In January 2026, the World Curling Federation banned a physical movement. Not a material or a piece of equipment. A specific way of moving your arms. The prohibited action was pushing a broom away from the body in a single downward stroke while keeping the brush head in contact with the ice on the return. The policy took effect immediately. The Milano Cortina Winter Olympics opened twenty-six days later.
This was the third time in a decade the WCF had significantly rewritten its sweeping rules. The official position, each time, was that legal brushes have limited influence on a stone’s trajectory. Athletes kept finding ways to prove it wrong.
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What Sweeping Does to a Curling Stone’s Path
When sweepers brush the ice ahead of a moving stone, friction from the broom generates heat that partially melts the pebble tops in the stone’s path. A thin water film forms. Friction between the stone and the ice drops, and the stone travels further and curls less than it would on untouched ice.
That is the mechanism. The details matter.
The ice surface in Olympic curling is pebbled — water is sprayed across the sheet and freezes into thousands of tiny bumps, each between one and two millimetres tall. A curling stone makes contact with those bumps through a narrow ring on its granite base called the running band: 130 millimetres in diameter, just three to eight millimetres wide. The stone’s full 20-kilogram weight is concentrated onto whatever pebble tips that ring touches at any given moment.
What makes the biggest difference during sweeping is not how fast the broom moves. A controlled study replicating conditions at the PyeongChang Olympic venue found that downward pressure on the brush is the dominant factor. More vertical force means more heat, more melt, and more distance gained. At maximum effort, that output pushes a sweeper’s heart rate to an average of 170 beats per minute, according to research published in the Journal of Sports Science and Medicine. Leads and seconds, the two positions who sweep the majority of stones in a game, train specifically for that cardiovascular load.
On an Olympic sheet where a centimetre separates a scoring stone from a wasted one, the distance effects of sweeping are decisive.
Why a Curling Stone Curls in the First Place
This is where the science gets uncomfortable: after nearly a century of research, physicists still cannot agree on why the stone curls at all.
A clockwise-rotating stone curls to the right. That is the opposite of what standard friction logic would predict. The long-held explanation rested on asymmetric pressure — the stone leans slightly forward as it travels, generating more melting at the leading edge, which reduces friction at the front compared to the back. The trailing edge’s stronger grip pulls the stone sideways. This held as the dominant theory for decades.
In 2012, researchers at Uppsala University in Sweden ran the calculations and found the problem. Curling stones rotate far too slowly, completing only one or two full rotations across the entire 28-metre sheet, for that effect to produce the lateral displacement of a metre or more that occurs in actual play.
The decade that followed produced competing explanations. The Uppsala team proposed that microscopic grooves carved by the stone’s running band direct its subsequent motion, demonstrating this by steering a stone along a deliberate zigzag path using pre-cut scratches in alternating directions. Researchers in Canada developed a pivot-slide model, arguing the stone moves through rapid tiny pivots around individual pebbles rather than a continuous slide, with each pivot shifting the path fractionally until the curl accumulates. A 2022 study in Scientific Reports, by physicist Jiro Murata of Rikkyo University, offered the most experimentally grounded attempt: precision measurements showed the friction coefficient varies between the faster-moving and slower-moving sides of the rotating running band, and the stone swings toward the slower friction points, producing drift in the direction observed on ice.
A 2020 paper, also in Scientific Reports, added a separate finding: curl distance is primarily governed by the running band’s own surface roughness and area, making the stone’s physical characteristics a larger factor than ice conditions alone.
No theory has universal acceptance. Sweepers are adjusting the outcome of something science has not fully explained.
How Sweepers Actually Steer the Stone
Sweeping does more than extend distance. With technique, it steers.
Brushing on the inside of the curl — to the right for a clockwise-rotating stone — reduces how much the stone curls. Brushing on the outside edge can add curl. The unswept side of the running band keeps its friction grip, and the stone drifts toward it. At the Olympic level, the skip reads the stone’s speed and line in real time and directs not just whether to sweep but how hard, on which side, and when to stop.
