Select an object. > Motion panel > Parameters > Key Info (Advanced) rollout
The Key Info (Advanced) rollout controls velocity in three ways:
You control the absolute velocity at a key using the Velocity Fields.
You average velocity over a specified amount of time using Normalize Time.
You force constant velocity from one component key to the next using Constant Velocity.
If you are creating Path animation, you can force constant velocity by placing a Normalize Spline modifier on the spline used as a path.
Procedure
To set normalize time for a key:
Select an object and choose a key to Normalize.
You can use the arrows in the Key Info (Basic) rollout to step through Keyframes.
In the Key Info (Advanced) rollout, click Normalize Time.
The key is moved in time to average the velocity through the key.
Interface
In/Out: The In field is the rate of change as the parameter approaches the key. The Out field is the rate of change as the parameter leaves the key.
These fields are only active for keys using the Custom tangent type.
The number in the field is the rate of change expressed as parameter units per tick. By changing the two values for X, Y, and Z you are changing the length and angle of the tangent handle.
Lock button: Changes one Custom tangent by changing the other an equal but opposite amount. For example, if you click the Lock button and the In value is 0.85, then the Out value is -0.85.
Normalize Time: Averages the position of the keys in time and applies them to any consecutive blocks of selected keys. Useful if you have an object that speeds up and slows down repeatedly, and you want to smooth out the motion.
Constant Velocity: When turned on for a key, values in between that key and the next one are interpolated so the object moves at a constant velocity across that curve segment.
Free Handle: Used for automatically updating the length of the tangent handle. When this is turned off, the length of the tangent is at a fixed percentage from its neighboring key. As you move a key, the handles adjust to stay the same percentage away from the neighboring keys. When this is turned on, the handle lengths are based on time lengths.