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Tonic Fall Guy
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Installed my mech on my new Scout (it's an XT) and it shifts fine throughout the range, but when I pedal backwards in the biggest gear (42) it drops gears after a few revolutions..Thinking maybe the B screw? The limits are right. Thanks!
 

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No. Anything you do below the cassette won't affect what's happening on top when backpedaling. Basically don't back pedal more than 1/2-3/4 of a crank revolution. The chain may start to drop to the next smaller cog but it'll pick up when you start pedaling forward.
 

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Have you checked alignment of your derailleur hanger? Or had it checked?

Odd, as I have used everything from 1x10 with a 42 expander to 1x11 with a 42/46 to Eagle 1x12 with a 50....now use an e*13 with a 9-46 12sp and haven't had that issue....

I can backpedal as much as I want and not drop a single gear. Sometimes do backpedal just to laugh at my Onyx hub lack of sound...

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My SRAM NX 1x11 and GX 1x12 do the same thing. It's normal, although annoying.
YES! my 2019 full stache 8 w/ sram gx 1x12 is doing the same thing. My lbs said they messaged trek and techs said its going to be normal on (high 2 gears) unless you push bottom bracket in such a few mm ( to evenly line up) , and or add an oval chain ring to prevent from chain
 

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Installed my mech on my new Scout (it's an XT) and it shifts fine throughout the range, but when I pedal backwards in the biggest gear (42) it drops gears after a few revolutions..Thinking maybe the B screw? The limits are right. Thanks!
my lbs basically told me not to back-pedal as much, (where it slaps back a gear).. or try adding an oval chain-ring?
 

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YES! my 2019 full stache 8 w/ sram gx 1x12 is doing the same thing. My lbs said they messaged trek and techs said its going to be normal on (high 2 gears) unless you push bottom bracket in such a few mm ( to evenly line up) , and or add an oval chain ring to prevent from chain
Well, if chainline is messed up, it'll definitely cause issues with this. If bottom bracket was improperly installed with respect to spacers for proper chainline, it could cause this.
I'd check your bottom bracket and make sure that the correct number of spacers on each side were used for the rear hub spacing and BB shell size.
I'm don't see how an oval chainring going to help that.
I've attached photos of my current setup. 148 boost chainline Niner Rip 9 AL. 73mm shell. Zero spacers on the drive side of my Wheels Mfg BB. SRAM X01 11sp RD converted to 12sp (e13 kit) and e13 9-46t cassette.
I can backpedal all day and not drop off the 46t ring.
Try measuring BB and check how many spacers (if there are any) are between the shells and frame.
If it is a 73 bb shell, shouldn't have any spacers inside it on it at all. A 68mm shell will usually have 1 spacer on each side for SRAM based drivetrains. Shimano usually indicates 2 on drive side, 1 on non-drive side (not sure why they indicate this, as 68+2.5+2.5+2.5 is NOT 73...it's 75.5, which would seem to make chainline incorrect).
Other thing to check is to make sure that your chainring that was installed is the correct spacing for your bike. If you get a non-boost chainring on a boost bike, that will cause issues with chainline as well, which could cause something like this.


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Well, if chainline is messed up, it'll definitely cause issues with this. If bottom bracket was improperly installed with respect to spacers for proper chainline, it could cause this.
I'd check your bottom bracket and make sure that the correct number of spacers on each side were used for the rear hub spacing and BB shell size.
I'm don't see how an oval chainring going to help that.
I've attached photos of my current setup. 148 boost chainline Niner Rip 9 AL. 73mm shell. Zero spacers on the drive side of my Wheels Mfg BB. SRAM X01 11sp RD converted to 12sp (e13 kit) and e13 9-46t cassette.
I can backpedal all day and not drop off the 46t ring.
Try measuring BB and check how many spacers (if there are any) are between the shells and frame.
If it is a 73 bb shell, shouldn't have any spacers inside it on it at all. A 68mm shell will usually have 1 spacer on each side for SRAM based drivetrains. Shimano usually indicates 2 on drive side, 1 on non-drive side (not sure why they indicate this, as 68+2.5+2.5+2.5 is NOT 73...it's 75.5, which would seem to make chainline incorrect).
Other thing to check is to make sure that your chainring that was installed is the correct spacing for your bike. If you get a non-boost chainring on a boost bike, that will cause issues with chainline as well, which could cause something like this.


