Looking for an edge on extreme terrain? Try overdriving the front axle.
Each TRX-4 axle has an 11-tooth pinion and a 34-tooth ring gear for a 3.09:1 ratio. Here’s the breakdown on where that figure comes from, if you’re not familiar with the concept of a gear ratio: 34 divided by 11 is 3.09. For every 3.09 revolutions of the pinion, the ring gear makes one complete revolution. The ratio between the pinion and ring gear is “3.09 to one,” or 3.09:1.
#8279R Standard Gearing
#8287 Overdrive Gearing
#8288 Underdrive Gearing
Note that 8279R duplicates the TRX-4’s stock axle ratio. But since overdriving the front axle places extra strain on the drivetrain, the machined-steel construction of 8279R is recommended.
Depending on how you gear each axle, you can achieve 11%, 13%, or 27% overdrive. That percentage reflects the greater speed of the front axle; for example, if you set up your TRX-4 for 13% overdrive, and your rear axle is spinning the tires at 100rpm, the front axle will spin its tires at 113rpm. This chart shows the gear combinations required to achieve each overdrive setup:
Overdrive chart
This graphic visualizes 13% overdrive
Installing the new ring and pinion gears isn’t difficult. To see the installation steps, take a look at Part 2 of our Ultimate TRX-4 build.
Overdrive in Action
What does overdrive mean for performance? Imagine a steep slope or an obstacle has your truck pointed nose-high. This transfers weight to the rear wheels, increasing their traction relative to the fronts. And of course, the more steeply the truck is angled, the closer it is to its tipping point. Applying the throttle transfers even more weight to the rear tires, increasing traction further, leaving very little grip for the fronts. It’s almost entirely up to the rear wheels to push the truck up the slope or over obstacle. And if the angle is sufficiently steep, you may just stand the truck up on its tailgate—either way, you’re not making any progress. Overdriving the front axle helps the front wheels pull the truck up slopes and over obstacles, instead of being pushed from the rear axle.
Better Steering
You can think of an overdriven front axle as having the same benefits as a front-wheel drive (FWD) car versus rear-wheel drive (RWD). Driving FWD car on a slippery road is much easier than driving a RWD car. Even when extreme climbing angles aren’t required, overdriving the front axle helps the front tires “bite” into corners and seams of the rocks in the trail, helping pull the truck through the terrain instead of pushing it.
Why Not Use Overdrive All the Time?
Overdriving the front axle offers maximum benefit when crawling extreme terrain, but it has downsides for all-around driving. Having the rear tires spin at a lower rate is like having a rear brake applied slightly as you drive, which makes the truck more prone to skidding and sliding on the trail when hustling in High gear. Overdrive also increases the load on the power system and drivetrain. They’re built to handle it, but you can expect a little less run time and higher operating temperatures for your motor and speed control when using an overdrive setup.
Experiment!
With three different overdrive options available, there’s plenty of tuning opportunity for improving extreme-terrain performance. If you’re more of an all-around trail-trucker and use High gear often (especially with a 3-cell LiPo), try the 11% setup first. For dedicated rock-crawling rigs, the 27% overdrive setting can dramatically improve the TRX-4’s ability to grab ledges and scale seemingly impossible obstacles. And if you want to really go all the way with a comp-ready rig, check out our TRX-4 Comp Crawler project.
For helpful Traxxas how-to videos, be sure visit the Traxxas Support channel, Like, and subscribe. Click here to go to the channel now.
