Motor Racing Photography

I have previously published my faltering steps into taking photos at Motor Racing, and benefitted (!?!) from the advice you gave me. But I am still not always getting good results.

My kit is a Canon EOS 500D with EFS 70-300mm IS USM lens with the IS set to position one. The centre sensor was chosen, with AI SERVO and AV mode.

I hope you will agree in the shots below that there are 2 "good" shots (well good enough for the equipment I used and my less than perfect panning). But there are also 4 POOR shots, and seek to do better with these next time.

The "good" shots have the car going horizontally passed me, making an easy pan to follow the car on a level plain. Unfortunately these shots are rather dull because there is only a single car in side profile.

The "poor" shots ought to be more interesting, because they give a 3/4 view of the car, and sometimes involve more than one car.

Since these shots often seemed to involve a diagonal pan, should I turn OFF the IS; is the IS getting confused by the diagonal pan, and would turning it off give better results ?

When there are 2 cars, how do you get sharpness for BOTH cars. This isn't strictly necessary, but when they are close then it would be good. I will have put my red spot on only one of the cars, hoping that one proved the most active - and my choice was only good about 50% of the time.

I give below small images, the full-size images unedited are availabe by links I list after>

Planning to go to Goodwood and Silverstone in the next couple of months so wanting your advice. Also thinking about upgrading to EOS 70D with 100-400mm, but only if I can mastch my skills to the technical ability of the new equipment.

Thanks

Diagonal pan of 2 cars, neither as sharp as I would like. Full-size is here: http://www.simoncars.co.uk/racing/big/Austin-Healeys.jpg

Diagonal pan of 2 cars, neither as sharp as I would like. Full-size is here: http://www.simoncars.co.uk/racing/big/DB3andAH100.jpg

Diagonal pan of Lotus and Austin-Healey. How to get BOTH cars sharp. Full-size is here:http://www.simoncars.co.uk/racing/big/lotus_healey.jpg

Only one car, but it is still a problematic diagonal pan, so not as sharp as I want. Full-size is here: http://www.simoncars.co.uk/racing/big/jaguarMkVII.jpg

Happy with the sharpness here, but then this is an easy horizontal pan - but only gives a side profile shot. Full size is here: http://www.simoncars.co.uk/racing/big/Bentley.jpg

Another 'satisfactory' horizontal pan, but again just a side profile not a 3/4 view. Would love to get this sharpness on a front three-quater shot. Full-size is here: http://www.simoncars.co.uk/racing/big/AstonMartinDB3.jpg