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[video=youtube]youtu.be/jK1MhjoUdtU[/video]

Thanks for the help.  I think I'll add magnets then weight to the faster car. 

Since you asked:
A generator on bike tire provides voltage to each lane.  Diodes in the circuit only allow 1 bike (kid/adult) to send power to a lane.  There are 2 kid bikes and 2 adult bikes.  A voltage regulator prevents too much power to lanes so cars don't spin out in corners.  (The generators provide plenty of power and must be regulated.)  A digital counter from Amazon counts laps via a magnet glued inside the car's roof.  The video is of the first version.

I cannot use different voltages for each lane as I am not there all the time.  The cars could be placed on different lanes as kids/adults experiment.

The new version still has the vs race option but adds an Arduino type controller that has a built in screen.  The controller is programmed to start a timer on the first pass under the sensor.  The lap counter goes to 10 and the timer stops.  This timed version has a different voltage regulator that is set to allow the cars to go a bit faster so they will spin out.  The timed version becomes a skill issue.  A switch and relay are used to go from vs mode to timed mode.  Racers will simply post their time on a white board.