A typical Jr Dragster has the following components that enter into the Gear
- 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
"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.