What Seat Height for Racing XC?

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I was just wondering what seat height some of you use when racing XC. I know that you want your seat high enough so that your legs are fully extended. I ride like this when training on the road. However, on trails a higher seat definately slows me down. I am much faster with a low seat, as I am sure most of you are. I usually lower my seat a bit when racing, but am I losing too much energy? Also, what about tire pressure when racing. Most of this is probably relative, but looking for some info.

Also, how do most of you train, and what kind of training program would make one competetive in sport? I've already proven myself in begginner, but I'm looking to move up. thanks for your time!

I was just wondering what seat height some of you use when racing XC. I know that you want your seat high enough so that your legs are fully extended. I ride like this when training on the road. However, on trails a higher seat definately slows me down. I am much faster with a low seat, as I am sure most of you are. I usually lower my seat a bit when racing, but am I losing too much energy? Also, what about tire pressure when racing. Most of this is probably relative, but looking for some info.

Also, how do most of you train, and what kind of training program would make one competetive in sport? I've already proven myself in begginner, but I'm looking to move up. thanks for your time!
I see this thread has been here unanswered for awhile, but it's a good question and worth replying to.
I use the same seat height between my road bike and XC bike, for both racing and training. I have found a significant difference in power between using a optimal seat height and dropping it a few inches, and very little difference in descending once I got used to the high seat position. If you ride all the time with the seat up high you will get used to it. I find it helps to grip the seat with my thighs while descending to help keep the bike stable. Also, when standing with your pedals level you will get 170+ mm of clearance between your butt and the seat, giving you plenty of room to get behind the saddle, move around, and use your legs as shock absorbers. Try looking at some photos and videos of pro XC racers to see how they get behind the saddle. The only reason I see for dropping the saddle is if you want to jump or do big drops (3 ft+), which are really not well suited for XC bikes anyway.
You do not want your legs completly straight at the bottom of the pedal stroke. The method I have has luck with is to adjust seat height so your heel is touching the pedal at the bottom of the stroke with your leg fully extended. When you clip-in this will give you a slight bend in the knee of about 15 degrees at the bottom of the stroke.