Two pulley chain tensioner?- Mtbr.com
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  1. #1
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    Two pulley chain tensioner?

    Singlespeeders, I come in peace.

    Currently I'm running no front derailleur and an 8 speed rear on my commuter, but I have two chainrings up front (road double crankset)...I keep it in the big ring 99% of the time, but I can drop it down manually if the need arises.

    I'm considering switching my rear casette to a singlespeed and ditching the derailleur/shifter, but I want to keep the two rings up front...so I'll basically have a manually shifted 2 speed.

    The problem with this is obviously chain tension. I need a tensioner that will allow for the difference between a 50 tooth and a 32 tooth front sprocket, but I'm thinking that I wouldn't get good engagement with the rear cog if I used a normal tensioner, because it would have to swing so far. I'm afraid the chain wouldn't contact much of the rear sprocket when in the lower gear up front...the 'wrap around' factor of the rear sprocket is my concern.

    ...which brings me to my question: Is there such an animal as a two-pully tensioner, like a derailleur with no side-swing? If so, where can I get me one?

    I know I could use a derailleur and just clamp way down on the limit screws, but I would like something cleaner/lighter if it exists.

    Thanks...
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  2. #2
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    Paul Components makes one.

    Here

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  4. #4
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    Ouch. Those both seem ideal, but that's pricy! Exactly what I'm looking for though, thanks.
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  5. #5
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    Why not run 2 gears in the back and 2 rings in the front, where the chainlength is the same, you just ditch the tensioner. It would be hard to make this work with such a big difference between front rings but if you change it up you could make it work something like this. To run 2 rings in the front and a cog on you'd have to run a huge chain in order to make it around the big ring. Longer chain means less efficiency.

    An example:
    36T ring to an 18T cog - 2.0 ratio, 108 link chain
    40T ring to a 14T cog - 2.87 ratio, 108 link chain.

    Then you could manually shift gears. Do you have horizontal dropouts where you adjust for slight changes in the rear wheel position?

  6. #6
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    I've run the sort of setup that GTscoob is suggesting and it works well. If you're not worried about the manual gear change, you may as well get rid of the tensioner too... I used to run a 36-16 and a 32-20 double setup on my 29er MTB. Same chain length, no tensioner, and it is simple. I really like plain ol' SS, but as mechanic for the US National MTB Devo team in Europe I needed a road gear to keep up with the fast geared kids on the way to the trails... hence my double.

    If you do want a tensioner, you should check out the Shimano Alfine tensioner. Jamis specs this tensioner on a bike of theirs with an internally-geared rear hub and two front rings.
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  7. #7
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    Good call Joey, the Alfine turns out to be much cheaper. Be aware that it is has a max capacity of 16t difference between rings; normally enough but not if you're using that 50t big ring

  8. #8
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    Quote Originally Posted by CommuterBoy
    Ouch. Those both seem ideal, but that's pricy! Exactly what I'm looking for though, thanks.
    y'know, you can just use your existing derailleur. Cut a piece of shifter cable that has the "button" on it (the metal stopper on the end that goes into the shift lever), then thread that through your derailleur so that the "button" or stopper is in the barrel adjuster, pull the derailluer into position over your cog, and fasten the free end of the cable to the derailleur, then use the barrel adjuster to fine tune the derailleur position.

  9. #9
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    Commuter boy, your avatar tears me up every time I see it!

  10. #10
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    All good ideas and options...thanks. Is there a chart somewhere that will allow me to compare gear ratios? Honestly I'm pretty ignorant about how much difference a tooth makes...

    A couple notes on my setup: this is my rural, two lane highway/dirt road commuter bike. When I pretend it's a singlespeed, I'm in the 50 tooth up front/16 tooth out back. This is the highest gear that still allows me to make the 1 mile climb at the end of my commute home. I'd like to ditch the added weight/winter maintenance of all those gears and the rear derailleur.

    That Alfine tensioner looks perfect. Does "16 tooth chainwrap capacity" mean that I can have sprockets up front that are 16 teeth apart? Like 50/34? Now we're getting close...

    I have vertical dropouts...but if I did two cogs up front and two out back, If I had to I could still use a tensioner to make up for a little difference in chain length, right? (as long as it allowed for enough play in the chainline?)

