use the limit point as your target 'stopping point' is wrong and potentially dangerous. Secondly, the fact that the bend is tightening does NOT mean you have to slow down, and the fact that it is widening does NOT mean you should speed up. That's a gross simplification which again will lead people into trouble imo.
Edited to add: my comments are mainly addressed to the original article and not a response to the specific suggestions in your post, which do make sense.
>> Edited by GreenV8S on Friday 17th March 16:54
GreenV8S said:
Edited to add: my comments are mainly addressed to the original article and not a response to the specific suggestions in your post, which do make sense.
I actually think we're in violent agreement! Your points are completely valid. Its just that when explaining the technique I have found it helpful to explain its two parts (shall we say) consecutively (within the same half hour session). I do "Limit Point Analysis" as part of a "Matching Speed To Vision" "lesson" that covers Limit Point, Surface Grip and the application of the System of Car Control to bends i.e. getting 'em to do their braking and gearchanging in a straight line before the corner then turning in at a speed that matches the curve (based on openness (vision) and severity (grip) and conditions (grip) such that they can stop in the distance they can see to be clear on their side of the road.
Generally if a speed adjustment is necessary mid-corner it was due to a failure to identify either the openness or severity of the corner prior to entry. i.e. going in way too hot without enough caution.
Had to get an Associate to brake hard only last night. He'd misread a bend due to a momentary loss of concentration...easily averted with a firm brake but if we'd actually gone in at that pace there would have been no reserve for dealing anything around the bend...scary...
Don said:
Generally if a speed adjustment is necessary mid-corner it was due to a failure to identify either the openness or severity of the corner prior to entry.
Presactly!
What the IAM are assuming is that the driver is comfortable with his/her speed while the vanishing point is being evaluated, therefore a tightening bend is likely to make them less comfortable and they should therefore slow down.
My problem with this is that rather than getting the speed right before the bend, they are suggesting that changing the speed whilst negotiating a bend is ok.
Surely this could result in weight-shifting during a bend....a big NO NO !
chim666 said:
Generally if a speed adjustment is necessary mid-corner it was due to a failure to identify either the openness or severity of the corner prior to entry.
Presactly!
What the IAM are assuming is that the driver is comfortable with his/her speed while the vanishing point is being evaluated, therefore a tightening bend is likely to make them less comfortable and they should therefore slow down.
My problem with this is that rather than getting the speed right before the bend, they are suggesting that changing the speed whilst negotiating a bend is ok.
Surely this could result in weight-shifting during a bend....a big NO NO !
You should always aim to be at the correct speed & in the correct gear before entry into the curved path.
It does not follow though that it is wrong to adjust speed whilst travelling in a curved path.
Ideally the speed should be constant, but there are times that you will have to brake for instance in a curved path & this wouldn't have to be as a result of poor assessment or any mistake, just the introduction of a new hazard for example. What is important, if you are going to be braking (or accelerating) whilst applying steering, is that you are aware of the inversely proportional relationship between these actions & the resultant forces they create with their affect on balance & grip. Provided your actions are smooth & progressive in their application (& you weren't way too hot to start with)it will not cause a great problem for safety or stability.
We should of course (on public roads) always be driving well within our limits, the vehicles limits & the conditions limits (not at 10/10ths for any of them) so that we have room for flexibility within our driving plans.
I think I'm going to be agreeing with vonhosen here (again) - perhaps apart from the very precise claim about the existence of an inverse proportional relationship.
I aim to be gently accelerating through my corners -- ie slowing to below the speed dictated by the limit point so that I can be comfortable stable and settled on the power, balanced as well as slightly accelerating up to the minimum limit point speed throughout the corner. This means that there is plenty in reserve (I'm not bang on the minimum limit point speed) should the corner do something funny (like have a tractor parked in it) and I'm confidently and positively on the power to settle the car early. It ideally ought to mean that I've got the entry speed not to hot. But then I did start this paragraph with "I aim"...
I'm not sure I understand the subtlety of GreenV8S's argument that the shape of the corner "out there" at the limit is more important or is in some way fundamentally different from it just being the furthest that you can see. Will have a think when I am out tomorrow - any pointers, GreenV8S?
I aim to be gently accelerating through my corners -- ie slowing to below the speed dictated by the limit point so that I can be comfortable stable and settled on the power, balanced as well as slightly accelerating up to the minimum limit point speed throughout the corner. This means that there is plenty in reserve (I'm not bang on the minimum limit point speed) should the corner do something funny (like have a tractor parked in it) and I'm confidently and positively on the power to settle the car early. It ideally ought to mean that I've got the entry speed not to hot. But then I did start this paragraph with "I aim"...
