Wednesday, January 18, 2012

Is Your Treadmil Lying To You? Treadmil Accuracy and Calibration

Tis the season when the ice gets the best of us, and we move some of those runs indoors.  We are lucky enough to have a treadmill in the basement OF OUR NEW HOUSE this year! That's good.  But...when I run on it, it seems a little fast...  Easy paces aren't so easy.  That's bad.  But is the computer lying to me?  Or am I big pansy?  Honestly, I don't know.  Either is possible, of course. But luckily for you, dear reader, I'm not just another pretty face dumb jock.  I'm also an engineer, and this is the kind of problem we live for!    As I write this, I have a notebook full of data, ready to be evaluated.  So is my treadmill accurate?  Let's find out.  Is your treadmill accurate?  Here's how to tell...

So, we need determine speed.  That's distance covered / time (except in the case of the kessel run, but we won't talk about that).  We will be figuring out how far the belt went in a given time. 

Step 1.  Determine how long your treadmills belt is.  The best way to do this is to tape some string onto the belt, and then push the belt around once, until the string is taught around the outside of the belt.  And then...measure the string.  My belt was 128" long.

Step 2.  Put some type of easily seen mark on the belt.  Turn on your treadmill and set it at your normal run pace.  Then grab your iPhone. 

Step 3.  Start your stop iPhone's Stop Watch as soon as the mark passes the bottom of the deck, then count 30 (seemed like a good number) passes of the mark on the bottom of the treadmill.  Do this three times.  You should get similar results all three times. 

My results:
30 Revs @ 6 Mph
Trial 1:  35.6 sec
Trial 2:  35.5 sec
Trial 3:  35.5 sec
Average: 35.5 seconds

You're free to stop right there, but I wanted to make sure the Treadmill didn't vary at higher or lower speeds, so I tested at 8 mph and 4 mph as well. 

30 Revs @ 8 Mph
Trial 1:  27.3 sec
Trial 2:  26.7 sec
Trial 3:  26.6 sec
Average:  seconds 26.8 sec

The belt count got more challenging at 8 mph, and that's about as fast as I run, so, good enough.

I also did 1 trial at 4 mph.  I got bored...

10 Revs @ 4 Mph
1.  16.2 sec 

Step 4.  So, now we can compute.  Lets start with 6 Mph.

So, the belt revolved 30 times and it's 128 inches long.  That means it moved:

30*128 = 3840 inches.  (Yes, we should be using the metric system, yes imperal measure is assinine, but I run in minutes/mile so if you don't like it...get your own blog.)

If we cover 3840 inches in 35.6 seconds, how many inches will we cover in 3600 seconds (1 hour)?  Enter algebra 1.

(3840 inch/35.6sec) = (x inches / 3600 sec)  Cross multiply and divide and we get...

388314.61 inches = 32359.55 feet (/5280 feet/mile)= 6.12 Miles.  But, wait...the treadmill was set at 6 Mph.  Houston, we have a problem. 

Checking 8 Mph we get:

(3840 inch/26.8sec) = (x inches / 3600 sec) = 506373.63 inches = 7.99 Mph.  I think we can call that 8. 

Checking 4 Mph we get:

(1280 inch/16.2 sec) = (x inches / 3600 sec) =4.48 Mph. 

Seeing the trend here?  It appears the that treadmill is pretty much right on (and maybe a little slow) at top end speeds but at the low end, it's quite a bit faster.   In my easy pace zone, it appears to be about 10 sec/mile fast.

So now what?  I dunno.   I think I'll run a few more trials to be sure and then call the manufacturer.  I can certainly just adjust my paces...but I'd prefer it if the computer's output = reality.  Story of my life.  Anyway, I guess stay tuned for part 2?

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