It's worth doing the sums and working out exactly what voltage you need - for some motors 50V would be fine, for others a big hindrance. Thankfully, @Irving2008 has put it all into a handy-dandy spreadsheet which ties together all the variables (apart from using pulleys/gears with a non 1:1 ratio) for which he deserves a beer - go looky here:

http://www.mycncuk.com/threads/1524-...tor+calculator