which gear motor is good to use. like i am going to ue wood as the base and some other things too with the wood. so i want to know which gear motor i best.
Both are good motors for a robot. GM3 would be slower but stronger, to carry more weight, GM9 would be faster but better for light weight robots as it is not as strong.
If you are using the wood paint stirring sticks, either motor could be used, as they can be light weight in constructing a robot.
If you are using a plank or other chunk of wood, using the GM3 might be better as it has more torque and would have a better chance of actually moving the robot due to its' heavier construction.
It is sort of like asking, "What screwdriver should I use?". It depends on what you are trying to do.
The 'Start here' robot post (notice how we keep referring you to that?) recommends a gear ratio somewhere between 120:1 to 210:1 for a beginner project. The higher the gear ratio, the slower and stronger the motor will be. A slower robot may be easier to control to start with, but if you need a faster robot, use a lower gear ratio.
The GM9 has a gear ratio of 143:1, while the GM3's ratio is 224:1. So the GM3 is slower with more torque, and the GM9 is faster with less torque.
my robot will be like fristl's. little changes: wood platform instead of battery pack( the battery pack will be under the wood platform) thats going to be the change and may be placing picaxe between the two gear motors not on them like fristl had one.
I will go for GM9.