Build Guide 3: Vehicle Prep & Mounting

Type: Build Guide

Time to move from the bench to the workshop. In this guide you'll prepare the TRX-4, mount the enclosure and electronics plate, and install the camera mount. No wiring yet — that's Guide 4. This is purely mechanical.

What You Need

Parts:

Traxxas TRX-4 Sport (fully assembled RTR)
Waterproof IP65 enclosure
Electronics mount plate
Camera mount bracket + 1/4" mini ball head
Long CSI ribbon cable (15-30cm)
Anker USB battery pack

Tools:

Hex driver set (metric — the TRX-4 uses 1.5mm, 2mm, 2.5mm hex)
Drill + small drill bits (if mount plate needs new holes)
Zip ties (assorted sizes)
Double-sided VHB mounting tape
Velcro straps
Marker/pencil for marking drill points
Ruler or calipers

Step 1: Get to Know the TRX-4

If this is your first time with the truck, take 10 minutes to drive it around with the stock radio. Seriously — it's fun, and you'll understand the steering range and speed characteristics you need to replicate with the Pi.

Things to note while driving:

Full steering lock left and right — how far do the wheels turn?
The slowest speed you can maintain — this is roughly your drag speed
Where the ESC is mounted and where the servo wires route
How the body shell attaches (clips) and comes off

Now remove the body shell and set it aside. You'll be working on the bare chassis.

Step 2: Plan the Layout

Before mounting anything, dry-fit everything on the chassis. Lay out the components and think about:

Weight distribution: The Pi enclosure and battery are your heaviest additions. Try to keep them centered and low. The TRX-4 has a good center of gravity — don't ruin it by mounting everything on one side.

Cable routing: The CSI camera cable is fragile. Plan a path from the Pi enclosure to the camera mount that avoids moving parts (suspension arms, driveshafts, steering linkages). The cable should have some slack for suspension travel but not so much that it snags.

Access: You'll need to open the enclosure regularly (to swap SD cards, adjust wiring, debug). Don't mount it somewhere you can't reach.

Suggested layout:

         [Front of truck]
              |
    ┌─────────────────────┐
    │                     │
    │   [Camera mount]    │  ← Behind cab area, looking back at cloth
    │         |           │
    │   [Pi enclosure]    │  ← Center of chassis, top of battery tray area
    │         |           │
    │   [USB battery]     │  ← Next to or under enclosure, velcro-strapped
    │                     │
    └─────────────────────┘
              |
         [Rear of truck]
              |
         [Drag bar attachment point]

Step 3: Mount the Electronics Plate

The electronics mount plate is your universal base. Everything attaches to the plate, and the plate attaches to the truck. This means you can remove the entire electronics package as one unit.

Mark the mounting holes. Hold the plate against the TRX-4 chassis and mark where existing screw holes line up. If none line up well, mark 2-4 new holes.
Drill if needed. Use a drill bit slightly smaller than your mounting screws. Deburr the holes.
Attach the plate. Use the TRX-4's existing body mount screws or new hardware. Use rubber washers or grommets to dampen vibration — the rover shakes a lot on rough ground.

Alternative approach: If you don't want to drill into the chassis, VHB tape + zip ties through the chassis rails works well. It's less permanent but plenty strong for our speeds.

Step 4: Mount the Waterproof Enclosure

The enclosure holds the Pi, PCA9685, ADS1115, and wiring. It needs cable entry points for:

Power cable (USB from battery)
Camera ribbon cable (CSI)
Servo cables (2-3 going out to ESC and steering)
GPS antenna cable (or the GPS module itself can mount outside)

Drill cable entry holes: Mark and drill holes in the enclosure for cable pass-throughs. Use cable glands if you have them — they maintain the waterproof seal. If not, silicone sealant around the cables after final wiring works fine.

Mount Pi inside the enclosure:

Use the brass standoffs that come with most enclosures
Orient the Pi so the GPIO pins face toward your cable exit holes
Leave room next to the Pi for the PCA9685 and ADS1115 boards

Mount enclosure to the electronics plate:

Bolt it down, or use VHB tape on the bottom
Make sure the enclosure lid opens freely once mounted

Step 5: Mount the Camera

The camera needs to look backward and down, toward where the drag cloth will be:

Attach the ball head to the camera bracket. Standard 1/4"-20 thread.
Mount the camera module to the ball head. You may need an adapter plate or a small custom bracket — some 3D-printed options exist, or just zip-tie the camera securely.
Attach the bracket to the chassis or electronics plate. The camera should be:
- Behind the cab area, pointing rearward
- High enough to see the full width of the cloth (~1m) at the drag distance
- Angled down at roughly 30-45 degrees

Test the field of view: Power up the Pi (on bench power for now), connect the long CSI cable to the camera, and capture a test image. Lay out your cloth material at the expected distance (behind the truck where it'll drag) and check framing.

# On the Pi
rpicam-still -o framing_test.jpg

Adjust the ball head angle until the cloth fills most of the frame. Lock it down tight — vibration will loosen things over time.

Step 6: GPS Antenna Placement

The GPS module needs a clear view of the sky. Options:

Mount on top of the enclosure: Simple, good sky view. Attach with VHB tape or a small bracket. Route the cable through a hole in the enclosure.
Mount on the body shell: Even better sky view, but the shell comes on and off a lot. You'd need a quick-disconnect.
Inside the enclosure with an external antenna: If your GPS module supports an external active antenna (SMA connector), you can keep the module inside and just mount the antenna on top.

Recommended: Mount the module itself on top of the enclosure with VHB tape. It's simple and works well enough for 2-3m precision.

Step 7: Battery Placement

The Anker USB battery pack:

Velcro-strap it next to or under the enclosure
Route the USB-C cable into the enclosure through a cable entry point
Make sure you can access the battery's power button without removing it
Make sure you can remove the battery for charging without disassembling everything

The TRX-4's main LiPo battery goes in its stock location — don't change this.

Step 8: Drag Bar Attachment Point

You won't build the full drag rig until Guide 6, but prep the attachment point now:

Identify a solid point at the rear of the chassis — the rear bumper mount or rear body post works well
The drag bar will be towed from here via a short rope or rigid link
Make sure there's nothing in the way of a bar swinging left-right by 30-40 degrees during turns

Checklist

Driven the TRX-4 with stock radio — understand its behavior
Body shell removed, chassis accessible
Electronics plate mounted to chassis
Waterproof enclosure mounted to plate with cable entry holes drilled
Pi mounted inside enclosure with room for PCA9685 and ADS1115
Camera mount installed, framing tested, angle locked
GPS module/antenna positioned with clear sky view
USB battery mounted and accessible
Rear attachment point identified for drag bar

Looking good. The chassis is ready for wiring — Guide 4: Wiring & Control Integration connects everything together.