Interaction of AC and DC waveforms in motorcycle frame and their effect on AC & DC components
In general, the frame or chassis of a vehicle is considered a very, very low resistance. 
If you are familiar with Ohm's law it tells us that the voltage drop along the frame will be given by \$ V = IR \$. Since R is very low V will be very low too - so low that the various circuits don't interfere.
It might help to consider the chassis as a big sea of mobile electrons. Current can enter and leave from any point on it without the local water level rising or falling. In electrical terms all points on the chassis are at the same potential (voltage).
Following on with the analogy, if we put a restriction into our sea - a small channel or pipe - then feeding in a large current would cause a rise on that side of the restriction. You may find parts of your motorbike frame have poor electrical connections with other parts and might be able to measure some voltages there.


In any case, your motorbike will run on DC with the exception of inside the alternator, if there is one, which will be alternating, as the name suggests, but rectified with diodes before connection to the DC circuits. The ignition circuit may also be considered AC.