On a track frame it usually a matter of preference but on a road conversion it can be a matter of safety. This is because the BB height is higher on a track frame than a road frame. This is not to say that longer cranks won't work for you, you just need to watch out when leaning into a turn. I have 170's on a road conversion and have only scraped a pedal once and luckily did not go down (scared the #$%$ out of me though). Hope this helps