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Old 05-24-08, 06:48 PM   #1
Little-Acorn
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Replacing front chainwheel on tandem for lower gear?

I have a Schwinn Twinn Sport 10sp tandem. It's sort of "one step up" from the basic Twinn tandem, having a top-bar front end and mixte rear, weighs about 12 lb. less than the older Twinn. It also came as a 10-speed from the factory.

Front (stoker's) chainwheels are 52-tooth and 39-tooth. I put a 14-34 cluster on the rear, to get that low hillclimbing gear. But I'd also like to replace that front 39-tooth chainwheel with a smaller one if possible. I don't want to replace the entire front crankset with a triple, cuz that's a major undertaking on a tandem, in time, effort, and $$ for parts.

Some have told me that it's impossible to put a chainwheel smaller than 39 teeth on the front of this particular bike, because the diameter of the bolt pattern pretty much dictates the smallest chainwheel that can go on and still fit outside those bolts.

But I noticed that right next to the 39-tooth chainwheel, is a 36-tooth chainwheel, that carries the synchronizing chain to an identical 36-tooth chainwheel on the front (captain's) crankset. And it is also bolted to those same bolts.

Is there any reason I couldn't get another one of those 36-tooth chainwheels usually used for the synchronizing chain, taken off the 39-tooth drive chainwheel, and put the 36 in its place, so I have one 52-tooth and two 36-tooth chainwheels side by side? One 36 chained to the captain's crankset and the other 36 chained to the rear cluster for hillclimbing?

Anybody ever tried this on a tandem? Especially a Schwinn?



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Old 05-24-08, 06:57 PM   #2
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It looks like the bolt circle on your crank is 110 mm so you can go as small as 34T for the inner drive chainring. The only problem I can see is if the inner drive and synchronizing chainrings are the same size or the inner drive ring is smaller, the two chains might not clear each other.

I've never seen a tandem set up like this before. Usually the synchronizing rings are on the other side of the bike. I guess this arrangement allowed them to use two conventional (and cheaper) cranks.
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Old 05-24-08, 09:42 PM   #3
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Originally Posted by HillRider View Post
It looks like the bolt circle on your crank is 110 mm so you can go as small as 34T for the inner drive chainring. The only problem I can see is if the inner drive and synchronizing chainrings are the same size or the inner drive ring is smaller, the two chains might not clear each other.
Hmmm, well, the chains cross close by each other now without interference. I don't see why they would clash due to closer gear size, they seem well enough separated now. Then again, I've never tried it

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I've never seen a tandem set up like this before. Usually the synchronizing rings are on the other side of the bike. I guess this arrangement allowed them to use two conventional (and cheaper) cranks.
From what I've heard, both arrangements are fairly common. I had a Fiore tandem, and have looked at other Schwinn Twinns, all were set up this way. All were low end bikes, of course. As you said, maybe that is what makes the difference.

Here's a silly question: How do you measure the bolt circle diameter? Distance from the center of one bolt, to the center of the crank, times two? Or is it the distance between two far-apart bolts (wouldn't be the mathematical diameter, but maybe tradition is stronger than math?)?
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Old 05-24-08, 11:47 PM   #4
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" Here's a silly question: How do you measure the bolt circle diameter? Distance from the center of one bolt, to the center of the crank, times two? Or is it the distance between two far-apart bolts (wouldn't be the mathematical diameter, but maybe tradition is stronger than math?)?

http://sheldonbrown.com/gloss_bo-z.html#bcd
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Old 05-25-08, 06:53 AM   #5
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" Here's a silly question: How do you measure the bolt circle diameter?
Measure the center-to-center distance between any two adjacent crank bolts.

32.9mm = 56mm BCD
33.4mm = 58mm
43.5mm = 74mm
55.4mm = 94mm
64.7mm = 110mm
76.4mm = 130mm
79.5mm = 135mm
84.6mm = 144mm
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Old 05-25-08, 07:14 AM   #6
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There are a couple of weird orphan BCDs that aren't on Retro Grouch's list, measure carefully.
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Old 05-25-08, 07:15 AM   #7
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Quote:
Originally Posted by Little-Acorn View Post
Hmmm, well, the chains cross close by each other now without interference. I don't see why they would clash due to closer gear size, they seem well enough separated now. Then again, I've never tried it
Currently the chain on the 39T drive chainring is always above (or "outside" of) the 36T synchronizing chain so they never really cross. If there is enough side clearance so they can be adjacent without interfering, then a smaller drive chainring will work but be sure they will clear.

