Join Date: Sep 2007
Location: Base of the Rocky Mountains, Canada. Wonderous things!
Bikes: 2010 Cannondale Hooligan 3
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After having worked in an electric bicycle specialty shop and dealt with this problem plenty enough times, I can tell you that the situation is more complicated than just what your forks are made of.
It also depends on the power of the motor, whether the controller has a soft start or pedal first start, and if it does not then how brutally hard is the rider going to accelerate?
In situations where people are using low - mid power motors designed for power assist and applying the throttle reasonably, most forks are quite fine. It's the high powered / overvolted motors and the riders who gun it straight off the line that have troubles with fork damage / failure. The most stress to the fork by far is generated when you gun the motor from the starting line.
Additionally, suspension forks are not optimal matches for electric motors either, generally. They are prone to damage during repeated heavy acceleration at the stanchions, developing play and becoming dangerously loose - potentially eventually failing. Cheap suspension forks already tend to do this a lot even for normal bicycles, electric ones with front hub driven motors just amplify the problem that much more. Again though, a decent quality suspension fork and a reasonable usage level from the rider given their particular motor setup can make it work alright. Some electric bicycle manufacturers such as eZee use a front motor mounted to a suspension fork, but it is a moderate power motor with a soft start controller.
A Chromoly fork IS preferrable, but others can work decently if you're not going to be building some kind of scooteresque super powered abomination.
If you have any doubt, go for a rear wheel drive system instead. The bicycle frame itself is capable of handling the forces generated by the motor to a greater degree than the fork is, and there are benefits in increased drive traction and keeping weight off the front wheel, which can make some bikes handle a bit wonky. The rear wheel motor kits aren't much harder to install or deal with.