Step Gear Teeth Ratio Rotations
1 Pinion
Gear 2 (DDG)
2 Gear 3 (DDG)
Gear 4
3 Gear 5 (Hop Up)
Gear 6 (Hop Up)
4 Gear 7
Gear 8
5 Gear 9
Gear 10
6 Gear 11
Gear 12
7 Gear 13
Gear 14
8 Gear 15
Gear 16
Final Gear Ratio:         



rc tank Impact gearbox transmissionOne of the great things about Impact's Transmission Unit (besides being extremely sturdy) is that it's so configurable, which makes it easy to fine tune your tank's scale speed.

This gadget shows the gear ratio calculations for Impact's transmission unit, based on the information from the table at their website. It lets you change the gears to observe the effect on the final output, expressed as "Final Gear Ratio" and "Rotations". Although you're not likely to change any of the "hard wired" gears, you can easily experiment with your motor's pinion, their drop-down gear, and Hop-Up gears.

Pinion Gear: Tamiya motors come with a brass 10-tooth pinion gear, Impact sells a steel 12-tooth replacement, and an 8-tooth brass pinion is currently (11/09) available at ETO Armor.

Drop Down Gear (DDG): This comes with the transmission unit. You can either install it or not. To remove it from the calculations, press the "No DDG" button. To reinstall it (and reset the other gears to their stock default values), press "Reset to Stock with DDG". (The pinion gear value won't be changed if you've changed it).

Hop-Up Gears: Impact has provided values for their three Hop-Up gear sets that offer alternatives to the stock values of Gear #5 (24T) and Gear #6 (34T), and allow you to fine tune your transmission unit; press the appropriate button to see the effect on the Final Gear Ratio. Their page shows the position and values of those gears, and has a very good explanation of factors that affect the actual speed of your tank. (Hey, they even "blessed" this page! Cool!)

To Use the Simulator: Press the "Calculate" button. (Duh!) This calculates the interim gear ratios ("Ratio") and the final output gear ratio.

The Rotations Column: The Final Gear Ratio wasn't terribly meaningful to me at the time, so I added the "Rotations" calculation column. This shows that if the pinion is rotated x number of times (which you can change), the other gears would rotate the outputted number of times. In other words, it takes 105.468 turns of the pinion to produce 1 turn at the final output, which happens to be the final gear ratio for a 10-tooth pinion with DDG. If you didn't know what "Final Gear Ratio" meant before, now you do.

If you imagine the final output to be scale MPH/KPH, you can experiment with gear changes to see the relative change in MPH. To do that, change the input Rotation in the first row to produce a plausible/known output MPH/KPH speed for the gear setup: I picked 1500 as a default start value, since it produces a ballpark and plausible MPH that's close to the result for the Jagdpanther w/DDG in *Impact's video. You can then change the pinion gear, remove the DDG, or experiment with different hop up gears to see how they affect the final MPH.

Note that this is a theoretical exercise only. In the real world, the motor's RPM, the diameter of the drive sprocket, friction, terrain, and other factors play a major role in determining the actual speed.

*Discrepancy? I noticed that the calculated scale MPHs in Impact's video didn't seem to jive with the theoretical results. If the TU with DDG (Test# 2) is calculated to be 14 MPH, then the TU without the DDG should be approximately 25 MPH, not 34 MPH (Test# 1). However, that test is of a stock Tamiya gearbox, not an Impact gearbox without DDG. The calculated results seem to match the results of other gear ratio/MPH calculators available on the WWW.

How It Works: Simple arithmetic, with interim calculations providing data for subsequent calculations, like a spreadsheet. The interim gear ratios are derived by dividing the gear's tooth count by the preceding gear's tooth count (ratio 1= gear 1/gear 0). The rotations are derived by dividing the last calculated rotations by the current gear's interim gear ratio (rpm 3 = rpm 1/ratio 3). The final gear ratio is derived by dividing the initial rotations by the final rotations (final rpm = rpm 0/rpm 15). Multiplying all the interim gear ratios produces the same result.

The "No DDG" function "removes" gears 2 and 3 by replacing their tooth value with "1". The calculated ratio and rotations at this step should be dimissed since the gears aren't really there. (This was a much easier solution than reconstructing the table without those gears.)

--11/03/09, rev. 11/08/09