A Special Builder's Notes


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30 October 2013

An Interesting Exercise.

Before the track day at Snetterton earlier this year, Learned Counsel spent a lot of time fiddling about with the corner weights of his racing car. I didn't quite understand what all the fuss was about until I got on the track; in cornering, the car's manners were impeccable. Prompted by the need to do something with the leaf springs on the Hillman - they're so stiff that there's almost no movement in them - and also the need to provide a few basic weights to the company who'll re-set them, I weighed the car this last weekend - it was interesting to see what the corner weights were.

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A few moments on the internet provided a method to calculate the horizontal Centre of Gravity (CG). I could have also calculated the vertical position of the CG but that would have involved jacking the car up some distance at the back to transfer weight to the scales underneath the front wheels. As the whole exercise was of mostly academic interest, I didn't care to take the risk of everything crashing to the floor.

The calcs have shown that the horizontal position of the CG lies almost exactly in the middle of the wheelbase; the addition of Miss X and myself makes very little difference to the result. I was thinking that this outcome seemed to have all the advantages of a mid-engined car without the drawback of having an engine awkwardly placed for maintenance. The good stuff makes for very pleasant reading - excellent weight distribution enabling easier and faster cornering, traction is more evenly distributed between the four wheels giving the brakes an easier time and, apparently, ride quality is also improved; no bad thing in a fast vintage tourer. So, I think that with all the bodywork and bits and pieces the all up weight is going to be just over a ton; somewhere around 1100kgs. I still have no idea what the performance is going to be like but with an engine rated at around 60hp in old money, it shouldn't be too bad.

It's nice to have a bit of luck with CG calcs. I remember that the Civil Aviation Authority (CAA) were particularly unhelpful when it came to establishing the operating CG range for the Avro. Their first mistake was to assume that the aircraft was flown solo only from the rear seat - like a Tiger Moth for instance. I produced photographic and written evidence to the contrary but, having made their decision, they were not prepared to be corrected. This had disastrous consequences for the paperwork. A CG range of 4.25" to 6.28" aft of the datum (the leading edge of the lower wing) was prescribed. The upshot of this was that with full tanks and the engine running, I could sit at the end of the runway for 5 minutes before the CG would exceed the rearward limit and flying the aircraft would become an offence for which I could be prosecuted. The CAA were made aware of this piece of nonsense but remained intransigent.

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The Avro never left the ground in a condition that corresponded to the paperwork but, her manners were always impeccable.

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