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PostPosted: Sun Feb 26, 2012 10:26 am 
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I have a SS screw-on freewheel type hub (DMR revolver) which gives me no chainline adjustment, so I need to make sure I get the correct length BB to give me a straight chainline. The cranks I have are ISIS (Raceface Prodigy) and I don't have any old BB's to try. The chainring will go in the outer position on the crankarm. So is there a simple way to ensure I buy the right length BB?


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PostPosted: Sun Feb 26, 2012 12:08 pm 
Gold Trader / rb Rider / Special
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Location: Rurally close.
Lewis1641 recently had similar trouble i think and was told you can space the freewheel somewhat.

Like you say though you will put it in the outer position so that means you could also use the inside.
Im tempted to stay start with a 113 but it can be hit and miss and if you did want to run the ring on outer you may need just a bit shorter than what the 'normal' one would be.

Do you have a previous bb from the frame as a starting reference?

What is the frame?


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PostPosted: Sun Feb 26, 2012 5:29 pm 
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The chainring i have only fits in the outer position. I was hoping someone would say that all screw-on freewheel type hubs line up with the outer ring if you run a chainline of say 50mm or something. That way all I'd have to do is find out what length BB gave that chainline for my crank. :?


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PostPosted: Sun Feb 26, 2012 7:04 pm 
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cyfa2809 wrote:
Do you have a previous bb from the frame as a starting reference?

What is the frame?


No old BB and it's a GT Peace (26).


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PostPosted: Mon Feb 27, 2012 8:41 am 
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what crank are you using? look at sheldon brown for recommended bb lengths and then calculate which bb you'll need to use to increase decrease the chainline.

http://sheldonbrown.com/chainline.html

http://sheldonbrown.com/bbsize.html

and yes you can redish the wheel


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PostPosted: Tue Feb 28, 2012 11:14 pm 
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Bingo!

http://raceface.com/instructions/CRANKS ... _crank.pdf

"All RF cranks are designed to have a 48.5-49.5mm chainline when installed on a 113mm ISIS crank"
This will be the distance of the middle ring from the centre line of the bike.
Working backwards, you need to find the chainline of the rear sprocket.
By measuring between the axle faces (that butt up to the dropouts) find the midpoint of the hub.
Then measurehow far the sprocket teeth (centres) are from the centre line.
Adjust the length of the BB axle to suit. eg, if the rear chainline is 53mm, the front has to go out to 53 too, so the BB needs to be8mm longer: (53-49) X2 .
This will move the pedals out a bit but it's no big deal.


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PostPosted: Wed Feb 29, 2012 12:28 am 
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Cool, cheers for that!!! 8)

So if I want to run the chainring in the outer position I then subtract 2 x the distance between the middle and the outer ring. Don't supose anyone knows what that is do they??? :lol:


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PostPosted: Wed Feb 29, 2012 12:38 pm 
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I have successfully run chainrings the wrong side - the counterbored parts to get the bolts to sit flush are only necessary to prevent then catch on the chain on multi-ring setups.


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PostPosted: Wed Feb 29, 2012 10:59 pm 
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My chainring (an FSA DH ring) is suposed to go in the outer position. It doesn't even fit in the middle.


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