Once your bike is upside down look at the derailleurs which are the small machines on the front and back gears that move the chain.
Fixed gear bike chain slipping.
This will allow you to clearly access all the parts and spin the pedals to see how the drivetrain is performing.
And explain what needs to be done to fix them.
Straightening a bent derailleur hanger how.
Press your shifter once.
Using a chain tool slip each of its ends through the links of your chain and check your chain against its wear indicators.
A chain that keeps falling off is either too loose in the case of a hub gear or fixed gear bike or the derailleurs are badly adjusted and move the chain right off the cogs when you change gears.
Look over the chain for excess wear or bent links.
Hub or fixed gear bike.
To fix a skip in the rear derailleur shift your chain into the smallest ring on your rear cassette the hardest gear and the middle or larger ring on your front derailleur.
If the chain is worn out.
To fix a slipped bike chain first turn your bike upside down as this will make it a lot easier to reattach the chain.
Place your bike in a stand.
They will also lead to one using more energy to cycle especially if the damaged gears are the heavier ones.
I show the most common causes for chains that skip slip jump gears while pedaling auto shifting phantom shifting derailleur especially while under torqu.
Note which gear these are on so you can place the chain back in the right place.
It is important that you take the time to inspect the condition of your gears and chain before any ride.