Junior Dragster Plus

Gear Grinder

These calculators will allow you to bench test various combinations of Engine RPM, Clutch Ratio, Gear Ratio, and Tires in a typical Jr Dragster environment. Usage Directions follow immediately after the calculators.

All Formulas are documented on this page.

Please read the Warning at the end of this page.

Gear Grinder Calculator
Engine RPM:
Clutch Ratio:
Jackshaft Axle RPM:
Front Sprocket:
Rear Sprocket:
Rollout (inches):
Max Speed (mph):

So you want to change Tires?
New Rollout (inches):
New Ratio:

Rollout Guesstimator
Tire Diameter (inches):
Rollout (inches):

"Gear Grinder"
A typical Jr Dragster has the following components that enter into the Gear Ratio decision: 

  • the power band of the motor as represented by RPM's. Of the most interest are the stall RPMs (the RPMs at which the clutch engages) and the target Maximum RPMs (of the motor),
  • the clutch ratio - enter the final ratio (typically 0.80) used to see how the setup is at the end of the track or enter the initial ratio (typically 2.5) to see how the setup affects how the car leaves,
  • the front and rear sprockets which together determine the Ratio,
  • the rollout of the tires used.

The clutch represents a twist in the calculation as it is typically a variable rate clutch ranging from an initial ratio of 2.5 : 1 to a final ratio of 0.8 : 1.  By setting the Final Clutch Ratio value to your initial clutch ratio and and setting the Engine RPM to your clutch Stall setting, you can see how hard your car is trying to pull off the line.  A higher resulting MPH represents a harder pull.

"So you want to change Tires?" estimates, with all else being the same, what ratio will you need with the new tires to run the same as you did with the old tires.

"Rollout Guesstimator" calculates the rollout (circumference) of tires based on the diameter. Rollout can be greatly affected by tire pressure (or lack thereof), and manufacturing discrepancies between stated size and actual size. Always measure your tires when and where possible. This calculator is in case you do not have the tires yet but are considering Brand X size whatever and want to get a general idea of how they might affect your overall ratio.

Formulas used:
Jackshaft Axle RPM = (1/ Final Clutch Ratio) * Engine RPM
Ratio = Rear Sprocket / Front Sprocket

             Jackshaft Axle RPM
mph = ( --------------------- * rollout / 1056) 

where 1056 represents the number of inches in a mile (63360) divided by the the number of minutes in an hour (60)

                                                   mph * 1056
New Ratio = the inverse of ( -----------------------)
                                             RPM * New Rollout

where 1056 represents the number of inches in a mile (63360) divided by the the number of minutes in an hour (60)

Rollout = Diameter * pi

where pi has been estimated to 3.1415962

Warning:  Use of this calculator is as is and at your own risk.  If you use ratios recommended by this calculator and it results in the over rev of your motor and the subsequent self destruction of your motor, the creator of this calculator and owner of any site on which this calculator appears can not and will not be held responsible. Validation of the results is ultimately up to you!  Start with a conservative gear ratio to protect your motor.  A conservative Ratio would be lower number.

Special thanks to Chris Schnick and Schnick Motorsports

Copyright 2003

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