Tach PM'd me to have a go at me for complaining about it. I'll reply here, in general terms, to illustrate what I was referring to. If the wheel is suspended on an axis and just spinning then I might agree. However the setup involves a wheel rolling on the ground (the no slip condition is explicitly mentioned somewhere in all of this). In that case you have both the rotational motion and the lateral motion. Hence my post above, where the front of the wheel basically acts like a normal mirror moving forwards at speed v towards the observer and emitter, which obviously then see a shift. If you lifted the wheel just off the ground and just let it spin at the same rate than obviously the motion of the bit of the wheel at the 'front' would now have no forward component, it would be at right angles to the direction of the photon bouncing back to the observer with the torch and thus no shift occurs. This is why I said it was poorly written (which Tach also complained about), because the picture doesn't mention anything to do with the motion of the wheel on the ground, yet there's been considerable discussion about it. It's easy to compute the motion of a particular piece of the wheel, it'll be the sum of it's rotational motion about the central axis and the motion of the axis, which you can express in terms of the angle the piece in question is making with some particular direction. When the piece lines up with the ground, so it's at the front or back of the wheel, wrt its motion, then it'll only have motion parallel to the ground due to the overall motion of the axis but that is non-zero. The only part of the wheel which is stationary at a given instant is the bit on the ground, that's what the no slip condition is! So, if the wheel is indeed rolling on the ground the claim of Tach's is false, trivially and obviously so. This obviously generalises to instances where the wheel's axis is moving in some arbitrary direction, so the only way Tach's claim could even possibly be true is if the axis, the emitter and the receiver are all at rest wrt one another. Can someone tell me if I'm just misremembering something? Did someone else mention the no slip condition has something to do with all of this? If it was Tach then his claim is immediately falsified. /edit Having just scrolled up in the thread I see the no slip conditon is mentioned in JamesR's openning post so if the debate involves a rolling wheel Tach is wrong. If he weren't wrong then speed cameras wouldn't work. I'd also point out he didn't actually address any of my previous post in his PM, he just complained about me posting it and how no one else had misunderstandings. It would seem my understanding was fine, as the rolling of the wheel is entirely relevant. Tach, if you plan to reply to me via PM or in the debate thread then I'd appreciate you addressing the explicit points I've raised rather than just saying what amounts to "Stop complaining and asking for people to clarify things!".