Discussion
Could anyone help with a little explanation of the limit point used when cornering.
I believe I understand the basic theory, however I am not sure about how it controls your actual speed into and through a corner. Roadcraft explains that you should reduce your speed when approaching the corner if the limit point comes towards you or remains stationary. As soon as it moves away, carry on at a constant speed, matching the speed it is moving away. The book states that will will help give the correct speed through the corner.
The bit I am unsure about is how can my approaching speed affect when the limit point moves away. If I give an example to help explain, you will see my confusion.
If I approach a corner at 5mph, when I am a certain distance away the limit point will start to move away. I could carry on at 5mph around the corner, maintaining the distance between me and the limit point. However, if I approached at 30mph, wouldn't the distance be the same when the limit point moved away. That means that the speed throughout the corner isn't affected by the limit point analysis.
I realise that it may be my understanding that is totally at fault, which is why I would appreciate any help you could give.
I believe I understand the basic theory, however I am not sure about how it controls your actual speed into and through a corner. Roadcraft explains that you should reduce your speed when approaching the corner if the limit point comes towards you or remains stationary. As soon as it moves away, carry on at a constant speed, matching the speed it is moving away. The book states that will will help give the correct speed through the corner.
The bit I am unsure about is how can my approaching speed affect when the limit point moves away. If I give an example to help explain, you will see my confusion.
If I approach a corner at 5mph, when I am a certain distance away the limit point will start to move away. I could carry on at 5mph around the corner, maintaining the distance between me and the limit point. However, if I approached at 30mph, wouldn't the distance be the same when the limit point moved away. That means that the speed throughout the corner isn't affected by the limit point analysis.
I realise that it may be my understanding that is totally at fault, which is why I would appreciate any help you could give.
Lady Godiva said:
If I approach a corner at 5mph, when I am a certain distance away the limit point will start to move away. I could carry on at 5mph around the corner, maintaining the distance between me and the limit point.
But if the bend is one that is safe to negotiate at say 50mph, you wont be maintaining the distance between you and the limit point, the limit point will "run away" from you, and you will be taking the bend too slow.
Lady Godiva said:
However, if I approached at 30mph, wouldn't the distance be the same when the limit point moved away.
No, if it is a 20mph bend, you may be "overtaking" the limit point, ie it is staying static, and you are approaching it too fast.
I am sure someone can explain it better than me, and I have seen good articles on the net about this, with pics, but I can't for the life of me find them now
I'm glad somebody has raised this one, as I've never really got to grips with this limit point business*. That's not to say I in any way question its merits, merely that I've never felt that I was understanding it and applying it properly on the road.
In the meantime I can assure you that I have no difficulty in confining myself to a safe speed when cornering, though it might be the case that I sometimes go unnecessarily slowly, but I doubt it. I wonder if what I am doing is actually using the limit point system without realising it - if such a thing is possible.
* Cue a contribution from my friend in the University of Sussex IT department, siezing on this evidence that I'm a complete numpty! Good morning MEF.
Best wishes all,
Dave.
>> Edited by TripleS on Sunday 19th February 11:28
In the meantime I can assure you that I have no difficulty in confining myself to a safe speed when cornering, though it might be the case that I sometimes go unnecessarily slowly, but I doubt it. I wonder if what I am doing is actually using the limit point system without realising it - if such a thing is possible.
* Cue a contribution from my friend in the University of Sussex IT department, siezing on this evidence that I'm a complete numpty! Good morning MEF.
Best wishes all,
Dave.
>> Edited by TripleS on Sunday 19th February 11:28
If the limit point is approaching you, the bend is tightening up. If it is receding, the bend is opening up. Hence if the limit point approaches you would normally want to slow down, when it starts receding you would consider starting to accelerate out of the corner. Bear in mind that the movement of the limit point tells you what is happening at the limit point and not where you are now, so think of this as advance warning and not an indication that you have to accelerate mmediately. It is more like an indication of whether your foot should be over the brake or the throttle, if you see what I mean.
Godiva - It's a good question. The confusion arises because you actually want the limit point to be a different distance, depending on your speed. If you are travelling faster, then you want the limit point further away.
