Curling clubs

The "tire rock" friction experiment:
Precision made Curlex Handle courtesy of Challenger Rink Supplies, Harry and Bruce Tingley. Contact Challenger: 1-800-665-2875 or E-mail: curling@kootenay.comchallenger web site http://www.curlingrink.com

Another experiment, not hypothetical, was carried out at the Nelson, BC, CC summer spiel this last July, 2000. What is shown is a "tire rock." Somewhere in my discussions on friction, I said a granite curling rock seems to defy friction laws by curling in the "wrong" direction?

The "tire rock" curls in the "expected" direction opposite to a granite rock exactly like the "gear" rock described on another web page. That is; if you spin the rock to the right, the tire rock will "curl" left as shown in the following picture. I let the tire rock "freeze down" just as a granite curling rock is frozen down before being put into play.

We are looking back in the hack direction. Right turn was applied at release and the tire rock went left. See the approximate 4" "curl" to the left from centerline. Even with a very hard push delivery, note the tire rock did not slide very far, only to the near hog line as it is in a very high friction environment with a running surface of non-wettable rough rubber on ice and not ice on ice as is the case with the granite curling rock.

"Tire rock," blue stones, travel sequence.

The "tire rock" has a running surface ring much like a granite curling rock. I deliberately roughened this 5/8th's inch "running surface" to try and get frost to adhere to the tire but it only made the friction factor higher and the rock pushed much harder and stopped sooner with more "curl" to the left and not right as in the granite rock keen friction scenario. As soon as the right rotation stopped, the tire rock skidded straight ahead in it's trajectory with no more curl to the left.

Just as in the rockfriction.htm "gear rock" hypothetical scenario, the tire rock "curls" in the expected direction. What frictional forces and direction of force vectors are involved in the tire rock scenario Vs. the granite rock scene?

Unlike the granite curling rock, the leading running surface edge of the tire rock is in a very high friction environment so a turn to the right makes the rubber "bite into" the pebble tops/frost providing a friction force, equal and opposite force vector, from right to left pushing the rock to the left. There is also a force vector at the rear "cup edge" running surface pushing the tire rock right. Why doesn't the tire rock then go straight down the sheet?

There is a net difference in lateral force to the left. The right friction vector force (Rf) minus the rear edge left vector force (Lf) leaves a positive smaller net (rf) vector force to the left. The front right to left friction vector force is pushing harder than the rear left to right friction vector force so the NET smaller force still pushes the tire rock left.

The "tire rock" is in a high friction environment AND while it may drag frost around to the "slow side" to be dumped out the back by rock vibration and pebble spaces like a granite rock with an ice coating, this force does not overwhelm the net of transverse/crosswise high leading edge friction force minus trailing edge transverse force vector; hence, the tire rock quickly stops rotation after the initial turn energy is expended by lateral friction which stops the turn. The tire rock stops rotation because the friction environment is much higher unlike the granite rock which takes longitudinal energy (down sheet kinetic energy) and transposes a small amount to rotational energy to keep the curling rock rotating.
This apparent anomaly for a granite rock is due to the fact, while the friction is higher by a small amount for the granite rock's, crosswise front minus crosswise back transverse/crosswise vectors, the friction force on the side of the rock (right side with right turn) in the longitudinal/down sheet direction is higher than the transverse net force and results in the rock continuing to turn in the original (direction of imparted turn). The tire rock quickly stopped rotating due to the VERY high transverse, crosswise, net friction force overwhelming the longitudinal net forces of the right side and left side of the granite rock. See "curling rock vector forces."

What causes this reaction?

One factor is the front "dive" down visual effect of a rapidly slowing object, a car. The "dive down" is a visual indicator only and is not the actual force in discussion as in the case of a car front end diving down with braking. It is the ROTATIONAL force about the objects, car or rock, center of gravity in relation to the distance from the surface, road or ice, that actually increases weight/normal load on the front end/tires or rock edge and hence applies more vertical "normal" load times friction factor to equal more overall stopping friction on the front tires. The dive down visual effect is the change of forward kinetic energy to rotational kinetic energy about a moment arm from the pavement surface to the car's center of gravity.

This same rotational torque translated to increased front edge load is in effect also with a curling rock even though it does not physically "dive" in the front upon slowing (it is solid). In the curling scenario, this slightly higher front edge load increase is very minimal and due to a rotational vector and due to the very low friction factor of ice on ice with minimum rock slowing friction per unit time. The major curling rock friction environment of a granite rock is that of physically striking tops of pebble causing the rock to vibrate up and down while shearing pebble tops on new pebble. On very keen ice the rock's running surface scraps a very thin build-up of frost from the pebble and valleys which is then dragged to the slow side to cause higher friction in the longitudinal/down ice direction resulting in "curl." This "curl" is due to the resultant net added torque on the rocks changed rotational center of gravity caused by snow on the outer leading edge. There is some snow on the cup's trailing "leading" edge, but it is much less with a lower torque arm distance to the center of gravity so while reducing curl, it does not stop curl.

1. Your car dives down (shocks and springs compressing) in the front with rapid braking which increases weight on the front tires/rock running surface while also decreasing the vertical "normal" load on the back tire/rock running surface. The law in physics says: As the vertical load is increased, so is the friction resistance force. The friction is only slightly higher on the front edge of the tire rock than the rear cup edge. On the other hand, the friction on the front tires of your car greatly increases versus the rear of car tire friction during quick stops (high friction environment and high center of gravity).

