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Thread: Crank Questions

  1. #1
    No Money and No Sense sillygolem's Avatar
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    Crank Questions

    What's the difference between a mountain bike crank and a road/touring crank? Is there any reason one wouldn't work in place of the other, provided the BB was the right length?

    What makes a crank suitable for single, double, or triple chainrings? I've seen some setups with a couple places to bolt chainrings with different BCD, and others that have the chainrings bolt to each other with only the largest bolted to the crank.

    Will a double chainring work with a triple derailleur?

    Can oval chainrings (Biopace et al.) be mixed with round ones?

    Is there an online database that lists BB sizes for old cranks?

    Is there anything particularly better about Octalink or ISIS over square taper cranks, aside from using a lighter hollow shaft?

    What's the big deal about outboard bearings?
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    I ride fixed because I'm mad at my parents. **** you Mom!

  2. #2
    coprolite fietsbob's Avatar
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    MTB now are quite different, the chainring sizes are much smaller tooth counts.
    Owing to the differences of the terrain and speeds involved.
    common race XC Mountain , 22.32,44 Road 30,& 42, or 39,, & 52, or 53t

    Current top end stuff is different than the low price stuff , so interchangeability is often an issue.

  3. #3
    Bicycle Repair Man !!! Sixty Fiver's Avatar
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    There used to be almost no difference between touring and mtb cranks as early mtb's used touring parts and before cassette drives this worked well with freewheels that would have been 13-x while modern mtb cranks are more compact because they can be run with 11-x cassettes and get the same gear range.

    An mtb crank with a 24/34/44 mated to a cassette with an 11-30 makes for a wonderful touring set up as you can run a closer spaced rear block and still have decent low gearing and more than enough up top.

    Road cranks often use larger bolt circles and will have a big ring of 52 or 53 and newer compact doubles run a lower small chainring to compliment 9 and 10 speed cassettes and offer sufficient climbing gears.

    With triples the outer two chain rings are bolted together and the inner ring has a smaller bolt circle to allow for a smaller chain ring... a compact triple will allow for a 20 tooth chain ring and would be used for some rather extreme climbing while most will be limited to 24 or 26 teeth.

    Biopace rings can be mixed with round rings... I prefer to use one or the other although a mismatched ring can be used in a pinch.

    The square taper bottom bracket was invented by Stronglight and used for many decades and is still a viable system... newer bottom brackets have tried to reduce weight and improve on the bearing quality of the old 3 piece that used cups, 1/4 inch loose balls and races and improve on the interface by making them splined.

    Problem with cartridge bottom brackets (most of them) is that they are not as smooth as the old style cup and cone assemblies and despite being virtually maintainence free their performance was lacking... an external bottom bracket addresses this by offering much better bearing support and materials that make for a very stiff and light bottom bracket.

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    If you switch from road to MTB you may need to alter the width of your BB spindle, ie a new BB cartridge.
    Octalink and Isis use wider, stiffer spindes so have to use smaller bearings that wear out more rapidly. I dont notice any improvement using a stiffer spindle.
    The current external BB units offer stiffer, lighter spindles, bigger bearings and ease of fitting BUT they are still trying to squeeze lightweight components into a space made for solid steel. At some point manufacturers will change to a new BB standard such as BB30 or similar (hopefully there will be lots of different standards to chose from). Till then the best options are square taper for durablity or external for lightness and performance.

    MTB chainrings are smaller so work best with a matched front mech with a tighter curve, BUT road gear levers from Shimano cannot index MTB derailleurs. The solutions are to use:
    - Tiagra triple mechs with a short tail. Beware that some versions of the triple mech interfere with the chainstays so cannot be lowered sufficiently.
    - Bar end levers that are non indexed.

    Campagnolo make shifters that micro-index at the front so work with any mech BUT the rear shifters do not play well with anything but Campy of correct "speed".

    If this sounds complicated, its because Mr Shimano doesn't tour.

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    Quote Originally Posted by MichaelW View Post
    If this sounds complicated, its because Mr Shimano doesn't tour.
    The LX groupset is now a touring groupset rather than MTB, and XT is also available with larger chainrings for touring

    For some of your questions

    Will a double chainring work with a triple derailleur? - no, you need a triple RD

    Can oval chainrings (Biopace et al.) be mixed with round ones? - Yes, but you can't get biopace anymore, it's been obsolete for 15+ years

    Is there an online database that lists BB sizes for old cranks? - no you will need to research this yourself, look at places like bikepedia

    Is there anything particularly better about Octalink or ISIS over square taper cranks, aside from using a lighter hollow shaft? - ISIS had issues with bearing not lasting, nothing wrong with Octalink, but both systems are obsolete, you can still get BB's, but no new cranks

    What's the big deal about outboard bearings? - cynically, a new systems to make money from; it is stiffer, probably cheaper to manufacture and install in a factory, very easy to maintain, your old crank won't work with it. this was been discussed before, and will be again, if buying new would go for this, if not, square taper still works well

  6. #6
    No Money and No Sense sillygolem's Avatar
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    Quote Originally Posted by Sixty Fiver View Post
    An mtb crank with a 24/34/44 mated to a cassette with an 11-30 makes for a wonderful touring set up as you can run a closer spaced rear block and still have decent low gearing and more than enough up top.

