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BMX crank sounds

Old 10-12-17, 09:18 PM
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BMX crank sounds

I'm having a problem with my BMX that hopefully someone can help with. I got it as a "do-up" bike as it needed some work so I was just going to rebuild the whole thing while I was at it. As I was riding it for the first time I heard a loud "clink" noise coming from the drive side crank that didn't happen every time I pedaled but when I applied force to go a bit faster and it sometimes happens when I'm just pedaling. The chain was worn, the sprocket was bent, the bottom bracket sealed cartridge bearings were a bit rusty so I thought maybe since the sprocket is bent its stressing the chain out when the chain goes around the bend. All of what was listed was replaced. I installed a brand new fiction savage bottom bracket, stolen 25-T sprocket and a gold KMC 1/8" chain. I took it for a test ride and it was still making that sound, just not as frequent. Everything has been greased and installed properly. I even thought maybe the bolts were too tight, I backed them off a bit to see if that was the problem but sure enough it wasn't. The next thing I'll be changing is the cranks and pedals since I'm doing a total rebuild of this bike but since I'm still going to be riding this in the mean time I want the problem sorted. I've tried everything I can think of. I'm a beginner bicycle mechanic so it's really infuriating when I can't solve the problem. Has anyone else experienced this problem? I need someone else's brain to help me solve this problem lol thanks!
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Old 10-12-17, 10:54 PM
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Just to make sure. Is the chain tensioned properly?

Before you answer ------- the right tension is NO TENSION.

Proper chain tension is sightly slack, and a tight chain can produce the kinds of noise you're hearing.

So, turn the cranks slowly observing the lower loop to make sure there's always some sag. The sag will vary as you turn the crank, and you want roughly 1/4" (1/2" total free up and down movement) of sag middle of the lower loop at the tightest place.

If after confirming that there's chain slack, it still makes noise, then we can look at other possibilities.
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Old 10-13-17, 07:50 AM
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Going along with the chain tension theory, is it a standard BMX sprocket in that it has one bolt holding it in place? Those can vary quite a bit in how they locate themselves on the spindle. Depending on the combination of parts and if it uses an adapter bushing you might have to play around to get it truly centered.

The other obvious culprit would be the center sleeve between the bottom bracket bearings. Very common to need a little custom fitting to get the length perfect, shimming a longer one or cutting/filing one down. I know some folks sort of set the crank tension with the bolts, but in my opinion if you can't tighten them down fully you got that center sleeve length wrong to begin with.
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Old 10-13-17, 12:58 PM
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So it wont make the noise all the time but only when force is applied? Is it one "clink" per revolution? Is it a clink or a thunk?
I've found that one clink per revolution is usually something loose or broken... for example from the poster above, a loose spyder or spyder bolt on the crank, or a loose pedal, or a bolt that wont tighten correctly because it is rusty and not lubed. You say you've replaced the bottom bracket. Is there any chance the bottom bracket could be shifting in the frame under torque? How are the pedals? worn? dry?
Parts are easy and usually noises go away when threads are lubed and tightened correctly. Not to be a downer but did you inspect the frame good? Ive also seen cracked frames make a noise on a pedal stroke also.
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Old 10-15-17, 11:48 PM
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Thanks for your reply's everyone. I did check the frame good before I installed the bearings as usual and everything was fine, the chain isn't tight and has slack. It is the standard one bolt sprocket. The sound doesn't happen for every revolution, it happens, I would say every 5-10 revolutions (I cant be sure with that number but it doesn't happen with every revolution. It is a loud CLINK sound coming from the drive side crank area
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