Ash for Cars
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Sir,
I was delighted to see the interest shown by your correspondence columns in the use of timber in the construction of cars. As an engineer using this material in different conditions of service it is pleasing to find so much appreciation of one of the oldest structural materials.
A characteristic of timber not so far mentioned in correspondence is its capacity to accept short-term loading up to about twice the value of the sustained loading which would cause failure. In the design of fendering for piers and jetties where impact loading from ships berthing is provided for this high impact strength is extremely useful. Surely much of the peak loading in a vehicle body must derive from impact as road shocks are absorbed, and hence this almost unique characteristic must be advantageous.
On the subject of decay, many timbers are more resistant than the ash, which is so popular with coachbuilders, to decay. However, if the timber is properly seasoned before use and properly protected in use, the moisture content should be below that required for decay to take place. If a timber is to be used green or unprotected a dock or harbour timber such as Greenheart would prove a better proposition. In terms of strength and other mechanical properties, tests on clear specimens published by the research organisations show ash to be similar to many other hard-woods generally available. All timber moves with changes in moisture content and the degree of movement normal to the growth rings is not the same as that tangential to the growth rings. The ratio of radial to tangential shrinkage is the characteristic which largely indicates the twisting and warping which will occur in service, and this must be of great importance to coachbuilders. I do not know the ratio for ash but observation leads me to believe it can scarcely be bettered. Many of those timbers which have a greater strength also have a large differential shrinkage factor and would not be considered.
Steel and concrete are manufactured and the process can be varied to produce characteristics called for by the designer. A naturally occurring material can only be selected, and a good grade of the wrong species will probably provide better services than a bad grade of the best species. With timber the craftsman is required to select as well as he can build.
I see that in “Grand Prix News for 1966” D.S.J. refers to the use of a balsa-aluminium sandwich sheet material in the McLaren single-seater. So timber is still being used in cars!
Can your readers tell me why so much highway furniture must be in concrete ? Modern techniques can produce light, strong, graceful units of timber which do not suspend lethal masses over the troubled motorist like the sword of Damocles.
Billericay. Brian Hall.
[This matter has moved here from “Vintage Postbag” as this letter concerns wood in modern car construction, another car using wood being the Marcos. The subject is now closed.—Ed.]