Long-winded, but the explanation below might help:-
Lets take you 5mph example even further.
Imagine walking around the corner - on the line that you would drive - and every yard you put down a marker saying how far the limit point is away.
At the end of the approaching straight, this number is getting smaller and smaller with each yard. In the corner it stays fairly constant -- going up or down a little bit acocrding to how the visibility and tightness of the corner progresses as you go through it. As the corner opens out onto the following straight, the number goes up and up.
You have written these numbers independently of the speed at which you have gone through the corner - and they will be true when you are next there, or true for me, when I come through the corner - they are a property of the corner, not your journey through it.
Now - on a particular day, you may decide that your safe stopping distance in yards is the same as your MPH (for simplicity). You look at all the markers in the corner and the lowest one is 20 yards to the limit point.
If you drive at exactly 20 down the approaching straight, and through the corner and out the far end, then you will never be going faster than your safe speed. You'll always be able to see far enough ahead to stop in. The limit point will come towards you and go away, being 20 yards away at its closest point.
You can clearly go through this corner quicker, and still be safe.
Suppose that you are now approaching at 60. As you approach the corner, the limit point get closer, at some point you pass a marker labelled 60 and the limit point is still coming towards you. It has got closer than the distance you can stop in. So you slow. you match your speed to the numbers on the markers, so that
AS THE LIMIT POINT GETS CLOSER, YOU ARE GOING SLOWER - slow enough to stop before you reach it.
You reach the 20 marker and the limit point starts to go away from you. So you speed up. But you can't speed up to 60 because the limit point is only at 30 yrds, so
AS THE LIMIT POINT GOES AWAY, YOU ACCELERATE - but only so much that you can stop before you reach it.
So you see it is natural that the limit point approaches and recedes, and you shouldn't try to hold it at a constant distance. Just make sure it is never closer than the distance you could stop in.
I like to imagine the limit point as the rear of the car in front, and then I know how far I like the car in front to be away for any given speed and I keep that distance. It must be a really good car, too, cos it gets out of the corners much faster than I can accelerate.
* * * * * * * *
It should be noted of course that there are other limits on how you corner, than the limit point. Traction is an important one, but equally getting a smooth application of power and a controlled smooth single application of deviation and unwinding are important, and might mean that you let the limit point slip away from you early on in the corner to allow you to catch it after the 20mph marker.
Long-winded, but the explanation below might help:-
Lets take you 5mph example even further.
Imagine walking around the corner - on the line that you would drive - and every yard you put down a marker saying how far the limit point is away.
At the end of the approaching straight, this number is getting smaller and smaller with each yard. In the corner it stays fairly constant -- going up or down a little bit acocrding to how the visibility and tightness of the corner progresses as you go through it. As the corner opens out onto the following straight, the number goes up and up.
You have written these numbers independently of the speed at which you have gone through the corner - and they will be true when you are next there, or true for me, when I come through the corner - they are a property of the corner, not your journey through it.
Now - on a particular day, you may decide that your safe stopping distance in yards is the same as your MPH (for simplicity). You look at all the markers in the corner and the lowest one is 20 yards to the limit point.
If you drive at exactly 20 down the approaching straight, and through the corner and out the far end, then you will never be going faster than your safe speed. You'll always be able to see far enough ahead to stop in. The limit point will come towards you and go away, being 20 yards away at its closest point.
You can clearly go through this corner quicker, and still be safe.
Suppose that you are now approaching at 60. As you approach the corner, the limit point get closer, at some point you pass a marker labelled 60 and the limit point is still coming towards you. It has got closer than the distance you can stop in. So you slow. you match your speed to the numbers on the markers, so that
AS THE LIMIT POINT GETS CLOSER, YOU ARE GOING SLOWER - slow enough to stop before you reach it.
You reach the 20 marker and the limit point starts to go away from you. So you speed up. But you can't speed up to 60 because the limit point is only at 30 yrds, so
AS THE LIMIT POINT GOES AWAY, YOU ACCELERATE - but only so much that you can stop before you reach it.
So you see it is natural that the limit point approaches and recedes, and you shouldn't try to hold it at a constant distance. Just make sure it is never closer than the distance you could stop in.
