I'm ordering my first cross bike soon and can choose the length of crank. I'm 5'7 with a relatively short inseam and longer torso. I'm thinking a shorter would be the way to go (170 mm) as the bike has a lower bottom bracket, but wanted to get some input from those who have raced cross before. My current road bike uses a 172.5 mm.

I would stick with the size you have on your road bike. Bottom line, it probably doesn't matter but I think I can feel the difference when change crank arm length. I have an 8cm BB drop on my cross bike and very rarely do I have a problems but I do notice the better handling all the time.

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2 Serotta's, 1 Spectrum, 1 Van Dessel, 1 Parlee & 1 Carl Strong, & 1 Titus.

My road and cross bikes have the same length.

Go shorter.

cx races are all about acceleration, short pedal length is a bad choice

Exactly my point, since a shorter crank will accelerate faster it's worth a try. I'd say it may even be an advantage on some courses where ther is a lot of sharp corners.

In my understanding this is totally wrong

Just keep it the same. Honestly, a mm or two isn't going to make any difference. If you really want it different, go slightly longer.

A really important part of cross a lot of prople ignore is torque. When conditions are crap and traction is low it is at times better to lower the cadanece and hopefully increase the torque to achieve better grip and power application to the tirers. A shorter crank would not be as benificial for this. Stay the same or longer if you want to try it