Assessment of "limit point" on double bends (or double apex)

[captainaverage]

I have been looking into advance driving techniques. If there is a bend that suddenly becomes tighter how do you anticipate/assess this? In this case is it ok to mid corner brake? Im guessing it's probably not. Do you trail brake in this case? What is the approach to this considering no car is infront of you? How do you assess a blind bend to find the limit point especially when it becomes tighter?

Thanks

[Rick101]

I think if your obs are good, 'suddenly' should be a rare thing, if ever.
The limit point would have stopped moving away from you, that should be an early clue. If it's stationary then you could reasonably expect the bend to be tightening. If you can't see the 2nd apex to due it being a double, I would have expected some signage to be in place in most cases.

With regard what to do, I really see no major issue with a little braking mid corner providing you are nowhere near the limit of the car, or more likely yourself. It will unbalance the car, but I'd rather that than end up in a ditch.

The best solution is to manage speed on approach. I think much like track driving, don't commit to the accelerator until you know you can keep your foot in.

Edited by Rick101 on Saturday 24th March 00:25

[S. Gonzales Esq.]

captainaverage said:

I have been looking into advance driving techniques. If there is a bend that suddenly becomes tighter how do you anticipate/assess this? In this case is it ok to mid corner brake? Im guessing it's probably not. Do you trail brake in this case? What is the approach to this considering no car is infront of you? How do you assess a blind bend to find the limit point especially when it becomes tighter?

Thanks

The term Limit Point refers to the furthest place on the surface of your side of the road that you have a clear view to at that particular moment. The distance to the Limit Point gives you your available stopping distance. Combine that with the road conditions and the capabilities of you and your vehicle, and there's your safe speed for that moment.

Assuming you don't have a clear view through a bend, then here's how you could use it:

If the Limit Point is getting closer to you, your stopping distance is decreasing and you should be losing speed.

If the Limit Point is a constant distance from you ('matched') then the bend is a constant radius and you should maintain speed (using a 'balanced throttle').

If the Limit Point is moving away from you then the radius of the bend is increasing, and as you start to remove steering lock you can accelerate.

To answer your specific question - if the bend continues to tighten, just keep slowing until the movement of the Limit Point is matched with yours. Getting the entry speed correct is the critical thing, and that comes from good observation and being prepared to use the brakes hard enough.


Things to note:

While you're practicing this it's easy to devote too much attention to looking at the Limit Point, and forget to actually look where you're going. Looking a long way ahead is still the best way to avoid surprises in bends, so keep your vision up.

When you start to pay attention to the Limit Point, you'll find that in most bends it'll be matched before you even start to turn the steering wheel. If you're still braking at this point, you've not slowed early enough.

Don't over-commit to accelerating if the view hasn't fully opened yet. Beware of double-apex bends that tighten unexpectedly.

If you get it wrong and have to brake mid-bend it shouldn't be a problem. Be as smooth as possible, and simultaneously ease off the steering. Weight transfer will both lighten the back end and make the car more responsive to steering input, so if you don't reduce it then you're more likely to experience lift-off oversteer.

If you have a clear view through the bend then trail-braking is a valid technique, but only when you're completely at home with how Limit Points work. Having to use it to cover up errors in bend assessment is not something to aim for.


Finally, if it's difficult to get your head around Limit Points you could try this:

• For a bend that you don't have a clear view through, your view of the road ahead will decrease, stabilise, and then increase.

• Your minimum speed should coincide with the place where you have the least vision.

• Look ahead, assess where that least vision may be, and get your braking (and gear change if necessary) done before you get there.

Edited by S. Gonzales Esq. on Saturday 24th March 13:59

[850R]

Can't really add to the fantastic stuff from the other replies, another linked observation might be on your approach, the line of hedgerows that follow the road up ahead and often give an indication of where the road may go, this is not gospel though as can be misleading for example a farmers access into his farm, also if a car is approaching you, I often look out for the speed to which it is exiting the bend, and angle at which it appears in my view, again please accept these are not hard and fast rules simply observations linked to the system. At least then you won't become part of the "suddenly" club

[johnao]

captainaverage said:

I have been looking into advance driving techniques. If there is a bend that suddenly becomes tighter how do you anticipate/assess this? In this case is it ok to mid corner brake? Im guessing it's probably not. Do you trail brake in this case? What is the approach to this considering no car is infront of you? How do you assess a blind bend to find the limit point especially when it becomes tighter?

Thanks

I agree with everything others have said.

I note that you pose the question "how do you anticipate this" in your second sentence. I have noticed a fairly common fault with "limit point newbies" in that they will, in the early stages of using limit point technique to exit a bend, anticipate that the limit point is going to run before it does so. This will lead to early accelerating towards the second apex if there is one, or accelerating too early whilst still in the bend. The answer to this is simple... never anticipate what the limit point is going to do; react only to what your eyes are actually telling you about what the limit point is doing.

[waremark]

captainaverage said:

I have been looking into advance driving techniques. If there is a bend that suddenly becomes tighter how do you anticipate/assess this? In this case is it ok to mid corner brake? Im guessing it's probably not. Do you trail brake in this case? What is the approach to this considering no car is infront of you? How do you assess a blind bend to find the limit point especially when it becomes tighter?