I like to imagine the limit point as the rear of the car in front, and then I know how far I like the car in front to be away for any given speed and I keep that distance. It must be a really good car, too, cos it gets out of the corners much faster than I can accelerate.
* * * * * * * *
It should be noted of course that there are other limits on how you corner, than the limit point. Traction is an important one, but equally getting a smooth application of power and a controlled smooth single application of deviation and unwinding are important, and might mean that you let the limit point slip away from you early on in the corner to allow you to catch it after the 20mph marker.
Dear Lady G - an excellent question, particularly from the gender most associated with the School Run!!!
Many drivers (even Advanced ones) struggle with the limit point theory. That is because they have misunderstood the basic principle. Even Observers have trouble explaining it.
The Limit Point CANNOT and DOES NOT give a safe cornering speed around a corner related to cornering forces. It can only give the safe speed based on stopping distances. There is a considerable difference between the two, and it's where most folk go wrong.
The best way to describe it is physically, then a verbal description.
Draw a large circle on a pice of paper, with a slightly smaller circle inside, to represent a bend in the road. Place an object in it to represent your car. Get a stick, or ruler, and place it from the car to the limit point. It will go from the car, touching the inside circle, then cross the outside circle (that is the limit point). Move the car and stick around the circle. Regardless of how fast you move around, the limit point will not change. So if you drove around there at 5mph, or 30mph, or 100mph, the distance to the limit point would remain the same. That proves that it is not the speed per se that it relates to.
Now use it in conjunction with stopping distances. You are approaching a right hand bend. The limit point is static, so you start to slow down, SO THAT YOU CAN STOP IN THE DISTANCE YOU CAN SEE TO BE CLEAR. Keeping it purely theoretical, lets say you are 53 metres away, you should only be going at 50pmh. The limit point is static, so you continue to slow down. Lets say that when you get to within 23 metres of the limit point, you are going at 30mph (this is the stopping distance for 30mph) and the limit point starts to move away. So, you are now travelling at the speed at which you can stop in the distance you can see to be clear. The limit point moves away at a constant rate, so keep at 30mph, and you can always stop in the distance you can see to be clear. However, as the bend straightens uot, you can see a bit further, so you can go a bit faster.
Remember, the above assumes you are in a vehicle capable of cornering at that speed. Nothing in the limit point theory tells you that. Keeping it theoretical, there are some corners where you can see 23 metres around it to the limit point, so you could do it at 30mph. Trouble is a TVR would do it easy, a Micra would spin off. Same limit point theory, same corner.
So, the limit point helps you with the speed based on stopping distances. As on observant driver you use the road conditions, and knowledge of the car, to adjust that speed, usually reducing it. You approach the corner, limit point sets you at 30mph, but observation tells you to go slower because of adverse camber and mud, reduce to 25mph, maintain this speed as long as the limit point moves away constantly (and the road conditions remain the same).
Hope this helps.
Phew!
P.S. 7db came in above while I was writing this. Do wot he sez, 'cos I think he sez the same as me, only better!!!
>> Edited by Big Fat F'er on Sunday 19th February 12:21
Many drivers (even Advanced ones) struggle with the limit point theory. That is because they have misunderstood the basic principle. Even Observers have trouble explaining it.
The Limit Point CANNOT and DOES NOT give a safe cornering speed around a corner related to cornering forces. It can only give the safe speed based on stopping distances. There is a considerable difference between the two, and it's where most folk go wrong.
The best way to describe it is physically, then a verbal description.
Draw a large circle on a pice of paper, with a slightly smaller circle inside, to represent a bend in the road. Place an object in it to represent your car. Get a stick, or ruler, and place it from the car to the limit point. It will go from the car, touching the inside circle, then cross the outside circle (that is the limit point). Move the car and stick around the circle. Regardless of how fast you move around, the limit point will not change. So if you drove around there at 5mph, or 30mph, or 100mph, the distance to the limit point would remain the same. That proves that it is not the speed per se that it relates to.
