The current National CX Champion Tim Johnson almost always pushes his bike on run-ups, Mark McCormack almost always carries it. Most racers will use both ways depending on the nature of the run-up section. See a variety of both techniques used by Elite Men and Elite Women on a short but extremely steep hill in 2004 CSI CX race in Northampton, MA:
At Granogue last year Powers/Wicks were riding the steep run up section of the course. Timmy shouldered the bike and pretty much killed it. They may gained ground on the initial up, but at the top, he was back in the bike and attacking, the riders didn't have the same pep. Generally I would say ride what you can, but in this case, Timmy was the most effiecent shouldering. (Timmy ended up winning the race I believe).
Gewilli's assement is pretty on. I think folks find what works best for them, and in the heat of battle that's what comes naturally.
a few other variables that can help determine the best choice can be the type of surface. i find when it's thick, goopy mud, you're so much better off picking it up to avoid accumulating all that crap on your bike.
but yet, when it's supper greasy, slick mud that doesn't stick to the bike, i've pushed the bike if only because it helped me avoid falling on my face and looking even more foolish than normal.
interesting that timmy was the runner in that granogue race...he doesn't really even run during training for cx and has great handling skills so could have probably ridden the climb if he wanted.
as most of you have mentioned, it all depends on the terrain for me.
when i push my bike, i sort of lean on it (weird...i know) to take some of the weight off my legs and give them a little relief.
i actually think i run faster while pushing the bike vs. shouldering it. but that's me.
i should experiment 1 day...
there are 2 times when i definitely don't push my bike:
1) when it takes me any type of extra effort to push the bike on the terrain
- my rule of thumb is if i'm running with the bike at my side and the bike is falling behind me, it's time to pick it up
- instances where this would happen would be heavy/sticky mud, steep run-ups, sand, etc.
2) when i want to block folks from coming around me
- bottlenecked sections that are easily rideable, but someone dabbed and everyone is off their bike. instead of pushing, i carry my bike to prevent the hoards from sneaking around the sides.
for instance, there was 1 race where the barriers were spaced about 40 or so feet apart. it wasn't really worth hopping back on the bike (since it was off-camber and rutted) and the surface was hardpacked dirt. so, the 1st lap, i shouldered the bike since there was a bottleneck and i didn't want the over anxious folks behind me passing all over the place. every lap after that, I pushed the bike since there was no extra effort required to roll the bike on the dirt. then, if someone wanted to pass me since the race settled in, i'd give them plenty of room to get by.
Personally, I like to toss my bike up the hill and then run up after it. I find it much easier to run without my bike. As a bonus, sometimes my bike will take out one or two of the guys I'm chasing, instantly improving my race position.
Seems like steeper, more uneven terrain is faster to shoulder it. Several of our courses have had short, not so steep run ups after barriers and it was easier to let the bike carry itself, but I think once the angle goes up and/or the bike becomes harder to push (up a hill, through sand, over bumpy terrain) it makes a lot more sense to shoulder it.