Thanks

My answer is a little different from most of the others. Best practice is to be on the gas enough to maintain speed once you start to see round the bend far enough for the limit point to remain a constant distance away from you (what some call limit point matched). Your speed must be slow enough that you can stop safely if there is a blockage just beyond the limit point. That includes dealing with any lack of stability from changing from gas to brakes.

Now, if the first section of the bend is constant radius, but further into the bend the radius tightens, you will see this as getting closer to the limit point, and you must change from gas to brakes. You won't need to brake as hard as if there was a complete blockage so it must be completely safe. If the radius tightens and you didn't need to brake, then ask yourself whether you slowed more than necessary before entering the bend.

Trail braking is a completely different approach, routinely starting to steer while still at least to some extent on the brakes. In a typical vision limited bend, it does not result in having the lowest speed where vision is shortest - and is noot consistent with the Roadcraft system used by the police, IAM and Rospa. It is probably the fastest way round a bend once you know for certain that you won't need to brake again, and hence is widely used in track driving.

[Pica-Pica]

850R said:

Can't really add to the fantastic stuff from the other replies, another linked observation might be on your approach, the line of hedgerows that follow the road up ahead and often give an indication of where the road may go, this is not gospel though as can be misleading for example a farmers access into his farm, also if a car is approaching you, I often look out for the speed to which it is exiting the bend, and angle at which it appears in my view, again please accept these are not hard and fast rules simply observations linked to the system. At least then you won't become part of the "suddenly" club

...and never go by telegraph poles either, they often veer across fields - after all, they are there to service the houses, not to indicate the road layout.

[captainaverage]

S. Gonzales Esq. said:

The term Limit Point refers to the furthest place on the surface of your side of the road that you have a clear view to at that particular moment. The distance to the Limit Point gives you your available stopping distance. Combine that with the road conditions and the capabilities of you and your vehicle, and there's your safe speed for that moment.

Assuming you don't have a clear view through a bend, then here's how you could use it:

If the Limit Point is getting closer to you, your stopping distance is decreasing and you should be losing speed.

If the Limit Point is a constant distance from you ('matched') then the bend is a constant radius and you should maintain speed (using a 'balanced throttle').

If the Limit Point is moving away from you then the radius of the bend is increasing, and as you start to remove steering lock you can accelerate.

To answer your specific question - if the bend continues to tighten, just keep slowing until the movement of the Limit Point is matched with yours. Getting the entry speed correct is the critical thing, and that comes from good observation and being prepared to use the brakes hard enough.


Things to note:

While you're practicing this it's easy to devote too much attention to looking at the Limit Point, and forget to actually look where you're going. Looking a long way ahead is still the best way to avoid surprises in bends, so keep your vision up.

When you start to pay attention to the Limit Point, you'll find that in most bends it'll be matched before you even start to turn the steering wheel. If you're still braking at this point, you've not slowed early enough.

Don't over-commit to accelerating if the view hasn't fully opened yet. Beware of double-apex bends that tighten unexpectedly.

If you get it wrong and have to brake mid-bend it shouldn't be a problem. Be as smooth as possible, and simultaneously ease off the steering. Weight transfer will both lighten the back end and make the car more responsive to steering input, so if you don't reduce it then you're more likely to experience lift-off oversteer.

If you have a clear view through the bend then trail-braking is a valid technique, but only when you're completely at home with how Limit Points work. Having to use it to cover up errors in bend assessment is not something to aim for.


Finally, if it's difficult to get your head around Limit Points you could try this:

• For a bend that you don't have a clear view through, your view of the road ahead will decrease, stabilise, and then increase.

• Your minimum speed should coincide with the place where you have the least vision.

• Look ahead, assess where that least vision may be, and get your braking (and gear change if necessary) done before you get there.

Edited by S. Gonzales Esq. on Saturday 24th March 13:59

Thank you to everyone who has given input.

S. Gonzales Esq:

I have got it in my head that the distance to limit point needs to be a straight, is this true or can it be a distance at a slight radius?

I can't get a link to work but on google maps there is a bend with coordinates 53.2604977,-1.7732784

If you look at the street view where the red van is going will the point where the road disappears behind the hedge on the right be the limit point or the one opposite that? If you drew a line perpendicular to the curve's tangent, they both look the same distance.

I would start to brake just a little before the 50 mph sign.

You say if the bend continues to tighten then just keep slowing. In this case if smoothly lifting off the throttle doesn't slow you down enough, it's okay to brake mid-corner? I am sorry I am asking this again but it's because I hear that you must never brake mid-corner, Is that to do with track only?

If you are fimiliar with snetterton, how would you tackle the Coram corner if the right hand side was completely covered by a wall. In that case would the only way to really achieve maximum speed would be to know the corner/experience?

What made me ask this question was a specific turn which is M53(N) meeting M56(E) (Cheshire motorway). The joining road is a bit tighter than the middle, the middle becomes less tight and you can see the road vanishing consistantly but then it becomes tight again and the road starts to vanish early.

