carleton

(This is a double post, but I figure both sub-forums would benefit)

You ever wanted to know your cadence and only have a computer that tracks speed? Fixed/track bikes are unique in that the cadence is directly related to the speed.

I'll let that sink in for a minute...

I used the Factor Label Method to figure this out. The bottom line is you need to know:
- Chainwheel Tooth count
- Rear Cog Tooth Count
- Speed (current or average)
- A magic number (that corresponds to variable tire sizes)

I won't bore you with the proof of the formula. Just make an excel spreadsheet with 4 fields plus a calculated one.

Field 1: Chainwheel Tooth Count
Field 2: Rear Cog Tooth Count
Field 3: Speed (in MPH)
Field 4: Magic Number Constant (use list below)
Field 5: Calculated as follows

(1,056 * RearCog * Speed) / (ChainWheel * MagicNumber * 3.14159)

That is your cadence!

For example:
Chainwheel: 48
Cog: 16
Speed from computer: 20 MPH
Magic Number: 26.28 (700c wheels with 23c tires)
Cadence = 85RPM


So how can you use this?

Cadence Training:
If you want to follow a cadence training program that says, 60RPM for 2 minutes then 90RPM for 30 seconds that would mean pedal up to 14MPH for 2 minutes then ratchet up to 21MPH for 30 seconds.

Average:
If you have a computer with an "auto" mode that only records average speed when the bike is moving then you can log your average cadence over your ride. Average speed of 15MPH = Average cadence of 64RPM.


Why did I do this?
Bike computers are pretty cheap ($15) even the wireless ones ($25). Wired cadence computers aren't that expensive (around $50), but wireless cadence computers start at $100+. So, using the following you can know your current and average cadence if you have a basic computer that records current and average speeds for about $25. That and I'm bored.





I got these magic numbers from Sheldon Brown's site:
29.13 = 700 X 56 / 56-622 / 29 inch
28.94 = 700 X 50 / 50-622 / 29 inch
27.86 = 700 X 44 / 44-622 / 29 inch
27.32 = 700 X 38 / 38-622
27.17 = 700 X 35 / 35-622
27 = 700 X 32 / 32-622
26.76 = 700 X 28 / 28-622
26.38 = 700 X 25 / 25-622
26.28 = 700 X 23 / 23-622
26.14 = 700 X 20 / 20-622
26.53 = Tubular / Wide
26.38 = Tubular / Narrow
28.15 = 28 X 1 1/2 / 40-635
27.18 = 27 X 1 3/8 / 35-630
27.08 = 27 X 1 1/4 / 32-630
27 = 27 X 1 1/8 / 28-630
26.88 = 27 X 1 / 25-630

andyarceye

Or an easier way is to place the sensor from your computer on the left side rear horizontal tube of your frame and fix the magnet to your left pedal crank arm so the magnet passes of the sensor, then set your computer to MPH and set wheel circumference to be 2682, now as you read the computer your speed in Mph is cadence / 10 so 8 Mph = 80 cadence 6 Mph = 60 cadence.

Ok, this method isn't 100% accurate at a speed of 8 Mph showing on the computer your real life cadence will be 80.007 which I would accept as close enough.

No more calculations to do

chas58

Now, that is an intriguing idea!

chas58

Carleton, Can you write an app for that?

carleton

Ha! Nice.


But then I'd have to use a forumula to calculate my speed...

I actually think that Cadence is a better training metric that speed. So this is actually very helpful.

I use this app called GearCalc.

https://itunes.apple.com/mt/app/gearc...412947619?mt=8

It was written by a track racer named Giovanni Rey.

andyarceye

Personally I feel a constant cadence creates a fluid riding style and forces a more efficient use of human energy, the speed is a by product so less important, but I do use a mobile phone app to show speed distance and where I have been anyway.
My reasons for doing things this way is ££, a cheap cycle computer £10 and I already have a phone that can run tracking apps.