Now use it in conjunction with stopping distances. You are approaching a right hand bend. The limit point is static, so you start to slow down, SO THAT YOU CAN STOP IN THE DISTANCE YOU CAN SEE TO BE CLEAR. Keeping it purely theoretical, lets say you are 53 metres away, you should only be going at 50pmh. The limit point is static, so you continue to slow down. Lets say that when you get to within 23 metres of the limit point, you are going at 30mph (this is the stopping distance for 30mph) and the limit point starts to move away. So, you are now travelling at the speed at which you can stop in the distance you can see to be clear. The limit point moves away at a constant rate, so keep at 30mph, and you can always stop in the distance you can see to be clear. However, as the bend straightens uot, you can see a bit further, so you can go a bit faster.
Remember, the above assumes you are in a vehicle capable of cornering at that speed. Nothing in the limit point theory tells you that. Keeping it theoretical, there are some corners where you can see 23 metres around it to the limit point, so you could do it at 30mph. Trouble is a TVR would do it easy, a Micra would spin off. Same limit point theory, same corner.
So, the limit point helps you with the speed based on stopping distances. As on observant driver you use the road conditions, and knowledge of the car, to adjust that speed, usually reducing it. You approach the corner, limit point sets you at 30mph, but observation tells you to go slower because of adverse camber and mud, reduce to 25mph, maintain this speed as long as the limit point moves away constantly (and the road conditions remain the same).
Hope this helps.
Phew!
P.S. 7db came in above while I was writing this. Do wot he sez, 'cos I think he sez the same as me, only better!!!
>> Edited by Big Fat F'er on Sunday 19th February 12:21
7db has presented a really good explanation.
I learnt about limit points for racing, rather than road driving, but it's fundamentally the same thing.
The basic idea is that you are never travelling into, through, and out of a bend, faster than the speed in which you can stop in a straight line.
As you approach the bend, there exists a distance (think of an imaginary line) between your vehicle and the farthest point of the bend your eyes can see in a straight line. This imaginary line must be straight and unobstructed -- it must not pass over anything that is a hazard or not part of the road surface itself.
At whatever speed you approach the start of the bend, this imaginary line must be at least as long as the minimum braking distance of your vehicle in a straight line. Therefore, at 100kph, this line may be 80m; or at 60kph, this line may be 40m. The faster your speed, the greater this imagery line must be, and therefore, the earlier you must begin your braking.
Therefore, we have the rule that your speed and the length of the imagary line is directly proportional to each other.
If the bend you're approaching is a tight one, this imaginary line will only be short. Therefore, your speed must be decreased in order to match the minimum braking distance of your vehicle. Logically, easier bends will have a longer imagery line, and therefore allows you higher speed since you can afford a longer distance in which to brake in a straight line.
After you enter the bend, the farthest point you can see into the bend will begin to move down the bend. When it does, you can ease off the brakes, by which time you will be travelling just at a speed that allows you to brake from your current position to the farthest point you can see, safely. Remember that you are still linked by an imaginary line from your vehicle to this farthest point, and that the line must be unobstructed. As this farthest point moves at a speed which matches your vehicle's speed, you can maintain this speed and travel through the bend at a safe pace. As I described before, the tigher the bend, the shorter the imaginary line between your vehicle and the farthest point, therefore dictating lower speed for your vehicle; vice versa for more easier bends.
As you approach the exit of the bend, the farthest point will begin to accelerate away from your vehicle. Therefore, the imaginary line between this farthest point and your vehicle becomes longer and longer, which affords greater braking distance, hence more speed for your vehicle. At this time, you are now chasing the farthest point as it moves further and further away into the horizon, or adjusting your speed for the next approaching bend or other hazard.
Sorry for the long-winded description. I like to think of limit point analysis as though my vehicle is connected by a straight piece of string, which is attached to the furthest unobstructed point of the road I can see. This way, I am always alert to adjust my speed for anything in my path -- be it a bend or a pothole or anything else.
The only trap is to not become fixated by the farthest point. You must still be alert for any other hazards. This is where peripheral vision and experience come in handy. Judging your minimum braking distance also requires experience, so unless you have ample race/rally experience or car control training, leave plenty of room for error.
(It looks like Big Fat F'er has posted while I was busy typing. Fortunately, his and 7db's explanations are probably more worthy than mine.
