Does brushing with a brush turned to 90 degrees really work?

Recently I’ve received numerous inquiries regarding teams rotating their brush heads 90 degrees when attempting to “carve” a stone (accentuate curl). Some tour players, as well as commentators, have adopted the term “knifing” for this tactic. As of the date of this post – 26 March 2022 – I’ve probably been asked the question at least twenty times and so, rather than continue to respond individually to each questioner, I decided to post this short article. As usual, this post is joint work with my research colleague John Newhook of Dalhousie University in Halifax.

Short answer

The question isn’t, really, whether or not using a brush head turned 90 degrees “works”. Applying brushing forces – and hence thermal energy – to the ice on one side of a stone over the other is going to have an effect on a stone’s trajectory. The actual question to be answered is under what conditions is brushing using a brush head turned 90 degrees going to be more impactful (or effective if you like) than the other techniques that are already known to impact the trajectory of a curling stone.

The short answer to this latter is question is, simply, that at the moment we don’t know. We are not aware of any published studies that attempt to quantify a difference in the effect of brushing with a brush head in its usual orientation versus one turned at 90 degrees. Nor have we had an opportunity to undertake our own testing, given the pandemic and a number of other constraints to our research that have arisen over the past two years. It appears that this technique came from Scotland (started with Team Mouat and subsequently adopted by Team Muirhead) relatively recently, perhaps within the last two years.

Aside – We believe there is a significant amount of confirmation bias amongst curling teams when it comes to brushing, because we believe very few teams undertake enough systematic experiments (“investigations” may be a better term as it sounds like a more accessible thing to do) to determine if a change in tactics actually makes a difference. We believe that there is a great deal of simply adopting what other teams are doing. One example of that is VERY slow brushing from the side of the stone in an attempt to “carve” draw weight shots, which we believe is not as effective as brushing at normal speed (although once again we are unaware of studies of that specific technique).

In the studies that I’ve personally participated in, we can experimentally measure and video about 8 shots per hour since we have to account for a variety of variables including the pebble wear on the sheet. Perhaps someone faster than I could double that to 15 shots per hour (and I’ve had up to four assistants helping). That would mean throwing, video, measuring and recording line to markers in the ice in 4 minutes for every shot, on average. I am aware of published studies in curling that provide results for over 200 shots, so you can see what I mean about the amount of effort involved. Well-planned, controlled investigations with enough trials to demonstrate that the effect is repeatable and attributable to the brushing technique and not a number of other factors that influence a stone’s trajectory is required in order to develop enough evidence, over a group of subjects, to have confidence in the result. End of Aside.

Brushing effectiveness depends on a large number of variables, including: ice conditions, condition of the stone’s running band, stone rotation, stone velocity, throwing tendencies of the athletes, the placement of the brush(es) in relation to the stone, the brushing angle of attack, the force profiles of the athletes, and whether one brusher is used or two. This is why we continue to stress that teams should experiment in practice and determine what techniques work for them with the athletes that they have, and develop a brushing strategy based on their abilities. At the end of the day, a team’s brushing effectiveness will be more dependent on their athleticism and ability to generate vertical force than on the location and orientation of the brush head.

A somewhat longer answer

Brushing forces applied to the ice change the characteristics of the ice in that location. As the brushing forces during a stroke are not uniform neither across the width of the brush pad nor along the length of the stroke, brushing creates interaction conditions beneath the stone’s running band that are not uniform. By understanding the impact of changing the interaction conditions beneath the running band, even if only temporarily, a team can select a brushing technique to achieve a desirable impact on the stone’s trajectory. We use the phrase “stone-ice interaction conditions” as the physics of this interaction between the stone and the ice is complex and not easily described by a single term or phenomena. There is clear evidence, going back more than a decade from multiple researchers (cf. references [1-3]), that a primary phenomenon in brushing is the temporary heating of the ice beneath the brush head. This effect is larger or smaller based on the amount of brushing force applied. It is proposed that any other stone-ice interaction effects that may exist are also proportional to the amount of force applied. It is also clear that precisely where the brushing forces are applied relative to the running band will also have a significant impact on the stone-ice interaction at that moment. Unfortunately, we are still some distance away from fully understanding, from a physics standpoint, precisely why a curling stone “curls” and hence we are unable to fully explain the impact of brushing a stone in motion. Nonetheless we have a great deal of observational evidence from which to draw. In general, brushing that creates more uniform stone-ice interaction conditions across the width of the running band is very effective for extending a stone’s travel distance, or carry. Brushing techniques that create more non-uniform (ie. more effect on one side than the other) stone-ice interactions across the width of the running band will impact the stone’s trajectory, accentuating or reducing curl. This latter effect is often referred to as directional effect, or directional brushing.

We restrict the discussion below to directionally influencing the trajectory of stone through “carving” (accentuating curl) only.

To accentuate curl, a common technique used by many teams is to use a single brusher and brush from a “snowplough” position behind the stone. From this position, the athlete angles the brush stroke so that it begins near the edge of the running band and proceeds across the face of the stone, with the “push” portion of the stroke in the direction of the stone’s rotation. The grey scale shading in the diagram illustrates the variation of pressure along the stroke with darker areas representing more pressure. From this diagram it is easy to visualize that the stone-ice interaction conditions will vary across the width of the running band, with the impact being higher in the darker areas.