Skips use a 1-to-10 number system to communicate where a stone will land, with 7 indicating it will reach the centre of the house. Sweepers call weight estimates back as it closes in. The commands have standard meanings across competitive curling:
| Skip’s Call | What Sweepers Do |
|---|---|
| “Hurry” or “Sweep” | Maximum pressure — extend the stone and hold the line |
| “Off” or “Whoa” | Stop completely — let the stone curl and slow on its own |
| “Sweep for line” | Hold the stone from over-curling, without pushing it further |
Research published in Sports Engineering in 2024 confirmed that directional sweeping with modern, WCF-approved brooms does affect curl, but the effect is smaller than most elite players expect or act on. Top athletes continue sweeping for directional control regardless. That gap between what the research measures and what the sport does has its roots in 2015.
The Frankenbroom Scandal That Changed Everything
The 2015 season broke curling’s relationship with its own brooms.
Brush heads with “directional fabric” appeared at the top levels of the sport. They did not work by generating heat. They worked by scratching the ice surface, augmenting friction in specific directions rather than reducing it. A stone that should have curled six feet could be redirected four feet against its natural curl direction. Sweepers were effectively steering stones that had already left the thrower’s hand, with an influence the sport had never seen before.
Fifty of the world’s ranked teams signed a joint statement refusing to use the brushes. Canadian women’s champion Chelsea Carey put it plainly after the season: “It was just no fun last year. There was a lot more animosity and tension than I’ve seen in a long time.”
The WCF declared a moratorium on directional fabric brush heads in November 2015. The following May, the federation convened the World Sweeping Summit at a curling facility in Kemptville, Ontario, bringing in scientists from the National Research Council of Canada. More than fifty different brush heads were tested under scientific conditions, with Olympic-level athletes including two-time world champion and gold medalist Ben Hebert sweeping while laser scanners mapped the ice surface after each stroke and sensors on stones tracked distance, speed, temperature, acceleration, and rotation.
Researchers at Western University later confirmed the mechanism: the banned brush heads left scratches in the ice four times deeper than those made by approved equipment.
Formal rules came in September 2016. All brush heads were required to be covered in a specific smooth fabric — 0420 Denier 100% Nylon — with no directional properties. The penalties were written to be severe: a team forfeits the game on a first violation within a competition, and a second offence within the same event carries a one-year ban from all WCF competitions.
Three Rule Changes and the Problem That Has Not Gone Away
The 2016 regulations did not close the issue.
| Year | What the WCF Changed |
|---|---|
| 2016 | Directional fabric banned. 0420 Denier Nylon mandated for all brush heads. Forfeit on first violation; one-year ban from WCF events on second offence within same competition. |
| 2025 | Regulations updated on component-style brooms — fabric sleeves over moulded foam — after several constructions were found “over effective” during the 2024-2025 season. The University of Saskatchewan conducted independent testing at Nutana Curling Club in Saskatoon, examining knife-style versus standard brushing and the effects of different foam densities. |
| January 2026 | Sweeping Technique Policy introduced — the first time specific arm movements were prohibited. Banned: pushing strokes with brush contact maintained on the return, and single strokes that lift the brush head before returning. Took effect less than a month before the Milano Cortina Games. |
Matt Dunstone, a member of the World Curling Athlete Commission involved in the January 2026 policy discussions, addressed the urgency directly: “We knew through discussions that this had to be done. If we were going to do it before the Olympics, this had to be done pronto.”
Skip Mike McEwen was more pointed: “I was just surprised at this stage that there was action, which traditionally, going back there, probably arguably, has been inaction and slow to do anything.”
The scientific consensus on modern, compliant brooms is that their influence on a curling stone’s curl is real but limited. The WCF’s own regulatory record across ten years describes something harder to contain than that.
This is not a settled argument.
The science says legal brushes produce modest directional effects. The governing body has rewritten the rules three times trying to define where modest becomes too much. Elite athletes have found the boundary every time and pushed past it. Each revision has arrived in a hurry — the most recent one with the Olympic countdown already running.
What sweepers do at 170 beats per minute on Olympic ice is apply heat and pressure to a phenomenon that physics has spent a century trying to fully explain, in a sport whose governing body is still working out, competition by competition, how much influence a broom should have.
The rules arrived in January. The argument started long before that.
