Devoted,
Exactly, between 50 and 90 9or 90 to 50 as you say), there is no change of the information, the Information is here the rotation in degrees. With that, the tire is supposed to stand still. This is how it should be!
Again, animation is change of values over time. No time change—no animation. No value change, no animation.
There is a keyframe on frame 50. So this keyframe value, is then all there is to it. Let me say it clearly:
Frame 50= 360º
Frame 51= 360º
Frame 52= 360º
Frame 53= 360º
etc
Frame 90= 360º
As after frame 50 comes no different information, that is all there is 360º
You can set the interpolation to continue after the last key frame, but that is a complete different idea, you project the animation change from the past to the “future”. But in your scene, after frame 50 (again) there is no information change. Hence there is a yellow line between the two dots (F-Curve window) to show case that, no line no change. (If you use “Track After” then the line will be black, and indicates an “extrapolation” of the keyframes before.
You need to set a keyframe to frame 90 with a new and DIFFERENT value than on frame 50, otherwise it will just be there, no P.R change an it is correctly doing nothing.
Let me know of that made sense. It is the basic idea of animation.
Years ago I had a classic animation artist on the Event booth, asking about a similar question. She was not certain how to tell the computer all the “in-between” poses. That is the interpolation. Inter as in between the key positions. After Frame 50 there is no Key position—so it repeats.
(BTW, to make a tire run one time around, the 0º and the 360º are the same.If you make a loop, this would introduce a small stop. In the moment we are clear with the Keyframes, I think it would be great if we could clear that as well.)