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Old 02-23-09, 08:17 AM   #1
bryroth
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Why are the sprockets on the crank inverse from the sprockets on the cassette?

If you start from the frame and count outwards, the cassette goes large gear, smaller gear, smaller gear, etc. On the crank, assuming you have a double or a triple, they go smaller, larger, larger.

I know there is a good and probably obvious reason, but I am not mechanical enough to figure it out by intuition. Could someone please tell me?

Thanks.
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Old 02-23-09, 08:23 AM   #2
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So it looks better.
So you don't have to crossover to get max high and low gears.
So there are fewer rings that can come in contact with your calf.
Howzat?
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Old 02-23-09, 08:46 AM   #3
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look at how the derailleur functions. it would be much more complicated to try to shift gears if you had to navigate between the crank arm and the big chain ring.
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Old 02-23-09, 09:06 AM   #4
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I've got another:
So the right side chainstay has got more clearance from the rings.
This is fun!
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Old 02-23-09, 09:25 AM   #5
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Better chainline.
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Old 02-23-09, 09:39 AM   #6
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Originally Posted by bryroth View Post
If you start from the frame and count outwards, the cassette goes large gear, smaller gear, smaller gear, etc. On the crank, assuming you have a double or a triple, they go smaller, larger, larger.

I know there is a good and probably obvious reason, but I am not mechanical enough to figure it out by intuition. Could someone please tell me?

Thanks.
While size wise, rear goes from big to small, gear wise both front and rear go from smaller gear to larger gear as you move to the right; its just that big ring up front=big gear while litte cog in back = big gear. So from a gearing stand point there is consistency in that for every shift outward, front or rear, you are increasing gear inches.
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Old 02-23-09, 09:42 AM   #7
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also try sticking a 34T where the 11T would usually go, you'll find that the cog will be grinding away at your stays.
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Old 02-23-09, 09:59 AM   #8
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The opposite arrangement would create poor chainlines and the big ring would hit the chainstay.
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Old 02-23-09, 11:03 AM   #9
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Envision the chain as it comes off a small outer chainring bumping into the bigger inner ring as you shift to a large inner sprocket on the rear.
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Old 02-23-09, 11:16 AM   #10
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There are some exceptions to the rule

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Old 02-23-09, 11:42 AM   #11
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There are some exceptions to the rule

Except the outside chain always remain on the same chain ring and isn't shifted. That's a weird synchronizing set-up. Why was it done that way?
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Old 02-23-09, 11:52 AM   #12
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Except the outside chain always remain on the same chain ring and isn't shifted. That's a weird synchronizing set-up. Why was it done that way?
Single-side drive doesn't require expensive tandem-specific cranksets, is a pretty good reason.

See here: http://www.bikexprt.com/bicycle/tancrank.htm
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Old 02-23-09, 11:56 AM   #13
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The chainline argument never quite made sense to me since to get to the middle of the total gear range, where presumably you're spending most of your time, you're going to be somewhat cross-chained. Of course, the tradeoff of the reversed setup is that your lowest and highest gears might be unusable, but you can likely work around that.

Seat/chainstay clearance is of course the big problem, and you can't easily do anything about that.
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Old 02-23-09, 12:01 PM   #14
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Except the outside chain always remain on the same chain ring and isn't shifted. That's a weird synchronizing set-up. Why was it done that way?
Actually, and although it's hard to see, there is an inner chain ring hidden behind the largest driving ring and a derailleur on this Bike Friday - Family Tandem.

Same-side sync drives are used for a number of reasons:
- They're economical in that they use two standard cranksets instead of a more expensive tandem cross-over crankset.
- They put less wear and tear on the rear bottom bracket, not that that's a big issue as bottom brackets are pretty beefy.
- They move all the chains to one side of the bike.
- Etc...

As to why the timing ring was put outboard instead of inboard for a same-side drive, there are some plusses and minuses on putting them inboard vs. outboard and it ultimately comes down to chain stay clearance, chain lines and things that are gemane to each tandem.
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Old 02-23-09, 12:05 PM   #15
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Except the outside chain always remain on the same chain ring and isn't shifted. That's a weird synchronizing set-up. Why was it done that way?
That is awfully cool. I suppose it would not be difficult to arrange it similarily but with the synch chain on the granny ring and with useful big and middle chainrings. Or even better - use one of the 4th chainring adapters that used to be available and have a useful triple...

...might get a little crowded around the f. derailleur, though.
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Old 02-23-09, 12:18 PM   #16
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Single-side drive doesn't require expensive tandem-specific cranksets, is a pretty good reason.