As a side note, it must be noted, as I am a mechanical engineer, that the car nose dive is BAD! Why? The "dive down" by itself (there is a variable mass x acceleration force in both the down and up cycle of a stopping vehicle) does not provide more overall braking effectiveness on the front tires and actually reduces the braking and control of the rear tires. The overall braking distance is more for a car that radically nose-dives versus a car that maintains a "higher/normal" more uniform profile to the ground and comes to a more level stop. There will always be a transfer of weight to the front tires but the car stops with much more control, i.e., directional stability weight/friction is maintained at the rear minimizing the chances of the car "swapping" ends. ALWAYS maintain shock absorbers/struts at new specifications!!

What causes this phenomenon? Again, simple physics! In a slowing mode, some of the kinetic energy of forward motion is transferred from forward kinetic energy acting through its "cg," center of gravity, to rotational kinetic energy due to the car's, relative to pavement, high center of gravity. The net change of kinetic energy from forward to angular/rotational momentum acting about the moment arm from the pavement/tire contact point/surface to the ever adjusting car center of gravity, cg (the car dives and then raises) makes the normal load momentarily higher in front with an actual decrease in normal load at the rear tires.

2. In the tire rock scenario, the above rotational momentum in the longitudinal plane works even with no "dive" since the tire rock is virtually solid like a granite rock. That force adds greatly to the friction component resulting from the fact that the load on the front edge is larger than the trailing cup edge. The radius of the leading edge is greater than the cup edge so has more "biting" edge. The projected area with more "grabbies" is greater in the front and as in the case of the tire rock slowing at a very rapid rate, there is more vertical normal force at work on the leading edge.

As stated above, the law of physics says a resistant friction force is solely a function of normal load, N, multiplied by the friction factor, u, (Nu) and NOT AREA of contact. This is a law and a "law" in physics can not be violated. Other friction factors become involved as in the "drag racer" scenario. Drag vehicles have wide tires. Right? Remember your bicycle days. Wasn't it easier to drag the rear wheel over the curb than bump over with the front tire. "Other" friction and line of force/vectors, factors are involved.

The very fact we are on a pebbled surface causes the tire rock and granite rock to vibrate. This vibration causes a pitching motion with leading and trailing edge hopping up and down increasing leading edge friction. In the granite rock scenario, it is this vibration that allows the granite rock to "dump and drag" frost from the slow side of the rock as it rotates and maximizes friction on that side (right side of right turning rock) resulting in curl.

ALSO, this vibration of the granite rock is what makes it slide so easily on pebbled ice and in contrast with no vibration -- almost "stick" to un-pebbled ice. Pretty simple, "Eh?" It is the high frequency vibration which breaks the molecular bond between ice on the rock and ice on the sliding surface/pebble tops that contributes to the rock traveling easier in a keen ice scenario. Clipping the ice before play by machine or dragging rocks does two things to increase ice "speed."

1. It shears off "grabbie/pointee" pebble tops so a delivered rock doesn't have to do the shearing work/energy per unit time job which would result in non-uniform weight/slide distance and curl reaction.
And.
2. It makes vibration causing ridges/bumps (slightly different heights of somewhat flat topped pebble --after shearing). How? When "clipping" the ice, the head/blade of the Ice King vibrates up and down as it nicks the tops off new pebble. This is GOOD. It acts to make the ice keen much like dragging curling rocks as they too vibrate/bounce up and down when pulled down the sheet in a raft to shear the pointee/drggie tops off pebble. This is why it is EXTREMELY important to keep the blade square with the sheet to not cause a delivered game rock to vibrate sideways/deflect resulting in non-uniform curl. On a gravel road, your car will do the same thing with traveling over uneven/angled wash-board ridges and could vibrate the vehicle sideways sending the car in a sideways skid!!

On drag race cars, why not use skinny tires (low area) as the law of physics, Nu, suggests? One "other" factor includes the friction reaction between "asperities" in the tire surface and pavement surface. These aspirates/bumps act like gear teeth on meshing gears. In the case of gears, 100-% of power is transferred from gear to gear -- no slipping. There is some slippage on a tire spinning over pavement but the wider tire provides more "grabbies" to interlock with the "grabbies" on the pavement. On start/launch, drag racers momentarily "spin" their tires to melt the running surface making the tire more "sticky" and to develop maximum engine RPM/horsepower/rotational momentum of engine and drive line parts at the initiation of the launch. With the same available horsepower, the driver that can regulate the amount of tire spin and then get to a higher tire friction environment of nearer no-spin while bringing the engine to maximum output power will get to the finish line first assuming their reaction time to the "tree" starting light is the same.

It must be noted, there is an empirical limit (trial and error) to the maximum effectiveness of wider tires. Because of lower normal load per unit area, a too wide tire may provide less friction than that of a somewhat narrower optimum width tire. A wide tire also has higher air drag at 200mph. Isn't friction interesting?
Another friction reducing factor for the "too wide" tire is the "air-plane" effect at speeds over 100MPH. We all have heard of the "hydro-plane" effect as a tire rolls over water. The water wedge lifts the tire and vehicle from the road surface. Water is 800 times more dense than air, but air at higher speeds also reduces the normal force on the tire surface. The wedge design of race cars counteracts some of this air wedge tire lift as a result of air pushing down on the car to increase friction on all tires with ever greater values at higher speeds.

Remember, to make the ice keener: Clipping the ice, like dragging a raft of rocks, makes flatter high area pebble tops to sustain the weight of the rock AND uneven tops/ridges that sets a vibration pattern that breaks the molecular bond of the rock's running surface ice coating with sheet ice.

SO ice technicians, keep the ice machine blade square with the sheet so you don't make angle windrows that WILL DEFLECT the rock unevenly depending on from which side of the sheet the rock is delivered and from which end/direction it is being delivered!