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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 09-23-06, 12:02 PM   #1
mander
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Conversion question: 48t chainring rubbing?

Hi everyone, first time poster, long time lurker with a conversion question. With a 42.2mm chainline, my '82 Marinoni road frame has less than 1/2 mm of space between a 50t chainring and the chainstay. I think this will definitely rub after the powdercoat goes on, so I am going to go with something smaller. Can anyone say offhand whether a 48t ring will give enough room? I am asking because I really like 48*18---two even numbers (for good wear), lots of teeth (ditto), and 70 inches (the gear inch of champions). I am ordering chainrings off the web and I want to get my bike on the road fast, so I would like to get something I for sure won't have to return. If no one knows, maybe I will just go ahead and do something with a 46 or 44.
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Old 09-23-06, 12:03 PM   #2
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Just FYI if you skid a lot 48 x 18 is not going to give you very many skid patches.
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Old 09-23-06, 12:30 PM   #3
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Just make the rear cog a prime number toother cog, and you'll have that many skid patches. Real nice. Just get a 17, and suck up the extra 2 or whatever gear inches. You probably won't even notice it.
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Old 09-23-06, 01:52 PM   #4
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a 48t ring will be about 1/3" smaller in diameter, or 1/6" smaller in radius. Measure in 1/6" from where your ring sits now and see how much more clearance you will have. On my first conversion, I couldn't run bigger than 44t without rubbing.
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Old 09-23-06, 02:07 PM   #5
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Thanks for the replies!

Sivat that's the stat I was looking for. I will measure and see. Can I assume that every two-tooth drop in chainring size will lose 1/3" total radius, or have i missed something?

Hrm, maybe 46*17 (71.1" with 700*23's) will work. I really want to keep to the low 70's because I have to climb 360 m on my 7 km commute every day. It's hard to reconcile the issue of skid patches with Sheldon Brown's rule about even numbers.
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Old 09-23-06, 02:15 PM   #6
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Quote:
Originally Posted by mander
Thanks for the replies!

Sivat that's the stat I was looking for. I will measure and see. Can I assume that every two-tooth drop in chainring size will lose 1/3" total radius, or have i missed something?
when are we gonna teach the kids math?

2 tooth drop = 1/Pi reduction in DIAMETER which is approx 1/3"

so you are loosing 1/6 of an inch RADIUS for every 2 tooth reduction
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Old 09-23-06, 02:16 PM   #7
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doh! thanks poopncow.
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Old 09-23-06, 02:18 PM   #8
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your welcome

1/6" is approx 5/32" with rounding
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Old 09-23-06, 02:18 PM   #9
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FWIW I really like 48X17
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Old 09-26-06, 12:59 AM   #10
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Breakthrough! A 47t chainring will clear the stays with mms to spare, AND 47 is a prime number so I can flip my flipflop and always have a huge number of skid patches. Yeah GirlAnachronism, 48*18 is definitely a bad choice---only three skid patches!

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Old 09-26-06, 01:09 AM   #11
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Quick question... does your BB spindle length fit with your crank arms? That could be something also. It should fit and then you could run whatever chainring (I think : /).
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Old 09-26-06, 01:16 AM   #12
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Not necessarily. Some frames, especially the bianchi touring frames, that were made for bigger tires, don't have the clearance for a big ring with a 42mm chainline. Since the frame was designed around a road triple, it was never expected that you would have any bigger than maybe a 34 tooth ring against the frame. So trying to run a big ring with a good chainline means you have chainstay clearance issues.
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Old 09-26-06, 01:20 AM   #13
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Ah ok. I see.

But in theory, if the threading is right, you could use a BB with a really long spindle length and get matching crankarms that would allow someone to fit anything on the frame... right?
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