bbroadside, on 27 May 2007 - 06:03 AM, said:
Is three-and-a-half a typical horsepower for the period? I imagine more would be needed if there were a sidecar but I don't know if those were common at the time. Does hp = bhp for these purposes or are there several types of horsepower?
Three and a half horsepower was typical at that time for a 500 cc motorcycle however it is not a rating of output.
This measure was instituted by the Royal Automobile Club
and was used to denote the power of early 20th century British cars
. Many cars took their names from this figure (hence the Austin
Seven and Riley
Nine), while others had names such as "40/50 hp", which indicated the RAC figure followed by the true measured power.Taxable horsepower does not reflect developed horsepower; rather, it is a calculated figure based on the engine's bore size, number of cylinders, and a (now archaic) presumption of engine efficiency. As new engines were designed with ever-increasing efficiency, it was no longer a useful measure, but was kept in use by UK regulations which used the rating for tax purposes
is the diameter (or bore) of the cylinder in inches
is the number of cylinders
This is equal to the displacement in cubic inches divided by 10π then divided again by the stroke in inches.
Since taxable horsepower was computed based on bore and number of cylinders, not based on actual displacement, it gave rise to engines with 'undersquare' dimensions (i.e., relatively small bore), but long stroke; this tended to impose an artificially low limit on rotational speed (rpm
), hampering the potential power output and efficiency of the engine.