ahmet sezgin

I have a Scott Aspect 10 (mountain bike) with 1 x drivetrain technology. I have an odd problem. When I pedal backwards there is a kind of thudding noise (pat pat pat). Sometimes when i pedal backwards it even shifts gear (without I touching the shifter). Is it something normal in 1 x drivetrain technology? Or does it tell me that there is problem about the adjustment of the Rear Derailleur?

cxwrench

I'll bet it only happens on the small cogs and the large cogs, not in the middle right? Take a look at your bike. Where is the rear derailleur? Below the cassette where it can do it's job and guide the chain on to the proper cog when pedaling forward, right? Do you see a derailleur above the cassette? Nope. That's why this happens. As cassettes have gained gears and cogs have gotten bigger the problem has become noticeable and gotten worse. To answer your question, no...there is no adjustment for that. How could there be w/ the derailleur below the cassette?

Andrew R Stewart 

When back pedaling there can be some rear der movement, or rotation towards the front ring. The cage slightly rotates clockwise to take up thus chain slack. The guide pulley becomes closer to the cassette's undersides and sometimes come into contact with the cog teeth. This is called "pulley knock". If your der has a "B" screw adjuster it could be run inwards a bit to further rock the der back and increase the guide pulley/cog gap.

Is this what's going on? We don't know. Andy

base2 

To add to cxwrench

What you are experiencing is a result of the angle the chain must take from the extreme ends of the cassette to the chainring.

Cassette cogs have both gotten bigger (42, 46, 50, & maybe even bigger ) & smaller (9,10) in recent years. The cassettes themselves have also gotten wider as well since designers realized that the large cogs have more room on account of the spokes are at an angle so have packed in even more cogs.

Combine both of those 2 factors with chainstays that are not necessarily any longer & the result is the chain rides a greater angle than drivetrains of yore; An angle steep enough to cause derailment.

Rotating in the forward direction, the derailleur handles the cassette & narrow/wide tooth profiles handle the chainring. Rotating backwards neither feature is engaged & you have the behavior you observe.

I wouldn't worry about it. It's the result of engineering compromises to achieve 1x simplicity & massive ratio changes with a simple thumb sweep at the shifter.

2x will exhibit similar behavior when cross chained, too. The solution: Don't pedal backwards.

FBinNY 

Agree with the above.

Short answer is it's normal and falls under the "nature of the beast" category.

The fix is simple ---- avoid backpedaling in the problem gears.