)
>> Edited by Jungles on Sunday 19th February 12:41
I learnt about limit points for racing, rather than road driving, but it's fundamentally the same thing.
The basic idea is that you are never travelling into, through, and out of a bend, faster than the speed in which you can stop in a straight line.
As you approach the bend, there exists a distance (think of an imaginary line) between your vehicle and the farthest point of the bend your eyes can see in a straight line. This imaginary line must be straight and unobstructed -- it must not pass over anything that is a hazard or not part of the road surface itself.
At whatever speed you approach the start of the bend, this imaginary line must be at least as long as the minimum braking distance of your vehicle in a straight line. Therefore, at 100kph, this line may be 80m; or at 60kph, this line may be 40m. The faster your speed, the greater this imagery line must be, and therefore, the earlier you must begin your braking.
Therefore, we have the rule that your speed and the length of the imagary line is directly proportional to each other.
If the bend you're approaching is a tight one, this imaginary line will only be short. Therefore, your speed must be decreased in order to match the minimum braking distance of your vehicle. Logically, easier bends will have a longer imagery line, and therefore allows you higher speed since you can afford a longer distance in which to brake in a straight line.
After you enter the bend, the farthest point you can see into the bend will begin to move down the bend. When it does, you can ease off the brakes, by which time you will be travelling just at a speed that allows you to brake from your current position to the farthest point you can see, safely. Remember that you are still linked by an imaginary line from your vehicle to this farthest point, and that the line must be unobstructed. As this farthest point moves at a speed which matches your vehicle's speed, you can maintain this speed and travel through the bend at a safe pace. As I described before, the tigher the bend, the shorter the imaginary line between your vehicle and the farthest point, therefore dictating lower speed for your vehicle; vice versa for more easier bends.
As you approach the exit of the bend, the farthest point will begin to accelerate away from your vehicle. Therefore, the imaginary line between this farthest point and your vehicle becomes longer and longer, which affords greater braking distance, hence more speed for your vehicle. At this time, you are now chasing the farthest point as it moves further and further away into the horizon, or adjusting your speed for the next approaching bend or other hazard.
Sorry for the long-winded description. I like to think of limit point analysis as though my vehicle is connected by a straight piece of string, which is attached to the furthest unobstructed point of the road I can see. This way, I am always alert to adjust my speed for anything in my path -- be it a bend or a pothole or anything else.
The only trap is to not become fixated by the farthest point. You must still be alert for any other hazards. This is where peripheral vision and experience come in handy. Judging your minimum braking distance also requires experience, so unless you have ample race/rally experience or car control training, leave plenty of room for error.
(It looks like Big Fat F'er has posted while I was busy typing. Fortunately, his and 7db's explanations are probably more worthy than mine.
) >> Edited by Jungles on Sunday 19th February 12:41
What you're all saying is good and true, but is not the limit point theory as I understand it.
You all seem to be saying that you should be able to stop within the distance you can see so your speed should be proportional to the distance you can see. And that's good (as long as you take into account that there is a corner so you have much less grip available for braking), but it isn't what the limit point theory is about.
The limit point theory tells you about the curvature of the road at the furthest point you can see. It tells you whether the curve is tightening or opening up, which is inportant in order to recognise where the apex is, what line you shoul take and where you should be accelerating or braking.
Simply noticing how far away the limit of vision is, is good stuff that you should be doing all the time. But it takes a special effort to also see whether the limit point is approaching or receding, and how fast it is doing it. Once you get the hang of this you will find you 'know' what the road is going to do before you can see it properly and to a passenger who is not usin this technique it will seem as if you have mystical powers of foresight. Similarly when you are passengering in a car you will find yourself wondering why the driver isn't accelerating since the corner is clearly opening up, but they haven't realized it yet.
You all seem to be saying that you should be able to stop within the distance you can see so your speed should be proportional to the distance you can see. And that's good (as long as you take into account that there is a corner so you have much less grip available for braking), but it isn't what the limit point theory is about.
The limit point theory tells you about the curvature of the road at the furthest point you can see. It tells you whether the curve is tightening or opening up, which is inportant in order to recognise where the apex is, what line you shoul take and where you should be accelerating or braking.