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I agree, it can be a # of things causing, you must go eliminating 1by1, which most will not do including myself. I wonder if its just my bikes geometry causing this, due to offset geo
Bicycle tire Tire Wheel Bicycle wheel Bicycle frame
 

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MY opinion is that the issue is coming from chain ring or/and bottom bracket area. I do not think it has to do with cassete or derailleur nor chain. I was a little unhappy that my lbs did not fix this issue, (or maybe they did not want to mess with it?)
 

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MY opinion is that the issue is coming from chain ring or/and bottom bracket area. I do not think it has to do with cassete or derailleur nor chain. I was a little unhappy that my lbs did not fix this issue, (or maybe they did not want to mess with it?)
If they're telling you to buy an oval chainring, sounds like they're trying to sell you more stuff.
It's why I build my bikes. Got tired of spending stupid $ for someone to do stuff that I could do with $50 worth of tools and some reading and patience.
Hell, I build my own wheels and that for my friends n family as well. Probably have a total of $300 in tools (including my work current work rack). Probably saved myself n my friends a few thousand so far...

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I agree, it can be a # of things causing, you must go eliminating 1by1, which most will not do including myself. I wonder if its just my bikes geometry causing this, due to offset geo
View attachment 1265225
Doubt it's your bike Geo causing that issue TBH.
You look to have a SRAM crankset on your bike? Frame listed as having a PressFit BB, so take a quick look and see if they put any spacers between the bottom bracket n chainring. If they did, that's likely an error and could affect your large cog.
Also could be from not setting b-limit while bike is sagged.
Your bike is also boost 148 spacing. Could be they put a non-boost chainring on the bike...which would shift chainring 3mm away from frame and could affect your large cog retention.
In the end, I'd be really upset at ANY LBS that basically felt that the fix on a stock bike having this issue was to buy more parts...

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No. Anything you do below the cassette won't affect what's happening on top when backpedaling. Basically don't back pedal more than 1/2-3/4 of a crank revolution. The chain may start to drop to the next smaller cog but it'll pick up when you start pedaling forward.
This ^^
I keep seeing people complain about bad things that happen when pedaling backwards. The only time I pedal backwards is to quickly align cranks to avoid pedal strikes on rocks/obstacles and that is no more than 1/2 a revolution. If it's performing well when pedaling and shifting I'm riding it and enjoying it.
 

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A man goes to the doctor and says, doctor every time I move my arm like this it hurts. The doctor said stop moving your arm like that........... . A man goes into a bike shop and tells the mechanic, every time I back paddle my chain drops to a smaller gear on my cassette. The mechanic said stop backpedaling...........
 

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Doubt it's your bike Geo causing that issue TBH.
You look to have a SRAM crankset on your bike? Frame listed as having a PressFit BB, so take a quick look and see if they put any spacers between the bottom bracket n chainring. If they did, that's likely an error and could affect your large cog.
Also could be from not setting b-limit while bike is sagged.
Your bike is also boost 148 spacing. Could be they put a non-boost chainring on the bike...which would shift chainring 3mm away from frame and could affect your large cog retention.
In the end, I'd be really upset at ANY LBS that basically felt that the fix on a stock bike having this issue was to buy more parts...

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thnx for the feedback!! I will take those steps to see if that is an issue. Believe me, it Sux only have 2 lbs in town.

NEw MOTTO : if you want it done right, then goin do it yourself !!!!!!!!!
 

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A man goes to the doctor and says, doctor every time I move my arm like this it hurts. The doctor said stop moving your arm like that........... . A man goes into a bike shop and tells the mechanic, every time I back paddle my chain drops to a smaller gear on my cassette. The mechanic said stop backpedaling...........
that doctor should be charged with medical negligence if you ask me ! ctfu
 
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thnx for the feedback!! I will take those steps to see if that is an issue. Believe me, it Sux only have 2 lbs in town.

NEw MOTTO : if you want it done right, then goin do it yourself !!!!!!!!!
Seriously...to me there's nothing more fun than bombing down a trail on a bike you build n maintain. It's pricey when you first get the tools, but once you have them, saves ya some $

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A man goes to the doctor and says, doctor every time I move my arm like this it hurts. The doctor said stop moving your arm like that........... . A man goes into a bike shop and tells the mechanic, every time I back paddle my chain drops to a smaller gear on my cassette. The mechanic said stop backpedaling...........
What's the purpose of backpedaling more than one crank revolution? Serious question, I don't get it.
 

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Not sure why you need to back pedal more than a few revs. (Ratchetting usually takes a half rev at most, right?) Anyways, it does have to do with chainline. I have a similar issue when I converted from 2x10 to 1x12. Chainline is severe by the time you get to the big cog. Go to 9:15 here:
 
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