Limit Points and Cornering

When approaching a bend, most drivers are thinking “how fast can I go round this bend?”, rather than “Can I stop if needed?”. 

 

This is the core reason we use limit points – to assess the severity of the bend so we can adjust our speed and consider the appropriate gear to take the bend safely.  We cover it in our Associate’s classroom session, but there is a lot to take in during that morning session.  This article is meant to be a guide to using limit points.

 

Cornering

First we need to set the scene with a few notes on cornering.  In the earlier versions of Roadcraft (1970/80s), the impact of forces acting on a vehicle during cornering were analysed.   Roadcraft stated “A vehicle is most stable when traveling on a straight and level course at a constant speed.  When it is driven around a curve certain forces are set up which affect its road holding capabilities and unless tyres retain sufficient grip on the road the driver will be unable to maintain his selected course”.

 

Momentum (m) (a combination of speed and weight of vehicle) combined with centrifugal (c) (caused by steering) force gives a resultant (r) force which is force that pushes our vehicle towards the outside of the bend.  The old Roadcraft diagram on the left shows the forces, whilst the one underneath from the current edition of Roadcraft demonstrates how these act at differing points whilst cornering.

By taking up the correct position on a bend, we not only give ourselves a better view of potential hazards, but also widen the radius of the bend your vehicle travels which reduces the centrifugal force on the vehicle.

 

Limit Points

Roadcraft states ‘The limit point gives you a systematic way of judging the correct speed to use though the bend

 

When approaching a bend, you will be taking in information such as road signs, road markings, where hedges indicate the road is going, tops of vehicles visible over hedges, telegraph poles etc.  These are all very useful aids, but can be misleading, for instance telegraph poles can deviate away from a road and go across fields; multiple chevron boards actually suggest there may have been drivers before you that have got the bend wrong, not the actual severity of the bend.

 

The limit point itself is the furthest point along a road that you have a clear and uninterrupted view of the road surface.  In other words the limit point is the furthest point at which you can see and be able to stop within.

limit 4

View 1 is the first view, and shows your limit point (where the verges merge, cutting off your view of the road).

limit 5

Watch your limit point as you drive closer to the bend.  In View 2, the distance to the bend has halved, but the limit point remains static.  If it remains static or comes closer, you should reduce your speed so you can still stop within the view that can be seen.

In this bend, you have to get quite close before the limit point starts to move (open out).

limit 6

If your speed matches the speed at which the limit point is opening out, then this is the right speed to continue round the bend (just to make matters more difficult, the bend here is also going downhill!)

In view 4 the view is opening up quite rapidly, so you can maintain your speed or think about gently accelerating.

limit 7

Finally, as you progress round the bend (view 5), the bend opens up completely, the road is clear and you can now accelerate away.

limit 8

Not all bends are constant.  As well as narrow country lanes, where the road direction/corners have evolved from meandering sheep paths (etc), some bends on major roads have been specifically designed with a tightening curve (double apex).  These bends will result in the initial limit point tightening up at some point on the bend, so be prepared to adjust your speed and steering if this happens.

 

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