netscriber said:
I know Kirby Palm has put a bit work into that site (the 2nd link) -- and I'm not shooting the messenger
-- but I really think it's rubbish: 21.6% of inseam is way too long, especially for riders over ~5ft8". I'm 6ft, and I tried 180mm cranks ,which ended up being too long, but Kirby Palm's formula would have me on 193mm!! Every time I type that, I realize more and more how TOTALLY rediculous it is. ANd he's NO athlete!!
http://www.nettally.com/palmk/kirby2.jpg
The first link, and the traditional advice is better. I tried all the long crank stuff for nearly 2 years, and it didn't work out.
to answer the question, going up a length by 5mm or more
should be a concern. Too long a crank can stuff up your position and/or your knees.
The easiest way to sort it out is to measure you're inseam, and it's not 'pants' inseam.
here's another traditional guide.
http://www.bikefitting.com/English/Theory/Crank.aspx
this chart is traditional, but a little less conservative than others
inseam ..........Crank Length
60 to 65cm..............150mm
66 to 70cm .............155mm
72 to 75cm .............160mm
75 to 78cm .............162.5mm
79 to 81cm .............165mm
82 to 83cm .............167.5mm
83 to 86cm ..............170mm to 172.5mm
87 to 90cm ..............175mm to 177.5mm
91 to 94cm ..............180mm
94cm plus ................185mm