Measuring your blade's rocker (geek alert)

[Bill_S]

Participating in recent threads about a blade's rocker and it's affect on skating, I became curious about my own Coronation Aces. Someone mentioned that MK and Wilson sometimes manufacture out-of-spec blades. Perhaps so, but it might be just a rumor that has persisted. Still, I wanted to ensure that my own Aces were OK, and not wildly different from their advertised rocker. Rocker is the curvature of the blade from front to back.

There are online radius calculators like http://mlmsolutions.biz/threepointcircle.php where you enter three sets of measurements, and it automatically calculates the results. I just needed accurate measurements for it - more than just a ruler and straightedge. Small errors can produce large changes in the calculated result.

I used my table saw as the "measuring fixture" because I know that my fence is fairly straight and true. I cobbled a sliding base for my dial indicator to allow it's measuring tip to follow the blade rocker. The dial indicator part was moved up along the fence, and at three points a little under 2" apart (arbitrarily decided), I took measurements.



With the three sets of x & y measurements, I plugged them into the calculator and got this result:



All my units were in inches, so the calculator gave me the answer in inches too. When I divided 88.741" by 12 to get feet, I arrived at a rocker of around 7.4 feet for my 7-year old Coronation Aces. Sharpening tends to flatten a rocker and increase the radius, so that doesn't surprise me. They probably did start life at their advertised 7' rocker.

Knowing this isn't going to make me a better skater (darn!). It also takes one more excuse off the table. The more I skate, the more I realize that it's just me, not the equipment, that's  responsible for how well I do.

[skategeek]

Cool!  I wonder if you could also do something like photograph/scan the blade and digitally make the measurements. (I have more digital toys than analog toys...)

[Bill_S]

I'm sure it could be done. You'd need good contrast to see the blade edge well, and sharp, non-distorting optics.

The rocker equation is very sensitive to measurement precision in one dimension (in depth, but not to the length between points along the blade). I found that a change of only 0.002" depth changed the rocker by a 1/4 foot in the online calculator. I wouldn't want to measure with less precision than 0.002" if possible.

To be able to resolve differences of 0.002" (the size of one pixel at 500 ppi), you'd have to have a minimum resolution of around 2 x 500 pixels per inch at 1:1 to find the edge well. Otherwise the edge could bisect a pixel at only 500ppi leading to an error greater than 0.002". But 1000 pixels per inch at 1:1 size is very achievable.

Photoshop has a ruler tool that could be pressed into service to find the x and y coordinates of three edge points. The rest is a piece of cake.

[Query]

The most common rocker measurement tool has several design flaws:

1. It has 3 very small points of contact, so it is very sensitive to extremely small scale variation.

2. It doesn't measure along the center of the blade, or along the edges. It is offset in such a way that side rail scratches, or the ends, can affect the measurement.

I like sighting a blade against calibrated curves. If you print the curves yourself, include a measured grid. If the grid line spacing isn't right throughout the print, or the grid angles aren't properly orthogonal, the printer isn't accurate or consistent enough. The same problem could apply to scanned images.

Many hockey skate sharpeners have a set of "rocker bars" with constant well calibrated radii. Do you know one well enough to let you borrow them?

In any event, be certain the blades are new with a factory sharpening. Most sharpeners, even mail order companies like Rainbo, modify the factory sharpening for MK and Wilson, and sometimes other companies.

Other things to check, that some sharpeners say are poor:

1. How warped is the blade - horizontally, vertically, or with a twist?

2. How symmetric and uniform is the plating? This is important because most sharpening machines grab the blade in one way or another, to affect centering.

3. How symmetric is the the honed portion of the blade at the bottom. Is the thickness removed the same on both sides?

4. Is the thickness of the honed portion uniform, up and down, and along the length (and is it meant to be?)

5. Are the countersinks centered on the holes?

But at some point, you may get tired of trying to look at your blades like a precision machinist.  If you skate, they won't keep their shape.

[lutefisk]

What rocker radii do hockey blades have?  Less than 7'?

[Bill_S]

Good question. I hope that someone can provide an answer.

Edit: I found this online...

http://werewolfbarmitzva.hubpages.com/hub/Impove-Your-Skating-Instantly-All-About-Radius-Pitch-and-Hollow

Sounds like 9 feet is common, with a range between 6 feet and 13 feet. I like the descriptions of how they perform at different rockers on this page, similar to this one for 13 feet...

A 13' radius is the longest on the market. The longer radius provides a greater amount of blade contact with the ice. More blade on the ice results in a higher top end speed, better balance (because the blade is long and flat), and better energy efficiency. The downside to a longer blade is a loss of maneuverability; a longer blade is harder to turn sharply and change directions.

[rd350]

Impressive and interesting!  I'm going to see if my fitter can check mine when I go next week, or the week after (I have a second appointment to work on my current skates since the customs will take a while).  My Coronation Blades blades are older than yours (around 20 years!) but I had about 10-12 years old the ice.

@Bill_S I agree, it's usually the skater, even when the equipment is flawed.  There are several skaters I know whose skates are practically falling apart and they still skate quite well!

[Query]

The hockey rocker radii in the link are for a central portion of the blade. Most hockey blades spiral up into shorter (more curved) rockers towards the ends, often in a series of discrete steps. For Olympic rink size use only, there may be a short un-rockered "flat" in the center. Expensive "CAG" machines are often used to create consistent complex profiles.

Large (less curved) rocker radii improve balance, speed, stops and direction changes; Small radii improve turns.

Perhaps figure skates have less complex rocker profiles to make it easier to maintain constant radius lobes??? (That's just a guess, and may be completely wrong.)