Choosing chainring configuration I'm interested in how I would choose a chainring configuration, supposing I was either replacing mine or building a new bike for a similar usage.

I have a 50/39/30 triple chainring (&amp; 10-speed cassette) on my Bianchi Volpe.  Rivendell sells a 46/36/24, with the argument that 50 is overkill, and if you're going to have a granny gear it should be smaller.

I buy that argument, but I spend 90% of my time riding 39 in the front and 15/17 in the back.  If I downgraded my middle to 36, then I'd have to ride smaller cogs in the back, which means it'll wear faster (I do ~2500 miles per year of city riding).  If I use the 46 then I'd be almost cross-chaining to get the same ratio.

So it sounds like this configuration would be bad for me.

However, I think a 46/30 double might be perfect.  That would pull the big ring in closer so when I'm riding on the bigger cogs in the back I hopefully wouldn't be cross-chaining.  It would wear slower because I'd be on more teeth.  I do a moderate bike tour about once a year and the 30 has worked fine for me so far (haven't done any mountains, but have done some steepish hills).

Does my rationale make sense?
There is a question here, it's just hidden in a misleading focus on details. 

How do you choose chainring setups?

You need to analyze the gear ratios and speeds that you ride at and figure out the correct
overlap for your riding. The way that I've found most useful to do this is to chart the gear ratios vs speed and use an error bar to represent the speed at a given rpm. 



What you want to avoid is having to "double shift" (i.e. shift both back and front) for the 
speeds that you typically ride at. IMHO, cross chaining is not an issue for modern gear. It
does absolutely no harm. However, double shifting gets to be very awkward. 

As a rough guide if your typical speed is 25 kph, having a couple gears higher and lower
on the same chainring at the rpms you find comfortable would be a good setup. A lot depends on what range of rpm you find comfortable to ride. A poor setup is one which
you have to double shift to get the next gear either higher or lower.

Having done a bunch of graphs and played with both triples and compact doubles over the years. I think if you are going compact double, you need to be able to do most of your flat/slightly rolling climbing in the big gear. And for somewhat average cyclists I agree with your reasoning. A 46/30 and something like 12-27 in the back would be a good replacement for the typical road triple. 

46/30s are made, just very hard to find. 


Velo Orange 46/30