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Old 01-29-08, 03:39 PM   #1
cmcanulty
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Chainring experiments

s there anyplace I can find out what size chainwheels are available
in the different Bolt Circle Diameters? I have looked a lot and short
of just browsing all the online bike shops can't find out. Like is it
possible to put a <38 tooth chainring on a 130BCD? Possibly by
redrilling the holes? I am trying to get a super low gear and super
high gear on my folding bicycle. Or is there a BCD that would allow me
to have 2 chainrings, 1 with 53 teeth and one with 22? I am NOT using
a front derailleur, will shift manually, so tooth difference doesn't
matter. I want to achieve a super wide gear range of sub 18 to 125 or
so. It has a 9sp cassette on rear. I know to avoid the extreme cross
combinations, thanks. This is one of the few times Sheldon Brown's
site didn't have the info or I didn't find it. It has to be possible!
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Old 01-29-08, 03:49 PM   #2
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Originally Posted by cmcanulty View Post
s there anyplace I can find out what size chainwheels are available
in the different Bolt Circle Diameters? I have looked a lot and short
of just browsing all the online bike shops can't find out. Like is it
possible to put a <38 tooth chainring on a 130BCD? Possibly by
redrilling the holes? I am trying to get a super low gear and super
high gear on my folding bicycle. Or is there a BCD that would allow me
to have 2 chainrings, 1 with 53 teeth and one with 22? I am NOT using
a front derailleur, will shift manually, so tooth difference doesn't
matter. I want to achieve a super wide gear range of sub 18 to 125 or
so. It has a 9sp cassette on rear. I know to avoid the extreme cross
combinations, thanks. This is one of the few times Sheldon Brown's
site didn't have the info or I didn't find it. It has to be possible!
really? I would just check what is available there and figure that is the best available
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Old 01-29-08, 05:11 PM   #3
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really? I would just check what is available there and figure that is the best available
+1

"There" being here.

I just looked at a ruler, and I suspect that a 22 tooth chainring has a smaller diameter than the BCD (Bolt Circle Diameter) of a 130 BCD crank. Maybe even smaller than a 110 BCD. So even re-drilling it wouldn't work.

It looks like a 110 BCD double crank would let you put a 33 tooth inner and a 61 tooth outer. With an 11x34 cassette and 20 inch wheels that would give you 19.4 to 110.9 inches.

Have you considered Capreo and a triple?

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Old 01-29-08, 05:27 PM   #4
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For something as small as 22T, you need a BCD of 74mm which is doable on a triple crankset.

On double cranksets with a BCD of 110, I think you only can go as low as 34T.



Edit: if you get a triple, you might also want to get a longer bottom bracket as well.

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Old 01-29-08, 05:32 PM   #5
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The only thing that just crossed my mind is a weird 50 BCD crank by TA Specialties ... http://www.specialites-ta.com/produits/trans_gb.htm#

If it works in the same fashion as other cranks, that should be able to get the gap you want.

I have no idea where to get it nor how much it would be.
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Old 01-29-08, 07:05 PM   #6
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Quote:
Originally Posted by cmcanulty View Post
This is one of the few times Sheldon Brown's
site didn't have the info or I didn't find it. It has to be possible!
He has it:
http://www.sheldonbrown.com/gloss_bo-z.html#bcd

You can't get 22t and 50+t on the same BCD unless you custom make a huge BCD ring. Your best option is probably the White Industries crank which uses a custom large ring and lets you attach any 5-bolt spider chainring as the smaller one:
http://www.whiteind.com/VBC.html

I think they make the large rings up to 50t.

One other thing to consider is that if you make too large of a jump that you'll likely have a gap between gears. You really don't want to have a difference in your chainrings that is larger than the range on the cassette or you'll be stopping to shift a lot. I like wide range doubles, but not with over a roughly 70% larger large ring.

alex
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Old 01-30-08, 06:52 AM   #7
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The Shimano 105 triple that I have, and no doubt many similar cranks by Shimano and others, have two big chainrings attached at the five 130 BCD holes, and a small chainring attached at a smaller set of holes drilled and tapped into the spider. The older 110 BCD MTB cranks have a similar arrangement. On my 105 triple the rings are 30, 42, 52; and I'm pretty sure you can find a smaller ring than the 30.

That said, I have a hard time imagining you'd ever need such a small ring. I know, what I can or can't imagine is not really relevant to the question, but I had to say it anyway.
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Old 01-30-08, 07:05 AM   #8
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That said, I have a hard time imagining you'd ever need such a small ring. I know, what I can or can't imagine is not really relevant to the question, but I had to say it anyway.
I love my 21 inch gear! It has totally changed my outlook on really steep hills.

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Old 01-30-08, 10:00 AM   #9
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That said, I have a hard time imagining you'd ever need such a small ring. I know, what I can or can't imagine is not really relevant to the question, but I had to say it anyway.
I am guessing that either he is planning fully loaded touring in the mountains or he probably doesn't NEED it. Then again he may have special circumstances or maybe he just WANTS to do it.

FWIW: A 39 tooth front ring and a 34 on the cassette yields a 23.3 inch gear with 20" wheels. I would find that adequate for anything I did on the TransAmerica in the Cascades or Rockies when fully loaded touring. In the Appalachians there were places where I would have wished for lower, but I personally am not likely to be doing fully loaded touring there on a folder. If I did I would be traveling very light and could get by with the 23.3. If I ever needed lower than 19 inch gear on any bike I would get off and walk, but that is just me.

Obviously everyone can and should do as they see fit.
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Old 01-30-08, 12:02 PM   #10
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I have an 18" gear on my mountain bike and love it for loaded hill climbing and sometimes would love a few inches lower, I have old knees. I also start out with 10lbs of dog with her basket then add all the other weight. I also like to spin fast instead of use a lot of power pedaling. I have a 11-32 cassette on the downtube with a 48 and 39 tooth front setup and I don't think a 34 rear cog can be retrofitted but not sure.
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Old 01-30-08, 01:55 PM   #11
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Theres a company in England called High Path.
They mess with drivetrains.

Im not overly keen on low gears...maybe 35".
Used to try to ride in top 100" even on steep hills.
Oh couldnt use that on a down hill now..

Have seen adaptors to fit cassette cogs to cranks.
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Old 01-30-08, 03:43 PM   #12
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I have a 11-32 cassette on the downtube with a 48 and 39 tooth front setup and I don't think a 34 rear cog can be retrofitted but not sure.
Depends on the derailer. If it is stock, what does Yan say?
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Old 01-30-08, 04:37 PM   #13
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Salsa makes a large chainring for what you want. They have a 110BCD 53 tooth. You can use a compact triple crank & mount a 22 with a 53. However, the rear derailleur may not be able to take up the chain slack if you are also using a 11-32 or 11-34 cassette. Also, without a front derailleur, the chain may come off the chainrings fairly easy.
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Old 02-02-08, 09:56 AM   #14
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What about drilling holes in the Downtube crank spider for a smaller BCD would this be safe for the crank?
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Old 02-02-08, 05:43 PM   #15
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What about drilling holes in the Downtube crank spider for a smaller BCD would this be safe for the crank?
What do you do about the spider? That is, how do you prevent the chain from rubbing against the spider?
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