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Radar Blog: Durometer

Hey everyone!

Whether you are an experienced skater or just beginning, you have probably heard wheels referred to by numbers, such as 88, 91, or 95.  This number is the hardness of the wheel.  It is an important piece of information for skaters to pay attention to when selecting a wheel because it gives a glimpse of what you can expect the wheel to feel like.  Before we get to that, let’s talk about what Durometer is, and how it is measured.

Duro-what?

Durometer is the international standard for measuring the hardness of objects like rubber and plastic.  There are a few different measurement scales, called Shores.  There is the OO Shore, the A Shore, the D Shore, and some others as well.  Why so many?  Different Shores are needed to measure different stuff. A racquet ball, for example is very soft and is measured on a different Shore than a bowling ball, which is very hard.  I’m not even sure if they measure the hardness of those things, though so think of it this way: If you are trying to weigh a paperclip very precisely, you would use a different scale than if you were trying to weigh a car precisely.  The same is true for measuring the Hardness of something.  The Shores overlap.  Soft things have low numbers, harder things have higher numbers.  Check out this cool chart:


For measuring the hardness of skate wheels, the Shore A is used.  As you can see in the chart, a pencil eraser is about 40A and a car tire tread is about 60A.  Okay, so how do they measure it?  Basically, they press on it.

  

So the guy is putting about the same amount of pressure on all three pieces of this rubbery plastic, but the finger presses furthest into the softest one.  That is the same concept used for more precise hardness measuring.  Here is a common example of a hardness measuring tool.  

This is a handheld device with a spring loaded pin on the bottom.  The measurement is all based on the depth of pin indentation.  That pin acts like the guy’s finger in the other image.  You press the pin onto the surface of an object and you get a measurement based on the material’s resistance.  Check out this illustration to see how it works:

Harder materials require more force to drive the pin into the surface, providing a higher measurement. It is a bit tricky to get a precise measurement.  If you hold the tool incorrectly, the tool can give a false reading.  Ever lurch up and down on your bathroom scale and watch your weight vary from 70 pounds to 250 pounds?  You can sort of do the same thing with these hand held tools.  To reduce this, the durometer is often mounted to a stand.  This reduces variable and prevents false readings.

  

 

The tools also need to be frequently tested to make sure they are measuring precisely.  We send ours to special labs for recalibration.

 

Righteous mustache.

Okay, so you get the basic idea of how hardness is measured.  What does any of this have to do with skating?  The hardness of the urethane on your wheels will drastically affect performance.  Softer wheels will compress into the surface more, giving them more grip, while the harder wheels will compress less, giving them less grip.  Hard wheels are said to have more Roll (speed) because there is less resistance to the surface.  Ultimately, you want enough grip to keep you in control and as much roll as you can get.  Skaters look for the balance between the Grip and Roll.  Swing too far one way or the other and certain aspects of skating will be compromised.

For (indoor) skate wheels, you will find anything from 84A all the way to 100A.  The 88A-95A range is the most common, though some skaters need a softer or harder wheel depending on the surface (we’ll get into that in a future post) and their specific needs.  Outdoor wheels are in the upper 70As and are made to roll over imperfections in the surface, small twigs, pebbles and the like without disrupting your skating motion and making street pizza out of your knees and other fleshy parts.

All of the fancy graphs and tools aside, people always ask: “What hardness of wheel should I buy?”  The answer is

It Depends.

There are a slew of variables that will affect the answer to this question.  The best thing to do is experiment.  Start with something in the low 90As, and work up or down from there.  If you are sliding around, try something softer.  If you are getting plenty of grip but feeling slow or having a hard time with plow or hockey stops, try something harder.  There are special fancy setups that you can try to. We’ll talk about those in an upcoming post.

By learning the durometer, you have a pretty clear idea of how the wheel will perform.  It is a great tool. But before you fall in love with Durometer, know that it is only one single piece of the puzzle.  There are other elements that determine how a wheel feels. Next time, we’ll discuss a few of those other variables and I’ll tell you why you MUST learn to look beyond Durometer when selecting a wheel.

15 Comments

Got something to say? Feel free, I want to hear from you! Leave a Comment

  1. Conan – you rock my radar lovin’ derby world *sigh*

  2. Love your blog! I am a wheel junkie myself! I love gear specs, and researching to discover more and more knowledge. Wheels are my favorite part of my setup. As someone who never skated before derby, I think understanding different wheels and what they can offer is priceless information for a beginning skater. I will make sure to direct our new girls to your blog!

    Trigger

  3. Chris AKA Army Corpse (Derby Ref) says:

    Great article, written in a nice, easy to understand style! Answered some lingering questions I’ve have about hardness versus softness. I am looking forward to the fancy split setups!!

  4. Stephen says:

    Thank you for your explanation of durometer. Being moderately new to skating, the barrage of wheel numbers and terminology is overwhelming at times. Your explanation of durometer was informative without being overly complex. It reduced the complexity of one “wheel variable” into an appropriate level of understanding for the layman. I look forward to your future posts.

  5. Tim says:

    In the graphic the scale is to 100A and you wrote “all the way to 100A”. Hardest wheels from Radar are “101A”. I’m a little bit puzzled. What means 101A?

    • Joshua "Conan" Pfenning says: (Author)

      Beyond 100, the measurement does get a bit sketchy. Keep an eye out for my next post and I’ll explain more about it.
      Thanks for the comments all!

  6. Mickispeedia says:

    Tricky business! I know I have a sense of what I expect a 93A wheel to feel like. But simply knowing what the mustachioed man’s fancy machine says about a particular wheel is not the whole story. A 93A in one type of wheel may feel very different than a 93A in another type of wheel. Looking forward to your next post.

  7. Cris Dobbins says:

    I’m in a consist internal battle trying to figure out my ultimate grip to roll ratio. I change my wheel hardness more then my hair styles! I’m also noticing that as I grow as a skater and up my skills my needs are different.

    Thanks for the great article! Now I can use fancy words like Durometer and Resilient Force and know what I’m taking about.

  8. Tripp N. Dale says:

    Nice job Conan.

    Would love for someone to tackle the advantages/disadvantages and differences between metal and plastic wheel hubs.

    Hope all is well!

    Tripp

    • Joshua "Conan" Pfenning says: (Author)

      Thanks Tripp! I plan to cover differences in hubs very soon! Thanks for reading.

  9. Great post Conan! Durometer is something often discussed but rarely understood by most skaters, way to break down all the relevent information for derby skaters to use. I love experimenting with various durometers of Radar Wheels to find the best fit for the skating surface in use. Additionally its great to understand durometer when it comes to the bushings in your plates, these are often overlooked but just as critical to great skating performance as is the durometer of the urethane on your wheels! I look forward to your next post!

    Ace

  10. Stephen says:

    The world is eagerly awaiting your next post about the science and reality of skate wheels. Hope you are planing to write another installment soon.

  11. kirk says:

    Im looking for a plastic hubbed wheel with the same urethane as the sting ray only.
    can you lead me in a correct direction.
    thanks kirk

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