    Road cranks often use larger bolt circles and will have a big ring of 52 or 53 and newer compact doubles run a lower small chainring to compliment 9 and 10 speed cassettes and offer sufficient climbing gears.

    Biopace rings can be mixed with round rings... I prefer to use one or the other although a mismatched ring can be used in a pinch.

    The square taper bottom bracket was invented by Stronglight and used for many decades and is still a viable system... newer bottom brackets have tried to reduce weight and improve on the bearing quality of the old 3 piece that used cups, 1/4 inch loose balls and races and improve on the interface by making them splined.

    Problem with cartridge bottom brackets (most of them) is that they are not as smooth as the old style cup and cone assemblies and despite being virtually maintainence free their performance was lacking... an external bottom bracket addresses this by offering much better bearing support and materials that make for a very stiff and light bottom bracket.
    This is going on a hybrid with a 3 x 6 setup. It has a 28/38/48 front crank with a 14-28 freewheel; this has so much overlap I could skip the middle chainring entirely. The current crank has had the drive-side pedal threading stripped out of it, so this is as good of time as any to switch cranks. I'm aiming to eventually turn this into a touring rig. My aim is to have relatively even gearing with a range between 25-30 and 80-90 gear inches.

    From what I understand, the major benefit knee-wise with Biopace and the like is with low gearing. Small rings are relatively cheap, and if I like it I may opt for a full set of NOS chainrings. If I do this, is it going to be disconcerting to switch between oval and round while riding?

    It has an early Tourney SIS derailleur on it now, and the only likely replacement would be a Tourney Megarange if I found a gear combination that would make the 34t low worthwhile. Both are triple RDs. The highest gear I can get on the back is 14t, or 13t if I squeeze a 7 speed freewheel onto the rear.

    It has a cup and cone BB right now, and it's in good shape. Is there any reason I should opt for a cartridge BB? Can I simply replace the shaft to match whatever crank I get, provided it's square taper?
    Quote Originally Posted by Scrodzilla View Post
    I ride fixed because I'm mad at my parents. **** you Mom!

  7. #7
    we be rollin' hybridbkrdr's Avatar
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    From what I understand, road cranksets place your feet closer to the frame while mountain bike cranksets place your feet farther away. I suppose this may help if you want to put one foot on the ground while biking off-road. But, I read riding long distances can be more comfortable with road cranksets.

    Also, if you try installing a road crankset with a 52T chairing on a mountain bike, it may hit the frame. And because the cranks are closer on a road crankset, they may also hit the frame. Anyway, that's from the stuff I read even though I haven't actually seen that or tried it.

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    Comments in RED:
    Quote Originally Posted by sillygolem View Post
    This is going on a hybrid with a 3 x 6 setup. It has a 28/38/48 front crank with a 14-28 freewheel; this has so much overlap I could skip the middle chainring entirely. The current crank has had the drive-side pedal threading stripped out of it, so this is as good of time as any to switch cranks. I'm aiming to eventually turn this into a touring rig. My aim is to have relatively even gearing with a range between 25-30 and 80-90 gear inches.

    Triple chainring setups give lots of overlap and there is no escaping it. You could try to set it up with a 28-48 double chainring (replace the middle ring with the large ring), but it might be difficult to get your front derailleur to handle such a giant jump in teeth... but then again, it might not be difficult. THe old-school touring setup is to have '1/2 step + granny,' which is a normal small chainring ~28 tooth or so, then two larger rings that are close in size.


    From what I understand, the major benefit knee-wise with Biopace and the like is with low gearing. Small rings are relatively cheap, and if I like it I may opt for a full set of NOS chainrings. If I do this, is it going to be disconcerting to switch between oval and round while riding?

    I could never feel much of a difference between round an oval rings. I think the benefits, even under ideal conditions, didn't actually do much. Oval rings are once again being used by racers, ironically being used completely differently than biopace were meant to be used.. anyhoo, if you are going this route it will likely not bother you at all.

    It has an early Tourney SIS derailleur on it now, and the only likely replacement would be a Tourney Megarange if I found a gear combination that would make the 34t low worthwhile. Both are triple RDs. The highest gear I can get on the back is 14t, or 13t if I squeeze a 7 speed freewheel onto the rear.

    Any Shimano rear derailleur designed for indexed shifting will work fine - with the exception of the 10 speed derailleurs, and a few old obsolete styles.

    It has a cup and cone BB right now, and it's in good shape. Is there any reason I should opt for a cartridge BB? Can I simply replace the shaft to match whatever crank I get, provided it's square taper?

    If you buy a new crank you will likely have to purchase a new BB to get correct spacing and chainline. New spindles for cup-and-cone BBs have not been commonly available for some time now, so you will likely wind up with a cartridge BB by default. I for one think good quality cartridge BBs are smoother than cup and cone... I think SixtyFiver has mistaken his own crustiness for bearing roughness . Cartridges are also zero maintenance until the time comes to replace - usually years and years for square taper models - and then replacement is a ten minute job and you pitch the old one into the recycling bin.

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