22/05/2024
*** TIPS ON TUESDAY *** Advanced Errors #4 The limitations of the Limit Point
Here's an absolutely basic riding rule. To avoid having to make an emergency stop, we should always aim to be able to stop WELL WITHIN in the distance we can SEE TO BE CLEAR and also EXPECT TO REMAIN CLEAR. This is a prime directive and explained in Motorcycle Roadcraft. This point where we lose an an uninterrupted view the road AND the road SURFACE is often called the Limit Point. Alternative names are the Vanishing Point, the Distance Point and the Convergence Point. Unless someone has some particular insight as to why they differ, I'd say they are all the same thing - the furthest point we can see the road ahead.
What often gets lost in translation is the absolute need to take account not just of the view of the road, but of the road surface. It's all too easy to think that we can ride into a corner where we can see the line of the bend and which we can see is clear of traffic, to be tripped up by something hidden.
On one memorable occasion my 'perfect' line though a corner revealed a low-loader parked up, just below my line-of-sight and perfectly concealed by a drystone wall, plus the associated mud from the digger that had been off-loaded and was now working in a field. Even though my line was wide enough to clear the parked low-loader, the surface controls our ability to steer and stop. Even long grass can conceal wet surfaces, mud, loose chippings left behind after resurfacing or a disintegrating surface in dire need of repair.
Motorcycle Roadcraft also talks about judging the sharpness of a corner using the 'arrowhead' formed where the two sides of the road meet. The more the arrowhead is turned towards us, the sharper the bend. I'd say that's a USEFUL technique but one that needs to be applied with caution since it's far from foolproof.
Firstly, we are told we can match speed to the bend by using ‘Limit Point Analysis’. The theory is that if the Limit Point is approaching (getting closer) the bend is tightening up and we should slow down. If it is receding (getting further away), the bend is opening up, and we can consider accelerating.
My thinking is that this advice was written firstly for cars, which don't suffer the same sort of ‘rocking-horse’ movement of the suspension that's caused by off-on throttle. And the softer the machine’s suspension, the more the bike pitches back and forth. My old NX650 Dominator with relatively soft, long travel suspension really wasn't suited to this approach.
The advice was also clearly written for police pursuit riders who needed to get through a corner as fast as they could. We simply don't need to 'chase the Limit Point' in the same way. If we enter at a more modest speed and aren't cornering anywhere near the limit of grip, we can adapt to small changes of radius by adding or taking away lean angle.
Two. We know NOTHING about the road BEYOND the Limit Point. We have to be alert to the presence of blind bends. Related to this, sometimes the Limit Point can move very rapidly towards us. The implication is a decreasing radius corner. There is an inevitable cognitive delay whilst we process this changing information and we could find it difficult to react promptly by slowing, steering or both.
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Thirdly, thanks to blind crests and hidden dips, sometimes the Limit Point simply vanishes. Take a look at the photo. We can see the road further ahead, but there's a totally blind area. Years ago, I topped a blind crest with the road clearly visible straight ahead, to discover a sharp right-hand bend leading down to a hairpin bend over a bridge with a climb to a second, equally sharp right-hander rejoining the line of the original road.
Four and often forgotten, on single track roads we may need to stop in much less than HALF the distance we can see to be clear - the approaching vehicle has to see us and stop too, and there's a fair chance the driver is also setting his or her speed to the furthest point they can see. That's why there are regular skidmarks and occasional piles of broken headlight glass on narrow lanes.
Fifthly - and my biggest issue - is that focussing on the Limit Point exclusively means we may not notice DEAD GROUND to the sides of our path, which is where scanning for VISION BLOCKERS and SURPRISE HORIZONS comes in very useful indeed. If we set out speed exclusively to the Limit Point, if a vehicle, pedestrian or even an animal appears from a Surprise Horizon between us and the Limit Point, we'll struggle to stop if an obstruction suddenly appears between our current position and the Limit Point.
Conclusion - in EVERY case, a good MARGIN FOR ERROR always makes sense. And don't forget to read the information being delivered to you via road signs and road markings. And if you don't know how to do that, can I politely suggest you book a short ONLINE COACHING SESSION with me? I'll take you through the efforts the Highways Engineers make to inform all road users about what's happening just ahead and potentially out of sight
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