Simply noticing how far away the limit of vision is, is good stuff that you should be doing all the time. But it takes a special effort to also see whether the limit point is approaching or receding, and how fast it is doing it. Once you get the hang of this you will find you 'know' what the road is going to do before you can see it properly and to a passenger who is not usin this technique it will seem as if you have mystical powers of foresight. Similarly when you are passengering in a car you will find yourself wondering why the driver isn't accelerating since the corner is clearly opening up, but they haven't realized it yet.
So far everybody seems to be singing from the same hymn sheet regarding limit point. The other thing you need to consider (which GreenV8S is sort of trying to bring out) is that the angle relative to the car that the limit point is gives a good measure of the curvature of the bend, i.e. if the limit point is at a 45 degree angle to your direction of travel, you now the bend is pretty tight.
But limit point is not the be-all and end-all of determining the appropriate speed, especially when you get away from modern roads and into those designed by the railway engineers of the 19th Century (they designed bends on a contracting radius because that's what you did on the railways to avoid spilling passengers' tea). Because the radius may tighten through such a bend and for stability you don't really want to be braking when you should have the car nicely balanced on the throttle.
For me, the process is:
1) Assess the severity of the bend using the limit point and everything else...
and calculate the maximum speed of cornering (this is a factor of both yourself and the car)
2) Reduce the speed to below this limit to allow:
* The entry, turn-in, sweep style of cornering
* Not to need to lose any speed mid-corner if the radius tightens sharply
3) Balance the car on the power (a whole topic all of its own)
4) Start cornering. If the radius does not deteriorate then you can add a little more power to start accelerating early up towards the maximum speed calculated in step 1.
5) Chase the limit point out of the corner using the throttle before you take any steering off
HTH
Dave
But limit point is not the be-all and end-all of determining the appropriate speed, especially when you get away from modern roads and into those designed by the railway engineers of the 19th Century (they designed bends on a contracting radius because that's what you did on the railways to avoid spilling passengers' tea). Because the radius may tighten through such a bend and for stability you don't really want to be braking when you should have the car nicely balanced on the throttle.
For me, the process is:
1) Assess the severity of the bend using the limit point and everything else...
and calculate the maximum speed of cornering (this is a factor of both yourself and the car) 2) Reduce the speed to below this limit to allow:
* The entry, turn-in, sweep style of cornering
* Not to need to lose any speed mid-corner if the radius tightens sharply
3) Balance the car on the power (a whole topic all of its own)
4) Start cornering. If the radius does not deteriorate then you can add a little more power to start accelerating early up towards the maximum speed calculated in step 1.
5) Chase the limit point out of the corner using the throttle before you take any steering off
HTH
Dave
That isn't quite it. The movement of the limit point tells you whether the curve is tightening or opening out. This is quite separate from any other clues you can get from the distance to the limit point, and what the angle is between the position of the limit point and your current bit of road. It's *almost* intuitive, and many of the posts have been about the same sort of effect, but the crucial point imo is that the rate of change of the distance between you and the limit point tells you about the curvature of the road at the limit point. If the limit point is getting closer the bend is tightening. If it is receding the bend is opening. You have to look for it, but once you see it the light bulb will go off. Suddenly you'll realize that you can anticipate what the road is going to do well before you can see it fully. Most people can judge what the road does half way to the limit point, and this technique roughly doubles your effective visual range.
>> Edited by GreenV8S on Sunday 19th February 15:05
>> Edited by GreenV8S on Sunday 19th February 15:05
Many thanks to everyone for their helpful comments.
I think that I understand it more now. When I initally started to study "Roadcraft" I thought it suggested that the limit point helped with appropriate speed based on cornering forces. I couldn't understand how the limit point could help. I realised that if I drove my Husband's Porche, I would have more grip, and could corner faster, than my Son's Punto. Yet the limit point would remain the same.
Having studied the comments above, I think I can now see how the limit point can give advance warning of what is happening to the bend. I can't say it's exactly clear, as I still think you could go around the corner (if it was a constant curve) at any speed, and the limit point would remain the same distance. It wouldn't come towards you, it wouldn't go away. So how would I actually know that 20mph, or 30mph, or even 60mph is the correct speed.
