Is this a bad idea - Mountain Biking??

I have been told that before a big race (running or biking, or even swimming) never introduce anything new. No new ANYTHING...no new shoes, clothing, etc. You need to break things in first and ensure it doesn't affect your performance or give you any chafing/blisters, make you uncomfortable, etc etc....

no clips man , you are looking for problems and or injury .. do yourself a favor and enjoy the race instead have fun

I was skeptical of how great they supposedly were at first, but they do make a world of difference, especially on the MTB - when you hit a rock, you stay clipped in and your feet don't go flailing. You're way more stable this way and you have much more power.

If you're not accustomed to riding clipped in adjust them to whatever the maximum float is, my PT suspects that changing pedals and cleats last year shortly before a metric century was a contributing factor to my IT band problems. It takes your body a while to adapt but your mechanical efficiency takes a big jump.

I am mainly a road rider, and the first week I got SPDs I fell off twice. Luckily I didn't *really* hurt myself, but mtb is a different matter.

***snicker*** been there, done that. It's mostly your dignity that is injured when you forget to unclip.

I recently switched from platform pedals to the shimano spd cliples pedals, and I got stuck in them and fell the first time i got on the bike. I realized that I needed to adjust the spring to be all the way loose. Once I did that, I had no problems at all with them. I can clip in and out really easily, and they make a huge difference - I can't imagine riding without them now.

I love mine, too. One bit of advice I got from my local bike shop was really simple and really helpful.

1. Find something you can hold yourself upright on the bike with - pole, post, fence, whatever. Make sure it's solid and you can get a grip on it.
2. Hop up on the bike, mash a brake, put your feet loosely on the pedals while holding on to the post, and clip one foot in (the one on the same side as the post - that way if you tip away from the post you can catch yourself with the loose foot).
3. Clip in and twist out at least ten times, so you start developing the muscle memory on that side. If it's hard to do, get off and loosen the tension.
4. Turn the bike around and repeat the process for the other foot.
5. When you're comfy, hold the post and clip both feet in and practice releasing them.

Now you've learned clipping and unclipping under circumstances that are actually harder to do than when you are riding, but less likely to hurt you. Don't be afraid to do this for a WHILE while practicing. The process should feel natural and instinctive to you - you should not have to think about it - it's just the way a foot comes off a pedal. Muscle memory is important.

That way, when things get ugly and you have to get your foot off the pedal, your instinctive mind can handle that little detail while you manage the bigger details like not crashing.

for me, mountain biking itself would be a bad idea (tried once, and ended up with a bruise covering more than half my thigh - guess i was lucky not to have broken anything).
anyhow, the idea of being attached to the pedals seems a bit scary, and i wouldn't even go on a 15k ride with them until i'd have gotten used to them properly. but i am clumsy, and the bike is so NOT my sport - I am still waiting for the day i manage to injure myself on a stationery bike.