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Originally Posted by Little-Acorn View Post
Here's a silly question: How do you measure the bolt circle diameter? Distance from the center of one bolt, to the center of the crank, times two? Or is it the distance between two far-apart bolts (wouldn't be the mathematical diameter, but maybe tradition is stronger than math?)?
With a 5-arm crank, you can't measure the bolt circle directly but RG's posting gives you the spacing between the centers of adjacent chainring bolts that corresponds to common bolt circles. If you want the trigonometry that goes into calculating these numbers, PM me.
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Old 05-26-08, 01:05 AM   #8
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What is the shifting limit of the FD? You might be limited by that rather than any BCD issues. I know on compact cranks that you need the compact FD to go 36/52 or 34/50. The 39/52 is probably the limit with your FD. You might have to keep the difference 13 teeth with the current FD or go with a compact FD.
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Old 06-01-08, 04:57 PM   #9
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Well, I got a 110mm 34-tooth chainwheel, and quickly found out that I should have measured a little more carefully. My chainrings bolt pattern is slightly larger than 110mm. They are maybe 117 or 118 mm. Is that a standard bolt pattern size? Ot do I have a weird one? Or maybe I'm just not measuring it right?

BTW, another silly question I should have asked earlier: Do I have to remove the left cranks and the bottom bracket, to replace the 39mm chainring with a 34 (or 36)? I can't see any other way to get the small chainring on and off.
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Old 06-02-08, 02:52 AM   #10
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Quote:
Originally Posted by Little-Acorn View Post
Well, I got a 110mm 34-tooth chainwheel, and quickly found out that I should have measured a little more carefully. My chainrings bolt pattern is slightly larger than 110mm. They are maybe 117 or 118 mm. Is that a standard bolt pattern size? Ot do I have a weird one? Or maybe I'm just not measuring it right?
I had a look into that a while ago, and AFAIK in that vicinity there are:

104
110
112
130

Haven't seen any around 117. Measuring can be a bit tricky, have you looked at the Sheldon Brown link?

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Originally Posted by Little-Acorn View Post
Do I have to remove the left cranks and the bottom bracket, to replace the 39mm chainring with a 34 (or 36)? I can't see any other way to get the small chainring on and off.
No need to mess with the left crank or the bottom bracket. If you remove the black cap at the center of the right crank (carefully, old ones are prone to cracking) you'll expose either a nut or a bolt depending on BB age and an internal thread in the crank. Remove nut/bolt, fit a crank puller http://www.worldcycling.com/merchant...ode=componentsPull right crank, undo the 5 chainring bolts(some come off easily only using an Allen key, others require a tool on the other side as well, improvised one may do the trick) Replace chainring, reassemble crank(torque it down well), check chain length and you're done.
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Old 06-03-08, 04:14 PM   #11
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A fellow on another forum mentioned that the Sakae crank arms on this bike sometimes came with a 118mm bolt pattern, and chainrings to match. Pretty hard to find nowadays, of course. Still, hope springs eternal.

The synchronizing chain on this bike rides on two 36-tooth chainwheels, both of which mount on this bolt pattern. I wouldn't be too unhappy if I could find a third such 36-tooth, and see if I can use it as a smaller drive chainring. Might look a little odd, but a fat guy on a skinny bike doesn't worry about things looking odd.
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Old 06-03-08, 05:11 PM   #12
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This looks like the type of crankset that you have to discard when the big ring wears out.
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Old 06-03-08, 06:40 PM   #13
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There are a few 118mm chainrings on Ebay. How often that happens, I don't know.

Worse comes to worst, when the present rings wear out, that's when I'll get out the wallet and put in a triple tandem crankset.
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Old 06-04-08, 08:02 AM   #14
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You could get a newer (and better) crank with a more common bolt circle and do one of two things:

1. If the crank is a triple with either a 110 or 130 mm bcd for the outer two rings and a 74 mm bcd for the granny, buy a 36T, 74 mm bcd granny ring. Harris Cyclery sells 36T, 74 mm BCD chainrings by TA.

2. If the crank is a double with a 110 mm bolt circle, buy two 36T 110 mm bcd chainrings and "tripilizer" (long) chainring bolts so you can mount all three chainrings on the same bolts the way your current crank does. The bolts on your current crank may work but new ones are available. Harris Cyclery sells these too.

I agree that the current crank is a low quality one and the outer chainring is not replacable. Might as well get something better while your at it.
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