I will try and apply this on my next run (Wednesday). Any more helpful comments before then would be very much appreciated, as I have now just confused myself, while writing that I was no longer confused.
I think that I understand it more now. When I initally started to study "Roadcraft" I thought it suggested that the limit point helped with appropriate speed based on cornering forces. I couldn't understand how the limit point could help. I realised that if I drove my Husband's Porche, I would have more grip, and could corner faster, than my Son's Punto. Yet the limit point would remain the same.
Having studied the comments above, I think I can now see how the limit point can give advance warning of what is happening to the bend. I can't say it's exactly clear, as I still think you could go around the corner (if it was a constant curve) at any speed, and the limit point would remain the same distance. It wouldn't come towards you, it wouldn't go away. So how would I actually know that 20mph, or 30mph, or even 60mph is the correct speed.
I will try and apply this on my next run (Wednesday). Any more helpful comments before then would be very much appreciated, as I have now just confused myself, while writing that I was no longer confused.
No, the limit point DOES give an indication of the appropriate speed.
Let's say it's a right hand bend. You are looking across an arc - the distance you can see (the chord of the arc) goes from your eye to the offsite verge and back to the nearside verge. From your viewpoint, the two appear to meet. The total chord length (eye to offside verge to nearside verge) is the "limit distance" if you like.
Now let's say it's a tight bend, and you are approaching at 60 miles per hour, or about 30 metres per second (if my maths is right). Let's say you are 100 metres from the bend.
So you start with 100 m visibilty to the limit point.
A second later, you've moved 30 m, but because the bend is tight, the limit point has moved say 10 m. So your visibility to the limit point is now 80 m (ie the original 100, less the 30 you've travelled, plus the 10 that you can see further round).
A second later, you've moved another 30 m, and because the bend is constant radius, the limit point has again moved 10 m. So now the visibility to the limit point is (80 less 30 plus 10) = 60 m
A second later, same maths, limit visibility is 40 metres.
So in that case, the limit point APPEARS to be moving towards you (even although it's you that's moving towards it) because the visibility distance is reducing.
THAT is what gives you the clue that you might want to think about slowing down a bit.
If the bend is a bigger radius, then the chord of the arc is longer so you move 30 m/s, but the furthest point also moves 30 metres in each second. Your visibility distance doesn't reduce on the approach to the bend - so you can tell it's more open... so your speed is probably right.
This is briefer than I meant it to be but my dinner is ready!
Let's say it's a right hand bend. You are looking across an arc - the distance you can see (the chord of the arc) goes from your eye to the offsite verge and back to the nearside verge. From your viewpoint, the two appear to meet. The total chord length (eye to offside verge to nearside verge) is the "limit distance" if you like.
Now let's say it's a tight bend, and you are approaching at 60 miles per hour, or about 30 metres per second (if my maths is right). Let's say you are 100 metres from the bend.
So you start with 100 m visibilty to the limit point.
A second later, you've moved 30 m, but because the bend is tight, the limit point has moved say 10 m. So your visibility to the limit point is now 80 m (ie the original 100, less the 30 you've travelled, plus the 10 that you can see further round).
A second later, you've moved another 30 m, and because the bend is constant radius, the limit point has again moved 10 m. So now the visibility to the limit point is (80 less 30 plus 10) = 60 m
A second later, same maths, limit visibility is 40 metres.
So in that case, the limit point APPEARS to be moving towards you (even although it's you that's moving towards it) because the visibility distance is reducing.
THAT is what gives you the clue that you might want to think about slowing down a bit.
If the bend is a bigger radius, then the chord of the arc is longer so you move 30 m/s, but the furthest point also moves 30 metres in each second. Your visibility distance doesn't reduce on the approach to the bend - so you can tell it's more open... so your speed is probably right.
This is briefer than I meant it to be but my dinner is ready!
Lady Godiva said:
I can't say it's exactly clear, as I still think you could go around the corner (if it was a constant curve) at any speed, and the limit point would remain the same distance. It wouldn't come towards you, it wouldn't go away. So how would I actually know that 20mph, or 30mph, or even 60mph is the correct speed.
