In my opinion, you don't need that much clearance over a road bike. With your cycling shoes and bike shorts on, can you safely and comfortably straddle the 51? If so, you're good to go. I don't have much room to spare on my road bike and it has never been an issue.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher