Servo and Radio Setup – Travel and Rates

When you set up a new model on your radio using default settings, it is likely that the maximum movement of the servo is very close to 45 degrees in each direction.  This is so for several reasons but the key facts are as follows:

• Most servos “out of the box” are made to  rotate to a maximum of around 60 degrees in each direction or 120 degrees overall.
• The radio defaults the Travel or End Point Adjustment to a value of 100 (brands vary as to the maximum allowed, typically either 125 or 150.
• The radio also defaults the Dual or Triple rate settings to 100 with the maximum typically being 125.

For purposes of this example I’ll use 125 as the maximum for the “Rates” and 150 as the maximum for the travel as that is what my Spektrum radio uses and that is what I have tested with.  Though the 100 value is not really 100 “percent” of anything (other than 100% of the default) most folks refer to it that way.  I feel like this creates an issue as many have trouble understanding how these functions work because they are thinking in terms of percentages and they really are not so I will just specify the value from this point on and avoid the confusing terminology.

Since the radio is set to a Travel maximum value of 100 out of a possible 150 (in each direction) the maximum rotation of the servo output shaft is 100/150 or 2/3rds of the 60 degrees max in each direction.  I.E. the servo rotates 45 degrees in each direction in this default configuration.  If we were to up the travel value to 150, we would then get the full 60 degree designed maximum rotation of the servo.  Simple enough.  This Travel setting is not commonly something assigned to a switch with the intention of changing it to allow for certain maneuvers or flying in certain conditions.  It is simply set and forgotten or never even looked at.

On the other hand a Rate switch is a commonly used setup.  Most folks will use the Rate switch to allow for lesser or greater travel as needed.  To understand how this works let’s return to the default setting for Travel (100) and adjust the rate settings only.  Let’s assume we have a dual rate switch assigned.  In position 1 it is set for a rate of 100 and at setting 2 it is set for our maximum setting of 125.  In position 1 we are at our standard default setting and the servo arm moves 45 degrees in each direction as we move the appropriate stick through the complete available motion.  When we switch to setting 2, we get a bit more travel.  As you might guess we get about 7.5 degrees more travel in each direction.  Not quite to our maximum, but half way between our default 45 and our maximum 60 degrees.

So now it gets interesting because there is some interaction between the two.  After doing some testing, here is what I found.  If you leave the Rate setting at 125 and set the Travel to 125, we can now reach our maximum of 60 degrees of motion.  Further increasing the Travel to 150 will not change (apparently) anything.  Maximum throw is still at the same 60 degrees.  But beware!  There is something a bit less obvious going on.  The motion is smooth and continuous with both set to 125 or with the Travel set to 150 and the Rate set to 100, but when you set both to maximum a “dead spot” is created at the high end of stick motion!!  The last 25% or so of stick motion results in no movement at the servo.  What is apparently happening is that the radio has interpreted the combination of settings such that the maximum output is reached before we run out of stick motion.  I can’t imagine why this would ever be a desirable outcome.  I would suggest that you either choose to limit your maximum Travel to 125 and use the Rate setting to reach the maximum travel, or limit your Rate setting to 100 and use the full Travel setting range as desired.