Now that's some serious panning
! Congratulations, I wouldn't even dare trying.
Now some comments...technique-wise...
First, I'd suggest using a lower (sensitivity) - ISO 100. As you seem to have been shooting in S (Tv for any Canon Eos users reading...shutter speed priority), this will give you a larger aperture, less noise in your pictures than shooting at f/25 and ISO 800...and sharper pictures obviously. Well, at the long end of a telephoto zoom lens pretty much every lens out there (except for the Nikkor 200-400 f/4 VR II monster) will benefit from stopping down a bit, but if the aperture gets too small you'll get diffraction (this is related to the lens as well as the pixel density of your sensor). Try avoiding apertures below f/11. That said, I have a perfectly usable picture taken at f/13 or f/14 and 230mm (Sony 70-400G SSM), so go try with your Tamron and find out what works for you
Secondly, I see the motorcycle is sharp, but the driver's head isn't. Picking a faster shutter speed helps but may obviously not give you the blurred background you're after. It's a bit of trial and error, I guess. I'm not exactly in this kind of photography, sadly.
Finally, I would like to give you a tip
. If you'd be shooting them head-on, pick a faster shutter speed. Someone has explained it to me some time ago - it has to do with physics...some parts (like the front of a car or motorcycle) are moving faster than the other, it appears. Something with the aspect or so. I hope Gordon reads this...he's got a degree in Physics from the University of Kent so he ought to know how this exactly works.
SSM/NEX-5/18-55 OSS/Lowepro Pro Trekker 600 AW/CompuTrekker AW/Nova 140 AW/Street & Field gear/Toploader Pro 75 AW. And a huge wishlist...My photos on Flickr...