I'm not sure I understand the subtlety of GreenV8S's argument that the shape of the corner "out there" at the limit is more important or is in some way fundamentally different from it just being the furthest that you can see. Will have a think when I am out tomorrow - any pointers, GreenV8S?
7db said:
I think I'm going to be agreeing with vonhosen here (again) - perhaps apart from the very precise claim about the existence of an inverse proportional relationship.
I aim to be gently accelerating through my corners -- ie slowing to below the speed dictated by the limit point so that I can be comfortable stable and settled on the power, balanced as well as slightly accelerating up to the minimum limit point speed throughout the corner. This means that there is plenty in reserve (I'm not bang on the minimum limit point speed) should the corner do something funny (like have a tractor parked in it) and I'm confidently and positively on the power to settle the car early. It ideally ought to mean that I've got the entry speed not to hot. But then I did start this paragraph with "I aim"...
I'm not sure I understand the subtlety of GreenV8S's argument that the shape of the corner "out there" at the limit is more important or is in some way fundamentally different from it just being the furthest that you can see. Will have a think when I am out tomorrow - any pointers, GreenV8S?
The inversely proportional reationship I am talking about is tyre grip trade off. There is only a finite amount of grip available for both steering & accelerating. The more you do of one the less you can do of the other if you don't wish to break traction & induce a skid.
>> Edited by vonhosen on Friday 17th March 22:24
Don said:
To say 'limit point getting nearer means slow down' is misleading and wrong.
Whoa there. I agree with everything up to that point. If your range of vision decreases you must slow down (if necessary) to ensure you can always stop within the distance you can see to be clear... I'm being pedantic...but when we "teach" limit point analysis we do need pithy phrases. Like: "Limit point rushing towards you, slow down, limit point rushing away speed up." and "Limit point rushing to the right or left, road is turning so slow, limit moving to the centre road is straightening speed up." etc etc
Limit point could be getting nearer with no turn in the road...e.g. a crest. Still means you need to slow down.
If the limit point moves towards you, but your spped is such you can still stop, indeed, there would be no need to slow down.
What is forgotten, is that the limit point works using the "stop in the distance you can see to be clear" technique. This will (apart from perhaps in snow/ice) mean that you have the grip to get round the bend.
It is however, not going to give you the max. speed you could physically corner in.
I've been using it for 15 years, it works every time if applied correctly.
Martin
7db said:
I'm not sure I understand the subtlety of GreenV8S's argument that the shape of the corner "out there" at the limit is more important or is in some way fundamentally different from it just being the furthest that you can see. Will have a think when I am out tomorrow - any pointers, GreenV8S?
The distance to the limit point tells you something about how much clear road you have in front of you. It doesn't always tell you the whole answer, and sometimes there will be other information which can tell you more. It seems common for discussions about the limit point to stop here: "the amount of 'known clear' road in front of you is reducing, manage your speed to ensure that you can stop in the distance that you know to be clear and can expect to remain clear". This isn't the limit point technique, it is just noticing how much road you can see.
The limit point technique tells you that when the limit point is approaching you, the curvature of the road between you and the limit point is increasing. This has nothing to do with your speed, or how fast you 'should' be going, or whether you need to speed up or slow down. It is simply giving you information about the shape of the road. When you look at a curve in front of you, you can normally judge the shape of the road some way ahead of you just using normal perspective. So what the movement of the limit point tells you is the shape of the road furthest away from you, the part that perspective doesn't tell you about.
Knowing the shape of the road doesn't tell you how fast you should go or whether you should be speeding up or slowing down, but it is fundamental to working those things out.
As input, you have an estimate of the furthest point where you *might* need to stop. Sometimes this will be before the limit point, sometimes it will be after. You can form an opinion about how much grip you expect to be available along that part of the road. And you know the shape of the road.
Based on these bits of information you can judge the maximum speed that would enable you to stop within your safe distance, and make sure you stay below that safe speed. The calculation is easy to do but hard to explain briefly, but it's a continuous ongoing calculation that most of us will do automatically. The movement of the limit point is a vitally important input to this calculation, but it's the outcome of the calculation that tells you whether you need to change your speed.
To reiterate: the limit point technique tells you about the curvature of the road out towards the limit of vision, which is an important *input* to your driving plan.
