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Cross-over to single-side conversion? Opinions? Experience? Help with compatibility?

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Cross-over to single-side conversion? Opinions? Experience? Help with compatibility?

Old 06-11-20, 11:44 AM
  #1  
cpunerd
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Cross-over to single-side conversion? Opinions? Experience? Help with compatibility?

Hi all, as part of our upgrade to electronic shifting (I'd post the thread but bikeforums says I'm still a n00b and not allowed to), my wife and I are contemplating the idea of switching from our current cross-over drive to single-side drive. We have a bunch of questions, most of which have to do with compatibility.

For starters, has anyone done a cross-over to single-side conversion before? What was your experience? What sort of setup do you now have? Would you do it again? If you were to buy a new bike, would you prefer one setup over another?

The rest of the questions we have are all on compatibility. Here's what our setup is like at the moment:
  • Captain
    • Eccentric bottom bracket
    • 73mm BB shell
    • Square taper BB
    • 175 mm cranks - 130 mm BCD
    • Non-drive side 42T single connected to timing chain
  • Stoker
    • 73 mm BB shell
    • Square taper BB
    • 150 mm cranks - 130 mm/74 mm BCD.
      • We have a desire to change this to 165 mm, this will also play a big part in our decision-making process.
    • Non-drive side 42T single connected to timing chain
    • Drive-side 52-39-30
      • The 30 *barely* clears the frame in the rear. We're hoping to upgrade to a 34, but we aren't sure if it'll fit.
  • (Soon-to-be-delivered) Electronic components
    • eTap front mech
    • eTap WiFli rear mech
    • eTap L/R shifters with cable-actuated braking
We have a few paths forward from here, but we have questions for all of them.
  1. Path of least resistance - Scrap the single-side conversion, stick with cross-over and our existing cranks, and order new stoker chainrings (be safe and go with 30-46-??)
    1. What do we do with the outer ring? We need *something* there.
      1. We (most likely) can't use the middle and outer ring due to the chainline capacity of the eTap front derailleur, we have to use the inner and middle rings.
      2. Can we just order spacers? A chain guard? A duplicate chainring of the middle ring?
    2. Does anyone have specific recommendations for chain rings? We contacted gebhardt.cz several days ago but still haven't received a reply.
  2. Single-side conversion
    1. Obviously the synchronization chain rings need to be of identical teeth.
    2. I assume that we can "simply" order two sets of 3x cranks, find appropriate chainrings, and link up.
      1. These can both be standard cranksets, right? That's the huge benefit of single-side drive, that all your parts are standard, it's simply the configuration that's a bit weird, right?
    3. What difficulties would we face if we were to order a 1x or 2x for the captain, as opposed to a 3x? Would the difference in chainline of the synchronization chain be enough to cause problems?
    4. If we order a 3x for the captain, and we intend to use the outer ring of the stoker's 3x for synchronization, is it okay to not install the middle and inner rings on the captain's chainset?
    5. Does anyone have experience with setting up a 3x chainset using a non-monotonically-increasing tooth count (e.g. 34-50-42)?
      1. What were your experiences?
      2. Did you have to do anything with spacers?
      3. Did you have "inner/middle/outer-specific" rings?
    6. Let's say we go with 30-46 for the inner and middle rings of the stoker.
      1. Should we go with a synchronization ring smaller than 46T?
        1. What do you think the maximum number of teeth would need to be to get good-enough clearance for an outer synchronization ring?
      2. Should we go with a synchronization ring larger than 46T?
        1. What do you think the maximum number of teeth would need to be to get good-enough clearance for an outer synchronization ring?
    7. Let's say we go with 34-50 for the inner and middle rings and it doesn't damage the frame.
      1. Same questions as 4.1.1 and 4.2.1, though replace 46T with 50T
    8. We've been pricing things out, and it really seems like it'd be way cheaper, and easier to source components, if we go with two "standard" bike cranksets as opposed to keeping the cross-over drive.
      1. We're based in Switzerland, so prices are a bit weird, especially with some places calculating VAT and some not, and shipping costs etc.
      2. Does that seem right? It seems like ordering new tandem cranks to deal with the change in BCD of the chain rings could be quite expensive, or just hard to find.
Phew... Okay, that's all that I can think of at the moment. I'll probably have more questions later.

Thanks so much for your help!
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Old 06-23-20, 04:39 PM
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conspiratemus1
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Originally Posted by cpunerd
Hi all, as part of our upgrade to electronic shifting (I'd post the thread but bikeforums says I'm still a n00b and not allowed to), my wife and I are contemplating the idea of switching from our current cross-over drive to single-side drive. We have a bunch of questions, most of which have to do with compatibility.
. . .

Thanks so much for your help!
The Achilles heel of same-side drive is fouling between the two chains in the event that the front derailer overshifts the main chain off the big ring (for a setup with the sync chain outboard) or the small ring (for a setup with the sync chain inboard.) In either case, you are left not with just a dropped chain (which if you are slick is often recoverable without stopping.) Instead you have two chains jammed together, one of which is under substantial tension, locked together by the humps and valleys in the contours of the two chains. You will be immobilized (in the heat, humidity, and mosquitoes) with one of you sourly standing by while the other of you, also sourly, attempts to unjam the two chains using a large thick screwdriver (not commonly brought along as an on-the-road bike-repair tool) after loosening the eccentric. You must also diagnose and correct, flawlessly and immediately, at roadside, whatever mal-adjustment caused the derailer to over-shift, else the problem will recur the very next time you try to shift the front as you struggle home.