Edited by captainaverage on Saturday 24th March 15:22

[captainaverage]

waremark said:

My answer is a little different from most of the others. Best practice is to be on the gas enough to maintain speed once you start to see round the bend far enough for the limit point to remain a constant distance away from you (what some call limit point matched). Your speed must be slow enough that you can stop safely if there is a blockage just beyond the limit point. That includes dealing with any lack of stability from changing from gas to brakes.

Now, if the first section of the bend is constant radius, but further into the bend the radius tightens, you will see this as getting closer to the limit point, and you must change from gas to brakes. You won't need to brake as hard as if there was a complete blockage so it must be completely safe. If the radius tightens and you didn't need to brake, then ask yourself whether you slowed more than necessary before entering the bend.

Trail braking is a completely different approach, routinely starting to steer while still at least to some extent on the brakes. In a typical vision limited bend, it does not result in having the lowest speed where vision is shortest - and is noot consistent with the Roadcraft system used by the police, IAM and Rospa. It is probably the fastest way round a bend once you know for certain that you won't need to brake again, and hence is widely used in track driving.

Thank you, this is something I wanted to know.

[captainaverage]

Rick101 said:

I think if your obs are good, 'suddenly' should be a rare thing, if ever.
The limit point would have stopped moving away from you, that should be an early clue. If it's stationary then you could reasonably expect the bend to be tightening. If you can't see the 2nd apex to due it being a double, I would have expected some signage to be in place in most cases.

With regard what to do, I really see no major issue with a little braking mid corner providing you are nowhere near the limit of the car, or more likely yourself. It will unbalance the car, but I'd rather that than end up in a ditch.

The best solution is to manage speed on approach. I think much like track driving, don't commit to the accelerator until you know you can keep your foot in.

Edited by Rick101 on Saturday 24th March 00:25

Other than the limit point, do you know what other techniques are there to adjust the entry speed correctly?

[Rick101]

I think a lot of it just comes down to observations.

A long way out, can you see where the road goes ahead? For example a valley where you can get an idea of the road up ahead.
Traffic ahead. Are they good drivers? Can you gauge their speed or even see brake lights a long way off?
Signage. One of my favourite subjects It is not just the speed board to look out for. There are often lots of signs which go un-noticed. Hazard line, chevrons, marker posts etc.
There may even be tyre marks on the road if it's a particularly bad spot.

It's a lot to take in. There was a comment earlier about actually looking at the road surface when you're working the limit point. I think there comes a point of information overload. Easy to do when you are stationary looking and learning, but people sometimes forget you are doing high speed and unless you are a particulally talented individual, it is near impossible to see everything. I think even at 60, you are covering about 30ft per second and in all honesty most of us will be covering quite a bit more than that.

[waremark]

Rick101 said:

I think even at 60, you are covering about 30ft per second and in all honesty most of us will be covering quite a bit more than that.

88 feet per second at 60 mph.

[Rick101]

Ahh yes, was thinking of my Railway distances 1760 per mile but that is yards not feet.

[Dr Jekyll]

Pica-Pica said:

...and never go by telegraph poles either, they often veer across fields - after all, they are there to service the houses, not to indicate the road layout.

On the other hand if you're approaching what seems to be a gentle bend and notice that the telegraph poles go off at 90 degrees there's no reason you can't take that into consideration.

[S. Gonzales Esq.]

I'll have a look at those links when I'm not on the phone, and will get back to you.

[S. Gonzales Esq.]

With acknowledgement of Reg Local's post of a while back that did something similar, here's a sequence of images running through that bend with the Limit Point indicated.

Which one of these is the place where vision (and available stopping distance) is the shortest?


1:


2:


3:


4:


5:


6:


7:


8:


9:

[captainaverage]

S. Gonzales Esq. said:

With acknowledgement of Reg Local's post of a while back that did something similar, here's a sequence of images running through that bend with the Limit Point indicated.

Which one of these is the place where vision (and available stopping distance) is the shortest?


1:


2:


3:


4:


5:


6:


7:


8:


9:

I would say image 6 shows the shortest stopping distance. I think there is less visibility to what is around the bend as well.

I also think the limit point doesn't move by much in image 1-4 but the stopping distance is more but I think image 6 would be the answer.

[Ardennes92]

Interesting discussion, but when did "vanishing point" become "limit point"?

[captainaverage]

Ardennes92 said:

Interesting discussion, but when did "vanishing point" become "limit point"?

I thought limit point was the point beyond which you can't see the road and your stopping distance keeps becoming shorter and shorter if it doesn't move away from you therefore vanishing point (road vanishing) = limit point?

I'm no driving expert so I might be wrong, I'm only going by what I've read and watched on YouTube.

[daz6215]

Don't over complicate things by becoming obsessed with the 'limit point', and don't fall into the trap of thinking you need to go fast around a corner on a public road, you don't! The limit point is a tool in the box, it's not the be all and end all, good bend assessment requires you to look at the 'big picture' and gather as much information as possible; this will help with your decision making. Keep it simple and drive into corners at a comfortable safe speed.

Edited by daz6215 on Sunday 25th March 18:31