HELP! Bent Crank or chainring?

[RMK!]

This is a brand new Rattan XL fresh off the boat. I waited 6 months and after setup I discovered a common problem, bent chainring. I removed and tried to take the bend out but every ride it continually drops the chain. from the front ring. Just received and bike stand and upon examination the geometry is totally off with the front crank set. Rattan has done nothing for 12 days now but I was able to find a replacement chainring although it is thicker and has an extra tooth (53 vs 52 orig) and after install exact same condition. Obviously I'm not bike (or other) mechanic . The front chainring causes a sever chain angle to the rear gear set. This is 8th gear, my highest gear) and shows the worst chain angle. Bike shops near me are all backed up (2 to 6 week waits for repairs). Any thought or suggestions are greatly appreciated?

 

[RMK!]

Thank you Timpo, I just dropped my ebike off at a local Bike shop. I had replaced the chainring and that isn't the problem. The bottom bracket on my new ebike is wide but apparently, not wide enough. The chain should not angle like mine does to reach the smallest gear. The photo on the right is the best illustration of the problem. Note the angle of the chain as it leaves the chainring ... not good. They are going to try things to see if they can get it to a point where it shifts. I'm beginning to think I am screwed.

 

[RMK!]

Good catch. Yes it was, that was because even when on the bike stand when I shift the chain comes off the ring. But the real problem is the top chainring exit to the rear gearset angle.

 

[Sierratim]

Good catch. Yes it was, that was because even when on the bike stand when I shift the chain comes off the ring. But the real problem is the top chainring exit to the rear gearset angle.

The chain line should be straight when the chain is in the middle cog of the cassette. The chain will be angled from the chain ring when it is in the smallest or largest cogs. What does it look like in the middle cog? If it's not straight then you probably need a different axle length in your bottom bracket.

If the chain line is straight with the chain in the middle cog then I'd suspect that the chain is not properly mounted on the front chain ring. Since you have a single chain ring it should have a wide/narrow tooth pattern where every other tooth is slightly different in shape and position. The function of this tooth pattern is to keep the chain from 'dropping' off the chain ring. You can test this by pulling your derailleur arm forward to get slack in the chain and then move the chain 1 tooth forward on the chain ring.

There are several online videos that cover wide/narrow chain rings as well as posts to other forums & threads that discuss this.

Good luck.

 

[RMK!]

Sierratim. thank you for the valuable input. I think you are spot on re the longer axle. The bike shop just called and said they think they have it much better if not perfect. I installed a new 53 tooth chainring yesterday and I hope it has the same or similar wide/narrow pattern. Besides 1 additional tooth, it is a bit thicker than the bent original chainring. I'll be picking it up tomorrow so fingers crossed.
Thanks again for the help!

 

[rich c]

That looks more like a frame welding problem than a chainring issue. Like the bottom bracket tube is crooked. I'd put a yardstick on that chainring and extend it back to the rear tire and see if the tire and chainring are parallel.

 

[RMK!]

I will be getting the bike back and some more details tomorrow. The bottom bracket looks to be correctly positioned but as they are not robotic welds and this was done as China was just emerging from their lockdown, anything's possible. Great feedback and suggestions. Thanks fellas.

 

[RMK!]

The bike shop tried reversing the chain guard and chainring and after adjusting the derailleur got it to shift on the stand. It seemed to be working but on my first ride it began dumping the chain when in small cog gears under pedal power. I think this bike is a lost leader. There are some serious design flaws unless I was just unlucky and got one of the lemons.

Trying to work something out with the Rattan but so far, radio silence ... Caveat Emptor!

 

[RMK!]

Thanks for the suggestions fellas ...

 

[Sierratim]

Thanks for the suggestions fellas ...

Does the chain ring have a wide/narrow tooth configuration?

 

[RMK!]

Looks like all the same size to me.

 

[Sierratim]

Looks like all the same size to me.

Could be the main issue here. There are a number of articles and videos on this topic. Here's a couple to start, https://bicycles.stackexchange.com/...nring-and-what-are-its-benefits-and-drawbacks, and https://www.worldwidecyclery.com/blogs/worldwide-cyclery-blog/narrow-wide-chainrings-guide .

The concept is to prevent the chain from dropping off the single chain ring when there is no front derailleur. Works well when properly installed.

Ride On!

 

[RMK!]

I was informed yesterday that I can return the ebike for a "full refund". I did take these photos of the issue. My first folder has gone 1600 miles with virtually no issues where the new ebike has the chainring alignment problem (and a few others). Thanks for the suggestions but apparently this mystery will go unsolved.