Originally Posted by twocicle

Thank you. I like that your calculation uses logic rather than trig functions. Simple

As a sanity check, I ran the same process for the standard tandem 69T rings & 250T/2000mm belt. Result came out to the correct 72.4cm tube length required.

Future spec:
The Gates rep I spoke with the other day indicated they are considering changing the tandem pitch to 11mm (same as singles) in the future, which would provide for many more ring options. In that case, the 2000mm belt would yield near 182 teeth (181.8181 to be more precise). The matching 11mm pitch rings for the standard bottom tube length would be 50T to acheive 72.6cm which is probably close enough to the current spec. The Santana spec would likely use the 55T/11mm rings (yields 69.85cm). So it unfortunately looks like there will still be no solution for the case (mine for example) where the current 71T rings are needed.

BTW, the only reasoning they had for no longer producing the 71T was that they were building only for current framebuilder/OEM "standard" sizes, regardless of losing the business of frame owners with all the previous builds. I doubt there is sufficient market to justify anyone licensing the ring spec from Gates and produce 3rd party versions / more sizes.