This section is dedicated to the history and technical detail of the inline skate.

A big thanks goes out to Fred Ahrens (Professional Engineer), in Turbine Engine Bearings, Seals & Systems Design & Analysis, for allowing replication of his research.

• History of inline skates
• Bearings
• Lubrication
• Types of inline skates
• Wheels

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History of inline skates

Inline skating can be an enjoyable and highly effective fitness training activity. It has approximately the same calorie burn rate as running with less than half the impact load.

The earliest known conceptualisation of an inline skate dates back to the 1700's when a Dutchman attempted to simulate ice skating by nailing wooden spools to strips of wood attached to his shoes. Versions of the inline skate resurfaced periodically, but the concept did not catch on. Given the technology of the time, inline skates could not be designed to function as well as conventional roller skates. By 1930, roller skates had found a permanent place in society. In 1980, two hockey players chanced upon a set of inline skates and found them to be ideal for off season training. They subsequently formed the Rollerblade company which subsequently reintroduced inline skates to the mainstream. Beginning in the 1990’s, inline skating has enjoyed immense, and growing, popularity and has moved to entirely new market segments. While still having a strong hockey following, the sport now has cross-over from skate boarders (aggressive) and, most importantly, has penetrated the fitness market.

Bearings, lubricants, and wheels are the three primary determinants of product performance. Other factors such as boot ergonomics, stiffness and frame geometry are not to be ignored however. There is a lot of mythology in the sport literature but not much product performance data.

 

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Bearings

This topic seems to have most geek skaters in a tizz. All seem to have a different opinion of which bearings and lubrication make them go faster.

Inline skates use rolling element radial ball bearings. The standard size is the “608” (common in vacuum cleaner motors) which is 8mm inner diameter, 22mm outer diameter by 7mm wide and are of a steel (typically 52100) construction. There are other sizes available like the “688” mini, but the 608 is the standard. When shopping for these bearings, the marketing “value proposition” usually centres around its ABEC rating. We are lead to believe that the higher the ABEC rating the better and faster it is.

 

The ABEC, annular bearing engineering committee, standards control the manufacturing tolerance of the bearing. The higher the ABEC rating the tighter the tolerance. For example, a 608 ABEC 1 bearing has an outer diameter tolerance of .0000 inches to -.0004 inches while an ABEC 7 is .0000 inches to -.0002 inches. Take note, this is not related to internal radial clearance, it is only the production tolerance band. The relevant question is whether a higher ABEC rating (and higher cost) equate to lower traction torque (what you feel when you blade) and the short answer is no.

 

For low nominal diameter (bore inner diameter, mm x RPM) bearings, less than 100,000 nominal diameter, the manufacturing tolerance does not have a significant impact. The ABEC rating becomes more important at high nominal diameter (around 1 million) where dynamic considerations (like ball excursion) come into play. For comparison, a inline skater doing around 12 mph on 80MM outer diameter wheels would be at 10,000 nominal diameter. This is essentially a static load case. The other case where a high ABEC rating is important would be for bearings used in high precision instruments where the bearings would add tolerance stack that degrades instrument precision.

 

The other choice to make is whether to buy the conventional steel bearings (the most common) or go to the extra expense of a hybrid ceramic. A typical bearing, for inline skates, will be made out of 52100 steel ( a high carbon steel) which is quite adequate. A hybrid ceramic has Silicone Nitride balls on a steel inner and outer race. Hybrids have far superior characteristics at higher speeds (> 1.5 million nominal diameter) than steel bearings. The reasons are that SiN is lighter than steel so the centrifugal loads (and hence contact stress) is lower resulting in a longer fatigue life. SiN has a lower thermal expansion coefficient so IRC does not reduce as much, reducing the thermal pre-load effect. As an added bonus SiN is impervious to corrosion and appears to be more damage tolerant. It is possible that hybrid ceramics may provide a marginal benefit in a skate application, in spite of the 50% to 300% cost increase. The marketing literature claims that hybrids require less lubrication. Test experience done by Fred Ahrens PE, with hybrid ceramics (for missile engine applications) does not support this assertion. It is likely that this claim is based on the fact that since hybrids can run at higher temperatures (lower thermal expansion) that it is simply more forgiving of lubrication deprivation. This would not be noticeable to an inline skater.

