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Converting triple to double crankset woes

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Converting triple to double crankset woes

Old 02-16-22, 10:17 PM
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Converting triple to double crankset woes

I’m building a tandem with a 2x11 Shimano drivetrain. I already have a set of FSA Gossamer tandem cranksets, which are designed for a 3x9 drivetrain. So I’m trying to convert the triple stoker’s crank to a double.

I got new chainrings for the task, 11-speed 48-32. I put them on the middle and inside position. But after I installed them, I noticed the gap between the chainrings was way too narrow. The outer ring’s chainring bolt almost rubs up against the inner ring.

How do I fix this—with chainring spacers? And if so is there a way to determine what size (and that it’ll still work with my front derailleur)? Or can I mount the chainring bolt the other way so it doesn’t stick out so much? I’m stumped.

And also, why is this happening? Is it because I’m using an inner ring that has more teeth (ie taller) than what the crankset is designed for?

thanks for any insight.
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Old 02-16-22, 11:52 PM
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Originally Posted by samkl
I’m building a tandem with a 2x11 Shimano drivetrain. I already have a set of FSA Gossamer tandem cranksets, which are designed for a 3x9 drivetrain. So I’m trying to convert the triple stoker’s crank to a double.

I got new chainrings for the task, 11-speed 48-32. I put them on the middle and inside position. But after I installed them, I noticed the gap between the chainrings was way too narrow. The outer ring’s chainring bolt almost rubs up against the inner ring.

How do I fix this—with chainring spacers? And if so is there a way to determine what size (and that it’ll still work with my front derailleur)? Or can I mount the chainring bolt the other way so it doesn’t stick out so much? I’m stumped.

And also, why is this happening? Is it because I’m using an inner ring that has more teeth (ie taller) than what the crankset is designed for?

thanks for any insight.
I have no experience with tandem-anything, so apologies if anything below is completely off, but looking at the aspect of going triple to double by using crankset originally meant for triple ... I wonder if that was a good idea because:

1) Your chainline will be different than usual (for a double) and wonder how the FD suitable for this setup would fit that.

2) I noticed lots of times chainrings are sold for specific crankset with specific position in mind. The 48 is something I would normally only consider fixing on the outer position. I suspect the ramps might have been optimised in a way that they expect e.g. the adjacent chainring be at least 34.

EDIT: Just want to add that I would totally expect an FD for triple to work with e.g. 48 on the outer and 34 on the middle ring (if it fits I wonder about your BCD now) if limit screw is properly set. I had a triple FD working fine for compact double 50/34 but that was a double crankset as such. I don't remember but I don't think I would have been able to put the 34 on the middle position due to the BCD on that triple meant for 39 middle ring.

Last edited by am8117; 02-16-22 at 11:59 PM.
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Old 02-17-22, 09:58 AM
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all the triples i've set up use chainring spacers for the smallest ring. your LBS should have on hand
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Old 02-17-22, 12:17 PM
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An outer ring will be designed for the bolts to inset on the outside, with the flats being flush on the inside.

By moving that ring to the inside, the bolts stick out slightly.

If it is a "modern" ring with ramps and pins, then it won't be reversible.

If you purchase either a vintage large ring, or a single speed large ring, then the ring will be reversible, and the inset for the chainring bolts would be on the proper side to allow the chainring bolts to be inset.

The drop stop pin (if you have one) should be threaded, and could be mounted on either side.

It is likely that shifting of a the single speed or vintage ring won't be quite as smooth as modern ramped and pinned rings, but you may be able cut a few short teeth. The lowest power (best place to shift) is with the cranks vertical, so trim some of those teeth.

You could also contact some chainring manufacturers and ask if the could make a ramped and pinned 48T "middle" ring. Perhaps Vuelta?
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Old 02-17-22, 01:02 PM
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Originally Posted by CliffordK
An outer ring will be designed for the bolts to inset on the outside, with the flats being flush on the inside.

By moving that ring to the inside, the bolts stick out slightly.

If it is a "modern" ring with ramps and pins, then it won't be reversible.

If you purchase either a vintage large ring, or a single speed large ring, then the ring will be reversible, and the inset for the chainring bolts would be on the proper side to allow the chainring bolts to be inset.

The drop stop pin (if you have one) should be threaded, and could be mounted on either side.

It is likely that shifting of a the single speed or vintage ring won't be quite as smooth as modern ramped and pinned rings, but you may be able cut a few short teeth. The lowest power (best place to shift) is with the cranks vertical, so trim some of those teeth.

You could also contact some chainring manufacturers and ask if the could make a ramped and pinned 48T "middle" ring. Perhaps Vuelta?
Interesting, thanks. So you're saying the bolts are backwards, and the ring as currently oriented may be backwards? I'll look into finding a ramped/pinned middle ring, too.
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Old 02-17-22, 01:42 PM
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Is the big ring on the wrong side of the spider arms?

Which might be what CliffordK is saying, but just in way more words.
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Old 02-17-22, 01:55 PM
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Originally Posted by samkl
Interesting, thanks. So you're saying the bolts are backwards, and the ring as currently oriented may be backwards? I'll look into finding a ramped/pinned middle ring, too.
I don't think the ring is on backwards. I see the drop stop pin in the right place (but very short due to being on the inside). You'll need either a longer pin, or perhaps just leave it off. Or, adding a bash guard instead.