See here: http://www.bikexprt.com/bicycle/tancrank.htm
Thanks for the reference, that was a very interesting article. The only downside I can see to the single-side drive is that the Q-factor for both cranks has to be quite large to keep the synchronizing chain's chainline straight.
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Old 02-23-09, 01:08 PM   #17
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The chainline argument never quite made sense to me since to get to the middle of the total gear range, where presumably you're spending most of your time, you're going to be somewhat cross-chained. Of course, the tradeoff of the reversed setup is that your lowest and highest gears might be unusable, but you can likely work around that.

Seat/chainstay clearance is of course the big problem, and you can't easily do anything about that.
What would be implication for rear derailleur of reversing the direction of the cogs? Would "high normal" become "low normal" and "low normal" become "high normal"

I am having a hard time imagining a rear cassette with big cog on the outside, even absent chainline and seatstay issues. Logically it just makes sense to me to move chain in same direction both front and rear to move to higher gears (i.e. to the right). Also, when shifting from small ring/cog to larger, because I am in essence lifting the chain up to get from smaller to larger, pushing the lever on my shifters in the same direction the chain is moving (i.e. to the left for right shifter/rear cogs and to the right for left shifter/front rings) also makes logical sense to my brain. Could be, however, I am just used to what I am used to and could get used to doing things backwards.
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Old 02-23-09, 01:26 PM   #18
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you can get a high rise RD which would reverse the low normal and high normal thing.
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Old 02-23-09, 01:34 PM   #19
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Also, when shifting from small ring/cog to larger, because I am in essence lifting the chain up to get from smaller to larger, pushing the lever on my shifters in the same direction the chain is moving (i.e. to the left for right shifter/rear cogs and to the right for left shifter/front rings) also makes logical sense to my brain. Could be, however, I am just used to what I am used to and could get used to doing things backwards.
The derailleurs have to do the exact opposite when you shift the other way, so on the surface it seems like it's possible. I imagine you would have to re-engineer the derailleurs to make it work well. Maybe if I get bored one day I'll reorder a few sprockets on a cassette and see what the derailleur does. If it is workable, I bet Shimano has a patent on it.

In any case, I don't deny that there are other, probably more important, reasons to do things the way we do now. I just don't think that improved chainline is one of them in most situations.
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Old 02-23-09, 01:45 PM   #20
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The only downside I can see to the single-side drive is that the Q-factor for both cranks has to be quite large to keep the synchronizing chain's chainline straight.
Since this is the entry level Bike Friday tandem, I'm sure the motivation was cost but I thought it was really smart.

The Q-factor for the stoker is exactly the same as if it were a standard triple - less than it would be for a triple with a synch chainring on the left side too.

I'm not sure how they got the synch chain line right though - maybe the BB shell is not symetrical, or maybe the synch chain is so long the chain line isn't that critical.




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Old 02-23-09, 01:45 PM   #21
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you can get a high rise RD which would reverse the low normal and high normal thing.
Shimano calls it "Rapid Rise" or "Low normal" and the spring tension pulls the cage toward the largest cog.

IIRC, Sun Tour made a front derailleur that was in effect "Rapid Rise", that is the spring tension moved the cage toward the big chainring and the cable tension pulled it toward the smaller rings. The reasoning here was to have both shifters do the same thing (upshift or downshift) while moving in the same direction. It never really caught on.
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Old 02-23-09, 01:46 PM   #22
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They same reason the chain is on the right?
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Old 02-23-09, 01:59 PM   #23
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Boy that's some height differential between front and stoker seats. Is that standard for tandems. It would seem to put stokers nose in a rather unpleasant location relative to the front rider's chamois.
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Old 02-23-09, 02:13 PM   #24
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Boy that's some height differential between front and stoker seats. Is that standard for tandems. It would seem to put stokers nose in a rather unpleasant location relative to the front rider's chamois.
Well, there's a big height difference between the captain and the stok-kid.



The ability to accomodate the height difference without any modifications was the reason I went with this Bike Friday tandem instead of something more conventional.
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Old 02-23-09, 02:52 PM   #25
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Not germane to chain line but many old tandems from the 1890s or so did have the lady up front as the old rules of courtesy included "ladies first". Most such setups included remote steering so that the stoker could steer the bike. Many had both capable of steering so that the strongest presumably won! Usually set up so the stoker had a higher saddle to look over the head of the captain. Added complication to meet the requirements of Victorian etiquette.
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