Measuring BCD & choosing right chainset I bought a used bike whose chain keeps slipping, and I believe it is because either the chainring or the bottom bracket is bent. I tried to show this as best as I could in the below pictures.


I am looking to replace the chainring, but I can't for the life of me figure out what its real BCD is. It's a 5 bolt, and the adjacent bolt distance seems to be ~91mm, with a measured BCD of ~166mm. The below pictures show the taking of these measurements with a tape.


If I calculate the BCD from the adjacent bolt distance, it comes out very differently from what I am measuring it directly with the tape, and, even if it didn't, either of the numbers seem way off from whatever chainset BCD sizes I am able to find out there - unless this one is a very uncommon one? I mention that the bike is a 20 inch foldable Ecosmo mountain bike, 6 gear shimano. I haven't been able to find manufacturer info on it, but the chainring does have &quot;9902-222-118-52T&quot; inscribed on it, and the brand seems to be &quot;Powerwheel&quot;. It has 52 teeth.
What am I doing wrong when measuring the BCD, and what chainring replacement sizes would be suitable for this bike?
It looks like you have a crankset with non-removable chainrings. The 'bolts' you are measuring the geometry of are just screws holding the chainguard on. The chainrings appear to be pressed steel and attached at the crank axle.
It's much more likely the pressed steel rings are bent than the bottom bracket axle, so replacing the entire crank is possibly all you need to do. The bottom bracket bearings may also be in bad shape which would require that to be replaced also.
Single chainring cranks for a square taper bottom bracket are relatively cheap, but there different model cranks require a different axle length to achieve the correct chainline. Sometimes it's easier to buy a new crank and matching bottom bracket than mess with trying to figure out what axle length a crank needs.