Build Guide 6: Drag Cloth & Sampling Rig

Type: Build Guide

The rover drives — now it needs something to drag. This guide covers building the sampling apparatus: the drag bar, cloth, weights, and attachment system. This is the one part of the build rooted in entomological research, so we follow established protocols.

What You Need

Parts:

PVC pipe or wooden dowel (~60-80cm long, ~2cm diameter) — the drag bar
White heavy flannel cloth (6.6 oz/sq yd, ~1m x 1m)
Permethrin spray (0.5% clothing treatment)
Trailing edge weights: metal chain or fishing sinkers (227-340g total)
Paracord or nylon rope (~50cm)
Carabiner or quick-link (small)

Tools:

Sewing machine or heavy needle + thread
Scissors or rotary cutter
PVC cutter or hacksaw
Drill (for the drag bar)
Measuring tape
Safety gloves (for permethrin handling)

Why These Specific Materials

This isn't arbitrary — each choice comes from tick sampling literature:

White flannel — Standard in every published tick drag study. The rough nap catches tick tarsal claws (they grab on and can't easily let go). White provides maximum contrast for visual counting and camera detection. Heavy weight (6.6 oz) resists bunching and lays flat.
1m x 1m — The standard size for calculating tick density per unit area. One square meter = one standardized sampling unit.
Permethrin treatment — Kills ticks on contact (<1 min for nymphs, 1-3 hrs for adults). This means ticks that grab the cloth stay on the cloth instead of crawling off before you photograph them. Per Kanten et al. (2020), permethrin-treated cloths significantly improve capture consistency.
Trailing edge weights — Without weights, the cloth lifts off the ground in grass, misses ground-level ticks, and bunches up during turns. Weights keep it flat and in contact with substrate.

Step 1: Cut the Cloth

Cut a rectangle approximately 1m wide x 1m long. The width should be slightly wider than the drag bar.

If your flannel came with two "right" sides (nap on both), pick the rougher side to face down — that's what contacts the ground and catches ticks.

Hem the edges if you have a sewing machine — raw flannel frays quickly, especially when dragged through grass and dirt. A simple fold-and-stitch on all four edges is plenty.

Step 2: Build the Drag Bar

The drag bar holds the leading edge of the cloth taut:

Cut the PVC pipe or dowel to ~70cm (slightly narrower than the cloth)
Drill two small holes near each end (for tying the tow rope)
Drill a small hole every 15-20cm along the length (for attaching the cloth)

Attach the cloth to the bar:

Fold the leading edge of the cloth over the bar by ~5cm
Sew or staple through the cloth and around the bar at each drilled hole
The cloth should hang behind the bar like a flag

Alternatively, use zip ties through the drilled holes and grommets in the cloth — this makes the cloth removable for washing/treatment.

Step 3: Add Trailing Edge Weights

The back edge of the cloth (the part dragging on the ground, farthest from the rover) needs weight to stay flat:

Option A — Chain weight:

Sew a channel/pocket along the trailing edge
Thread a light metal chain through it
Target total weight: 227-340g (about 0.5-0.75 lbs)

Option B — Fishing sinkers:

Sew small pockets every 15-20cm along the trailing edge
Drop a sinker into each pocket
Distribute weight evenly

Option C — Simplest:

Fold the trailing edge over and sew a hem pocket
Fill with a few handfuls of small pebbles or BB pellets
Seal the ends

Test the weight: hold the bar at waist height and let the cloth hang. It should drape straight down without too much billowing. On the ground, it should lay flat with light pressure.

Step 4: Attach to the Rover

Connect the drag bar to the truck's rear attachment point:

Tie paracord to both ends of the drag bar (through the drilled holes)
Join the two cords at a single point ~20-30cm behind the bar (forms a V or bridle)
Attach to the rover's rear point via a carabiner or quick-link

The bridle matters: A single attachment point makes the cloth track behind the rover through turns. Two separate ropes would cause the cloth to swing wide on turns.

Distance from rover: The bar should be about 30-50cm behind the rear bumper. Too close and the cloth bunches under the truck during stops. Too far and the leverage makes steering harder.

[Rover] ←30-50cm→ [Drag Bar]
                      │
                      │ cloth (1m)
                      │
              [Trailing edge + weights]

Step 5: Test the Drag

This is a physical test — no electronics needed beyond driving:

Lay the cloth out flat behind the truck on grass
Drive forward slowly (manual control from Guide 5)
Watch the cloth as the truck moves:

What you're checking:

Does the cloth lay flat and maintain ground contact?
Does it bunch up or fold over itself?
Do the weights keep the trailing edge down?
Does it survive turns without tangling?
Does it snag on anything? (rocks, sticks, uneven ground)

Common problems and fixes:

Problem	Fix
Cloth bunches up in the middle	Add weight, or stiffen the bar
Cloth flips over during turns	Reduce speed during turns, add more trailing weight
Cloth lifts off ground in grass	Add weight — you probably don't have enough
Bar drags/catches on ground	Shorten the tow rope slightly so the bar lifts a few cm
Cloth snags on obstacles	Keep to mowed areas; trim sticks from test path
Cloth wraps around wheels on tight turns	Lengthen tow rope, reduce steering angle during drag runs

Step 6: Camera Framing Check

With the cloth dragging behind the rover, stop and check your camera framing:

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

Transfer and inspect. You should see:

The full width of the cloth (or as much as possible)
Good contrast between white cloth and green grass
Sharp focus at the cloth distance
No part of the rover blocking the view

If the framing is off, adjust the camera mount angle (Guide 3). This is why we used a ball head — loosen, reframe, retighten.

Step 7: Permethrin Treatment

Do this outdoors. Wear gloves. Keep away from cats (permethrin is toxic to cats).

Lay the cloth flat on a surface you don't care about (cardboard, old tarp)
Spray permethrin evenly across the entire ground-contact side
Let it dry completely (2-4 hours depending on humidity)
The treatment lasts through several drag sessions before needing reapplication

When to re-treat:

After washing the cloth
After ~5-6 drag sessions
If you notice ticks crawling off the cloth during inspections (they should be dead or immobilized)

Step 8: Full Drag Run Test

Put it all together for a real test run:

Treat the cloth (or skip if just testing mechanics)
Attach the rig to the rover
Drive a 50-meter path through grass at drag speed
Stop every 15 meters (walk alongside and tell the rover to stop, or pace it)
At each stop, visually inspect the cloth:
- Is it still flat and in contact with the ground?
- Are there any visible ticks? (Even without permethrin, in tick season you might catch some)
- How's the camera frame?

This simulates the autonomous run we'll build in Guides 7-8, just with manual driving.

Maintenance Notes

Washing: Wash the cloth after each session to remove debris. Air dry. Re-treat with permethrin after washing.
Storage: Store the cloth in a sealed bag between sessions. Don't store treated cloth near cat-accessible areas.
Inspection: Check the tow ropes and bar attachment for wear before each session. Flannel wears through eventually — replace when it thins or tears.
Spare cloth: Cut and hem a second cloth. You'll want a backup, and it lets you wash one while using the other.

Checklist

Cloth cut to 1m x 1m and hemmed
Drag bar built with attachment points
Cloth secured to drag bar
Trailing edge weights installed (227-340g)
Bridle/tow rope connects bar to rover rear
Drag test on grass — cloth lays flat and tracks through turns
Camera framing confirmed with cloth deployed
Permethrin treatment applied (first application)
Full 50m test drag completed

The hardware build is essentially done. Everything from here is software. Guide 7: Capture & Navigation Software turns this into an autonomous system.