The TRX-4 comes standard with a 2-speed High/Low transmission with 32 pitch gears, mated to a 1:1 transfer case also utilizing three 27T 32P gears. The transmission is mated to a 45T spur with twin plate slipper clutch. There are two important things to note about this transmission:
1. The difference in final drive ratios between high gear and low gear is much greater than most (any?) other 2-speeds on the market. With the stock spur and pinion, the final drive ratio in low gear is 64.64:1 compared to 25.85:1 in high gear. That's a biiiiiig difference.
2. The 25.85:1 final drive ratio in high gear is very tall for a scaler/crawler, which explains why Traxxas recommends using a 550-can motor instead of a 540-can motor if you are going with a brushed motor (see motor and ESC below).
The first thing you should do before running your TRX-4 is check your spur/pinion mesh and pinion gear alignment. Proper gear mesh is important for prolonging the life of your spur and pinion gears and keeping temperatures down. The TRX-4 motor plate uses fixed motor mounting positions designed to take the guess-work out of adjusting gear mesh. The holes on one side of the plate are labeled with the letters A-H:
The manual includes a table specifying which holes to use for different pinion and spur combinations:
This is a nice idea for newcomers, but there is a problem in the execution. The recommended motor placement for the stock gearing (hole "C" in the motor plate) leaves spur/pinion mesh too tight, which can cause premature wear of the spur gear and undue strain on the motor (which can in turn cause overheating). Moving the motor one hole further out to hole "D" provides much better mesh for stock gearing:
My testing of several other pinion sizes indicates a similar issue with the recommended motor placement for other spur/pinion combinations. It thus appears that the proper motor placement should be one hole further out than indicated in the table for most, if not all spur/pinion combinations. Given these issues, I suspect it won't take long for the aftermarket to provide a motor plate with more adjustability for experienced builders.
While you have your spur gear cover off, you should also check your pinion/spur alignment. On some early production models (including mine), the pinion gear is installed too far out on the motor shaft, causing the pinion and spur teeth to be offset:
This in turn can cause premature spur gear wear. The fix is simple...move the pinion closer to the motor so that the teeth align better with the spur gear:
With respect to choosing a gear ratio for your setup, if you use the stock motor and ESC, Traxxas recommends no larger than a 12T pinion with the stock 45T spur, and that?s sound advice in my book. The stock Titan 21T 550-can motor running on 3S has plenty of juice in high gear, while low gear provides gobs of usable torque and excellent low-speed resolution.
If you need to service your transmission or transfer case, Harley posted an excellent video showing how to disassemble the tranny and t-case:
Stock driveshafts use hardened steel U-joints connected by plastic male and female shafts. If the durability of the Traxxas Maxx shafts is any indication, these driveshafts should hold up well.
3. Motor and ESCThe TRX-4 comes standard with the Traxxas XL-5 HV ESC and Titan 550 21T reverse direction motor. The XL5 supports up to 3S Lipo, and I definitely prefer 3S to 2S with the stock setup. Although a 21T motor on 3S might seem a bit fast and perhaps lacking in torque for a scaler, the combined gear reduction of the drivetrain and axles more than compensates. Low-speed resolution and torque are very good, particularly on 3S in low gear, and there is plenty of wheel speed in top gear when needed or for giggles when the trail flattens out.
Traxxas and motor guru John Holmes of Holmes Hobbies don't recommend 540-sized brushed motors in the TRX-4 due to the tall 25.85:1 final drive ratio in high gear. So when choosing a motor/ESC combo, keep in mind that you will want to stick with a 550-can brushed motor or a lower kv brushless setup than you would normally choose in other rigs to keep wear and motor temps under control. Holmes Hobbies offers the TorqueMaster Pro 550 in 21T and a limited edition 30T. They also offer the Crawlmaster Pro 550 (which is a 5-slot design) in 10T and 12T flavors, which would be comparable to a 21T and 27T 3-slot motor, respectively.
If you plan to go brushless, a 2200 kv 540-sized sensored brushless 4-pole motor should be a good baseline to get performance similar to the stock 21T motor. My current favorite 4-pole sensored brushless motors are the Puller Pro line (stubby or standard 540 size) and the new Castle 4-pole crawler motors. Matched with a good ESC like the Holmes Hobbies Trailmaster BLE or the Castle Mamba X, both offer very smooth startup and low speed resolution combined with lots of torque and plenty of top end (relative to the kv rating). My word of caution on brushless motors with the TRX-4 is not to overdo it either with kv selection or driving style this early in the game. The TRX-4 is a very new platform, too new for us to know its true weak points, much less for the aftermarket to have addressed them. The last two major releases (the SCX10 II and the Ascender) had weak points at initial launch (transfer case gears and pinion/driveshafts) that seemed fine with a stock RTR setup but didn't hold up so well to brushless power, especially when pushed hard. Eventually, the manufacturers and/or aftermarket developed more durable alternatives, but it took a little while.
One neat feature of the stock ESC when used with the stock radio is the ability to program it through the optional Traxxas Link module. But as noted in the electronics section below, it would be nice to see Traxxas add more programming options through Traxxas Link.