But, the holes are countersunk on the wrong side, and it is doubtful there is enough material to countersink them on the inside without filling and redrilling.

Thus, you need a ramped and pinned middle ring, or mounting a ring without the ramps and pins backwards.

The tooth profile for modern rings is wrong to simply reverse them.

I am seeing a few 42T "middle" chainrings on E-Bay.

Thinking back, there was an interesting recent thread by @jonwvara who makes custom tripilizer chainrings.

He might have some ideas of reverse countersunk larger "middle" chainrings.
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Old 02-17-22, 02:00 PM
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Originally Posted by Iride01
Is the big ring on the wrong side of the spider arms?

Which might be what CliffordK is saying, but just in way more words.
Apparently intentionally put there so a 48/32 could be mounted on 130 BCD.

48/34 would be the easy solution, but would require 110 BCD crankset.

Or, even hunting down a 33T 110 BCD ring.
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Old 02-17-22, 02:04 PM
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I know OP wants 2x11 but here is a thought: consider keeping the 3 rings upfront even it you have to friction shift it or find a left hand 3 speed shifter (they were around for 9 speed) and use an 11 speed right hand shifter.

may be less hassle.

it does seem from recent posts, tandems are not high on the priority list for manufacturers
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Old 02-17-22, 02:10 PM
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Maybe I'm misunderstanding the OP. Is this a 3x crank with the big ring removed to make the 2x or is this a crank that is intended to be 2x?

I'm also having an issue with the chainring nuts being so close to the small ring. Which I guess gets back to what you were saying about BCD/PCD's.

I've never really played around with changing rings. I've always just bought a new crank. Though I do switch out 2x 50/34 and 2x 52/36 rings depending on where I'm riding. But making a 3x crank a 2x crank, I wouldn't. (though I do realize there isn't a large variety of components made for tandems)

Last edited by Iride01; 02-17-22 at 02:14 PM.
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Old 02-17-22, 02:48 PM
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Originally Posted by squirtdad
I know OP wants 2x11 but here is a thought: consider keeping the 3 rings upfront even it you have to friction shift it or find a left hand 3 speed shifter (they were around for 9 speed) and use an 11 speed right hand shifter.

may be less hassle.

it does seem from recent posts, tandems are not high on the priority list for manufacturers
I did think about that, but I don't think my Ultegra mid-cage derailleur has enough capacity for a triple. Or if any road derailleur can do 3x11.

Otherwise I could keep the triple, but would have to replace the rest of the drivetrain :/. And unfortunately all the tandem double cranksets out there are insanely expensive.
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Old 02-17-22, 02:49 PM
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Originally Posted by Iride01
Maybe I'm misunderstanding the OP. Is this a 3x crank with the big ring removed to make the 2x or is this a crank that is intended to be 2x?

I'm also having an issue with the chainring nuts being so close to the small ring. Which I guess gets back to what you were saying about BCD/PCD's.

I've never really played around with changing rings. I've always just bought a new crank. Though I do switch out 2x 50/34 and 2x 52/36 rings depending on where I'm riding. But making a 3x crank a 2x crank, I wouldn't. (though I do realize there isn't a large variety of components made for tandems)
This is a 3x crank with the big ring removed, and with new, replacement rings in the middle and inner positions.
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Old 02-17-22, 03:59 PM
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Originally Posted by samkl
I did think about that, but I don't think my Ultegra mid-cage derailleur has enough capacity for a triple. Or if any road derailleur can do 3x11.
That is a good point. Make sure that your derailleur has the adjustment on the inside to go down to the inner ring.

I thought the fd-cx70 did a triple, but maybe not.

The fd-6703 will do a triple.

Using a 6703 left shifter and a 6800 right shifter might not look too bad.
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Old 02-20-22, 08:39 PM
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Well, I took it to a respected bike shop and the head mechanic said it wouldn't work. So, onto plan B...
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Old 02-20-22, 09:35 PM
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Ask that 'head mechanic' to tell you the smallest chainring that will fit the following bolt circle cranks

110mm

130mm

144mm

if he says 33 - 38 - 41 without looking them up ! ....( he should be able to tell you by rote ) ....then he may be close to knowing his head from a hole in the ground when it comes to bicycles

if he indiscriminately uses the word 'bike' rather than 'bicycle' ....be very cautious as this indicates a lack of thought and attention to detail
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Old 02-21-22, 05:21 PM
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CliffordK has identified the issue correctly, I believe.

I just "doublized" a 50-40-30 triple to 46-30. It went well, but it did depend on finding a 10-speed compatible, ramped and pinned 46T middle ring to deal with the counterbore issue. Mine is a 135 BCD ring from Specialities TA, and meets all the requirements plus is beautifully machined and finished. A quick check suggests that they do offer 46T middle rings in 110 and 130 BCD, but not 48T. The other place to look, if only to drool over the amazing choices, is Spa Cycles. Not sure if they have resumed shipping to Canada/States.

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