    I want to do this on the cheap....so buying a bunch of chainrings and two rear cogs makes it seem like I might as well just buy that $70 Paul's tensioner.

    Good idea on using the cable stop to make the derailleur adjustable. That's the cheapest/easiest option of all...wouldn't even have to mess with the limit screws... but I'd just love to get rid of that thing in favor of something as simple as that alfine...

    And as long as I'm asking questions... am I going to destroy my shimano cassette compatible hub by putting all of that stress on the skinny little part occupied by one little chainring? Are there singlespeed conversion kits that spread that load out over a wider area of the hub?

    Thanks folks. You arent as bad as they say...
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  11. #11
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    Quote Originally Posted by CommuterBoy
    All good ideas and options...thanks. Is there a chart somewhere that will allow me to compare gear ratios? Honestly I'm pretty ignorant about how much difference a tooth makes...

    A couple notes on my setup: this is my rural, two lane highway/dirt road commuter bike. When I pretend it's a singlespeed, I'm in the 50 tooth up front/16 tooth out back. This is the highest gear that still allows me to make the 1 mile climb at the end of my commute home. I'd like to ditch the added weight/winter maintenance of all those gears and the rear derailleur.

    That Alfine tensioner looks perfect. Does "16 tooth chainwrap capacity" mean that I can have sprockets up front that are 16 teeth apart? Like 50/34? Now we're getting close...

    I have vertical dropouts...but if I did two cogs up front and two out back, If I had to I could still use a tensioner to make up for a little difference in chain length, right? (as long as it allowed for enough play in the chainline?)

    I want to do this on the cheap....so buying a bunch of chainrings and two rear cogs makes it seem like I might as well just buy that $70 Paul's tensioner.

    Good idea on using the cable stop to make the derailleur adjustable. That's the cheapest/easiest option of all...wouldn't even have to mess with the limit screws... but I'd just love to get rid of that thing in favor of something as simple as that alfine...

    And as long as I'm asking questions... am I going to destroy my shimano cassette compatible hub by putting all of that stress on the skinny little part occupied by one little chainring? Are there singlespeed conversion kits that spread that load out over a wider area of the hub?

    Thanks folks. You arent as bad as they say...
    Sheldon Brown's Gear Calculator will let you calculate all kinds of gear ratios.

    I believe you are correct on the "chainwrap capacity"

    What hub do you have? If it has a steel freehub body you might not have any problems with that cog. If the freehub body is aluminum you will need something with a wider base, such as this Surly SS cog.

  12. #12
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    It's a shimano Deore hub...I thought it was aluminum, but I'll have to check.

    That gear calculator helps, thanks... I could make a couple changes and get the same ratio...but then we're back to buying chainrings.... I wonder if that alfine tensioner could handle just two more little teeth of difference to accomodate my 50/32 road double...
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  13. #13
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    Quote Originally Posted by CommuterBoy
    It's a shimano Deore hub...I thought it was aluminum, but I'll have to check.

    That gear calculator helps, thanks... I could make a couple changes and get the same ratio...but then we're back to buying chainrings.... I wonder if that alfine tensioner could handle just two more little teeth of difference to accomodate my 50/32 road double...
    Deore is steel. The only Shimano hub that isn't steel is the XTR (titanium)

  14. #14
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    Sweet. Still makes me a little nervous, but better than aluminum...
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  15. #15
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    Quote Originally Posted by CommuterBoy
    Sweet. Still makes me a little nervous, but better than aluminum...
    If its anything like these stamped SS cogs it shouldn't be a problem. In fact you could even buy those instead of a more expensive Surly if you want a cog with taller teeth. I've used them a lot for SS setups on Shimano and other steel freehubs

  16. #16
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    ^^ perfect. That's like what comes with the kits I'm seeing with the spacers, right?

    I just realized I think I have a 34 tooth sprocket in my shop that will fit my FSA crankset on this bike... that would put me in the 'acceptable range' of that alfine tensioner. I might have to jump on that.

    The only reason to avoid two rear cogs is that I can shift into the smaller ring in the front 'on the fly' when I get to the bottom of a steep hill...just a little push with the toe and the chain will drop down. If I had two rear cogs I'd have to stop to shift.
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  17. #17
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    Yep, same cogs

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