CrowSeph

The series of the strange problems is not over yet.
On my mountainbike if you turn the crank backwards you will notice a strange thing , basically you can feel a sort of friction somewhere.
To give you an idea , with a normal bike and a dynometer you can use 1,5kg of force to swipe around the crankset but on my mountainbike you had to push until 2,8kg!
i removed the wheel and checked the cassette , once in a ride the cassette became loose. But the cassette is super smooth right now!
the derailleur works fine , the crankset works fine , the chain is still new (dirty but still new). The bottom bracket bearing seems working properly too.

what else i can cheeck? everything seems fine but when i mount the bike back i can feel the problem

i'am using a deore m4100 2x10 26-36/11-42
and mostly i notice the friction when i use the 11+26

Troul 

bad hub assembly. I'd start looking for a replacement.

Bill Kapaun

You are cross chaining BADLY.
Put it on 26-11.
Stand behind the bike and look at the angle of the chain. BAD!!!
It should be obvious why you don't use that combination.

Iride01 

Too long a chain and when pedaling backwards in the 26F/11R the top run of chain is going slack and getting mixed up with other stuff.

Don't pedal backwards. More useless than coasting. <grin>

You really put your bikes on a dynamometer? Don't know what to tell you about those readings.

dsbrantjr

At least the OP presented some actual data, which as an engineer I appreciate.

Moisture

Clean and re lube the rear derailleur pivot points and pulleys

Trakhak

This sufficiently accounts for the (non)problem, not that that ever stops anyone posting other hypotheses.

Proof: run the chain up to the middle chainring and middle sprocket and measure again.

andrewclaus

A bent rear derailleur or derailleur hanger could be making it worse too.

oldbobcat

Dirty or worn freehub pushing back on your chain. To test, shift to your lowest gearing and try it. If this is the problem, pedaling backward will be significantly easier.

CrowSeph

okay guys i'll keep updated after i clean my chain and make a new test with the lowest gearing.

base2 

Freehub grease.
Disassemble.
Clean.
Reapply.
Reassemble.
Ride.

Trakhak

Cleaning is good. But your new test with the lowest gearing will prove nothing. The test that will prove that the problem is nonexistent will be to use the durometer with the chain running in as straight a line a possible from the middle of the cassette to the center chainring. It's a given for every derailleur-equipped bike ever built that friction losses increase as the chain angle from the cassette to the chainring increases.

And, as noted by other posters above, if you can't resist the urge to backpedal, please keep it to a minimum. The best that can happen is that you can get away with backpedaling once in a while without damaging the bike. But be forewarned that, with some bikes in some gear combinations, breaking the rear derailleur or chain or rear dropout is a real possibility. Why risk it for no benefit?

CrowSeph

Seems i found the problem. The chain were in a wrong position. Originally were under that metal plate, causing the chain itself pushing into the metal.
Seems more easy to show in the photo than explaining.

Andrew R Stewart 

We see this problem every so often as more try to fix their bikes at home. But usually when the chain is running on the wrong side of that cage tab it also makes some noise and has a rough/notchy feel. Andy

Dan Burkhart 

I would love to meet a pro mechanic who has never done this. As I always say, there are two kinds of mechanics, those who have incorrectly routed the chain through a derailleur and liars.

Andrew R Stewart 

Well, I don't lie if I can help it. I've done this too. But the difference is that an experienced wrench will, usually..., realize pretty quickly what they've done and correct it before the bike leaves the repair stand.

One of my favorite statements about what separates a pro from others is that the pro knows how to hide their mistakes before anyone knows it. This from the mouth of the current generation's grandfather frame builder, Al Eisentraut. Andy

Dan Burkhart 

Yeah I’m sure I never let one go out the door like that but I have done it a time or two. It’s better if there is a quick link involved.