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Chain drop a cog while back pedaling in big big gears

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Chain drop a cog while back pedaling in big big gears

Old 12-30-20, 10:36 AM
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Ridemehard
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Chain drop a cog while back pedaling in big big gears

Recently got into road cycling and learning to work on my own bike. I have a question to the above title and can't seem to an answer. I have a Trek Domane SL6 with R8000 groupset. The bike was working perfectly until I installed a set of new Prime carbon 50 wheelset I got over Christmas. When the chain is in the large chainring and largest cog (while on the bike stand running through all the gears) and if I back pedal the chain would drop a cog. I readjusted the rear derailleur thinking the new wheel might have off set it. The derailleurs (F&R) are aligned and shifting through all gears fine. I tried cleaning the chain/cog and lube chain but same thing. I don't ride in the big and big gears but it bothers me and I don't recall this happening while in the original wheelset. Before I swap back to my original wheelset is this normal? TIA
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Old 12-30-20, 10:48 AM
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dsaul
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It's normal.
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Old 12-30-20, 11:10 AM
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One solution is to always pedal forwards.
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Old 12-30-20, 11:27 AM
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When you pedal forwards the rear derailleur guides the chain onto the cog. When you pedal backwards there is nothing guiding the chain onto the top of the cog so that in cross chained situations the chain will tend to drop off onto the next smaller cog. My bet is that your old wheel might also exhibit the same symptom
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Old 12-30-20, 11:28 AM
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Originally Posted by Koyote View Post
One solution is to always pedal forwards.
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Old 12-30-20, 11:30 AM
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Originally Posted by alcjphil View Post
When you pedal forwards the rear derailleur guides the chain onto the cog. When you pedal backwards there is nothing guiding the chain onto the top of the cog so that in cross chained situations the chain will tend to drop off onto the next smaller cog. My bet is that your old wheel might also exhibit the same symptom
Great info. Thanks!
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Old 12-30-20, 11:30 AM
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Originally Posted by dsaul View Post
It's normal.
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Old 12-30-20, 11:33 AM
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+1. Normal. You're doing two things that bike drivetrains don't like; running big-big and backpedaling.
You can kinda get away with it on old 5-6 speed bikes, but on a modern 10-sp, short chainstay road bike, what you describe will dump the chain pretty much every time.
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Old 12-30-20, 01:12 PM
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Originally Posted by Ridemehard View Post
The derailleurs (F&R) are aligned and shifting through all gears fine.
When you say "aligned", do you mean aligned or adjusted?
Have you actually checked the Hanger Alignment?

Barry
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Old 12-30-20, 02:16 PM
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I had this happen once but it was because I did not have the front derailleur trimmed properly. I had the front too far to the right so when I backpedaled, the front acted as a chain guide for the rear cogs. I rarely cross chain at all but once in a blue moon, I will get taken by surprise, then stuff happens.
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Old 12-30-20, 03:22 PM
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Ridemehard
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Originally Posted by Barry2 View Post
When you say "aligned", do you mean aligned or adjusted?
Have you actually checked the Hanger Alignment?

Barry
The FD & RD were adjusted the best of my ability and shifts good. The little jockey pully on the RD are aligned when on the the smallest cog and when on largest cog. I can't confirm if the hanger is straight without the proper tool but it looks straight to me. Again when I had my original rear wheel on I don't recall experiencing this issue.
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Old 01-01-21, 02:25 AM
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My Domane 50/34 11-34 does it. Normal.
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Old 01-01-21, 10:06 AM
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I would say, do the same thing with your old wheelset and see if the chain drops. If not, there is something different in your new wheel's cassette. In that case try putting your old cassette on the new wheel and see what it does.

Probably you should verify that the rear triangle of the frame is in-line using the string method for frame alignment (Google that, it's easy), before aligning the dropout. You could also borrow a Park Dropout Alignment Gauge tool and see if you can align the derailleur hanger better than it is. There was a Model 1 and now there's a Model 2, both will work fine. I don't really see how this could pull the chain sideways, but if you are out of alignment it will be better.

It's also possible your rear triangle is misaligned, with the rear wheel offset toward the non-drive side. If this is the case then in big-big the chain angularity at the back is more pronounced. The big chainring tends to pull the chain off of the biggest cassette sprocket more strongly than if the wheel was sitting more toward the drive side. Essentially the rear chainline and the front chainline might not match.

It could also be that the rear triangle is correctly aligned and the wheel is sitting in the same plane as the main triangle, and has the correct axle spacers. If the chainline is greater in the front, you could have the wrong chainset, wrong BB, or wrong spacers in the BB.
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Old 01-01-21, 10:24 AM
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It's a carbon frame. It HAS to be perfectly aligned. There is no way it can get out of alingment either. You can't 'bend' it. They come out of molds, so they're all the same. What is happening to the OP is perfectly normal.
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Old 01-01-21, 01:23 PM
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My take of this is that the new wheel may have the cassette a little more inboard (closer to the centerline of the frame) and is enough to cause this. If the OP needed to adjust the RD's low limit screw looser to get shifting to the largest cog, this is the case. This would also may mean that the high limit was also tightened.

If comparing new vs old wheel, does the smallest cog sit the same distance from the inside of the dropout? Don't know what specific hub and cassette configurations the OP has so can't confirm.
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Old 01-01-21, 06:21 PM
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Ridemehard
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Originally Posted by GAtkins View Post
My Domane 50/34 11-34 does it. Normal.
thanks for confirming this. I have the exact factory setup.
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Old 01-01-21, 06:36 PM
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Originally Posted by KCT1986 View Post
My take of this is that the new wheel may have the cassette a little more inboard (closer to the centerline of the frame) and is enough to cause this. If the OP needed to adjust the RD's low limit screw looser to get shifting to the largest cog, this is the case. This would also may mean that the high limit was also tightened.

If comparing new vs old wheel, does the smallest cog sit the same distance from the inside of the dropout? Don't know what specific hub and cassette configurations the OP has so can't confirm.
The wheelset is prime blackedition 50mm. I googled the hub spec for the wheel and is by Novatec. Im having clunking sound from the hub during my first ride. I think the hub is having engaging issues (pawls being sticky from the grease?) but the clunking sound has gotten less after riding 10 miles. Still waiting to hear back from CRC if the clunking sound will fix itself, like a break in period.
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Old 01-02-21, 01:38 PM
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Originally Posted by Barry2 View Post
When you say "aligned", do you mean aligned or adjusted?
Have you actually checked the Hanger Alignment?

Barry
Hanger alignment has nothing to do with backpedaling. The chain will fall of the top of the cog when backpedaling. It doesn't matter how you align the RD or RD hanger, it's not going to change what happens at the top of the cog.
Short wheelbased derailleur bikes are not designed for backpedaling.
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