Everybody’s Scalin’ – Overdrive / Underdrive and You
Whenever you wanna get really serious about fine-tuning the way your scale crawler handles, overdrive and underdrive gearing is something you absolutely should look into. I know that this can be confusing for some, so here’s a quick explanation for how it works.
Overdrive (OD) and underdrive (UD) are measured by using the stock ring and pinion as a baseline. OD means that an axle spins faster than stock. UD means that it spins slower. This is achieved by swapping your stock axle gears to specific ones made for the pertinent ratio you require. This mod is most common on Axial trucks given how the OD and UD gear sets have been available forever, but nowadays you can find them for other popular scalers as well.
For most crawling trucks, putting UD gears in the rear and leaving the front stock will suffice. This lets the front spin slightly faster, thereby letting the front pull the rear. This effect is absolutely invaluable for climbing hills. By just swapping out one set of gears, you won’t get that much additional stress on your drivetrain. Yeah, the front wheels will move a tad faster than the rear, but it’s not that pronounced.
If you want to get a bit more extreme, you can also add OD gears to the front. This REALLY allows the front to pull, and if you enjoy extreme hill climbing (like me!), this is a set-up to try. You need to be careful here though, as your front wheels will now be spinning noticeably faster than your rear, increasing the potential for breakage. You will want to have other upgraded components (outdrives, gears and driveshafts) if you want to roll like this.
One other option you have is to run UD in both the front and rear axles, which will lessen torque twist. Any loss of actual speed you get can be compensated for by increasing your motor pinion size. Putting the taller gearing on the transmission/motor versus axles will allow for a rig that feels smoother.
And hey, whenever you upgrade to aftermarket ring and pinion gears, they are usually made of a better material than pot metal stockers, so you will be bulletproofing the axle as well as getting the desired gearing.
Hopefully that demystifies this! If you’ve ever been on the fence about trying this, you should!