Safely Switch Bicycles before a Century? I did an unsupported century last year on my 30 pound "shop" mountain bike. The first 55 miles were easy but the last 45 or so were pretty horrible. This year, I want to try it again but with _more_ hills. I really don't want to lug my 30 pound mountain bike up the hills, but I don't have the money for a decent road bike. If I follow a reasonable training plan on my 30 pound mountain bike this year, will I be "safe" moving into high-dollar 15 pound _rental_ road bike 24 hours before an unsupported century? **WARNING: This question was edited from its original version. It originally used the term "homemade" century to denote an unsupported century. Also, the question _originally_ asked if it would be okay to switch to a road bike the _day of_ the mentioned century. Now, the question asks if it is safe to switch 24 hours before the mentioned century.**

I've done pretty much exactly what you describe: training on a (really terrible old beat up MTB) and then using a rented road bike (totally different configuration, weight, etc.) for a century (RideLondon-Surrey 100) with no problems. I can't see how I wold have survived riding that distance (on those hills!) in my MTB. So I strongly recommend switching!

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The answers so far sound like good general advice, but it's not clear that any of those answering have actually tried what they are advising against.