Sprocket size for Goodwood and Dunsfold

edited April 2015 in IET Formula 24+
Hi, I was just interested to know what size sprockets people run for Goodwood and Dunsfold, I know there is the calculations that I can do but I was just interested to know what size people use, as I assume that the sprocket that comes with the kit car won't be used often,


  • Don't suppose you mind telling me these calculations? Could be quite useful for us...


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  • edited April 2015
    There's a great opportunity for application of simple physics to Greenpower cars, the Rotary Racer team have made some great documentation on this, found here: http://www.greenpower.beamweb.co.uk/files/TechnicalInfo/GreenpowerScience/
    You'll need to measure characteristics for your car and estimate the air drag coefficient using either lap times, current draw or CFD if you can accurately model your car (CFD is always a bit optimistic though).

    An alternative way to do this is to take lots of sprocket sizes to the race and during the practice laps adjusting the gear ratio until you pull about 17A consistently around the track (for F24, F24+ should be about 22A).

    From memory the kit car comes with a 72T and 12T sprocket, meaning a ratio of 6, our old kit car ran on a ratio of 4.2 for most of the season, and we finished all our races on the edge of battery life (a good thing). So perhaps a good starting point? If you aren't making any changes to the rear axle, I would leave the 72T in position and increase the motor sprocket teeth, as these sprockets are cheaper and easier to change in the paddock/pit.

    Shown below is one our own tools (excel) for calculating this, but Rotary Racer have a similar thing on their website. Take note of the "ratio effect on distance vs speed" there is a narrow band for the perfect ratio.

  • Hi here is the gearing calculations

    Circumference of Tyre = Tyre Diameter x π
    Wheel Speed = Motor Speed x Teeth on Motor Sprocket Teeth on Wheel Sprocket
    Wheel Speed x Tyre Circumference = Distance travelled per minute Multiply by 60 and divide by 5280 to get Miles per Hour
    Example for 16”wheel, 14T drive sprocket, 50T wheel sprocket. Circumference = 410mm x 3.143 = 1.289m = 4.229ft Wheel Speed = 1850rpm x 14 = 518 rpm
    (Motor 1850rpm approx equals 15 Amps)
    518 X 4.229 = 2190.6 ft/min X 60 = 131437 ft/hr ÷ 5280 = 24.9 mph
    The above example is for a single gear, reasonably aerodynamic car. The car will probably go faster than this with a downhill/tailwind and slower with an uphill/headwind. With this gearing, if the car goes slower than 18mph, for even a short period, it will be drawing 45Amps and in danger of cooking the motor and killing the batteries. Go on the safe side and gear your car for a slower speed (bigger wheel sprocket) which will keep the motor revs high and the current draw low. When you have successfully completed a full race distance on your batteries you can then convert excess energy into a little more speed. Work upwards from a conservative speed. Aiming for an unrealistic top speed will only bring disappointment.
    If using a layshaft between the motor and the wheel, multiply the primary gear ratio by the secondary gear ratio to get the overall ratio. E.g. 3:1 + 5:1 = 15:1
  • Good advice Zachy2110. Also, if you begin to notice your batteries getting discharged to less than 10.5 V each, then stop using them ASAP. If you continue crawling round they will lose capacity to store charge and replacements will cost you dearly. Log each lap time and plot it as you go. Try and predict when you should come in for a battery change. You will learn a lot about your drivers, your batteries and you might get some surprises.....
  • OldTimer, I think you'll find there are no longer any battery changes.
  • True, during a race. Many teams used 'old' batteries for practice which had been damaged through over-discharge. Lead acid batteries used to be good for 1500 charge/discharge cycles - which is a lot of motoring. Looking after the batteries carefully, not over-discharging them or mechanically shocking them too much will enable maximum life. Plotting lap times shows the "cliff-edge" characteristics which need to be avoided. If you approach the cliff-edge too soon then its game over very quickly - and you will have to change your batteries and maybe buy new ones. Best do it once during practice to see the effect.
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