While this angle of attack is commonplace it is not the only possibility. One side effect of such a stroke is that it can lead to additional “carry” of the stone because a greater proportion of the stone’s running band will have improvement in the stone-ice interaction conditions from brushing, even if they are greater on the right-hand side of the stone than the left-hand side. To avoid greater carry and only accentuate curl, one might propose that having maximum influence on the stone-ice interaction on the high-side of the running band, and minimum influence on the low-side, is the best approach. To do this, one option employed by teams is to use a lower angle of attack and brush across the face of the stone. Yet another option is to “corner sweep” and brush the high-side edge of the running band only. In all of these options, the orientation and location of the brush head and brush stroke is very important, and misplacement of the brush head or stroke by even a few inches from the desired location dramatically influences the effectiveness of the technique, and can even promote the opposite effect on trajectory.

This “corner sweeping” technique, with an emphasis on precise positioning of the brush stroke and maximizing the impact only on the high-side of the running band, gives rise to the idea of turning the brush head 90 degrees in order to narrow the area in front of the stone impacted by brushing, potentially offering more precise placement and hence the application of heat on one side of the stone’s running band or the other (see Figure B below).

As we stated above, brushing this way will impact the trajectory of the stone but its effectiveness will greatly depend on the force profile of the brusher, the stone’s velocity, and to a lesser extent, its rotation. However, as one can see from Figure B, a significant proportion of the stone’s running band will be moving over unbrushed ice and so this technique will yield less carry. However, our observations of how competitive teams have actually used a 90-degree orientation from this year’s Olympic, Scotties, Brier, and World Women’s competitions are that many teams are actually combining the two ideas (Figure C).

Overall, the stroke in Figure C is very similar to that in Figure A but there are two important differences. The first to note is that the location of the brush head at the beginning of the stroke is entirely to the right of the centre of the stone. The second is that with the brush in that orientation there will be less ice coverage than the stroke in Figure A as the stone moves down the sheet. There will be less ice coverage during the bout because of the “zig-zag” nature of the brush stroke as the player moves forward with the brush in that orientation. We can assume that less ice coverage will yield a reduction in carry, but to what degree the “carving” effect may be improved or degraded is unknown. Unfortunately, however, without extensive testing we do not have a good idea regarding the nature of these trade-offs, nor to what extent is the effectiveness of this technique dependent on factors such as a player’s force profile.

Summary

Without comprehensive testing we are not in a position to recommend, or advocate for, any specific technique – again we believe it is critically important that teams test for themselves in practice what techniques work for them. However, we can state the following:

• brushing with the head rotated 90 degrees does have the potential to impact a stone’s trajectory with greater asymmetry of the stone-ice interaction conditions on an edge of the running band;
• a consequence is that there is less ice coverage in front of the entire running band, so there will be less carry, and hence a need for more precise weight tolerance;
• carving a draw-weight shot from a snowplough position with the brush in the usual orientation moving across the top of the stone is used by many teams to accentuate curl;
• without testing, it is unclear if trying to accentuate curl using a 90-degree brush orientation is better than the typical straight-on approach;
• we know from our own research that while all competitive teams use WCF-approved equipment, the pressure distribution across the brush pad varies considerably amongst the various manufacturers and brush models. Our sense is that it is very likely that different brushes will be more, or less, effective if the head is turned at 90 degrees as a consequence of different product designs. Progress in our research on brush design has been largely halted by the pandemic but we hope to restart our investigations this spring.

We are pleased to see teams experiment with different tactics with their brushing, especially when those tactics are chosen through testing so that the tactics fit the capabilities of the respective players. If you watch carefully, some of the top teams will employ a variety of techniques related to one or two brushers, snowplough vs. an across-the-face brush stroke, high-side vs. low-side brush placement, and (even) normal or 90 degree orientation of the brush head. They change tactics not just from shot to shot but sometimes during the same shot. We believe that the teams that are able to achieve the desired brushing impact consistently across these options have spent the significant time experimenting, evaluating and establishing what works for their team. They then practice it continually and also adjust their weight and line of delivery choices to match their brushing effectiveness.

Our hope is that even though the number of variables is great, we will have an opportunity in the future to try to quantify the effectiveness of these different tactics in order to better present guidance to competitive teams and their coaches.

References

[1] John L. Bradley (December 2009). The sports science of curling: A practical review. Journal of Sports Science and Medicine (8), pp. 495-500.

[2] Marmo, B., Buckingham, M-P. and Blackford, J. (2006). Optimising sweeping techniques for Olympic Curlers. International Sports Engineering Association Conference, Munich, Germany. Abstract: Sports Engineering 9(4), pp. 249.

[3] Marmo, B. A., I. S. Farrow, M-P Buckingham, and J. R. Blackford (2006). Frictional heat generated by sweeping in curling and its effects on ice friction. Proceedings of the Institution of Mechanical Engineers, Part L.: Journal of materials: Design and Applications 220(4), pp. 189-197.