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Many users here regularly run their Crystalyte 4XX wheels at 72 volts. However, you may need to upgrade your controller to handle the higher voltage depending on what controller you currently have. With my 24V-48V controller, too high a voltage would definitely blow a mosfet as those things are what limits the voltage. See http://www.ebikes.ca/store/ for a list of controllers. Also, running at such a high voltage tends to incur higher resistive losses in the 4XX so you'll notice the motor getting significantly warmer. At higher voltages, one way to reduce losses is to upgrade your motor to a 5xxx but you'd also need a large-amp controller to handle the extra amps it draws.