I was looking for small gear motors and I found two nice ones: https://www.robotshop.com/ca/solarbotics-gm7-gear-motor-7.html and https://www.robotshop.com/ca/solarbotics-gm10-geared-pager-motor.html. The first one has a 120:1 gear ratio and the second 81:1. Are these ratios too slow for a robot about 6x6 cm big?
Depends on the size of
Depends on the size of wheels etc.
I checked your first link:
The RPM is 145, so your wheels will turn 145 rounds per minute, without load.
Say, your wheel’s outer size is 10 centimeters, you know it will travel 10 centimeters in one round. If the wheel would have no load, it would then travel 1450 centimeters in one minute, or 14,5 meters, which calculates to a bit under 1km/h.
Now, if you put on really big wheels, like 30 centimeters outer size, the speed will get highter, but the load will also be bigger, so it isn’t really just adding it up.
The other link says 370rpm, so you’d go 37 meters per minute, that’s alot faster, a bit over 2km/h, so more than double the speed.
Don’t just look at the gear
Don’t just look at the gear ratio; look at the other specs as well. The gm10 is a tiny motor with very little torque. You can only use these motors to move a very light robot. Frits used these motors to run the drum sticks for the Yellow Drum Machine, and it bearly has enough umph to do that. I’ve used one for a drum stick on Blind Lemon as well. They are tiny, tiny, tiny.
Depends what you wanna
Depends what you wanna build: For a racer it’s too slow but an explorer it might be good enough. My LMouseR is running (ok, not really running 
) on two gear motors with 120:1 and for me it’s fast enough…since it’s not about speed but steady movement
Oh whoops how stupid of my
Oh whoops how stupid of my part. Didn’t understand rpm was “rotations per minute”. Well thanks a lot guys! Oh and by the way it’s a really small robot I’m planning to build.
solid numbers
One rule of thumb I have been using for a while is the 110100 rule. I made it up.
I generally stick to about 1’ per second for a travel speed. Walter has 2 “standard” speeds --3/4FPS “regular” and 1/2fps “slow”. At these speeds, I can average about 10 sonar readings per foot. I figure this is pretty good “resolution” for a robot. As for the 100, I have found that when sending data --lets say the data needed for RC control, if you can send 100 updates of data per second, your joystick will “feel” instantanious --no delay or herky-jerky.
These numbers should give you a good ball-park to start from.
It’s not about the size of a
It’s not about the size of a robot but the task he/she/it has to perform.
Example:
Look at my LMouseR It’s using very slow gear motors and I like that slow movement. But now just think there would be four smaller wheels instead of these two very big ones. The bot would lookm like a small racer. with a lower gear ratio…say 2:1 it would be very fast. So…please tell us what you wanna do and we can tell you if that ratio is ok. (Size does not matter - don’t get this sentence wrong )