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My experiences using an 11-40t cassette with an Ultegra R8050 rear derailleur

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My experiences using an 11-40t cassette with an Ultegra R8050 rear derailleur

Old 06-22-19, 08:06 PM
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MoreLowGears
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My experiences using an 11-40t cassette with an Ultegra R8050 rear derailleur

This is a cross-post from ******, but I'm not allowed to post links yet so I copied it here.

tl;dr: My setup with a Roadlink and shorter chain (unchanged from the 11-32t cassette) works pretty well. The lower gear is a godsend when climbing 8%+ sections and makes them a lot more enjoyable. The shifting and gap size work well enough for regular riding, though I don't do fast club rides. The only significant downside is that the chain will derail when backpedaling in my tallest gear. This is really annoying, to the point where I'm tempted to try swapping back to my 11-32t for rides without steep climbs. I'm learning to forward pedal instead of backpedaling during descents, but it's easy to forget and honestly could be a little dangerous.

Wanting some lower gearing on my road bike, I decided to put a wider-range CS-M7000 11-40t cassette on my bike running the most recent Ultegra Di2 groupset. I found a number of articles and videos boasting positive experiences online. I enjoy having two extra low gears for my rides that go up into the steep local mountains, but I've had a few mechanical problems that I thought I'd share.

My bike originally had a CS-R8000 11-32t cassette. I really liked the gearing it provided, having comfortable, evenly-spaced gaps between cogs and providing a low enough gear with a compact crankset for hills and easy mountain rides. The 11-40 has a 31t instead of a 32t cog, with 35t and 40t cogs providing a lower gears. For the two lower gears, it trades the small jumps of 8-10% over four gears with an 18% and a 15% jump at the top end of the cassette. Since I only use my 11t and 12t cogs on steep mountain descents, where the large jump between the 11t and 13t doesn't matter, this is a pretty good compromise.

The cassette was easy to install and...it worked! I replaced the cassette and cut a new chain with two more links than the old one and readjusted my rear derailleur; the mountain bike cassette plus spacer sat a tiny bit closer to the hub, by maybe 1mm. I adjusted the B-screw to keep tension on the chain, giving the 40t cog a few more mm of clearance than it needed.

Despite that there were some significant issues. Shifting performance had noticeably degraded. Upshifts were more jarring, even when accounting for the larger gaps between cogs, causing the pedals to jump a bit further. Downshifts got rougher when shifting through multiple cogs at once. This seemed to be explained by the upper jockey wheel position: in the small cogs it was about 3" away from the cogs, and in the big cogs the pulley was 1-2" further back. The derailleur cage curve was noticeably shallower, though still safe. Finally , when in my tallest gear (50/11), the chain was extremely prone to dropping when backpedaling, to the point where it'd happen once a ride on steep descents.

I tried a few things to improve shift quality. I installed a Wolftooth Roadlink DM so I wouldn't have to crank the B-screw as much. This didn't affect capacity at all but made the pulley move in a straighter line up and down the cassette, and improved shifting slightly. I removed two links from the chain (back to my original length), which allowed me to take out the B-screw even more and bring the jockey wheel a lot closer to the cogs. The B-screw position was still being determined by slack in the chain the little-little, but it was pretty close to ideal and noticeably improved shifting. When cross-chaining big-big the chain had more tension between the pulleys and got a little louder, but the derailleur arm had enough extra swing that it seems safe. I decided to not use big-big to avoid the extra friction and wear on the pulleys.

Shifting was now significantly better, though not as good as it was with 11-32. I don't have a Shimano mountain bike derailleur lying around to compare it to but I would guess the performance isn't too far off. Downshifts were smooth and upshifts were manageable even under full load. The chain derailment proved more vexing.

The chain derailment seemed to have two causes. First, there was a lot of friction in the drivetrain. Once the wheel was up to speed, even moderate backpedaling would cause the chain to go completely slack for a second. If I backpedaled a little less than half a crank revolution, it would introduce enough slack that the natural jerkiness of the chain would throw it onto a lower cog, or off the cassette entirely. Second, one of the teeth in the 13t cog was angled in such a way that it would briefly catch the chain while backpedaling (forward pedaling was fine). With a little speed this would dramatically throw the chain off the cogs and derail it. This exacerbated the first cause, though I suspect that it wouldn't be enough to derail the chain without the extra friction.

