Stan Bradbury's Speedway Training Notes
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GEAR RATIOS
Some of you may be aware of the great importance of using the correct gear ratio on your machine. The gear ratio is the number of times your engine turns in comparison to 1 turn of your rear wheel. You may be familiar with how simple it is to calculate these gear ratios or interpret the charts where the gear ratios have already been calculated for your convenience. For the benefit of those who do not know, here is some information on the subject.
Top flight professional racers frequently make changes to their gear ratios by as little as one tooth on the rear wheel sprocket, two or three times during a racing, program. This is to cope with changing track conditions, grip in the starting area or elsewhere on the track or simply a feeling that the previous gear ratio was not "spot on." In some cases, the improvement can be purely psychological. In extreme cases a ratio which is too low can cause a damaged engine, due to over-reving or, in the case of a gear ratio which is too high, failure to hold the best course through the turn, resulting in a "lay-down" as an alternative to a crash into the safety fence. As speedway machines do not have multiple-speed transmissions (discounting the 2-speed long-track transmission) it is not possible to shift up or down if the gear being used is incorrect. This makes the choice of the correct gear ratio as important as a reliable motor or oil in the tank. Not all riders require the same gear ratios on the same track. In order to calculate a gear ratio, here is how to proceed:
First multiply the number of teeth on the countershaft (jack-shaft or layshaft) sprocket by the number of teeth on the engine sprocket. Next, multiply the number of teeth on your rear wheel sprocket by the number of teeth on your clutch sprocket. Then, divide the second answer by your first answer and the result will be your gear ratio. Here is an example: Let's say you have a 19 tooth engine sprocket with a fairly common 16T. on the countershaft: 19 x 16 = 304.
If you have a 44 tooth clutch (Jawa) and a 59T. rear wheel sprocket, the two multiplied together: 59 x 44 =2,596. Next, find the gear ratio by dividing 304 into 2,596: 2,596 / 304= 8.53 (to 1) or nearly 8.54 : 1.
Now 8.53 might be all right for a Weslake or Godden with a 19" rear wheel on a 1/4 mile track, but it would be too high for the high revving 2 valve Jawa.
Going down 1 tooth on the engine sprocket will give 18 x 16 = 288, and divide this into 2,596 = 9.01, which would be too low for a 1/4 mile track. So you would also have to change the rear wheel sprocket to get a gear ratio of about 8.7 (to 1) for the Jawa with a 19" rear wheel while in this case, a Weslake or Godden rider may only need to put a 60T. on his rear wheel to drop his gear ratio down to 8.68:1 which could be a bit low for a "big handful" type of rider if the track is "slick" (not much dirt or cushion). Usually, you can get a fairly close idea of what gear ratio to start with, by the size of the track. A 1/4 mile (440 yards) track is probably going to need a gear ratio between 8.5 to 8.86 to 1. A track of say 1/8 mile (220 yards) may require a ratio of around 10 to 1 and so on.
Jawa's & NEB clutches are 44T. A 16T. countershaft sprocket is usually fitted behind the clutch.
If you ask an experienced rider what gear ratio he is using or what size sprockets he has on his machine (or simply take a look at his bike) you will soon be able to figure out what gear ratio he is using and what sprockets you need to use in order to have a similar gear ratio. Obviously, as you become more experienced, a selection of rear wheel sprockets plus a few engine sprockets will become necessary as you cannot always count on being able to borrow the right one. On the so called 1/2 mile or long track, the importance of selecting a satisfactory gear ratio becomes much more important for the reasons outlined earlier in this chapter.
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