Curling Rock Friction Explained

CHAPTER TS 2, page2.10 of The Book which is now free on the web.

WHY ROCKS MAY CURL/MOVE IN THE "WRONG" DIRECTION

Update: Nov.14, 2000. The president of the Grande Prairie Curling Club advised me that their Tilesert inserted rocks were re-surfaced in place under the direction of Mr.Thompson himself AND if I did not apologize to Thompson, I could not curl at the GPCC anymore. He inferred Thompson was heading home to start a lawsuit against me!!
The Thompson Co. has in fact attempted to honor their warranty to try and make their rocks curl properly (I have been advised). After Jan., 2001, I will re-evaluate these porcelain inserts after 1 month's use. I am very dubious that an in-place fix cannot do what the factory cannot do?? It now strikes me that if I had not raised the question of poor performance and notified the curling community, would the rocks been resurfaced. It did take more than 9 months of knowing they were not performing like blue hone inserts would have before a resurfacing technique was performed??? During this period, the salesman, KEVIN MARTIN, told the club to change their pebbling technique. Kevin Martin has Bluhone Granite inserted rocks at "his" club - Avonair , Edmonton.

I did curl again at GPCC and the rocks again "wore out" soon after the "in-place" sand paper job. I will now curl at the Valleyview club this year, competitive league, because they will not curl at a club,GPCC, with such "bad rocks."

Destroy your club, Buy porcelain rocks
Analog rock: Consider your rock a 12" porcelain gear lying on its side with teeth around the edges (like a gear). Porcelain has similar thermal conductivity as granite and can be roughed to be "wettable." Whether porcelain will remain rough enough to remain wettable after use is to be evaluated.

    We don't want to introduce thermal factors to the surface so we use a porcelain gear with similar insulation/thermal conductivity qualities of granite. No analog rock that is made of a non-wettable substance/surface will work like granite.
Back to more science:
    Let's give that gear/rock a push like a granite rock with TURN to the right. Which way will the gear/rock curl/move? The teeth in our gear rock are sharper/"bitier" than a granite rock's front edge even with its pits and normal roughness. The rear edge/momentary inner "cup" is asmoother edge as are most rocks due to a shallow relief cup angle. The rear left transverese friction is also less because the inside edge projection is less than the leading edge projection.
    So, the net lateral force is going to be opposite the direction of rotation IN A HIGH FRICTION ENVIRONMENT and the gear rock "curls" opposite to a curling rock. In this case, the gear rock goes left.
    The gear rock teeth would grab the pebble causing the gear/rock to move left! It would deviate left until all imparted turn energy is used up and then it would dead handle moving more or less in a straight/random direction! The friction environment of the gear/rock is much higher than that of the granite rock so the teeth grab and shear the pebble with the net of front and rear edge tangential lateral force pushing the gear/rock left.
The major friction factor is generated by the physical grabbing and shearing of pebble tops. A tire tossed flatways onto a highway with the same turn would generate more friction as the coefficient of actual rubbing friction is higher as it skids on the largely immovable asphalt pavement asperities.

A rubber tire tossed flat ways down a highway spinning right would do the same. It would move left, laterally opposite to the normal curling rock turn/curl direction, due to the high friction environment until the initial turn energy had dissipated and then it would skid ahead in the projected direction.

In a game situation, after the first two ends, (pebble tops knocked/sheared off/down) a curling rock will move in the direction of rotation because the snowplow slow side friction is higher than the net of front and rear cup edge lateral shear friction. When you brush a rock, you minimize snowplow slow side friction and the remaining shear friction has a greater influence and may stop curl or even cause a lateral rock movement opposite to the original "curl" direction. The curl is reduced. You "illegal" corner brushers can do your job now that you know where to brush? The so-called illegal or short or corner brushing is no longer illegal.

Analog Rock/Gear

DIRECTION of TRAVEL

"Gear" teeth grab/"shear" pebble and make the gear/rock move left in the high friction environment -- like a granite rock's leading and cup edges biting new pebble in the first two ends. It is clear that the net of leading edge less/minus trailing cup edge lateral friction is great enough to cause the rock to move left contra to normal curl. The net effect is less because of two reasons. 1. Normal load is higher in front (nose dive effect -rotational torque/force about cg. due to slowing) and more importantly -- 2. The projected radial contact surface is less for the trailing cup edge Vs. leading edge giving less "biting" edge and hence less friction for the rear. Vibration from uneven pebble heights dumps friction causing snow out the momentary trailing edge and vibration breaks the molecular bond of ice on the rocks running surface and sheet ice and is THE reason the rock slides so easily on pebbled ice. Without pebble, there is no curl. Without some frost on pebble tops and valleys, there is no curl. This fact is why you can slow or even stop curl by brushing!!

Side thrust opposite to curl direction, "shear friction," can overcome slow side snowplow friction when the ice is very cold (pebble hard) and when new tall pebble is present giving the rock edges friction material to bite on causing the rock to curl minimally or even "backup.".
The rock will run straighter or may even "curl" to the left in a rock turning right scenario. Brushers can level and polish the tops of pebble and valleys reducing shear friction enabling the rock to travel further and with more curl (Especially in the EARLY TWO ENDS) when the rock is in a high friction new pebble (pebble not clipped) environment.

After the pebble reaches a more uniform height (tops are flatter) to support rock weight, the net lateral forces of leading and cup edge pebble shearing is reduced allowing curl. The rock will continue to "shave" the frost buildup and drag it to the slow side to effect -- curl.
Across the face brushing will make the rock slide further and make it run straighter (reducing net shear force difference of leading edge and cup edge -- shearing which act in opposite directions) and slow side friction become more equal because slow side friction is reduced (due to momentary brushing of large crystals/frost into small/ballbearing frost) in the new "low friction" environment of "keen" ice. Shortside brushing on the slow side of a "draw" can stop curl and can make the rock actually curl in the opposite direction (backup) by up to 6". Brushing the fast side will increase curl!!