Sounds like the addition of some external low ESR caps might be beneficial. Might even be a good idea to build in some type of "surge supressor" (using zerer diodes or something) to keep the spikes closer to the working voltage of those caps.
Inductive kickback under braking can be fairly high, especially with higher turn motors. The exact amount depends on the inductance of the motor and the speed. I've generated over 2,000v pulses (measuring equipment didn't measure high enough) using a 9v battery on a 7 Henry inductor. Of course, that's quite a bit more inductance than these motors, but you get the idea.
Inductive kickback under braking can be fairly high, especially with higher turn motors. The exact amount depends on the inductance of the motor and the speed. I've generated over 2,000v pulses (measuring equipment didn't measure high enough) using a 9v battery on a 7 Henry inductor. Of course, that's quite a bit more inductance than these motors, but you get the idea.
To generate similar voltage kick back high turn motor need to spin faster then low turn, that is way I was reffering as safer.