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Rick Appleton, CANAM Observer writes...

Wednesday, 8 August 2012

In the 'readers letters' section of the winter edition of the IAM Magazine, there was a letter from a Michael Mangan which commented on the previous issue's article on Information, Position, Speed, Gear, Acceleration. Mr Mangan wrote that he finds that the 'acceleration' part of IPSGA 'is unnecessary and can be actively confusing'.

I agree that it can be confusing but it is far from unnecessary. The IPSGA article in the same issue helps to put it into perspective, stressing that all the 5 aspects are intimately interrelated.

However, I think a problem can arise from a misunderstanding of the term acceleration, in this context. As Mr Mangan states, although it comes at the end of ISPGA, the 'A' does NOT apply 'just to the final stage' of the system - something that happens after the hazard has been negotiated - rather, it is a continuous process. It is sometimes referred to as 'acceleration sense' or 'balancing the car with the throttle'. This concept may require some explanation; in my opinion a proper understanding is fundamental to an appreciation of the importance of the whole system.

To grasp it fully, it is helpful to have some knowledge of physics, in particular of the forces which influence any lump of matter whilst it is in motion and the effect that these forces have on the way it behaves.

A car travelling in a straight line has momentum, which is a product of its weight and its speed.

Momentum wants to travel in a straight line (↑).
[The greater the momentum (speed x weight) the more difficult it is to change its direction.]

Other forces affecting the car's straight line course are
a. engine drive (↑).
b. wheel friction with the road surface (↓) - this is influenced by the quality of the tyres and the 'slipperiness' of the road surface.
c. air resistance (↓) and
d. wind resistance (which depends on wind direction)

The driver subconsciously balances all these forces with the throttle to maintain a steady straight line speed. In a straight line all the forces (apart from wind effect) act in line with the tyre tread.

What happens when the steering wheel is turned?

In a left-hand turn the friction effect (↓) is diverted across the tread (¡ú). It also increases in size. This slows the car's speed slightly and acts with the momentum to create deviation:

Momentum ↑ + Friction ↙ = Deviation ↖

At the same time centrifugal force comes into effect. Everyone should be familiar with the concept of a motorcycle leaning over in a corner to counteract this force. The same force acts on a car, but a car cannot compensate like a motorcycle. The heavy upper part of the car wants to continue in a straight line, whilst the tyres (at the bottom) are in effect trying to trip the car up; the result is that the car tends to lean towards the outer side of the curve and its weight is transferred from the inner-side wheels to the outer-side wheels resulting in loss of adhesion and potential under-steer and skidding. This uncomfortable 'yawing' effect will be increased at higher speeds and with tightening of the curve, and will also be accentuated by further steering adjustment as will happen in the majority of situations. It will be greatly increased if the brakes are applied.

Hence the importance of the 'P' and 'S' parts of IPSGA: ALL excess speed should be taken off before entering the bend.

Fortunately there is a very simple way of correcting the problem. This is achieved by applying more turning force ↖ using engine drive (which always acts in the same direction as the tyre tread) by the simple expedient of applying gentle positive throttle. The car is now drawn into the curve, centrifugal force is counteracted, and the car remains stable with weight distributed equally through all four road wheels. The car can be balanced in a level position through a series of bends simply by use of the throttle.

This is called 'acceleration sense': in order to be able to use acceleration sense it is essential that the car is in the correct gear. This means that the engine is operating within the power band, giving maximum flexibility. This varies from car to car but is usually around 2,000 to 3,500 revs. Knowing the exact revs is unimportant - one needs to learn to 'sense it'.

In simple terms

Any curve will act to reduce speed. The aim of acceleration sense is to apply sufficient positive throttle to just counteract this reduction, plus a little bit more. When done correctly this gives a feeling 'as if the car is negotiating the corner on rails'. When driving an automatic gearbox, this technique can only be used if the gearbox is locked into the appropriate gear - when 'making progress' through a series of bends using acceleration sense, the last thing you want is a sudden gear change in the middle.

Post Script

Many years ago when I was a young motorcyclist I borrowed a motorcycle combination. The sidecar was mounted on the left, and before I set off the owner explained to me that this prevented the normal technique of leaning a bike over to the inside of a left hand bend to counteract centrifugal force. He explained that instead, one has to 'accelerate the bike round the sidecar'.

A short while later I was travelling down an incline towards a left hand bend, and facing a large lorry travelling in the other direction. The following few seconds is etched on my brain like a video.

I tried to turn and absolutely nothing happened - I just kept going straight towards the lorry. In the face of imminent disaster it took some courage to open the throttle hard, but my friend's advice overcame my fear and the increased engine power floated me easily round the corner.

The only way a left mounted combination can negotiate a left-hand bend at even moderate pace is to apply 'position, speed, gear, acceleration'. Failure to do this or application of the brakes in the corner will result in the side car wheel leaving the ground and the unit travelling straight on. At worst it will flip over.

Once learned, the technique is never forgotten. The principal is exactly the same for a car.