A Fraer-Nash m.p.h./r.p.m. Ready Reckoner
by Lt. J. P. Shenton, R.N. CEen,. CLUTTON’S extremely interesting article on gear ratios has no -doubt prompted many people to investigate the gear ratios of their particular cars and discover to what extent they comply with some of the . principles mentioned. In the case of Frazer-Nashes and G.N.s this is of especial interest owing to the ease with which the ratios can he
This “ready reckoner” Was actually drawn out for use with a Frazer-Nash, but some parts of it may be useful in principle for other, more conventional, ears.
The object of the diagram is to give ” at a glance” relationship between m.p.h., r.p.m., gear ratio, and sprocket teeth, without having to resort to endless pages of scribbled calculation which, all too often, prove to be entirely devoid of meaning ! To draw the diagram, proceed as
follows :— .
(1) Take a sheet of graph paper of the type which has a thick line every tenth line. Preferably it should not be less than 14 large squares long by 10 deep.
(2) On the left mark off m.p.h. vertiCally on the basis of one large square to 10 m.p.h.
(3) Along the bottom, from left to right, mark off engine r.p.m. on the basis of one large square to 500 r.p.m. (4) Ca ‘ciliate the speed of the car on an imaginary gear ratio of 1 : 1 at 500 r.p.m. This is : Overall diameter of tyre 22 500
x x . m.p.h. 88 X 60 x 12 7 1
For 4.50-in. by 19-in. tyres this wcirks out at 41.67 m.p.h.
(5) Mark the point corresponding to 500 r.p.m. and 41.67 m.p.h.
(6) Draw a horizontal line through the point. Mark the point itself as “1,” and continue numbering to the right at intervals of large squares. This line is the gear ratio scale. (7) Calculate the theoretical number of rear axle sprocket teeth used in conjunction with a 17-tooth bevel shaft sprocket to give an overall gear ratio of 1 : 1. This is : 17 x (bevel pinion teeth) bevel teeth
and in the case of the :normal 3.5 bevel, ratio works out at 4.86 teeth. (8) Draw a line from the origin of the m.p.h. and r.p.m. scales up through the ” 1 ” on the gear ratio scale. Draw a line parallel to the m.p.h. scale and 4.86 small squares from it. Through the point where these two lines intersect draw a horizontal line and number it from left
to right, starting with the m.p.h. scale line as “0 ” and continuing at intervals of large squares, counting each large square as 10. This is the 17-tooth bevel shaft sprocket scale.
(9) Repeat 7 and 8 for the eases of 11, 12, 22 and any other bevel shaft sprockets that may be considered. For the sake of clearness only the 17-tooth scale has been included in the description.
In order to obtain greater accuracy it is as well to make use of larger scales, e.g., instead of marking the spot corresponding to 41.67 m.p.h. and 5(10 r.p.m., mark the spot corresponding to 83.34 m.p.h. and 1,000 r.p.m., and find the former point by means of proportion. Use of thediagram. -Suppose it is desired to find out the circa of fitting a 43 rear axle sprocket. with a 17-bevel shaft sprocket. Draw a line from the
origin of the if 1.1 mid r.p.m. scales up through t he ” 43 ” mark on the 17-tooth bevel shaft sprocket scale, and continue it upwards.
This line then gives the direct relation between m.p.h. and r.p.m. for that particular sprocket combination, and shows that 47 m.p.h. will correspond to 5,000 r.p.m., for instance. The overall gear ratio is indicated by the point at which the line cuts the gear ratio scale, in this ease 8.9 to 1.