Problem with Leg IK on Robot Rig

I’m trying to rig a Gundam model, but I’ve been stuck for 2 days now. When I try to make the leg Ik the legs keep bending in reverse. I’ve changed the rotation of the bones and it still happens. Anyone know what’s causing this? Here’s the file if you want to look. (note: the Ik is taken off in the file, but only takes a second to added it back) Sometimes I can get one leg to go the correct way but the other is reverse.


Your leg bones are bent the wrong way, they need to be bent forward. Like your own leg. So just make a smooth arc where the first two upper leg bones go forward and the lower leg goes backwards. For the pole targets they need to be set to -90 degrees.

I’m not sure I understand. I mean I know how legs bend, but the way you said I don’t understand. You say the top 2 are forward and the back a little back. That would mean the leg would still be straight. Do you have a pic to explain?

The bend doesn’t need to be this extreme, it just has to have a slight bend for the IK to bend the leg the right way.

I see. To me the bones are in the correct place for the model. But the IK doesn’t understand that and wants to bend back. Ah so I ned to bend it some. I’ll try that.

Ok that did work. But the leg wont raise straight up. The knee goes off to the side. Plus if I make the chain length 5, it wont grab the correct bones. It needs to be the whole leg. It’ll grab the first 3 just fine, then parts of the skirt. This is my first time doing an IK rig. I think it being a machine with extra bones is what’s making this so hard.


So what is the whole leg then? I’m guessing hips/crotch aren’t parts of the legs in humans and I don’t see why it would be different for a robot rig. The IK constraint wants to line up every bone so if you’ve got bones next to each other (like the hips and crotch bones) you will get strange rotations for the leg.

Then you’ve got some constraints for the skirt bones which copy the rotations of those bones (that are also being affected by the IK) so they will move as well. By the way, if you’re using constraints you should probably use local space instead of world space to make it behave better when the whole rig is moving.

Here’s an example that looks fine to me: Gundam_0079_Color a.blend (990 KB) I removed a bunch of stuff to make the file size smaller. It’s a bit hard to see what’s wrong when you’ve not bound to mesh to the rig so maybe you could parent the mesh to the rig so it’s easier to see how it should work.

I checked the file and still not right. The IK needs to affect 5 bones in the legs. Thanks for looking at it. Should I make the hip/crotch bones differently? I original had the crotch bone on the x connected to the hip. Think of an upside down L shape. But I figured this other way would rotate better then the original twist action. I added some pics to show what should be happening.


I don’t know what your twist bone is supposed to do but it’s possible to do what you want if you add a bone at the end of the chain (the bone doesn’t do anything): Gundam_0079_Color b.blend (990 KB)

The twist bone is the bone that actually lets the leg move forward. Look at the last pic. See the crotch hip area. Bone areas 1 and 3 can only move forwards and backwards. Bone 4 can only move side to side and lastly bond 5 twist to make the whole leg move.

Look here that whole hip area has to twist to make the leg move forward. Also how to put a rotation limited on a IK chain? Some bones I need to put a rotate limit on.

There’s the Inverse Kinematics tab in the bone panel, you can set rotation limits there. But it sounds you may be better off not using IK at all.

Do you think I have too many bones for IK to work correctly? Again this is my first time ever setting them up. But I’ve used them before with normal human models.

Not really too many bones, but IK is designed to work one way (all bones turn to reach a target location) whereas you have some restrictions for certain bones that don’t seem to work that well with an IK rig, so maybe it’s better to just use regular FK for this.

Yeah that’s kinda what I meant lol. I really thank you for the help. I did learn some new things from this.

Seem to have a new problem. I’m almost finish with the rig, but hit another problem. When I rotate the arm up, it slowly twist out of place. The bone is straight, so not sure why it’s doing this. You can see in the pic. Updated file


It’s not straight (0.445Y and 0.430Y). I don’t know if that’s the issue you’re having though. Doesn’t look that strange to me…

Ok it looks 100% center to me, but still twist a little. If you look at it from the side and the front it’s straight. Something is forcing it to twist. It only twist when rotating forward.

Update: I figured it out. But not sure why it’s doing it. If I lock the y or use a copy rotation it stops. If I take off the copy rotation at the should area and just move both bones together it works fine. Locking the y makes it twist.

Update 2: To fix it, I made a master shoulder bone and parented them both to it. That fixed the problem…for now lol.