1

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). Apr 24, 2012 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, 2012 at 11:00
  • I just put a road bike setup on the chainring with my moutain bike cassette. I'll try it soon. I'm very new to bikes. The bike shop daud it turned out great...
    – JMarks
    Apr 11, 2022 at 2:55
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    95% of "I want my MTB to go faster" questions are answered correctly by "learn to pedal faster". Unless you spinning out at a cadence of over 100, bigger gears are not going to help much. MTBs have settled around the 42-44 max chainring because very few riders are capable of pushing fat tires and straight bars fast enough to need bigger.
    – mattnz
    Apr 15, 2022 at 11:20
  • If you spin out at 100rpm, you need to work on your technique. There are a lot of tests where maximum power happens around 120rpm.
    – ojs
    Apr 15, 2022 at 19:27

3 Answers 3

3

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, 2012 at 6:35
  • Also, please review my update, My intention is to create a super fast MTB, where possible
    – Starx
    Apr 24, 2012 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, 2012 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, 2012 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, 2012 at 6:10
3

In addition to the points about whether a road crankset will fit an MTB—which are important points—consider this: you may not be strong enough to take advantage of the higher gearing. If you've got a bike with a 53x11 top gear, and you're never in a gear higher than 53x17, all you've done is limit your usable gear range. Be realistic about how high a gear you can really use.

If your goal is to use an MTB on the road, where maybe you could use those higher gears (53x11 is a very tall gear even on the road), then maybe what you want is…not an MTB.

1
  • I use my 11 tooth cog perhaps once a month while commuting. Very rarely even that's not enough, when its blowing a decent tailwind and I feel I can go faster. I'm happy to carry about a small cog that is not used often, but the larger dinner plate cogs not-so-much.
    – Criggie
    Apr 15, 2022 at 6:12
0

The chainstay is designed to accommodate only certain sized chainrings. I've seen this on some road bikes which were converted to tri bikes, who use 55-56 tooth rings.

Even then, I bet most mountain cranksets would not accept a chainring larger than 46 (this opens the BCD fitment issue). Then if you were to switch to a road crankset, you'd have to make sure it was compatible with your mountain BB shell. Many road bikes are still using 68mm width BB's while mountain are up to 73 (and probably changing too!)

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    Are you sure triathlon bikes use larger chainrings? I was under the impression that compact (smaller) cranksets came into existence to meet demand from the triathlon community
    – Paul H
    Apr 15, 2022 at 21:56

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