 

In conclusion, for aggressive and hockey players, an ABEC 1 is more than sufficient. Also, the quality levels on all the reputable brands is at ABEC 3 or better in any case (even if marked ABEC 1). For a casual fitness inline skater (20 minutes typical session) an ABEC 3 or 5 will do. For the hardcore fitness types (>1 hrs per trip) an ABEC 5 or hybrid would be suitable. The primary drivers for traction torque will be the internal geometry like race/ball finish, ball diameter to race curvature, number of rolling elements, etc. These are not controlled by the ABEC rating.

 

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Lubrication

The purpose of a lubricant in the bearing is to provide a fluid film for elesto-hydrodynamic lubrication (EHL), to convey heat out of the bearings and guard against contaminants. For inline skating there are two choices (if not buying pre-packed grease bearings); grease or oil lube. Each has it’s advantages.

Grease lubrication in the bearings has the benefit that it is really cheap and last a long time between maintenance intervals. Greases also provides superior contamination resistance which is good for those who inline skate in wet or sandy conditions. The downside is that a grease packed bearing feels as though they slow down quickly, and make skating hard work.

Oil lubrication is best for those who need the absolute lowest running torque and are willing to tolerate a higher maintenance level for their hobby. The question is what lubricant to select? The lubricants available that are labeled for inline skating applications typically sell for £3.50 per half oz, compared to less than £0.05 per oz for a premium motor or gear oil. What is the difference? Typically for a low load, low speed application such as inline skating one would want a high viscosity oil with a tackiness additive and perhaps a demulsifier (to get water out of the oil). The high viscosity is mandatory since the EHL film thickness is proportional to the ball entrainment velocity (not very high) and viscosity. Should the film be too thin then adhesive wear, from boundary lubrication, will cause accelerated bearing wear and lower life. Stickyness is important for lubrication retention in the bearing. Unfortunately, this data is not available for lubrications marketed for the inline skate market. It is available for the popular motor and gear oils. So what to do?

The bottom line is that each user will have to evaluate their own skating style and choose according to their needs. Those desiring low maintenance and high contamination resistance should use grease. Most others use oil.

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Types of inline skates

There are 5 basic types of skates on the market today. They are characterised by the style in which the inline skater chooses to skate.

• Aggressive - This style is characterized by brazen stunts including half pipe (a U-shaped trough that aggressive skaters jump in, on or through), sliding down railings, jumping stairs and wall riding. These blades are generally very rugged, have grind plates on the frame (to protect the frame and allow easier sliding on railings) and smaller softer wheels. The boots offer very firm support and are well padded. These skates never come with heel brakes.

 

• Hockey - As noted, this was where the inline skate really evolved from. It allows hockey players to train off season and without recourse to an indoor ice skating rink. Again these skates do not come with heel brakes. The boot normally only have waxed laces to tie up, and are very ridged. Most times somewhat uncomfortable till broken in.

 

• Recreational/Fitness - This is the largest market segment because it captures the fitness crowd. When ones goes to a sporting stores, the majority of the skates available usually fall into this category. Theses skates come with heel brakes, and vary greatly in ankle support.

 

• Speed skating - Speed skating is really a separate category because the equipment is more specialized ( and expensive). For example, a speed blade will have 1 additional wheel (5 wheels instead of 4) on each frame, larger wheels and mid cut boots. It is rare to find a heel brake on these types of skates.

 

• Kite blading - This is a fairly new style of inline skating. It involves inline skates with 2 to 3 large, fat, inflatable wheels per skate. The idea is to use them on grass or hard sand while flying a power kite (40 square feet upwards). This is mostly for the extreme skater that enjoys going faster than 30 miles an hour on skates.
  

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Wheels

This is a simpler topic. There are only 2 things that need consideration and the rest is marketing. That is wheel size and durometer. Make sure the wheel is polyurethane (most are) or one could take a ride on the fleecing railroad!

Durometer refers to the wheels hardness, very important! Softer wheels like 78A provide better traction but wear much quicker and have a higher rolling resistance. Harder wheels last longer, have a lower rolling resistance but have slightly less traction. So for distance and speed the higher durometer wheels (85a or 88a) are better. But for a hockey or aggressive skater, the softer (75A durometer or lower) is better.

 

Size refers to the wheels outer diameter. A speed or fitness inline skaters will want a larger wheel (preferably 80mm) since these offer a lower rolling resistance. This is true because the larger wheels have a smaller contact area for a given load and have the added benefit that you can plow through debris easier. But keep in mind, an inline skate that can accept this large of a wheel will have to have a longer frame. This is good for speed and distance since it is more stable but makes them sluggish for hockey or aggressive skating.

 

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