Chain cracking - need help

[Byte]

Since my build is finished it drives well, has enough power, but there's one thing that I can't fix: the chain makes a cracking noise under full power. When riding normally it doesn't make any bad sounds, however when giving a little more power the chain makes cracking sounds. First I used a Wipperman MTB chain. I thought it might be a good idea to switch chains, so I bought the ultra strong Wipperman 1G8 BMX chain. Didn't solve the problem though... I aligned the chain as best as I could and tried a lot of other possibilities but none worked.

I use a LH-side drive, running from a 16T freewheel to a 39T sprocket. When accelerating really hard - and the chain makes the cracking sound - the chain visibly vibrates on the bottom side. Is this because the chain links don't engage properly with the rear sprocket?

Picture to make it more clear:

Black: Sprockets
Red: Chain
Green: Chain direction
Blue: Visible vibrating from chain, when accelerating hard

Where should I search for the problem? Should I try mounting a guide on the bottom side? Notice: The chain only makes cracking sounds when under full power. I can make wheelies without the chain cracking. However, when leaning forward on the handlebar and accelerating hard it makes a few cracking sounds.


Help me please!

 

[kfong]

Check if your chain still meets specs. And not overstretched with use. Remove any play. If that does not solve it, then a tensioning guide would be my next approach.

 

[turbo1889]

Almost sounds like you have a fundamental frequency oscillation engineering problem. Its one of those nasty little gremlins that pokes its ugly head up sometimes when you least expect it. Notable historical examples include that one suspension bridge that was just the right length for its fundamental frequency to be exactly the same as the sway caused by the wind that normally blew over the river it was built across and with each sway of the bridge the sway gained a little until the bridge literally tore itself apart or that jet airliner that had just the right length of wings for the fundamental frequency of the wings to be the same as the vibrations of the jet engines when at full power and it would tear its own wings off so they had to under-rate the engines and back the throttles off slightly so that at full throttle the engines would run a little slower so the vibrations didn't match the fundamental frequency created by the length of its wings.

If you shorten or lengthen the chain-line the problem will go away if this is what is happening since shortening or lengthening changes the fundamental frequency (actually its plural as in frequencies but in many cases you only have to worry about one scale that is causing the trouble). If you can't move your motor to accomplish that because your motor can't be moved due to your mounting set-up then putting an idler tensioner into the system could work so long as you don't put it dead center where it will just be in the middle of the wave frequency and just serve as a pivot point for the wave form. Mount the tensioner off center preferably about a 1/4 of the way down the run so it suppresses the wave right at a crest/trough point.

 

[crossbreak]

an spring loaded idler with some friction damping (rubber for example) should solve your problem. Does not look so sexy though. The fundamental frequiency can be increased up by using a smaller chain like a #219 and sprockets with more teeth, this may solve your problem without an idler. The chain preload influences fundamental only marginal, anyway this would be th first thing to try: Just engage your tensioner a bit harder.

 

[Byte]

I will try the spring loaded idler and update this topic with results. Will try to make a video too, if possible. Thank you for the help