SAFER OVERTAKING

We received a letter from Dr JP Barnard of Stellenbosch, expressing concern that the conclusion of the Tech Feature in the December 2021 issue could be misleading with regard to gear selection and optimum engine speed to maximise acceleration for a safer overtaking manoeuvre. This was certainly not the intention; however, Dr Barnard’s comment is valid and, hopefully, the following explanation will clarify in a more practical way.

Critical to safe overtaking is to minimise time spent facing oncoming traffic by maximising acceleration and completing the overtake as quickly as possible. The key to doing so is the selection of the correct gear to ensure the combination of selected gear (gear ratio) and engine speed provides the highest level of gross tractive force through as much of the overtaking manoeuvre as possible.

With a manual transmission, there is no substitute for a good “feel” for the characteristics of the engine and the spacing of the gear ratios. Selecting too low a gear, as often happens in a more pressured situation, immediately increases engine speed towards the rev limiter at which point the gross tractive force will be declining rapidly. An almost immediate upshift will also be required, further delaying the overtake. This is even more critical on diesel engines where the torque drops off a cliff as the engine approaches maximum power. A good guideline would be to select a gear to begin the overtake at about max torque revs before accelerating, the gross tractive force steadily increasing and countering running resistance. Acceleration will gradually taper off approaching max power revs and the driver will need to judge the optimum gearshift point; waiting too long means a steady decline of tractive force but upshifting too early could drop engine speed to below max torque revs and off the boil – tech editor.