Why do riders crash in corners? After all, had they not successfully negotiated a lot of other corners before they reached the one that got them? Was there something about certain corners that made them more likely to be accident sites? Why did these riders select a particular corner entry speed that proved to be so spectacularly incorrect? What is the process that we use for judging the severity of a corner and selecting a suitable entry speed? Do we all use the same method, or are there a number of ways in which we can analyse a corner before we reach it?
Some years ago a team of scientists wanted to answer these questions, so they wired up some drivers with a device that overlays the point of a drivers gaze onto a video, so that they could see where they were looking as they entered and negotiated a corner. Their findings were quite interesting as it seemed that their initial gaze was directed towards something called the Tangent Point (TP) which is the point of the inner lane boundary bearing the most curvature on the retina. Looking at a 2D picture of a road, we see that this point of maximum curvature forms a hook in the kerb.
The scientists found that most of their test drivers naturally fixated on the TP as they negotiated a bend and in fact about 75% of their time was spent looking at this point rather than any other point on the road. They also found that drivers on their way into a corner glanced at the TP about two to three seconds before turning the wheel and then reacquired it at around half a second before making a steering input. The degree of hook at the tangent point will give a fair estimation of the severity of the upcoming bend and the rate at which it is coming towards a driver will give a good indication of by how much they might need to reduce their speed.
It seems then that the degree of curvature at the Tangent Point is the crucial piece of information we all use to set ourselves up for a corner. If this is the case, then if we get the corner wrong, we must have misinterpreted the curvature at the Tangent Point. This then is where the left hand bend finally begins to offer up its secrets.
Most of us think of a corner as being a constant curve linking two straights going in different directions. For most of the time, we would be right in this assumption, but not always. We now need to study a bit of history and strangely not of road transport, but of the railways. When speeds on the railways were very slow, straights were indeed connected together by constant curvature bends as this was by far the easiest way of planning the layout of the tracks. It soon became apparent that as speeds increased, the passengers were getting thrown to one side of the carriage as the train went directly from the straight onto the curve. This sudden onset of ‘G’ in the corner was considered to be a bad thing for the passengers, so the railway companies started to look for a way round the problem. They soon came up with the idea of the spiral transition, or clothoid easement curve. The easement curve as its name suggests, eases the carriage from going straight to going round the ultimate curve. By using this track layout, the railway companies discovered that the amount of ‘G’ could be increased gradually, thus allowing the passengers to feel its gentle onset and brace against it so no more spilt tea and passengers heaping up on top of each other in bends, a perfect result!
All would have been well had the spiral transition or easement curve stayed where it was on the railways, but somehow it escaped. Those readers who have done an advanced riding course will be well aware of the quest to get riders to turn the bike quickly. Turning quickly is a very good thing to do, but no matter how quickly you turn a bike or a car, the path you take from going straight ahead to turning will ALWAYS be a spiral. If you can imagine turning slowly, then at the beginning of the turn the bike will not be banked over very far so it describes quite a wide arc. As the angle of bank increases, the radius of the arc gets smaller and smaller until it matches the arc we need to negotiate the corner, this then is a classic spiral. We can’t help but do this as it’s the laws of physics working away unnoticed and unheeded.
Back in the mists of time a chap called Henry Criswell who was the county surveyor of Devon, decided that in order to promote passenger comfort; roads should be designed with easement or transition spirals installed at the beginning of bends. His book “Highway Spirals, Banking and Vertical Curves” Became the seminal work for all highway design and construction engineers and is still in use today. What he singularly failed to realise at the time was that the driver already had control over the rate of spiralling into a bend by dint of their ability to steer the vehicle, which is something that a railway train, constrained by the track, simply cannot do.
Thus the spiral easement became part of our modern roads landscape and everybody went on their way rejoicing in improved passenger comfort. Except that it didn’t improve passenger comfort as that was under the control of the driver not the road. What it did do was to fool riders and drivers into the possibility of making a very serious error. Let’s hark back to the beginning of this little piece when we discovered that the rate of curvature at the Tangent Point was critical in our understanding of the severity of an upcoming corner. The clothoid spiral easement throws an enormous spanner in the works by showing a radius of curvature that is far less severe than the radius of the actual curve!
If we judge the corner by the curvature at the Tangent Point and the curvature is not that of the actual corner, we are going to get it wrong. If we get a left hand bend wrong, then we will run wide into the oncoming traffic, which unless we are very lucky, will kill us stone dead.
Is this then the secret of the single vehicle accident on rural left hand bends? Many years ago, back in 1977, a young engineer called Doug Stewart who was working for Aberdeenshire Highways Department noticed a significant number of accidents were occurring on just a few bends in the County. Being of an enquiring mind, he wanted to know what the difference was between these bends and other equally severe bends that had noticeably fewer accidents. The bends in question, quite naturally featured our old friend the easement curve or spiral transition. As a Highways Engineer, he was lucky when one of the killer curves was scheduled for re-surfacing. He persuaded the Council to re-align the curve from a spiral to a circular curve so that they could see if the realignment made any difference to the accident statistics. As you might have guessed, the number of accidents fell away significantly after the change.
So now we have identified the problem what can we do about it?
With the best will in the world it would take forever to realign every spiral transition in the country and that is supposing that the powers that be admit that they are a significant a problem in the first place. Once again it is down to us riders to work out a strategy for handling transitional spirals so that we can identify and negotiate them without running wide. First thing I did when I learnt about these horrid things was to go out and see if I could find any in the local area. That proved to be an extremely difficult task as the easement curves are very well disguised, which I suppose is the reason we have the problem in the first place! I could only identify a transitional spiral for certain after I had gone round it and not before. The key identifier of a transitional spiral is the fact that you have to make two steering inputs in order to negotiate it. The first carries you round the spiral and the second to carry you round the actual corner. This, sadly, is thruppeny bit-ing, yet unless we do it, we cannot safely negotiate a bend with a spiral transition, this is a terrible conundrum, but it is the lesser of two evils, either steer once and run wide or steer twice and make it round.
As we can’t tell by looking whether or not a corner has a spiral transition, we must be prepared to make two steering inputs at any corner, especially those we are unfamiliar with. Being prepared in this way means that we don’t have to work out a strategy on the fly, we can simply go to a pre-planned response once we discover the true nature of the bend. This does mean that we have to keep a fair bit of bank in the bank, but all riders should have no problem with doing that.