What the sports mode does is freeze the action. This is done by setting a high shutter speed so that the subject is sharp and in focus, unfortunately what this creates is an image that looks like time has stopped. What you need to do is use the TV - shutter priority setting on the dial. This then give you control of the shutter speed and allows you to take pictures with a slower shutter speed creating that blur you crave.
It depends on the shot as to what shutter speed you set and what the subject is, bikes are faster than cars, so you need a slightly higher shutter speed. It also depends what part of the circuit you're on, slow corner pan, slower shutter speeds, pan on a fast straight, faster shutter speed.
Now there are people who'll suggest shutter speeds of 1/125 sec or slower, but you've never done this before so you need to get comfortable with the settings and also a smooth motion as you pan with the subject.
Personally, I would start at say 1/400-500 sec for a bike and slightly less for cars until you start to get constant results then slow you shutter speed down. For panned shots of bikes, I get constant results at between 1/320 and 1/200 sec, cars slightly slower.
People think panning with a camera is easy, it's not, that's why most of the motorsports shots in magazines are head on shots, panning requires a smooth action from focusing on the subject some distance away, moving with the action, keep subject in focus, until you get the shot you want, then taking a 1 - 3 image burst and continuing with the action once its finished. This takes lots of practise to develop a smooth panning action to constantly get in focus
, sharp results time after time. You don't need the slowest shutter speeds to get motion blur.
