Monday, May 24, 2010

25 in 5 - aka, roller derby physics

Besides roller derby, physics is one of my other casual hobbies. Today I ask the question:

How fast do you need to skate in order to complete 25 laps in five minutes?

The circumference of a WFTDA regulation track is:
148.5 feet on the inside
236.5 feet on the outside.

For my calculations, I took the average of these numbers: 192.5 feet.

The number of feet in a mile is 5,280. The number of laps in one mile is (5,280 ft /192.5 ft) = 27.4 laps.

25 laps is equal to 0.91 miles (25 laps /27.4 laps).

Therefore, the skater must travel 0.91 miles in 5 minutes.

We all know that velocity is distance divided by time.

5 minutes is equal to 0.083 hours (5 minutes / 60 minutes).

Therefore, a skater must travel 0.91 miles in 0.083 hours.

0.91 miles / 0.083 hours = 10.9 miles per hour.

In order to skate 25 laps in 5 minutes, a skater must travel at an average velocity of 10.9 miles per hour.

Many girls on the team can skate over 25 laps. My personal record is 31. Looking at our team's time trial data, it looks like the maximum number anyone has done (recently) is 34. MPH equivalents for other laps below:

# of laps -- MPH
25 ---------- 10.9
26 ---------- 11.4
27 ---------- 11.8
28 ---------- 12.3
29 ---------- 12.7
30 ---------- 13.1
31 ---------- 13.6
34 ---------- 14.9

**Please note these are just quick and dirty calculations. If any other super nerds want to add to or continue this conversation, leave a comment!


  1. Hey, Lady LaLa can go 10.9 mph ??? Holy cow !

  2. Technically, we count the track as 160' long. As a physics and probably math geek, this is a good measurement to go by because there are 16 10' marks on an official track.

    People also tend to hug the inside border and not average in the middle, especially sprinting and in time trials.

    It takes 33 laps to go a mile. So if you could do 33 laps in 5 minutes, that would be 12mph.

    As a skater, you'll be focusing on those 10' marks and it is a good idea to not only know about speed, but grasp how many seconds it takes your various skaters (especially jammers) to cover 10', 20' and 30'.

    Here's a Derby math problem for your fans - if DeRanged (RMRG) averaged 8.1 laps per minute during the final jam of the RMRG vs. Philly bout at nationals last year, how fast was she skating? (That was against an entire pack)

  3. Bitches! This is exactly the type of conversation I was hoping for in the comments of this post. :D

    Regarding the 16 10' marks... looking at the diagram ( - page 4) it looks like there 18 10' marks -- unless I am reading it wrong? I am only beginning to understand how the track is actually laid out. I think it's really interesting that the outside track isn't parallel with the inside oval.

  4. Derby math and physics are two of the essential building blocks to derby strategy!

    It's really awesome to read about a new skater thinking about this and analyzing and it's not that your calculations are wrong - because they're not. I like to simplify things as much as possible and there's kinda this built in tool.

    And, damn, you're right. There are 18 10' segments. Here all along everyone's been believing me there are only 16! I think mentally I forget about the 4 10' segments between the pivot and jammer lines (including the imaginary pivot and jammer lines on the back side straight away) and the corners.

    That means there are 29 1/3 laps in a mile.

    Thinking about the big picture is really cool. Then also think about the little picture - how many seconds does it take a pack to make a lap, or a jammer? How many seconds to cover 10 or 20 or 30 feet? If I knock this opponent over and she takes 2 seconds to recover, can my jammer pass her before she gets up?

    When we bout, we tend to go a lot slower than our fastest speeds - even our jammers.

    But it is no joke about DeRanged's speed in the final jam of the RMRG vs. Philly bout. You can watch it in my youtube account. She scored the 4 points in that jam that won the game!

  5. Oh, yeah, and the two "ovals" are not parallel. The track was initially designed by Electra Blu for the Texas Rollergirls. She used CAD to "flatten" a banked track AND to make it fit in the original Texas venue.

    So some of it is physics and some of it is arbitrary based on one very early league's venue restrictions.

    The even corners (think of a derby track like any track, corner 1, 2 backside straightaway, corner 3, 4, frontside straightaway) are wider than the odd ones to account for the natural forces of skating in a circle on a flat surface. On "average" the track is 14' wide. However, at the pivot line it is 13' wide and coming out of the corner it is 14' wide.

    All of the 10' marks, including the pivot and jammer line, are set either parallel to the inside "oval" in the straight away or as a radius from the inside "oval" arcs center point. The center point for the outside oval is offset from the inside oval by 1'.

    Rock and Roll on, Temper!

  6. ah, I learn something new everyday!

  7. On the roller_girls Yahoo Group, someone actually figured the diameter of the optimum path, since that's how we skate 25 in 5.

  8. OMG you are all awesome! this is covalent bondage from central arkansas roller derby. i think about the physics of skating and the angles of the track all the TIME while at practice. my buddy slaymaker and i have improved our jamming by using centrifugal force (i think i used that right) and finding ways to work against it while dodging through the pack. i have found that using this kind of language is very helpful when trying to explain to a skater tot why their body isn't doing what they want to do and therefore how to overcome their own physics.
    yeah science!

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