Static variables are class-level variables; there is only one copy of the variable which is shared by all instances of that class.
In line 6, you set i to 55, but remember that there is only one copy of i shared by all instances of class SerTest. So i will be 55 also for the new SerTest object that you read in from the serialized file in line 14 - that's to be expected, and does not demonstrate that the static variable is serialized.
When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables. In the case of the Bicycle class, the instance variables are cadence, gear, and speed. Each Bicycle object has its own values for these variables, stored in different memory locations.
Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables.