Here is a common question about using long cranks on a bike that has a standard height bottom bracket:
CUSTOMER QUESTION: Lennard; I’ve taken my Medici to the bike shop to have the cranks replaced. I believe the bottom bracket is 10 1/2 inches from the deck. I believe the old cranks that come off of it were 175 mm and are still in the shop. I’d consider getting longer ones. What length would you say are “safe” with that bracket height?
Well, the conventional wisdom for a racing bike is that you not put longer than 175mm on that bike.
The question of “safe” is an individual one dependent on how you will ride the bike.
I would strongly advise against using cranks any longer than 200mm with that bike.
I’ve ridden bikes with a 267mm BB height like that Medici has with 200mm cranks for years without problems, and we have plenty of customers doing the same thing. But given that the bike we built for you has a higher BB to clear for the long cranks, you would have to ride the Medici differently than you ride the Zinn, were you to put longer than 175mm cranks on the Medici.
My own personal feeling is that, if you’re not racing, there is no reason to pedal as far into a corner or to start pedaling so soon out of it that you would hit with a crank as long as 200mm. But you have to take the reduced pedaling clearance into account and make sure you don’t have your pedal down in corners.
On the other hand, at your age, and, particularly given your recent broken hip, you don’t want to risk falling off of your bike, so I’d recommend being very careful about what you choose. If there are places you ride, like off-camber straight sections and wide corners, where you think there’s a risk of pedaling when the bike is leaned relative to the road, I’d recommend you stick with 175mm or maybe 180mm.