The version of single-side we tried several years ago used the approach described by John Allen on the late Sheldon Brown's website (which Mr. Allen is maintaining.) The difficulty is you can have hundreds of flawless shifts, especially on a double, week in, week out, and you think you are pretty smart...then one day the derailer adjustment screw retracts that last fraction of a tenth of a millimetre and the overshift wrecks your day. (In my case, the chain jamming occurred on the first (and only) test ride. Fortunately stoker was with me, to provide moral force against ever trying that again.)

My suspicion is that, like much of the Internet, single-sides are only "vapour-ware": concepts that look do-able and "should" work, but are never actually built and tested in real life, even if claimed otherwise. The nice photo of a 42 bolted outboard of a standard double or triple crankset looks so clever. Not. The only way I can see a same-side being reliable would be if the aft sync ring was bigger than the the outer ring, and mounted far enough outboard to make room for the outer cage plate of the derailer, yet not so far as to interfere with the crank arm or with stoker's ankle bone (medial malleolus) as she pedals. A similar argument might apply to putting a large aft sync ring in the granny position of a triple crank, but then the chainstay gets in the way and being in Switzerland you might not want to give up a traditional granny inner ring. (I admit I didn't read every word of your post and I don't know if Di2 is immune to over-shifting.)

I do recognize the advantages of same-side drive. Our tandem with stock cross-over drive chewed up a new cup-and-cone bottom bracket during a two-week mountain tour in Austria in 2006 -- the balls came out looking like lentils and American/Canadian footballs -- but continues to behave to this day with cartridge BBs. The advantage of being able to use ordinary cranks, with their greater variety of lengths is a plus, yes, although my stoker can't tell the difference between 170 and 165 on our two tandems. But unless you have a sure-fire way to prevent over-shifts (and the certainty of chain jams), they will prove your undoing.

Last edited by conspiratemus1; 06-23-20 at 04:47 PM.
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Old 06-23-20, 10:39 PM
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Thanks, that all makes a lot of sense. If we are to go with single-side, I'm now thinking of going with 30/46 and then something *really* big on the outside, like 54 or 56. something that's going to provide a lot of clearance. But you also bring up a good point about the outer ring not getting in the way of the front derailleur. More to research it seems. A pity if it wouldn't work because it'd be basically €150-200 for two normal chainsets that could link up. It'll cost almost that much just for the stoker cranks alone if we switch from 150 mm to 165 mm.

My wife definitely feels the difference between 170 and 165. She's continuously tried cranks over 170 mm and above and after about two hours gets really bad hip pain. 165 seems to be the perfect threshold for her. We thought it was smaller but when get got a Canyon Ultimate solo bike that came with 165s, she kept them on and was quite pleased.
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Old 06-24-20, 12:25 PM
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conspiratemus1
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Originally Posted by cpunerd
Thanks, that all makes a lot of sense. If we are to go with single-side, I'm now thinking of going with 30/46 and then something *really* big on the outside, like 54 or 56. something that's going to provide a lot of clearance. But you also bring up a good point about the outer ring not getting in the way of the front derailleur. More to research it seems. A pity if it wouldn't work because it'd be basically €150-200 for two normal chainsets that could link up. It'll cost almost that much just for the stoker cranks alone if we switch from 150 mm to 165 mm.

My wife definitely feels the difference between 170 and 165. She's continuously tried cranks over 170 mm and above and after about two hours gets really bad hip pain. 165 seems to be the perfect threshold for her. We thought it was smaller but when get got a Canyon Ultimate solo bike that came with 165s, she kept them on and was quite pleased.
See Chris W (post #7) in this thread: https://www.bikeforums.net/tandem-cy...ide-drive.html
My previous comment about ideas from the internet doesn't apply to Chris. I don't know him personally but if he says he's built something and it works, you can take it as given that he has and it does. He's in Switzerland, too, and might have ideas about availability of parts in country.
Good luck.
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Old 07-07-20, 11:36 PM
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cpunerd
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FYI we installed eTap derailleurs and levers on our current setup (works absolutely wonderfully). Having a large outer ring while using the inner and middle rings is a no-go though, the large ring interferes with the the front derailleur. We were able to put an 11-40 in the rear with a Wolf Tooth Road link and it shifts perfectly through all gears, including completely cross chained (I was quite surprised, but we don't intend to cross chain). But at the moment we have 39-52 in the front, as well as a decorative, unusable 30. We'd still ideally like to switch to single side, but we also just happened across a great deal on some stoker cranks, so we'll probably end up sticking with cross-over for now and swapping the front for a 2x semi-compact.
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Old 07-08-20, 03:13 AM
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If you wanted to keep the triple one option is to go with bar end or even downtube shifter for front deraileur. The double vs triple is of course a whole other oft-discussed topic.
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Old 07-08-20, 03:51 AM
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cpunerd
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We were far more interested in electronic shifting than in switching to a single-side drive, we just through that we might be able to do both at once. I still think we can but we're no longer as interested in it.
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