Can someone explain me how a 2 speed trnny work ? web site or explain...

Are you asking about the auto 2 speed often found nitro cars, or the manually shifted ones from the controller?

so basically when the engine is moving slow, it's using the first gear. Then when the RPM goes up, the "centripital force" increases and engages the second gear.

http://www.rctek.com/technical/trans...ny_design.html

yea basicly as said above ... the 1st gear is on a oneway bearing . the second gear has basicly a clutch sord of like the use in the engine(not exsactly but its the same princital). as the car speeds up the 2nd gear engages becouse of the centrital force pushes the shoes out. sence the 2nd gear is going faster then the 1st the first gear no longer contributes power to the car becouse its on a oneway bearing

Sugafree gave a good link with a good description of how perfomance 2-speed gearsets work.

As for the more run-of-the-mill twospeeds, they work on a disc with deploying pawl system.
http://www3.towerhobbies.com/cgi-bin...&I=LXKLU8&P=FR

Look at this PDF link, page 4, the exploded diagram of the 3speed Savage gearset. In particular, look at the discs, PN 87216 & 87217
http://www.hpieurope.com/manuals/pdf/87218.pdf

This cute little disk has a little finger pawl that deploys from the disc (like how a cat deploys his claws to scratch up your furniture) except the high RPMs cause the pawl to come out by its weight from centripital forces. The gear of the higher gear speed has a pin for this pawl to grab a hold of so that it locks onto that gear, thus the higher gear is engaged.

The "grub" setscrews with the spring determine when that pawl deploys. Tighter, and the pawl comes out later (at higher RPMs). Looser, and the pawl comes out sooner (lower RPMs). So, you can tighten or loosen the grub with teh spring untill you feel your gearset shifting at the right RPM.