I would suggest magnetic ones.
There aren't any restrictions on types of magnet in <R92>. As such, any type is fair game--just use caution if it's mounted on something; whatever it's mounted on also needs to be legal.
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Past teams:
2003-2007: FRC0330 BeachBots
2008: FRC1135 Shmoebotics
2012: FRC4046 Schroedinger's Dragons
"Rockets are tricky..."--Elon Musk
