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  1. PO
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    <p>The rotation part of your code looks ok. However, you have an if/else block that returns the same thing in both cases, and as mentioned by @joojaa, you can avoid <code>getAttr -time</code> if you cache the translation values. In fact you should avoid <code>getAttr</code> and <code>setAttr</code> completely in expressions. </p> <p>Instead, refer to the attributes you want directly and Maya will create connections for you. This is much faster and less prone to errors when you rename nodes and so on.</p> <p>To cache the translation values, and calculate change in position you can add attributes to the node and use them in the expression.</p> <p>Let's say you have a cylinder called wheel that rotates around its local X and is parented to a group node called control:</p> <p>Add a vector attribute: <code>control.lastTranslate</code><br> Add a vector attribute: <code>control.deltaTranslate</code><br> Add a float attribute: <code>control.distance</code> </p> <p>Here's an expression that will store the change in translation, then rotate the wheel based on the distance travelled.</p> <pre><code>// When deltaTranslate is calculated, lastTranslate still has its previous value. control.deltaTranslateX = control.translateX - control.lastTranslateX; control.deltaTranslateY = control.translateY - control.lastTranslateY; control.deltaTranslateZ = control.translateZ - control.lastTranslateZ; control.lastTranslateX = control.translateX; control.lastTranslateY = control.translateY; control.lastTranslateZ = control.translateZ; control.distance = mag(&lt;&lt;control.deltaTranslateX,control.deltaTranslateY,control.deltaTranslateZ&gt;&gt;); // Get radius from history node (or somewhere) and move the wheel's hub off the floor. wheel.translateY = polyCylinder1.radius; // add rotation to the wheel float $tau = 6.283185307179586; wheel.rotateX = wheel.rotateX + ( control.distance* -360.0) / (polyCylinder1.radius * $tau ); </code></pre> <p>It's best to test this kind of thing by animating rather than dragging nodes around in the view.</p> <p>If you wanted to make the wheel aim to the direction of travel, you could add a locator at translate + deltaTranslate and hook up an aim constraint.</p> <p>e.g.</p> <pre><code>aimLocator.translateX = (control.deltaTranslateX / control.distance) + control.translateX; aimLocator.translateY = (control.deltaTranslateY / control.distance) + control.translateY; aimLocator.translateZ = (control.deltaTranslateZ / control.distance) + control.translateZ; </code></pre> <p>Dividing by distance will normalize the offset. You should probably check that distance is not zero.</p>
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    1. PORotation Automation
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    1. USJulian Mann
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      1. VTUpMod
    1. COThis seems quite promising @julian! I've had a bit of a play with it, but how can I "switch" or calculate the direction with flipping the wheel so to speak with an aim constraint. I don't fully understand, but what I have seems to work quite nicely thanks to you!
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    2. COI see what you mean. The expression above only knows how far you went, and not whether you are going forward or back. There are a couple of options. To automatically shift from forward to reverse, you can compare the direction you are facing to the direction you are moving. This implies that you manually control the direction you are facing. Alternatively you can set the orientation automatically to the direction you are moving. In this case the wheels are always rolling forward and you have no manual control. I'll edit my answer with details of each as soon as I get a chance.
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    3. COThankyou heaps! That would be great!
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