lower gearing using mtb crank
I got a 520 about a little over a year ago and want to replace the standard 50/39/30 tiagra crankset. I've looked at the shimano lx and other cranksets that have 44/32/22, but I want a inner chainring of 20. Are there any cranksets out there or can I buy a 64mm 20 tooth chainring and switch out the 22 tooth?