383 Setting intermediate gear ratios

Ratios between top and bottom gears should be spaced in such a way that they will provide the tractive effort-speed characteristics as close to the ideal as possible. Intermediate ratios can be best selected as a first approximation by using a geometric progression. This method of obtaining the gear ratios requires the engine to operate within the same speed range in each gear, which is normally selected to provide the best fuel economy.

Consider the engine to vehicle speed characteristics for each gear ratio as shown (Fig. 3.35). When changing gear the engine speed will drop from the highest NH to the lowest NL without any change in road speed, i.e. V1, V2, V3 etc.

Let G1 = 1st overall gear ratio G2 = 2nd overall gear ratio G3 = 3rd overall gear ratio G4 = 4th overall gear ratio G5 = 5th overall gear ratio where Overall

Engine speed (rev/min)

gear ratio Road wheel speed (rev/min)

Wheel speed when engine is on the high limit Nh in

Wheel speed when engine is on the low limit NL in second gear G2 = Nl (rev/min) G2

These wheel speeds must be equal for true rolling

Nl Nh

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