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The Vehicle A-Pillar

A-pillar of Vauxhall Astra

 

The pillar between the windscreen and front door of a car is called the A-pillar. This component is often reasonably large as it may be required to 1. conceal wiring, 2. house an airbag, 3. allow easy access for welding robots and 4. ensure that the windscreen can be replaced easily if necessary. The A-pillar also has the adverse effect of reducing visibility. European legislation allows for an "obscuration angle" of 6 degrees. As a result, manufacturers have taken advantage of this generous specification and in recent decades A-pillar thickness has increased significantly.

 

 

 

In 2002 a study conducted by the Minnesota Department of Transportation (MDT) found a correlation between lack of visibility and risk of collision. Amongst their conclusions was the following statement: “…the vehicle itself must be re-engineered to provide for better vision...”. But, Safety is a thorny issue. A British DfT study entitled "Investigation into 'A' Pillar Obscuration - A Study to Quantify the Problem Using Real World Data" was published in September 2006 and as Dr Stephen Ladyman stated in the House of Commons on 26th October 2006 the final report was inconclusive.

 

 

Graph showing risk of collision

 

 

Stereo camera set-up

 

 

Safety aside, obscured vision reduces driving pleasure. To help illustrate how things have changed over the years, the view from a classic vehicle - a Jaguar E-type - was captured using two cameras. These cameras had lenses, similar in performance to the human eye, mounted with a separation corresponding to average interpupillary distance. In a single photo from one of the cameras, the A-pillar is clearly apparent. However, when photos from both cameras are superimposed and the background is lined up it can be seen that the A-pillar effectively disappears. What one eye cannot see the other eye can and at distances greater than a few feet from the vehicle there is effectively no obstruction to vision.

 

 

Driver's view of A-pillar of Jaguar E-type

 

 

Composite photo illustrating how an E-type's A-pillar does not obstruct vision

 

 

It is standard practice to quantify engineering matters. A-pillar thickness has increased, typically, from around 5cm in the 1960s to 10cm or more today. Dividing 10cm by 5cm allows us to deduce that A-pillars have generally doubled in thickness. Quantifying the increase in A-pillar obscuration is quite a different matter. Today all cars exhibit some form of A-pillar blind-spot. Older vehicles often had an A-pillar blind spot that was effectively zero. Dividing a finite quantity by zero quantifies the increase in A-pillar obscuration as infinity!

 

 

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