I’m currently trying to animate with an r15 roblox rig in blender, its going well so far, i just dont know how to use ik constraints. i tried putting ik constraints on the feet so they wouldnt moove but i just coulnt figure it out. i used the blender importer/exporter roblox plugin. i know that it has a built in ik constraint feature, yet i have no idea how to use it or how to use ik constraints…
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So with Blender animating it’s typically better to not use IK constraints as the physics are wonky if you want to do anything flashy or animate anything more than a small shuffle. But once you have your rig imported to blender. (which you need a special plugin for) You want to Press N and that’ll open up a menu on the side where you can see the blender plugin. Select Rebuild rig, select the rig, go into pose mode, click the bottom bone that’s connected to the foot and press “Create IK Constraint” in the plugin menu. Here’s a video to help.
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I see that, I tried it out and it worked. But is there a way to sort of limit the rotation for the bones? In the video when you move the leg, the leg bends and moves in weird ways.
The IK constraints are automatically set to be fairly realistic but yes there is a way you can edit it. If you click the yellow bone that has the IK constraint applied to it then you can press the bone icon in the properties menu on the right and you should see a small dropdown that says “Inverse Kinematics” and that lets you adjust rotational degrees on each axis. You will have to change each bone that has IK individually though, it’s not a global change.
i added this to the foot but it doesnt make any nice bends. it moves the leg in a straight line.
If the Yellow bone is the foot itself it isn’t going to properly IK. The yellow bone should be the ankle which would be the center for most of the rotation. And if you changed the rotational degrees manually then you just have to play with the degrees until it allows for uniform movement. Though I suppose it also depends on where the extra bone created from the IK constraints is located. The foot bone that appears after applying IK is the actual controller and it isn’t locked to the ankle, if you move that foot bone too far from the rest of the leg, it will hyperextend and become straight.
I haven’t encountered a leg just locking up for any other reason when I’ve done this in the past so that’s about as much as I can say atm 
I actually found a pretty simple fix to the bending issue.
My issue was that the knee bent the wrong direction:
The fix was to simply select the knee bone (its called “RightLowerLeg” or “LeftLowerLeg”, in pose mode), press N to open the sidebar, select the “Item” tab (usually top most tab) and change the Rotation “X” from 0.000 to 0.5.
Now your knee should rotate correctly. Don’t forget to record this keyframe and set it at frame 1, for it to be applied through the entire animation.
Here’s a screenshot of what it should look like:
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