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Some trig help needed
It’s 1am and I can’t think straight any more… I’ve got a graphic of a tandem and a trailer, which I’m animating using nulls so that the wheels always touch the ground. I got the tandem wheels working, but am having trouble with the trailer… basically I think this could be summarised as “from the diagram below, what’s the formula to make B always touch the line between C and D?” The positions of A, C and D are known, as is the length ab. (A is the “hinge” connecting the trailer to the bike, C is the bike’s rear wheel contact point, B is the trailer wheel’s contact point defined by a point control that doesn’t move, and D is a null that can pretty much go wherever it wants to.)
At this point I’m just going round in circles, any help would be appreciated!
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1 Reply
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I *think* I’ve done it (well, it’s good enough, at least). Expression for the rotation:
wo = effect("WheelOffset")("Angle"); // arbitrary angle control for fine tuning
Cxy = thisComp.layer("Tandem").effect("RearWheelContactPoint")("Point"); // C in the diagram
Axy = transform.position; // hinge point: A
Bxy = effect("WheelPoint")("Point"); // Rear wheel contact point: B
Dxy = thisComp.layer("TrailerWheelGuide").transform.position; // trailer wheel guide: Dc = length(Axy, Bxy);
theta = Math.atan2(Dxy[1]-Cxy[1], Dxy[0]-Cxy[0]);
sintheta = Math.sin(theta);
costheta = Math.cos(theta);
xs = Axy[0] - Cxy[0];
ys = Axy[1] - Cxy[1];alpha = Math.asin(((sintheta * xs) - (costheta * ys)) / c) + theta;
radiansToDegrees( alpha ) + wo;
Now to figure out why the guy’s shins are coming out of his shoes whenever I move the bike around….
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