The drivetrain is what takes the power created by the engine and transfers it to the wheels. Generally, drivetrain components don’t need much maintenance besides a good cleaning every now and then. Sometimes a drivetrain component will break, which is annoying because that often means taking apart most of the vehicle to find the issue and replace the broken part.
Drivetrain components may break for the following reasons:
- The nitro RC model has a known weak point in the drivetrain that is prone to breaking.
- You’ve installed a more powerful motor that adds more stress than what the stock drivetrain was designed for.
- You’ve installed a different size spur gear, clutch bell, or wheels/tires that change the gear ratio of the drivetrain and places additional stress on the drivetrain.
- You are pushing the nitro RC very hard, going from no throttle to full throttle often.
- You are jumping the nitro RC and landing with the throttle applied.
- You crash the nitro RC or have a high impact landing.
When something in the drivetrain breaks, do some research to see if it is a common issue and if there is a recommended upgrade. Be careful though. When you beef up one component in the drivetrain so it can withstand more abuse, you are simply moving that stress to other components and those components may be more difficult to access or more expensive to replace. Read accounts online to decide which upgrades make sense and which don’t.
Drivetrain Components
The manual is a great place to start to learn about the drivetrain on your nitro RC. There are often sections in the manual about maintaining the various components in the drivetrain. You will also find exploded views that show you exactly how the components of the drivetrain are assembled.
Spur gear/slipper
The spur gear is meshed with the clutch bell to transfer power from the engine to the transmission. It is critical to have a proper mesh between the clutch bell and spur gear to prevent damage to the spur gear or clutch bell. On many RTR nitro RCs, the spur gear is plastic and the clutch bell is metal, so the spur gear typically will be destroyed, or “stripped”, if the mesh is not correct. It is also critical that the engine mount screws are tight with thread locker applied (e.g. blue loctite) to prevent the engine mount from moving while driving and causing the mesh to change.
The slipper is a thin pad on the face of the spur gear that allows the spur gear to slip if too much force is applied on a high traction surface. This prevents excessive stress on the drivetrain. Read your manual on how to properly set the tension on the slipper, but in general you screw down the slipper nut as tight as you can and then back it off a 1/2 turn. In looser conditions, you might want to set the slipper looser to reduce wheel spin. In higher traction conditions, you can set it tighter for better acceleration, but you will also place more stress on the drivetrain.
Some people remove the slipper assembly entirely from their nitro RC for maximum performance on high traction surfaces. This places more stress on the drivetrain and can lead to broken parts if proper throttle control is not utilized.
Transmission
The transmission transfers the rotational energy from the spur gear to the driveshafts and adjusts the gear ratio properly for the vehicle. The transmission is generally closed in a box and is run dry without grease or oil (check your manual).
Some transmissions have a 2-speed or 3-speed system that will activate at certain RPMs and change the gear ratio of the vehicle. This is generally a performance upgrade for nitro RCs because it allows the nitro RC to get the more power from the engine at both low and high RPMs compared to a single speed transmission. Multiple speed transmissions have adjustment screws that can change the shift point of the transmission.
- 2-Speed Shift Point Adjustment (Traxxas)
- HPI Savage X 2Speed Adjusting (YouTube)
- HPI Savage 3 Speed Transmission tip for adjusting the shift points (YouTube)
Differentials
Typically, if an axle receives power from the engine, it will have a differential installed that allows variable amounts of power to be applied to each wheel. This helps the nitro RC handle and accelerate better under various conditions. 2WD vehicles will have one differential on the rear axle and 4WD vehicles will have two differentials, one on each axle. Sometimes, 4WD vehicles will have a 3rd differential on the center driveshaft that allows varying amounts of power to be sent to the front versus the rear.
If you don’t know what differentials do or how they work, I highly suggest watching this video from 1937.
Differentials are typically sealed and filled with oil. The weight of the oil controls how effective the differential is. Extremely high weight oils are used to effectively lock up the differential and make the nitro RC drive as if there was no differential installed. Some enthusiasts like this as it provides more throttle response and a more challenging driving experience.
It is a good idea to stick with the stock differential oil and do some research before experimenting with different weight oils.
Drive shafts
The drive shafts take power from the differential and send it to the wheels. There are two kinds of driveshafts: universal and constant velocity (CV or CVD). Universal driveshafts are cheaper, typically made of plastic, and provide variable velocity to the wheels at different orientations of the suspension. Constant velocity driveshafts are more expensive, made of metal, and provide constant velocity to the wheels regardless of suspension orientation. Universal driveshafts work just fine for bashing, but CVDs will provide a smoother throttle response and are sometimes more durable than the cheaper universals.
Center Driveshaft
If you have a 4WD nitro RC, it will likely come equipped with a center driveshaft which sends the power from the front of the transmission up to the front differential. Some 4WD nitro RCs come with a drive belt instead, which is used to transfer power to the front wheels and will need to be replaced more often than a metal shaft.
Brakes
Every nitro RC comes equipped with brakes which are positioned somewhere along the drivetrain and are connected to the throttle servo so they can be controlled with the throttle trigger.
Adjusting brakes
Brakes have multiple points at which they can be tweaked:
- The throttle/brake linkage can usually be mechanically adjusted to apply more or less brakes.
- The end point on the transmitter controls how far the servo moves when the brakes are fully applied.
- A screw on the brake assembly itself can usually be adjusted to control how responsive the brakes are.
When diagnosing a brake problem, always confirm the brake components are in good condition before adjust the various brake settings. Adjust the mechanical linkage first, then the end point on the transmitter, and finally the screw on the brake assembly (if needed).
Upgrading brakes
The most basic system is a single disc composite brake, found on vehicles like the Traxxas Nitro Rustler.
The brake material itself can be upgraded to fiber or metal for better braking performance. Sometimes the number of discs can be upgraded to a dual brake system as well for better braking.
Recommended Resources on Brakes
- Throttle and Brake Linkage Adjustment (Traxxas)
- HPI Savage Nitro Car Brake Adjustment (YouTube)
If you have a Traxxas Jato 3.3, the model is known for having terrible brakes, which isn’t helped by the fact that the model is extremely fast. A common modification is the “jato brake flip”, which flips the position of the brakes so they can engage more fully. Look it up and watch this video for more information.