Speed controllers isn't really that much of a physics area :P all you need to know is a few simple amp ratings and what other people use.

The only two bits of information you really need to know are:

- the stall current of the motors you intend on using (for drill motors it can vary, but in any case you're unlikely to stall the motors unless you manage to break the gearboxes in combat as usually the wheels will slip first...I've heard 20 to 30 amps quoted as a ballpark figure for 12 to 18 volt drill motors run at their rated voltages, so that can be used for now)

- the maximum current that the speed controller you're looking at can handle. It's worth noting here that radio controlled car speed controllers - usually recognised by a massive coloured heatsink and an 'edgy' name - tend to use the raw specification of the FETs (the thingymajigs that do the speed controlling) for this number and so you get quite weedy devices claiming to provide control over 180 or 200 amp currents - they won't, and it's usualy good to avoid R/C car speedos entirely for this and other reasons.

Ideally you want the second number to be higher than the first. In practice you are unlikely to see the stall current, unless you break the motor, for more than a fraction of a second, so you can *probably* get away with a 50A maximum controller for a motor that draws 60A at stall. (bear in mind that I've never fought with anything more than antweights so yu may want to check this with someone with wider experience, I'm not an engineer :P)

The best gauge of what works though is what other people use For drill motors, the following have all been used:

- Sabertooth 2x25 (this is a dual controller that combines control of two motors, and control mixing, in one unit - this only handles 45A peak though, and if run with overvolted motors has been known to cut out under sudden acceleration due to the high amp draw)

- Scorpion XL and XXL (also dual controllers, not heard of any problems with these)

- IBC (also a dual controller, which has another channel from which a weapon relay can be activated)

- Electronize 15A and 30A (these are single channel controllers, so you need two - if you're proficient with electronics though you can add external relays to improve the current handling, Jamie will know more than me about this as he's done it quite often)

- RobotPower Sidewinder (also a dual controller, handles 80A continuously and is really overkill for featherweights)

- RS80E (I think that's what it's called - Alex Marler used one of these, ask him for more information but I believe it's also a dual controller)

- 'blue boat controllers' (these are single channel but seem to work well, aside from that I know nothing about them, sorry :P)

Hope that all helps, other people who know more than me will be able to correct or add to this ramble :P