Measuring BCD & choosing right chainset

Question

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 "9902-222-118-52T" inscribed on it, and the brand seems to be "Powerwheel". It has 52 teeth.

What am I doing wrong when measuring the BCD, and what chainring replacement sizes would be suitable for this bike?

Answer 1

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.

Answer 2

The numbers don't tally because you're measuring wrongly, I've marked up your photo with the circle (in yellow) you should be measuring, it's more like 147mm, or whatever irl without camera perspective distortion. That's much closer to the calculated value - 88 mm adjacent = 149mm calculated BCD. Measuring guide: Terminology index - a list of bike part names and cycling concepts

Measuring with a tape instead of a caliper is doing it the hard way. I don't trust either of the measurements with the tape/camera combo, for lack of accuracy where accuracy is vital.

That's all moot though because the chainset is one piece, those bolts are screws holding the plastic chainguard in place, nothing more. You need to replace the whole crankset together, if you want to replace it. It seems skewed as a whole to me though, I can't tell from the photo's properly but I wonder if the bottom bracket is skewed in the shell, and the chainring is a symptom.

Think carefully how much money you want to spend correcting a 20 inch folding mountain bike, remember the sunk cost fallacy. But also ignore bike snobs who should know better than tell you how to live your life ;)

Answer 3

Before buying a replacement ring I would recommend removing the crank assembly. This requires a "square taper crank tool"along with a socket and ratchet most likely 14mm. Once off the bike edit your question to include photos of both sides of the assemble. Try to determine if the crank is bent by laying the crank on a flat surface (pedal side up) and see if it sit flat. While the crank is off spin the left side pedal arm while checking the exposed axle for wobble. Your crank appears to be a one piece design meaning it is likely riveted together and may need to be replaced as a unit.