For first, my motors are :
and my robot is a bit like this whit the same wheel and motor , and a similar frame .. but i don t have speed encoder:
my problem is that my robot isn t going straight .. do you have any ideeas?
lori3:
my problem is that my robot isn t going straight .. do you have any ideeas?
Yes, get motors with encoders.
Otherwise you could try to calibrate the motors' relative speed.... run one at X% of the other and see how that goes. I doubt if that would be too successful though.
Small gear motors like that are dominated by friction, so no they won't run identically,
and encoders are the only good solution.
OK.. but you can say me for what is good the speed encoder.. and how it works ? i know it has something in its name but i understand only the speed word i don t know what is encoder 
Its not a speed encoder, its a shaft-encoder and it produces a certain number of pulses
per revolution (ie its a position encoder), allowing the distance to be monitored for
each wheel separately.
A feedback loop is then needed to adjust the relative drive strength to the two motors
to keep the two distances in step with each other (or at any fixed ratio you want to turn
a circle).
For small robots typically a computer-mouse-style optical encoder is employed, perhaps
a few dozen pulses per revolution. Something like this: http://www.psurobotics.org/wiki/images/0/0c/SampleEncoder.jpg
For big industrial servo motors shaft-encoders are available that give position to fractions of
a second of arc, but they are expensive and bulky.