Low friction is the whole job of a runner: sand, scrape and polish the bottom to a glassy finish, then either coat it with a thin shell of ice in freezing weather, shoe it with steel strips that resist wear on packed snow, or screw on HDPE plastic, which is extremely slippery on snow. On a 150 kg sled load, a pull of about 147 N on waxed wood over wet snow falls to roughly 44 N on ice, and plastic shoes need slotted screw holes because a 1.5 m strip changes length by about 9 mm over a 40 K temperature swing.
Part of Sled & Travois Construction
Runner design is the shaping, smoothing and surfacing of a sled’s runner bottoms to give the least friction on snow or ice.
What The Coating Is Worth
Pull on level ground is the friction coefficient times the load weight. For a 150 kg sled load (1,472 N), with coefficients recalled from textbook tables and offered as orders of magnitude, not measurements:
| Runner surface | Friction coefficient | Pull |
|---|---|---|
| Waxed wood on wet snow | about 0.1 | 147 N |
| Waxed wood on dry snow | about 0.04 | 59 N |
| Ice shell on ice | about 0.03 | 44 N |
Snow friction depends on temperature. In general terms, a thin film of meltwater, produced by the sliding itself, lubricates the runner, and in very cold dry snow that film cannot form and friction rises; in snow near freezing the water can instead grip the runner and slow it (recalled behaviour, not measured here). So the best coating changes with the day: see Snow Conditions.
What You Need
- Runners, hardwood, smooth-planed along the grain (Basic Sled Construction)
- Sandpaper, a scraper and a polishing cloth
- Water and a cloth for icing, in freezing weather
- Metal strips: salvaged steel strapping, angle iron or flat bar
- HDPE: cutting boards or barrel plastic
- Screws or bolts and an auger or drill
Method: Surface The Runners
Step 1: Smooth the bottom. Sand, scrape and polish to glassy smoothness, with the final strokes along the grain.
Step 2: Choose the surface.
- Ice shell: in freezing conditions, apply a thin layer of water to the runner bottom. It freezes into an ice shell, the lowest-friction surface available. Reapply periodically, because it wears off.
- Steel shoes: screw strips of strapping, angle iron or flat bar to the bottom. They resist wear and slide well on packed snow.
- HDPE: screw plastic from cutting boards or barrels to the bottom of the wooden runners. It is extremely low-friction on snow.
Step 3: Sink the fasteners. Countersink every screw head flush or below the surface, so none protrudes to catch.
Step 4: Slot the plastic. An HDPE strip grows and shrinks far more than wood or steel. With an expansion coefficient of roughly 150 × 10⁻⁶ per kelvin (a recalled figure for polyethylene, which varies with grade), a 1.5 m strip warmed by 40 K lengthens by 1,500 mm × 150 × 10⁻⁶ × 40 = 9 mm. A steel strip the same length grows 1,500 × 12 × 10⁻⁶ × 40 = 0.7 mm. So drill the plastic’s screw holes as slots lengthwise, or leave the ends free, otherwise it buckles or shears the fasteners when the temperature changes.
Step 5: Ice carefully. Apply water with a cloth in thin even coats, one at a time, and let each freeze; scrape off lumps. Re-ice at stops.
Common Mistakes
- Rough or splintered wood — the coating takes its shape from the surface.
- Thick, lumpy ice — a lumpy surface drags more than a smooth one.
- Tightly fixed plastic — it buckles in the cold.
- Protruding screw heads — they catch and tear the surface.
Safety
Wet hands in the cold, and steel edges
Applying water in freezing weather wets gloves and hands quickly, and frostbite follows fast: see Frostbite: Warm Water Soak Treatment and Hypothermia Recognition and Rewarming. Cut strapping and angle iron have sharp edges, so wear gloves and file the edges. A well-prepared runner is fast: keep control of a loaded sled on any slope.
What’s Next
- Learn next: Snow Conditions — matching the surface to the day
- Combine with: Sled Frame Construction — the frame the runners carry