When the limit point is initially explained within "Roadcraft" teachings, it is in a simple way.
Inevitably as people practice it, they may find situations where they find themselves questioning matters beyond that simple explaination & they would have to be dealt with on an individual basis relative to those circumstances. Inquisitive people seeking understanding after all will always ask "But what if...?"
In simple terms, for the purposes of "Roadcraft" the limit point is the furthest point along the road that you have an uninterupted view of the surface. It is the point where on the flat level road the nearside & offside edges intersect. With your line of sight to that point though & the curvature of the road, that is not acurately the position you will have to be able to stop within the distance you can see to be clear on your side of the road. Acurately it will be slightly shorter than that point in some cases, or some distance beyond it in others. (but you shouldn't have been close to the limit on it anyway as we should be comfortably within it.)
The limit point is not just the furthest point on the road you can see, if that view of the surface is interrupted.
The limit point is not the only information we have to concern ourselves with though. We should start with it on seeing the bend, take in all other available information including surface etc as already talked about by others. Lateral scans that may assist in further revealing the severity of the curved path or hazards beyond the limit point & we then come back to the limit point before entry, having first ensured that speed & gear are correct prior to entry. You should be holding the strong line in the bend, at a constant speed (some throttle to negate the effect of steering which wishes to take you off that strong line) not chasing the limit point, just going with it. Only come off the strong line when you can see the whole road (both sides of the road)is safe & on our roads that is mostly where you see the road breaking the other way. If you start to chase the limit point too early you risk getting yourself into problems if there is a double apex. Confirm what the road & a limit point are doing before accelerating too much, You should only be actively chasing it where it is confirmed to be racing away & you will be progressively building drive relative to & as you do take steering off.
Apexing is another totally different subject, but in "Roadcraft" we primarily don't apex because it is quicker (though it may be slightly so in use) but because of the reduction in cornering forces it places on the vehicle aiding stability. Apex is about safety & smoothness more so than carrying extra speed, on public roads.
>> Edited by vonhosen on Saturday 18th March 17:05
Inevitably as people practice it, they may find situations where they find themselves questioning matters beyond that simple explaination & they would have to be dealt with on an individual basis relative to those circumstances. Inquisitive people seeking understanding after all will always ask "But what if...?"
In simple terms, for the purposes of "Roadcraft" the limit point is the furthest point along the road that you have an uninterupted view of the surface. It is the point where on the flat level road the nearside & offside edges intersect. With your line of sight to that point though & the curvature of the road, that is not acurately the position you will have to be able to stop within the distance you can see to be clear on your side of the road. Acurately it will be slightly shorter than that point in some cases, or some distance beyond it in others. (but you shouldn't have been close to the limit on it anyway as we should be comfortably within it.)
The limit point is not just the furthest point on the road you can see, if that view of the surface is interrupted.
The limit point is not the only information we have to concern ourselves with though. We should start with it on seeing the bend, take in all other available information including surface etc as already talked about by others. Lateral scans that may assist in further revealing the severity of the curved path or hazards beyond the limit point & we then come back to the limit point before entry, having first ensured that speed & gear are correct prior to entry. You should be holding the strong line in the bend, at a constant speed (some throttle to negate the effect of steering which wishes to take you off that strong line) not chasing the limit point, just going with it. Only come off the strong line when you can see the whole road (both sides of the road)is safe & on our roads that is mostly where you see the road breaking the other way. If you start to chase the limit point too early you risk getting yourself into problems if there is a double apex. Confirm what the road & a limit point are doing before accelerating too much, You should only be actively chasing it where it is confirmed to be racing away & you will be progressively building drive relative to & as you do take steering off.
Apexing is another totally different subject, but in "Roadcraft" we primarily don't apex because it is quicker (though it may be slightly so in use) but because of the reduction in cornering forces it places on the vehicle aiding stability. Apex is about safety & smoothness more so than carrying extra speed, on public roads.
>> Edited by vonhosen on Saturday 18th March 17:05
dilbert said:
With all this talk of treelines and the width of the road, I'm left wondering if it's possible to create a bend that is an optical illusion.
The bend cues one way and goes the other!
See one of my old threads for discussion of just such a corner.
www.pistonheads.com/gassing/topic.asp?t=216775&f=154&h=0
Complete with map of a nasty corner that threw me. Notice that on knowing the corner, the visual clues are there, but there are stronger visual clues suggesting other routes which you need to ignore. In practice, that's hard to do first time in.
