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Old 11-01-09, 05:03 PM   #1
billyymc
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38 or 40 Tooth chainring with 74mm BCD?

Have a Trek MTB from around the early to mid-90's...this is the bike right here (although this isn't mine):

http://www.thesamba.com/vw/classifie....php?id=588890

I had been riding it until a couple years ago when the threads on the fork/headset stripped and I put it up for a while. Got a new fork, headset, etc.

Would like to take off the front derailer and just have a 1/7. I could try using the middle chainring, but would prefer to get a SS ring of the right size - would like a 38 -42 tooth, somewhere in that range.

Problem is, the chainrings on the bike all mount as 74mm BCD, and I can't find a 74mm BCD ring with that tooth count (that's SS specific with no ramps).

Any ideas where I can find one? THanks.
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Old 11-01-09, 05:07 PM   #2
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The two large rings have a BCD of 110mm. There are plenty of rings made for that size.
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Old 11-01-09, 05:39 PM   #3
billyymc
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davidad - if it were that easy, even I could have figured it out!!

No, all three rings on this bike are 74mm BCD. The crank/spider is that size. The three rings stack -- the smallest to mid have spacers between them, and then the mid to largest are sort of pressed into an offset configuration so they stack with the proper spacing between them.

So I really do need a SS ring with 74MM BCD with anywhere from 38-42 teeth. I just don't think they make one like that.

Is there any kind of adapter I could use to take a 110mm BCD ring and be able to use is on this crank?
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Old 11-01-09, 06:46 PM   #4
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Can't you just find shorter bolts and use the existing middle or outer chainrings then?

Failing that, you could get a "triplizer" 39 or 42 chainring and use it in reverse. It is kind of hard to find a triplizer these days though. Probably cheaper to get a larger b.c.d. crank.
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Old 11-01-09, 06:49 PM   #5
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Just looking at the pictures now, are you sure that's not a 94mm BCD?
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Old 11-01-09, 06:56 PM   #6
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Billy post a few good closeups of the crank? http://www.cycleunderground.com.au/chainringdesigns.htm
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Old 11-01-09, 06:57 PM   #7
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Well, I measured it from center to center of two adjacent holes -- 44mm as best I could tell. Which when I look at SB's page, tells me it's 43.5 C to C or 74 BCD, right?

I wanted to avoid using the existing middle ring since I think it might be more apt to throw the chain than a ring made without intending to be shifted, no?

Not sure what triplizer is...but thanks.
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Old 11-01-09, 07:24 PM   #8
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Harris Cyclery lists TA 74 mm bcd rings up to 36T. Look here:

http://www.sheldonbrown.com/harris/chainrings/74.html

Most 74 mm bcd rings are flat as granny rings typically don't have ramps and pins.

A "triplizer" is an inner ring for a double crank that has tabs on the inside circumference that project inward and are drilled for chainring bolts in a 74 mm bcd. They allow fitting a granny ring to a double crank that wasn't drilled for a triple. What zzyzx_xyzzy was proposing is that you get one of these in 39 or 42T form and use the inner tabs to bolt it to your crank. I've seen them advertised but Harris Cyclery no lomger has them .
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Old 11-01-09, 08:41 PM   #9
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Quote:
Originally Posted by Bianchigirll View Post
Billy post a few good closeups of the crank? http://www.cycleunderground.com.au/chainringdesigns.htm
Here's a couple pics.



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Old 11-01-09, 08:44 PM   #10
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Thanks Hillrider -- that 36 tooth TA might be my best choice. Would like a 38 or 40, but can't be too choosy apparently.

Bianchigirl - thanks for the link also.

Last edited by billyymc; 11-01-09 at 09:04 PM.
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Old 11-01-09, 09:29 PM   #11
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Your big ring has 42 teeth. I know it's not a "singlespeed specific" ring and it has pins (which won't come into play) and shaped teeth, but it should be fine in a singlespeed setup.
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Old 11-01-09, 11:07 PM   #12
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Originally Posted by timcupery View Post
Your big ring has 42 teeth. I know it's not a "singlespeed specific" ring and it has pins (which won't come into play) and shaped teeth, but it should be fine in a singlespeed setup.
I agree. Just bolt it on the crank with short bolts and go ride. You'll probably want a shorter spindle in the BB to align the chainring with the middle of the cassette.
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Old 11-02-09, 04:31 AM   #13
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The tripleizer is not going to be any better than your current 42 tooth big ring, so I would forget about the tripleizer.

Last edited by Chris_W; 11-02-09 at 07:09 AM.
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Old 11-02-09, 07:06 AM   #14
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I agree. Just bolt it on the crank with short bolts and go ride. You'll probably want a shorter spindle in the BB to align the chainring with the middle of the cassette.
Sounds like the way to go. I already spent more on this bike (will be my beater) than I wanted to with a new fork and threadless headset. Will save the new ring until this one has no teeth left!
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