My training bike has a 30/42/52 crankset and a 12-26 cassette. In case you’re wondering the triple chainring is because it’s one of Scott’s flatbar road bikes (hence the FB designation). The wheels are a pretty standard 700C x 28 size. If you’re not sure of your wheel circumference and don’t want to measure it you can use this page as a guide.
The formula for calculating the metres of development for a specific gear is as follows:’
Metres of development (mod) = circumference of drive wheel in metres (wc) x number of teeth in chainring (ct) / number of teeth in rear sprocket (rt)
I’m going to refer to the abbreviations from here on. For example, let’s assume the following:
Wheel circumference for a 700C x 28 road wheel = 2.136m
Front chainring = 42 teeth
Rear sprocket = 15 teeth
Using the formula above (mod = c * ct / rt) we can calculate the metres of development using the following:
mod = 2.136 * 42 / 15
This results in a metres of development value of 5.98. From here it’s a relatively simple matter of taking the average cadence (c) and the training session time in minutes (t) to work out the average speed (s) and distance (d). The formula for working out the average speed in km/h is as follows:
s = ((c * mod) * t * (60 / t)) / 1000
My short ride today to test this formula was 30 minutes @ an average cadence of 91rpm and using a 42-15 gear. The formula looks like this:
s = ((91 * 5.98) * 30 * (60 / 30)) / 1000
The resulting average speed is 32.66 km/h … yeah yeah slow I know but I’m traditionally a mountain biker so I’m never gonna be up there with the TDF riders. 😉
For distance you can use the formula:
d = s * t
This example uses distance measured in km, speed measured in km/h and time measured in hours). For the test ride I’m using in this article that comes out as follows:
d = 32.66 * 0.5 = 16.33km
I’m also a bit of a numbers geek (although I’m rubbish at mathematics) when it comes to my own training so I’ve thrown together an Excel spreadsheet that works it all out for me simply by entering the time (in minutes) and average cadence (in rpm) for a trainer session. If you’re interested in checking out these Excel spreadsheets (Excel 2007 and 2003) please let me know and I’ll look at publishing them on this site.
Hope that is interesting for or helps someone!