As Ed said ...spend a few pounds more and get a speed controller that is commonly used in the Robot community and gives a wide voltage spread.

Ed mentioned the Electronize ...Technobots do a range of controllers e.g.

Sabertooth Dual 25A Motor Controller Universal.

http://www.technobots.co.uk/acatalog/Online_Catalogue_Dimension_Engineering_Controllers _34.htmlhttp://www.technobots.co.uk/acatalog...gineering_Cont rollers_34.html


Double the price of your two Viper Mtroniks Escs
http://www.mtroniks.net/mtroniks_SC_products.asp?CategoryID=1&SubCategoryI D=3http://www.mtroniks.net/mtroniks_SC_...ubCategoryID=3
.... but probably worth it in the long term.