carleton

(I screwed up the headline. Should be your cadence. Sorry)

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

larsalan

https://software.bareknucklebrigade.c...it.applet.html

gkelley

^^^beat me to it

Syscrush

True, and cool.

But a computer that measures and displays cadence is what - $20?

Of course, you might not want those extra wires & sensor messing up the lines of your pretty fixie, and knowing the cadences for a few key/common speeds might be all you need.

urodacus

now do that in your head, on the bike.

or just use a watch and count pedal strokes while a minute elapses.

or don't bother. c'mon now, how many hipsters even know what cadence is, let alone care about it? and if you want to impress others while sinking that bottle of Pabst, it's far more effective to lie through your teeth anyway.

gkelley

Post me a link to the 20 dollar bike computer with cadence that isn't a complete POS

carleton

I figured that somebody did it somewhere. As they say, there's nothing new under the sun.

That's what prompted all of this. I've been riding seriously a lot and wanted to know my cadence. I installed a wired cadence computer and it looks like sh*t with zip ties everywhere. Looked into a wireless one that does cadence and they start at $100. That's a lot of money to spend to see how slow I'm going so I came up with this.

You are right about the few common speeds. Just take your written training guide and convert all RPMs to MPH and remember those.

blankgen

I installed my cadence/speedo wire by taping it to the underside of the down tube. Other than two zip ties on the chainstay for the sensor it looks pretty clean.