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Quick Math Question for Smart People

Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Quick Math Question for Smart People

Old 10-06-18, 10:27 AM
  #1  
Paul Barnard
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Quick Math Question for Smart People

I'll start out by saying this is not one of those questions where the person asking it knows the answer. I suck really badly at math. I put a new computer on my new bike. I am a Luddite, so it's a regular 8 function wheel magnet computer. I accidentally programmed in a circumference of 2136 instead of 2124. I didn't realize it until after my ride. During the ride, each time I looked down at the computer, I thought that speed on the display is reading faster than it felt like I was going. When my computer showed 20MPH, how fast was I really going? Is there a flat percentage I was off? I lied, that was two questions.
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Old 10-06-18, 10:33 AM
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still rounds to twenty so, good job...
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Old 10-06-18, 10:55 AM
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About .1 mph off.

We'll let it go this time, but don't let it happen again
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Old 10-06-18, 11:18 AM
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Pump up your tires a bit more to compensate
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Old 10-06-18, 11:18 AM
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Your actual speed would be:

(Actual Speed) = (Recorded Speed) * 2124 / 2136

So, if the computer read 20 MPH, your actual speed would have been about 19.9 MPH.

Off by about 0.1 MPH.
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Old 10-06-18, 11:43 AM
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Originally Posted by Paul Barnard View Post
I'll start out by saying this is not one of those questions where the person asking it knows the answer. I suck really badly at math. I put a new computer on my new bike. I am a Luddite, so it's a regular 8 function wheel magnet computer. I accidentally programmed in a circumference of 2136 instead of 2124. I didn't realize it until after my ride. During the ride, each time I looked down at the computer, I thought that speed on the display is reading faster than it felt like I was going. When my computer showed 20MPH, how fast was I really going? Is there a flat percentage I was off? I lied, that was two questions.

0.00% difference - Strava ignores wheel sensors and if the ride isn't on Strava, it didn't happen anyway.

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Old 10-06-18, 12:48 PM
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Originally Posted by WhyFi View Post
0.00% difference - Strava ignores wheel sensors and if the ride isn't on Strava, it didn't happen anyway.

I enjoy good technology. I am worse with tech than I am with math though. I doubt I could get Strava to work.
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Old 10-06-18, 09:26 PM
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Strava has a new premium feature where somebody will follow behind recording your ride for you.
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Old 10-07-18, 05:07 AM
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Nobody answered your second question, but yes, you were off by a flat percentage: 2136/2124 - 1 which is half a percent no matter how fast you're going. On many bike computers that have wheel circumference settings in centimeters instead of millimeters, 0.5% is the normal margin of error. Also, if you could sense your speedometer was off by 0.5%, you have an incredible sense of speed. The only way I would notice something like that is via a known distance. Say, for example, that I know a specific house is exactly 10.00 miles on my route. If it measured 10.05 miles, then I would know my computer is off by 0.5%.
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Old 10-07-18, 06:01 AM
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Originally Posted by paul barnard View Post
............... When my computer showed 20mph, how fast was i really going?......
braggart !!!!
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Old 10-07-18, 08:30 AM
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Originally Posted by Seattle Forrest View Post
Strava has a new premium feature where somebody will follow behind recording your ride for you.
Most of them Strava kind of riders don't like to go slow.
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Old 10-07-18, 09:00 AM
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There's another thread floating around on this forum where a member is reporting a 3MPH gain switching from a touring bike to a racing bike. I went from a Trek 1500 to Specialized Tarmac to my Litespeed T6. I have never had a power meter, but I have owned and ridden a lot of bikes. You develop a comfort zone for each bike. In my case the terrain is flat. On the 1500, my comfort zone would find me very close to 18MPH. With the Tarmac, it would find me at about 17. I did a lot of back to back runs where I would move from the Tarmac to the 1500 and back. I'd ignore the computer until I had been in my happy place for a while. I'd then look down to see what my speed was. I also checked the computers on the same loop and they matched each other. It looks like the Litespeed is going to bump me back up.
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Old 10-07-18, 10:56 AM
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Originally Posted by Paul Barnard View Post
There's another thread floating around on this forum where a member is reporting a 3MPH gain switching from a touring bike to a racing bike....

.
I've experienced the same variability riding the same bike over the same course-- there seems so many factors involved in average mph, in addition to the bike being ridden, that... it's hard to be objective. Even so, my feeling is that it's all wind-related. My guess is that the confusion comes about because, it's possible that, given the small amount of horsepower we have at our disposal, a 7% decrease in avg mph may actually represent a 3% increase in performance compared to a previous ride if we understood how to factor in the 20 degree difference in the angle of a prevailing wind.
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Old 10-13-18, 10:22 PM
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Originally Posted by Paul Barnard View Post
I'll start out by saying this is not one of those questions where the person asking it knows the answer. I suck really badly at math. I put a new computer on my new bike. I am a Luddite, so it's a regular 8 function wheel magnet computer. I accidentally programmed in a circumference of 2136 instead of 2124. I didn't realize it until after my ride. During the ride, each time I looked down at the computer, I thought that speed on the display is reading faster than it felt like I was going. When my computer showed 20MPH, how fast was I really going? Is there a flat percentage I was off? I lied, that was two questions.
2136 / 2124 = 100.56%, so you overstated your speed by .56%, or roughly 1/200th. At 20mph, this is a 0.1 mph difference.
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Old 10-13-18, 11:25 PM
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I'm not bad at math. One time a farmer asked me to round up his 68 sheep, so I said sure: 70.
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Old 10-14-18, 01:52 AM
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Originally Posted by rousseau View Post
I'm not bad at math. One time a farmer asked me to round up his 68 sheep, so I said sure: 70.
lucky it wasn't multiplication, division & squareroots...!
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Old 10-14-18, 10:45 PM
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Originally Posted by Paul Barnard View Post
I accidentally programmed in a circumference of 2136 instead of 2124.
How sure are you that 2124 is the right number?
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Old 10-15-18, 05:41 AM
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Paul Barnard
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Originally Posted by DiabloScott View Post
How sure are you that 2124 is the right number?

Pretty sure. I have run a lot of different tires, on a lot of different bikes, using a lot of different computers. I have compared those to my friend's computers and to mapping programs for determining distance. There's nothing to lead me to believe my numbers are off.
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Old 10-15-18, 07:27 AM
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Originally Posted by WhyFi View Post
0.00% difference - Strava ignores wheel sensors and if the ride isn't on Strava, it didn't happen anyway.

I don't think this is true.

Strava has an option to override your distance with theirs if you have a wheel sensor. I've had a ride were the battery in my sensor started to go. My average speed would go from 30 kph to 5 to 18 to 27 to 12 etc. When I got home, my distance was way under reported. I used this feature in Strava to override with the GPS data and my ride distance and average speed was significantly increased.
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Old 10-15-18, 09:44 AM
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Originally Posted by OBoile View Post
I don't think this is true.

Strava has an option to override your distance with theirs if you have a wheel sensor. I've had a ride were the battery in my sensor started to go. My average speed would go from 30 kph to 5 to 18 to 27 to 12 etc. When I got home, my distance was way under reported. I used this feature in Strava to override with the GPS data and my ride distance and average speed was significantly increased.
I was thinking of how it pertains to segment results specifically, but way to roll with the joke.
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Old 10-15-18, 11:31 AM
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Originally Posted by WhyFi View Post
I was thinking of how it pertains to segment results specifically, but way to roll with the joke.
Okay. My bad.
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