RL>Diodes or capacitors?  The only place a diode is needed (if
RL>at all) is
RL>reverse-biased across the leads.  Generally, all you want is
RL>three
RL>capacitors.  One across the leads and one connected from each
RL>lead to
RL>the can.   I've seen diodes connected across relays and
RL>motors in
RL>other applications, but never RC cars.
Disk or Elctrolytic, Metalized Paper or ceramic? you have to state which kind 
you need and the value, because it could be dangerous to have something like 
a 12,000 Mfd electrolytic instead of something like a .1 Mfd disk.
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