Rotating circular feathered gradients

Hello everybody,

I am trying to mimic this Formula 1 intro motion graphics: http://www.youtube.com/watch?v=YhZ9-N5wcD0&feature=related

Here's where I got so far:

Now, if you watch the original video closely, you can see that the colored circles have sharp start and long feathered end. I want to achieve this effect.

Here's how I've been working so far.

I have animated the 'Start' and 'End' options of the circle:

Then I added some motion blur to it. And it resulted in feathering out both ends of the circle.

And I need it look like that:

Any suggestions?

I would appreciate any help and thought on this matter.

Cheers,

DissidentPJ