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MTB's cranks usually have a 42 or 44T where as Road bikes range up to 53T.

So far I am under a impression that more tooth, means more speed. Same goes for cassettes, lesser the tooth more the speed.

But, nothing is as it seems, I want to know what are bad things I expect when I take this move? Is it that bad, that it would be simply better to forget about it and move on?

My main intention is to create a super fast MTB, where possible.

Please Note: I am not a expert on crank & cassette

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The higher the gear ratio, the more speed you have. It's the combo between front and rear sprocket -- the larger the tooth ratio between the two, the faster (for a given cadence and wheel diameter). –  Daniel R Hicks Apr 24 '12 at 10:57
    
@DanielRHicks, I am certain... I was trying to mean that, but didn't know the how to put them into sentence like yours :P –  Starx Apr 24 '12 at 11:00

1 Answer 1

There are several things to consider:

  1. Chain ring clearance to the frame, because a larger tooth count has a larger diameter. Since the chain stay is at an angle, and designed for a certain diameter, too large a ring will come too close to the frame. Anything closer than 5 mm, is not a good idea, since a larger chain ring will also flex more.
  2. Maximum tooth count for front derailleur. A front derailleur is designed to work and shift well with a certain size of ring. There is usually a small range above that which can be used. (i.e. Shimano's 3x XT FD-M785 is designed for a 44t, but can accept up to 48t.)
  3. Cable pull ratio for front shifter/Chainring spacing

There are special shifters designed to allow a road bike drive train to run with a flat bar brake and shifter. But there is no easy and guaranteed way to run a road crank set with a mountain bike drive train, unless you replace all of the drive train parts, and buy road touring flat bar shifters.

It is usually more cost effective to buy a road bike.

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Can you explain the point no 1? –  Starx Apr 24 '12 at 6:35
    
Also, please review my update, My intention is to create a super fast MTB, where possible –  Starx Apr 24 '12 at 6:42
    
Answer updated. Where possible will be key. Are you riding it off road? It doesn't appear so from the questions you ask. –  zenbike Apr 24 '12 at 9:04
    
Not neccessarily off road, but whenever I approach a smoother terrain, where I could pump for extra speed. I want the extra speed on these cases. –  Starx Apr 24 '12 at 9:09
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If you must have a moutain bike frame, then Point 1 is the most relivent. You may be able to change the bottom bracket if the frame interfares with the larger chainring, however that will upset the chain alignment to the cluster - defeating the purpose of the exercise. Remember that 53 Road chainrings typically were used with 12 (smallest) on the rear, so a 48/11 combo is, to all practical purposes the same. As far as "more tooth, more speed" (on the chainring) - that assumes you have the brute strength to push it and that you want to regardless of issues like damage to your knees. –  mattnz Apr 25 '12 at 6:10

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