How long are your legs? How tall are you? What do you prefer?
It's a minefield Adam.
If you google it then there are all kinds of calculations you can do and whether any of them are any good, who knows? They'd probably tell you to get 173.8 cranks
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If you are average height (5'10" ish) then I think you'd normally use 172.5 cranks. That doesn't take in to account your leg length though as you may have the body of someone 5'5" and the legs of someone 6'2"!
After you get round that it then can depend on whether you are a spinner or a grinder, shorter cranks for a spinner. Ullrich always rode big gears and despite not being that tall used long cranks, very long for TTs.
As it's off season, and the race bike will be tucked up for winter, you could try the 172.5s on your training bike and see how you find it. Is there a chance it could help souplesse (sp?) with more spinning? Bike set up could become a nightmare though as you won't just be able to transfer the measurements from one to the other.
Being short has it's advantages, you just stick 170s on anything
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Money on birds and booze is never wasted money Huw.