You are right - the limit point would be a constant distance. The insight is that that distance chooses the speed for you -- it's the speed at which you are comfortable you can stop in that distance.
Imagine the limit point is the back of another car, and don't tailgate it.
As others have said -- it might be wise to drop back from the limit point speed for reasons of traction, confidence and stability.
Lady Godiva said:
I can't say it's exactly clear, as I still think you could go around the corner (if it was a constant curve) at any speed, and the limit point would remain the same distance. It wouldn't come towards you, it wouldn't go away. So how would I actually know that 20mph, or 30mph, or even 60mph is the correct speed.
You're right, the distance would be constant when you were ON the curve. Where the limit point is helpful is on the straight(er) APPROACH to the curve, where it helps you determine the severity of the curve and hence the appropriate speed, before you get onto the bend.
I have an easy one:
Man on a motorbike is in front of you "at the limit point" (assuming he's on some stupidly quick bike)
If he pulls away from you, you can accelerate
If your approaching him, decelerate
If your going up his *ss, get on the brakes
Simple & very effective, easy for newbies to understand too.
Your road positioning also effects his position, and its clearly seen how effective this can be seen to increase his distance on you (yor safety margin) By moving over 1ft to right on left hand sweeper he suddenly appears to have gained 200yards on you. Thats 200yards of visibilty you've gained just by positioning your car differently. So you are now controlling that limit point too.
>> Edited by cptsideways on Sunday 19th February 19:05
Man on a motorbike is in front of you "at the limit point" (assuming he's on some stupidly quick bike)
If he pulls away from you, you can accelerate
If your approaching him, decelerate
If your going up his *ss, get on the brakes
Simple & very effective, easy for newbies to understand too.
Your road positioning also effects his position, and its clearly seen how effective this can be seen to increase his distance on you (yor safety margin) By moving over 1ft to right on left hand sweeper he suddenly appears to have gained 200yards on you. Thats 200yards of visibilty you've gained just by positioning your car differently. So you are now controlling that limit point too.
>> Edited by cptsideways on Sunday 19th February 19:05
Lady Godiva said:In the distance between yourself and the limit point, you should be able to detect, assess, and act upon a hazard (people will just say "brake in the distance you can see", but that is a gross simplification). If the bend is a nice easy one, you should have a limit point that is a long distance away from you, which affords you the ability to travel faster due to having more distance in which to react. If the bend is tight, the limit point is closer, so you must travel more slowly to offset the decrease in the available distance in which you can take action.
I can't say it's exactly clear, as I still think you could go around the corner (if it was a constant curve) at any speed, and the limit point would remain the same distance. It wouldn't come towards you, it wouldn't go away. So how would I actually know that 20mph, or 30mph, or even 60mph is the correct speed.
It's a bit like determining the safe distance in which to travel behind another vehicle. Refer to Cpt Sideways' post.
That's only a general idea, of course. As GreenV8 reminds us, there are more factors to determining correct cornering speeds, such as road surface type, camber, rate of change of curvature, etc. But for most drivers under most conditions, limit points and room for error will serve well enough.
>> Edited by Jungles on Sunday 19th February 21:26
Here's another technique to explore for blind bends.
If you are approaching a blind bend and want to use the limit point, you can raise your vision above the road surface. If you're in a left-hand bend, look at the right-hand hedge/tree row. If the hedge is moving towards you, ease off. When the right-hand hedge row starts to move away from you, you can accelerate.
Obviously, look at the left-hand hedge row for right-hand blind bends.
Personally, I struggled with using the Limit Point for ages until a police driving school instructor suggested this technique.
HTH
If you are approaching a blind bend and want to use the limit point, you can raise your vision above the road surface. If you're in a left-hand bend, look at the right-hand hedge/tree row. If the hedge is moving towards you, ease off. When the right-hand hedge row starts to move away from you, you can accelerate.
Obviously, look at the left-hand hedge row for right-hand blind bends.
Personally, I struggled with using the Limit Point for ages until a police driving school instructor suggested this technique.
HTH
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