Crossing chaining hasn't been any problem at all since the invention of bushless chains 20+ years ago and wasn't even a real problem back in the ancient days. It's a persistent myth that just won't die... 

Your bike should leave the shop capable of shifting into any combo of gears possible and riding any amount of time you like in that gear. At most I would accept a slight extra noise in the extreme combos(big/big and little/little), but those components were designed to work in all the gear combos. 

If you are mixing/matching components or running odd size gears, you might run into some problems with derailluer capacity, but any bike stock from the shop should be able to use all it's gears. 

Mechanic says not to use top three/bottom two gears or the chain will throw
I am making a battery powered project which involves bi-directional motor control. However motor needs to turn rarely (about 10-20 seconds in 24 hour period). I am using L298 motor driver. However the quiescent current is in mA range. To reduce current consumption, I am planning to use a mosfet switch as shown in the figure below:

VM = 9 V

VCC = 4.5 V or 6 V (I will decide after a little experiment)

Motor current consumption &lt; 1 A



Whenever I need to use the motor, I will turn the mosfet ON and keep it OFF while the motor is not in use. (I am using logic level mosfet.) The quiescent current of mosfet is in uA range and that's how I am planning to save a lot of battery power.

Will it work?

Any suggestions/improvements are welcome.
