Bike Pittsburgh

Interpreting crank/cassette numbers

Hi all! I have a question that I'm sure many of you could answer in your sleep. The thing is I don't know hugely much about the building or maintaining of bikes (though I hope to learn more). However, for a mechanical engineering class, I'm doing a final project where I analyze the forces in a bike being ridden up a hill.

So I need gear ratios.

In memoriam of my Cannondale R500 that was stolen recently, I decided to use that as the bike for the report. I am having trouble interpreting the numbers I found on Cannondale's webpage, though.

The crank is this:
TruVatiV Touro Triple, 30/42/52

And the rear cogs are this:
Shimano Tiagra 9 speed 12-15

on a site that sells cassettes, I found more info about this one. The numbers given are this:
12-25 - 12.13.14.15.17.19.21.23.25

So what do all these numbers mean? The numbers for the rear cogs look like they could be diameters in cm, but the three numbers for the crank are definitely not - I don't think my crank was 52 cm!

Could someone help me garner some diameters and therefore gear ratios from these?

Thanks,
Aln

RE: Interpreting crank/cassette numbers

The numbers are the number of teeth on the chainring or cassette.

For chainrings the number is the exact number of teeth (say 30 or whatever).

For cassettes it's the smallest and biggest cog, so for instance a 12-25 would have a 12 tooth cog on one end and a 25 tooth cog on the other end. What's inbetween there varies based on the number of "speeds" (eg 7-8-9) and the manufacturer to some extent.

To calculate the ratios you can use this page here
to figure that out. It's also handy because it takes most of the guess work out about what other cogs your cassette has.

What you're doing is pretty well studied. I don't know what else you're looking for but (or at all)
Martin, J.C., D.L. Milliken, J.E. Cobb, K.L. McFadden, and A.R. Coggan. Validation of
a mathematical model for road-cycling power. Journal of Applied Biomechanics, 14(3):
276-291; 1998 (cited 50 times; 4th most highly cited paper ever published by Journal of
Applied Biomechanics).

and
NERD!

and I have a powermeter (eg on bike watt measuring device) so if you wanted to get very specific you could choose a hill and I could ride it to see how your model matches up.

Good luck.

RE: Interpreting crank/cassette numbers

Thank you! And sorry for the double post (I just realised)

RE: Interpreting crank/cassette numbers

Man, double posting is easy to do, isn't it?

RE: Interpreting crank/cassette numbers

That power meter sounds very interesting. I don't think I need to go that in-depth, but out of my own curiosity, I might look into those some day.

RE: Interpreting crank/cassette numbers

I don't know that it's that in depth. It would certainly validate your analysis, since hills are w/kg and it would be pretty easy to nail down both of those and then get the grade of the hill. Then it's just a matter of back calculating torque based on the gears.