A few days ago I posted & got some excellent advice on shifting with a 20/36/46 chainring setup I am considering for my new touring bike (Mongoose Randonneur - haven't got it yet). People seemed to think the setup was feasible. It started me thinking about how wide you could go & I played around a bit with Sheldon Brown's gear calculator and I've come up with my dream ratios: 20/38/48 chainrings with an 11-34 cassette. Gives wonderful half-step gearing on the two top rings and as low as I like for climbing up mountains laden & tired - I have mild chronic fatigue so I do get tired sometimes. (Interestingly the 16.1 inch low gear gives 4.6 kph at 60 rpm & 6.2 kph at 80 rpm - with 700x38 tyres - which is a lot faster than I'd walk a laden bike up a steep mountain, contrary to the seeming popular myth that anything lower than a 20 inch gear is no faster than walking).
But is my dream setup of 20/38/48 technically feasible? I'm prepared to live with the three or four click rear shift when shifting down to the granny, but would the front derailleur get the chain back on the middle ring, and if so, would it take an age? Would such a setup leave me with such tetchy front derailleur adjustments that I'd have to fiddle with it every 100km? I've heard there are gadgets that attach to the seatpost to stop the chain falling off when shifting to small granny rings - anyone know the name of these gadgets & would they help?
I'd really appreciate any thoughts on these questions - I just know the LBS mechanic will not be keen on installing a 20/38/48 setup & may not have the time to discuss it at length with me. It's so great having such a forum to ask these questions in. I promise to post a ride report & pictures of the bike when I finally get it! 🙂