At first I thought the friction was caused by my freehub, so I serviced it myself then had my local bike shop do it. This made it a little better but neither attempt fixed the problem. The bike shop claimed the problem stemmed from the top pully being so far away from the cog, but I was suspicious of this. Temporarily loosening the B-screw did help a little bit, but that wasn't an option because it created slack in little-little. I can't tell where the friction was coming from but I guess it's mostly from somewhere along the pulleys. It's a lot more friction than I got when cross-chaining with my original setup.

I tried installing a clutch derailleur (the Ultegra RX one). This helped with the derailments, with the clutch transferring the resistance to the pedals while it held the chain in place, but it was still possible to derail the chain if I didn't attempt to slow down my feet in time after noticing the resistance to backpedaling. It also created problems with downshifts, making them take seconds to complete in the middle gears even under minimal load. Because the pulley was further away from the cogs, the chain could bend more and the pulley can't push the chain against the cassette ramps very hard. Rather than release some slack to let the chain climb the ramp, the derailleur held in place and simply pulled the chain back to its original cog. This was much worse for me than the derailments. I considered riding with the clutch disengaged and stopping to engaging it before steep descents, but this didn't seem like it justified the cost of the new derailleur so I plan to return it.

So, that's what I'm working with today. The lower gearing is too good to pass up! I'm training for the Buff Epic century in Colorado (100mi, 7.8k ft) so I'm using it a lot, but for shorter rides I'll consider swapping out cassettes and using the 11-32 with the roadlink and a different b-screw setting. Thanks for reading. If this post gains traction I'll post videos of the chain derailing and the chain catching on the 12t cog.
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Old 06-22-19, 08:43 PM
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pdoege
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I think you may be able to do better on the chain derailment. I run the same cassette on two bikes with Ultegra. One with the clutch and one without. My chain(s) stay out on road without the clutch. The clutch removes all the slop.

I'm going to buy Shimano's gravel Di2 rear mech when it comes out just to see what it brings to the table.

Good luck! I really like mine in the mountains around town.
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Old 06-23-19, 02:20 AM
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Other option would be to with a go sub-compact crank. I'm running Absoluteblack 46/30 rings which are great but super expensive and you spin out at 33ish mph. Shimano coming out with their GRX line lets you do it with less of a compromise thanks to their 48/31 chainrings and it is cheaper. Put a 11-34 cassette on the back and you have close enough to that compact + 11-40 with tighter spacing.

My bike is my road/gravel bike and I didn't want to deal with messing with things every time I swapped wheelsets hence the subcompact. I run 11-32 cassettes on both to keep my tighter spacing and can still climb everything on the gravel rides around here, doesn't hurt on some of the road climbs either.

Last edited by Canker; 06-23-19 at 02:25 AM.
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Old 06-23-19, 01:44 PM
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MoreLowGears
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Originally Posted by pdoege View Post
I'm going to buy Shimano's gravel Di2 rear mech when it comes out just to see what it brings to the table.
I'm really interested in that, I'm glad Shimano is going in that direction. I'm not sure it'll work though: the RX812-GS (the one that supports large cassettes) only has a 31t capacity, which is 8 less than the Ultegra GS derailleur. It's designed for 1x and I suspect it won't have enough capacity to wrap the extra 16t from the front. If it's possible to swap the derailleur cage with the 2x version though, that would be pretty sweet.

Originally Posted by Canker View Post
Other option would be to with a go sub-compact crank. I'm running Absoluteblack 46/30 rings which are great but super expensive and you spin out at 33ish mph. Shimano coming out with their GRX line lets you do it with less of a compromise thanks to their 48/31 chainrings and it is cheaper. Put a 11-34 cassette on the back and you have close enough to that compact + 11-40 with tighter spacing.
I'd really like to go subcompact, but it's not feasible for me right now. 46/30 x 11-36 would provide the same low gear while losing a high gear at the top I don't really need. I have Ultegra R8000 cranks with a dual-sided Stages power meter, and unfortunately the power meter takes up enough space that the AbsoluteBlack 46/30 oval chainrings wouldn't fit. I'm not sure it's possible to buy a 46/30 crankset with a power meter right now, but power cranks are pretty expensive and I'm not willing to shell out for another set.
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