A bug in the rear suspension simulation has been fixed in Beta9, so bikes created for Beta8 may need physics data tuning.
First of all, enable the physics debug mode:
http://forum.piboso.com/index.php?topic=3767.0
Then:
1) make sure that the natural frequencies of the front and rear are correct ( the last two purple lines in the bottom left of the screen, F for front and R for rear ).
As a VERY ROUGH reference, a road bike should be between 1 and 2 hz, and racing bike between 2 and 3 hz.
2) check the front and rear sag against real data, with and without the rider using the keys 1 and 2.
3) make sure the critical damping ratios for both the front and rear are below 1 ( SB: slow bump; FB: fast bump; SR: slow rebound; FR: fast rebound ).
The stock bikes can give a starting point for racing bikes. For road bikes, you probably need lower values.