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Discussion Starter · #1 ·
I have a Soma Rush with Phil Wood track hubs, a Shimano singlespeed freewheel, and Ultegra double cranks. I'm using a 45t chainring on the front and a 16t on the rear. I would get the ideal chainline with the front chainring mounted on the inside of the crank. However, I do not have clearance from the chainstay on the inside...so I mounted it on the outside. I think my chainline is off by about 3-4mm. It seems to work fine with this set-up but is a little noisy.

First, is this a problem? Second, if it is, is there a way to space the rear freewheel out by 3-4 mm? Thanks for your help.
 

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chainline...

FWIW I also have a Rush w/ a PW track hub, and there is plenty of clearance for a 49t chainring w/ the correct BB and chainline.....your chainline should be close to the track standard of 42mm in the back w/ that hub, to check the front measure from the center of the seat tube to the center of the chainring...if you want to check the cog chainline, subtract the distance from the inside of the dropout to the center of the cog from 60mm....e.g. 60mm - 18mm = 42mm
 

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Discussion Starter · #5 ·
More info...

Thanks to Sheldon Brown, I've found that the Shimano standard front chainline for an Ultegra double with a 109.5 bottom bracket is 43.5mm to the center of the two rings. To the inside ring it's 41 and to the outside it's 46. Thus, if my rear chainline is 42, I will probably never get an ideal chainline with the Ultegra road double cranks (as I mentioned, the inside is very close to being spot on, but I don't have clearance with a 45t chainring). Any other thoughts or suggestions?
 

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Discussion Starter · #8 ·
42 16

Thanks for the suggestion, Dave. A 42 will work on the inside. However, it was simply too easy of a gear for me. Also, it is my understanding that single-speed freewheels do not come in sizes smaller than 16 due to the pawls. I think "asterisk" thought I was talking about a fixed set-up.
 

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With a standard hub, you can use certain 15t freewheels. Check danscomp.com. They have a 15T that works. I think it's ACS but call to be sure. Avoid small tooth freewheels that say for flip flop hubs. Those are the ones that need a hub that has a smaller diameter on one side to accomodate small tooth freewheels(not the same as a fixed/free flip flop)
 

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Go back to Sheldon Brown's chain line discussion and scan down the Chainline page for the charts that give chainline numbers for hubs, track cogs and single speed freewheels. The chainline numbers for your PW hub and Shimano freewheel combination are 44.64 and 45.42 depending on whether the fw is 3/32" or 1/8". Either number is close to the outer 46 chainline for your crank. So if you are using the same components that are listed in the charts, you should be good on the outside ring.

I'm running the same crank with ring on the inside, just installed this week. I used a 1mm spacer behind the lip of the drive side bottom bracket cup to gain extra chainring / chainstay clearance on my old road frame with 126mm rear spacing. I also used a spacer behind my fixed cog to match the bb spacer so chainline remained constant.

I don't think you mentioned whether you physically measured your front and rear chainline. Again, go to Sheldon - he has easy instructions - and this will let you know if you are off and by how much.
 

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something isn't right....

GFocker said:
Thanks to Sheldon Brown, I've found that the Shimano standard front chainline for an Ultegra double with a 109.5 bottom bracket is 43.5mm to the center of the two rings. To the inside ring it's 41 and to the outside it's 46. Thus, if my rear chainline is 42, I will probably never get an ideal chainline with the Ultegra road double cranks (as I mentioned, the inside is very close to being spot on, but I don't have clearance with a 45t chainring). Any other thoughts or suggestions?

41mm up front is good, 1mm error is plenty close enough, +/- 3mm (about 1/8") is generally considered acceptable from what I've read (and experienced on my bikes), but no way is your 45t ring at 41mm (from the bikes centerline) now, you would have lots of chainstay clearance if it was
 

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Chorus BB...

I run a 102mm Campy Chorus BB that came with my frame, and some no-name 130mm BCD cranks that seem to fit right. With chainring on the outside my chainline's about 2mm off. It's all working fine but that said I'm running 3/32" with a connex 8sp shifty chain, which has a little sideways play.
 
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