You did a pretty good job of panning - the tractor is sharp, but the blurred tire and blurred background both sugges a lot of speed. The direct side-view, however, makes it seem fairly static and removes a lot of the human interest.
Depending on what effect you're after, there's a couple of different things you can try. One is to shoot at about a thirty degree angle relative to the track. You're generally far enough to the side to be safe, but you can get more of the set-up to show what's going on. You also get a more 'personal' interaction with the driver (and sled operator, if there is one) that will 'human interest' to the shot. If you wait till you're at about a sixty degree angle, on the other hand, you'll still have a lot of the 'human' factor, but you can get more of the blurred-background effect you have here.
The other thing you can do is vary shutter speed. You've used a relatively slow shutter here to get the motion blur, and as mentioned above that's good for giving the sense of speed. I like to use a fast shutter, though, to give a better feel for the power these things produce. If you get the timing right you can 'freeze' the rear tire and get a good shot of the stress deformation bands these things generate. It may just be the techo-geek in me, but I think those stress-band shots are totally cool!