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-   -   Double to single (!) with right sided drive and SRAM Eagle (https://www.bikeforums.net/tandem-cycling/1098920-double-single-right-sided-drive-sram-eagle.html)

 willhs 02-25-17 10:11 AM

Double to single (!) with right sided drive and SRAM Eagle

I'm looking to put together another tandem, and I think the tech is there to do a 1x chainring build now with SRAM Eagle. Hear me out. I would love to have any thoughts from the experienced users here. This will be my third tandem build. Here is a comparison of the current 10 speed 11-36 cassette that I have now compared to a SRAM Eagle setup:

HTML5 Gear Calculator

They are functionally the same. Sure, there are smaller gaps if you are constantly switching between the front chainrings, but in practice we shift the RD for the vast majority of shifts, and if we are going to hit a climb that will require easier gears I shift the FD ahead of time so we have that additional range when we need it (to avoid shifting the FD in situations of high torque). So, all of my gear shifts are a 12-17% jump on the cassette of the 11-36, and they remain in that range on the SRAM Eagle. But, the Eagle cassette has a 500% range and my current setup is 445%, so if anything there is a gain from the change, and there is never a concern about when and how to shift the front derailleur.

Actually, if you exclude the last 2 bailout gears and the 10t "bombing down the hill" gear, the average jump between the major gears is 14.8% on the Eagle, and 13.7% on the 11-36. I'm not sure if the difference is big enough to be perceivable.

This also makes it very feasible to make a right sided drive, use "regular" cranks," lighten up and simplify the setup.

Then we get to chainline for the drivetrain and timing belt. Fortunately, SRAM has a really excellent specifications document with chainline and chainstay clearance info.

SRAM Eagle wants a chainline of about 49mm +/- 2. The rear dropouts are 135 OLD. So, for the rear crank (see page 33 of the above), the wide axle 2x crank seems to be perfect for clearing the chainstays with the crank and giving 42.5mm from BB center to inner aspect of the inner chainring and 50.5mm to the big ring (within spec for Eagle, but could use an offset narrow/wide chainring to get it just right). Then, a 1x crank in the front (see page 47) would give a 43.7mm from BB center to inner aspect of chainring. This would make it an inner ring right sided timing chain, which would keep it out of the way.

To make it shift, I plan to use the X-shifter wireless electronic shifter with the "road" shifters. I'm a backer on kickstarter. I think this is the only way to integrate Eagle with a road bar setup.

https://www.kickstarter.com/projects...less-smart-shi

I haven't seen any example of this being done, so I wanted to outline the build and see if anyone has any thoughts.

 racefacelefty 02-25-17 11:58 AM

I agree this is highly feasible, particularly with the ratio jumps you already have. What blade will you run on the front?

I too backed X-shifter (new name coming) with a view to using it on a 3x11 Red22 system for the Hydro brakes component and smart mapping of the shifts to get the close ratios I crave. Due to delays I went 2x11 using a SRAM FD with yaw and a short cage RD as I run 10-25. with same side timing chain. The 2x11 is working as expected, the X-shifter might improve shifts under load but not much more. Although I could conceivably add the 13 that is missing on my cluster and do 12 speed like that.

 willhs 02-27-17 10:37 AM

Originally Posted by racefacelefty (Post 19402293)
I agree this is highly feasible, particularly with the ratio jumps you already have. What blade will you run on the front?

I too backed X-shifter (new name coming) with a view to using it on a 3x11 Red22 system for the Hydro brakes component and smart mapping of the shifts to get the close ratios I crave. Due to delays I went 2x11 using a SRAM FD with yaw and a short cage RD as I run 10-25. with same side timing chain. The 2x11 is working as expected, the X-shifter might improve shifts under load but not much more. Although I could conceivably add the 13 that is missing on my cluster and do 12 speed like that.

That's too funny. Yeah, I think the X-shifter could be a big help for the franken-gear setups that tandems can end up with. Thanks for your help. I'll report back on the build once I get all the parts.

 bomberesque 01-20-18 01:26 AM

Hi, bit of a zombie thread, but did you get this working? I have a new tandem frame inbound and am thinking of eagle. I'll have flat bars so shifter is not a problem but wondering if you got the chain line right with 1 double and 1 single crankset

 willhs 01-28-18 03:13 PM

Originally Posted by bomberesque (Post 20120341)
Hi, bit of a zombie thread, but did you get this working? I have a new tandem frame inbound and am thinking of eagle. I'll have flat bars so shifter is not a problem but wondering if you got the chain line right with 1 double and 1 single crankset

I did work it out which parts to buy but didn't do the drivetrain rebuild because the stock drivetrain was good enough (it was for a friend) and they might not be using it enough to justify adding bling.

You can Google the tech documents for these parts yourself if you want to verify. I already did. The chainline should definitely work, and with the new Eagle I really think this is a viable tandem option.

Pasted from my build list:

Front crank: SRAM Force CX1 Crank at either 172.5mm or 175mm length depending on what you are used to on your bike, with 110BCD (not 130) and a 50T narrow wide chainring or no chainring (GXP version not BB30): Prices around \$200 without ring and \$250 with.

Rear crank: SRAM S902 or S952 crankset at anywhere from 160mm to 172.5mm length with the compact 110 BCD as well. It can either have the 50/34 rings attached, or if you can find a cheaper one without the rings that is fine because you'll need to replace the big ring anyway and the small ring will be cheap (again get the GXP version not BB30).

