Need help using flash for cycling photography

[primalcarl]

I've only recently got a 580EXII and I'm not sure of the best mode to use for taking photos of professional cyclists in poor daylight.

What mode and settings should I be looking for to freeze the action?

[AB8ND]

When shooting mountain bike races, I have 2 scenarios I'm walking the trails or I have a set spot that I want to shoot. If I'm walking the I'll shoot manual, adjusting the exposure as needed, then just use my strobe on ETTL to fill in any shadows. If I find an interesting corner or climb, I will try to set up a couple strobes triggered by Pocket Wizards, one main the other for fill. I'll try to let the strobes be dominate by maybe a 1 or 1 1/2 step, what ever looks as natural as possible. Sometimes I just can't get 2 strobes set up so I'll just use one and adjust it closer to the ambient. I like to get the strobe light to look like I found a spot here the sun was shining through the trees. Aiming the strobes so they shoot through a small bush or branches will casting some shadows on the subject can make the light appear even more natural.
Above all ask the riders if the flashes bother them, when I rode I never noticed when someone did use flash and anyone I ask says they don't noticed it, but please respect the riders.

Jack