GreenV8S said:
Lots of good stuff
Thanks GreenV8S. If I've understood you correctly, you are saying that the limit point technique should be used to provide a speed -- ie that speed at which you can stop before reaching the limit point -- which is then compared with all the other speeds which limit progress (legal speed limit, limit of traction, limit of confidence etc etc), and the tightest restriction observed?
i.e. - that the technique as described is correct, but the automatic "comes towards go slower, goes away go faster" fails to take into account other limits?
If so then I'm fully agreeing with you. I was concerned that I was missing something in limit point technique. That knowing the curve of the road - "out there" - was in someway different from it just being a certain distance to where you might need to stop.
Incidentally, I like to think of the limit point as being the tail-lights of (a high performance!) car in front, and aiming not to tailgate it.
You might find this article interesting, talking about limit points in general, not just limits on cornering:-
www.5ive-o.org/forum/showthread.php?t=10394
Congrats on 10,000 posts, by the way.
7db said:
I was concerned that I was missing something in limit point technique. That knowing the curve of the road - "out there" - was in someway different from it just being a certain distance to where you might need to stop.
What I've been trying to say is that it is fundamentally different.
The limit point is NOT the stopping point (i.e. the point where you plan to be able to stop if necessary). On single track roads you may need to be able to stop in less than half the distance to the limit point. In other cases where you have a clear view above the road you may be confident that you don't need to stop for hundreds of yards past the limit point.
What the limit point technique does is give you advanced notice about the shape of the road. And you need this in order to plan what line to take, judge how much grip needs to be used for cornering at a given speed and hence how much grip will be available for braking, and hence how fast you can afford to approach the bend and still stop by your stopping point. This is all input to your driving plan which determines how fast you actually go, and what line you take. There are other inputs of course.
GreenV8S said:
What I've been trying to say is that it is fundamentally different.
I was afraid of that. I'm all ears, however.
Are you saying that you use it to help choose where to end braking, turn-in point, etc?
I'd be interested to hear how that works -- for simplicity we can assume that we are on a two-lane country road approaching a left-hander with tall hedges restricting the view on both sides, close to the edge of the carriageway (so we don't need all the other "there are other things" caveats).
I have a drive planned later - I'd love to be thinking about this as I do some corners.
7db said:
Are you saying that you use it to help choose where to end braking, turn-in point, etc?
The limit point technique tells you about the curvature of the road out towards the limit point. That's all, there's nothing magical about it. If the distance to the limit point is reducing then the bend is tightening up. If it is increasing then the bend is opening out. The eureka comes because this limit point technique lets you know the shape of the road far, far sooner than you could tell just using perspective. Once you have cracked it, it's like having somebody in the car in front giving you a running commentary on what they can see ahead.
What the limit point does not do is tell you how fast to go, where your stopping point is, what line you should be on, whether you should be braking or accelerating, where the hazards are and where you should be looking. These are all things that you need to work out based on information from the limit point technique and from other sources.
Good stuff from the IAM, this sort of thing should get wider exposure.
On similar lines, and looking a little further ahead than the limiting point, its always a good idea to check the near horizon for clues that the road direction may be about to change. A line of trees, hedges, or road sign smack bang in the middle of where the curvature is heading is something of a clue that the curve is about to tighten or switchback.
I agree totally with the "fog vision" comments earlier. You can always tell this sort of driver even when its not foggy by the fact that they will drive an equal distance behind cars, 4x4's, vans, trucks, whatever. A dead giveaway that they drive and react based on what is exactly in front of them. You can guarantee that they will only ever brake AFTER the vehicle in front of them. Just shifting their gaze a car or two (or preferably three oor four) ahead would give them much better reactions and minimize the domino effect in heavy traffic.
On similar lines, and looking a little further ahead than the limiting point, its always a good idea to check the near horizon for clues that the road direction may be about to change. A line of trees, hedges, or road sign smack bang in the middle of where the curvature is heading is something of a clue that the curve is about to tighten or switchback.
I agree totally with the "fog vision" comments earlier. You can always tell this sort of driver even when its not foggy by the fact that they will drive an equal distance behind cars, 4x4's, vans, trucks, whatever. A dead giveaway that they drive and react based on what is exactly in front of them. You can guarantee that they will only ever brake AFTER the vehicle in front of them. Just shifting their gaze a car or two (or preferably three oor four) ahead would give them much better reactions and minimize the domino effect in heavy traffic.
Gassing Station | Advanced Driving | Top of Page | What's New | My Stuff