I want to triple a bike for my wife. I have noticed that some road bikes use mountain components, and I wonder why.

As an example, my wife's current bike, a 91 Sirrus has Deore DX cranks and derailleurs. I have seen bikes from the same late 80's early 90's era use 105 and RSX triple components.

I am curious as to why one would pick one or the other--- can you tell me why? Are there functional differences? And are they generally cross compatible? Can you use a mountain cranks with road derailleurs?

Actually, I know there is at least one difference. The mountain cranks use 110 bcd chainrings. The road cranks use 130 bcd. I guess that could make a difference as to which chainrings are available.

For my wife's new bike I have the option of using the Deore DX from her old bike. That was the original plan when I thought I was going to be buying a bare frame. But I may not want to dismantle that bike now, as I may only need cranks and derailleurs to make this new bike a triple.

So If I want to go in search of triple cranks and derailleurs, should I look for road or mountain?

Jared