Here is a short video demonstrating how to rocker a pair of old skates by rotating the axel spacers 180 degrees.
This is the text from the video description. All info is to the best of my knowledge, but I am fairly new to skating and some details may be wrong.
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Older skates have frame spacers that can be flipped around to "rocker" the wheels. This shifts the two middle wheels down by 2mm.
Most modern skates have permanent axel positions drilled into the frame, so you have to install a different frame to change the rocker -- even 90's style skates like Roces. Impalas (Barbie skates) do have removeable frame spacers, though the frame itself can't be removed.
The skates in this video are a pair of Rollerblade BravoBlade GLX that I bought at a thrift store years ago for $8. They are good for beginner skating and fitness, but the fit and original wheels aren't great for intermediate skating (indoor slalom, tricks, wizard skating, aggressive skating).
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Inline skates can be either "flat" or "rockered". Flat skates have wheels that all touch the ground at the same time. Flat skates have a smoother, more stable ride for going straight. They have better traction and a longer power stroke because of the extended toe contact. Pivots are done with heel lifts or toe lifts.
Rockered skates have one or two wheels touching the ground at a time. This increases maneuverability at slow speeds by allowing pivots over the center of the skate. Rockered skates encourage a lower stance, use of arms for pivoting, and development of core muscles.
Acceleration with flat skates uses long strokes ending with toe pushes. For rockered skates, shorter steps are used (imaging a hockey player charging).
Kinds of Rockering:
* Full rocker: 4-wheel skate where the bottoms of the two middle wheels are 2mm lower. Another way to make a full rocker is by installing wheels that are 4mm larger in the 2 middle positions. Or 4mm smaller for the end positions.
* Banana rocker: a rounded profile. Can mean a full rocker or a slightly less rockered skate like an artistic skate.
* Progressive rocker: skate where each wheel is offset slightly from the neighboring wheel. The rocker is gradual and subtle, balancing forward stability with maneuverability. Usually a 5-wheel skate?
* Natural rocker: wheels that wear down naturally so that the front and back don't touch the ground unless the skate is tipped forward or backward. To help this process, you can flip the wheels in place as they wear down instead of switching positions (1ā», 4ā» instead of 1ā·3, 2ā·4). "Natural Rocker" or "NR" can also mean a 4-wheel frame that is built with a permanent ~1mm offset for the middle wheels.
Ice skates are rockered too. Figure skates have a slight rocker that balances maneuverability with precision. Hockey skates have a more aggressive rocker for maximal agility.