You will need a total of two 34t 110BCD chainrings and one 50t 110 BCD narrow-wide 1x chainring (also called a CX1 chainring). So if you don't get them packaged with the above cranks, you need to buy them separately. You'll definitely need to buy at least one 34t ring in addition.

 twocicle 02-02-18 03:29 PM

Originally Posted by willhs (Post 20136912)
I did work it out which parts to buy but didn't do the drivetrain rebuild because the stock drivetrain was good enough (it was for a friend) and they might not be using it enough to justify adding bling.

...

You will need a total of two 34t 110BCD chainrings and one 50t 110 BCD narrow-wide 1x chainring (also called a CX1 chainring). So if you don't get them packaged with the above cranks, you need to buy them separately. You'll definitely need to buy at least one 34t ring in addition.

If you had tried the above, a big problem would have been with that choice of 50t "big" ring. Narrow-wide 1x chainrings are not intended to allow a chain to derail (ie: shift) and so do not have ramps, pins, ramping or tooth profiling to assist with chainring shifting. With the 1x chainring, this setup would simply not shift reliably to/from the big ring.

Solution is simple, just get a matched set of regular 50/34 compact crank 110bcd chainrings, then slap the 34t timing ring on the outside.

You would obviously need to add significant spacers to provide a gap to the timing ring, sufficient enough to prevent chain to chain contact when shifting up to the big ring (chains frequently ride on top of the big ring and a bit to the outside and so can tangle with a SSD timing chain).

 willhs 02-04-18 01:37 PM

Originally Posted by twocicle (Post 20146946)
If you had tried the above, a big problem would have been with that choice of 50t "big" ring. Narrow-wide 1x chainrings are not intended to allow a chain to derail (ie: shift) and so do not have ramps, pins, ramping or tooth profiling to assist with chainring shifting. With the 1x chainring, this setup would simply not shift reliably to/from the big ring.

Solution is simple, just get a matched set of regular 50/34 compact crank 110bcd chainrings, then slap the 34t timing ring on the outside.

You would obviously need to add significant spacers to provide a gap to the timing ring, sufficient enough to prevent chain to chain contact when shifting up to the big ring (chains frequently ride on top of the big ring and a bit to the outside and so can tangle with a SSD timing chain).

Actually that's the point. It's intentionally a 1x drivetrain to go with an Eagle cassette. The chain line of the front/captain crankset matches up within a mm of the inner chainring of the rear/stoker crankset (in other words it is a better match than the outer chainring approach you mention), and the 50t narrow wide chainring lines up perfectly with the chainline of SRAM Eagle. It would be the lightest tandem drivetrain without sacrificing any gear range and it would be pretty cheap. Some of the shifts would be a few percent larger than what I am currently using but I don't think it would make an issue.

Thanks,
Will

 twocicle 02-07-18 09:55 AM

Ah, yes. I missed that you were going for a 1x.

 pickettt 02-07-18 01:09 PM

My gearing is just fine, but this is intriguing, I wanna do it just to do it.

 kevrider 02-10-18 10:34 PM

pardon my ignorance, but did SRAM change their cable pull for Eagle? that mix-n-matchability between road and mtn components was one of the benefits of SRAM over Shimano. could be moot with a 1x12 road group on the horizon.

 twocicle 02-14-18 12:02 PM

Just a word of caution,

Although my single bikes (mtb & Fat) both have Eagle 10-50 and so far have performed great, our LBS has seen quite a few broken teeth issues on these cassettes. My cassettes show some marring but nothing broken to date. Tandem use/forces are a lot more significant, so extrapolate...

 willhs 02-16-18 12:25 PM

Originally Posted by twocicle (Post 20169285)
Just a word of caution,

Although my single bikes (mtb & Fat) both have Eagle 10-50 and so far have performed great, our LBS has seen quite a few broken teeth issues on these cassettes. My cassettes show some marring but nothing broken to date. Tandem use/forces are a lot more significant, so extrapolate...

Interesting... I wonder if the GX heavier/cheaper version would also be more durable.

And with regards to cable pull differences, that's the reason for using X-Shifter. The wireless electronic shifting would also save weight. They are just starting to ship to Kickstarter backers.

Will

 twocicle 02-17-18 12:19 PM

Also try to get a XD steel driver body, not AL. The latter tends to crack under high load/torque (as seen from single bikes with ultra low gearing).

 oldacura 02-19-18 03:52 PM

Originally Posted by twocicle (Post 20169285)
Just a word of caution,

Although my single bikes (mtb & Fat) both have Eagle 10-50 and so far have performed great, our LBS has seen quite a few broken teeth issues on these cassettes. My cassettes show some marring but nothing broken to date. Tandem use/forces are a lot more significant, so extrapolate...

Seems like the teeth most likely to break would be on the smallest (10 tooth) cog. You might have 5 teeth at most in mesh with the chain. On the 50 tooth cog, you should have at least 20 teeth in mesh.

 twocicle 02-20-18 09:55 AM

Originally Posted by oldacura (Post 20178475)
Seems like the teeth most likely to break would be on the smallest (10 tooth) cog. You might have 5 teeth at most in mesh with the chain. On the 50 tooth cog, you should have at least 20 teeth in mesh.

It is the largest 3 cogs that are a problem, but especially the AL 50T + sharp chain angle. Tandem chainstays may not be as short as on the singles that have thrashed the cassette, but it does demonstrate a certain amount of fragility. Not sure why the 2nd and 3rd cogs are having an issue as well. I would have assumed no problem as they are not AL. Anyway, obvious rule for tandems is to try avoiding AL for cogs and freehub body.

 PMK 02-22-18 06:14 PM

FWIW, many of the teams on tandem mountain bikes run very similar with good results. Assuming you get enough gearing and the jumps from gear to gear fit your spin, it should reliably work well.

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