one of the biggest problems i see on this forum is the ol' engine vibration issue.
the usual suspects are an unbalanced crank, bad engine mounts, or out-of-true rear sprockets.
here's some observations and solutions based on my real-world experiments.
culprit number 1:
exhaust pipes
the stock exhaust pipe comes with a lousy hanger-mount that doesn't seem to fit most bikes, so it's seldom used. the pipe hangs free, starts to vibrate, and can increase as speed increases, which can cause the whole bike to shake.
my current bike, i built 2 small, custom straps from a sheet of 1/16" aluminum. strapped one around the pipe, and it bolts up to one strapped around the frame. the frame strap has a cushion of innertube underneath, to prevent slipping, wear, and some dampening effects, as well as providing a small amount of flex.
my bike developed a vibration it never had before, and after i got home i looked it over and noticed the strap had broken. i made another, beefier one, and after a test ride, the vibration was gone.
you can check if your pipe is causing a vibration by holding your foot against it while riding. it might melt your shoe a little, but so what. on some bikes, it's visibly noticeable. you can see the pipe vibrating.
culprit number 2:
gas tanks
when i was building the bike, i hadn't fabricated a tank yet, so i put the stock peanut on for a few rides. since it was only temporary, i just slapped it on the bare frame.
the bike had a severe vibration at almost WOT, and then would power through it.
also, when the tank was full, the vibration wasn't as bad. at half-tank, it was pretty bad, and at almost empty, the vibration was severe.
i found it was the tank sorta by accident. i tucked my knees in, hugging the tank, and the vibration went away.
after wrapping some innertube around the frame and remounting the tank, the vibration lessened, but still noticeable. some more cushioning woulda got rid of it completely.
now that the bike has a custom in-frame tank, there are no more problems at all.
i've also had bikes with custom tanks that seemed to resonate at a certain speed, causing a vibration. i know Jeff (culvercityclassic) has had the same problem.
culprit number 3:
chain rub.
a bike i built for a friend of mine has little clearance for the rear wheel, and the chain was rubbing the frame at the chainstay when i first got it together. i didn't have any thick washers to space the wheel out at the time, and we weren't worried about the paint so i left it like that for a few days.
i got the bike back to do some tuning on it and he was complaining about a vibration around WOT. i had to adjust the clutch on it, and since i was already working on it, i didn't bother to test ride it. i did some minor things, adjusted the clutch and front drum brake, and added some washers to the inside rear drive side.
when i test rode it, there was no vibration at all. the only thing that i did that would affect it was the washers.
it seems logical that a rubbing chain could cause a buzz felt through the whole bike.
it can also cut through your frame if it's bad enough and goes on long enough.
so there's three things to check. if your bike suddenly starts vibrating, and never did it before, odds are something's loose or broken. my very first bike ran smooth, then started buzzing like crazy, and i found out the frame had cracked. so pay attention if it suddenly develops.
if you have a vibration problem, don't be so quick to assume it's a bad motor and you need your crank balanced. it could be something much simpler.
the usual suspects are an unbalanced crank, bad engine mounts, or out-of-true rear sprockets.
here's some observations and solutions based on my real-world experiments.
culprit number 1:
exhaust pipes
the stock exhaust pipe comes with a lousy hanger-mount that doesn't seem to fit most bikes, so it's seldom used. the pipe hangs free, starts to vibrate, and can increase as speed increases, which can cause the whole bike to shake.
my current bike, i built 2 small, custom straps from a sheet of 1/16" aluminum. strapped one around the pipe, and it bolts up to one strapped around the frame. the frame strap has a cushion of innertube underneath, to prevent slipping, wear, and some dampening effects, as well as providing a small amount of flex.
my bike developed a vibration it never had before, and after i got home i looked it over and noticed the strap had broken. i made another, beefier one, and after a test ride, the vibration was gone.
you can check if your pipe is causing a vibration by holding your foot against it while riding. it might melt your shoe a little, but so what. on some bikes, it's visibly noticeable. you can see the pipe vibrating.
culprit number 2:
gas tanks
when i was building the bike, i hadn't fabricated a tank yet, so i put the stock peanut on for a few rides. since it was only temporary, i just slapped it on the bare frame.
the bike had a severe vibration at almost WOT, and then would power through it.
also, when the tank was full, the vibration wasn't as bad. at half-tank, it was pretty bad, and at almost empty, the vibration was severe.
i found it was the tank sorta by accident. i tucked my knees in, hugging the tank, and the vibration went away.
after wrapping some innertube around the frame and remounting the tank, the vibration lessened, but still noticeable. some more cushioning woulda got rid of it completely.
now that the bike has a custom in-frame tank, there are no more problems at all.
i've also had bikes with custom tanks that seemed to resonate at a certain speed, causing a vibration. i know Jeff (culvercityclassic) has had the same problem.
culprit number 3:
chain rub.
a bike i built for a friend of mine has little clearance for the rear wheel, and the chain was rubbing the frame at the chainstay when i first got it together. i didn't have any thick washers to space the wheel out at the time, and we weren't worried about the paint so i left it like that for a few days.
i got the bike back to do some tuning on it and he was complaining about a vibration around WOT. i had to adjust the clutch on it, and since i was already working on it, i didn't bother to test ride it. i did some minor things, adjusted the clutch and front drum brake, and added some washers to the inside rear drive side.
when i test rode it, there was no vibration at all. the only thing that i did that would affect it was the washers.
it seems logical that a rubbing chain could cause a buzz felt through the whole bike.
it can also cut through your frame if it's bad enough and goes on long enough.
so there's three things to check. if your bike suddenly starts vibrating, and never did it before, odds are something's loose or broken. my very first bike ran smooth, then started buzzing like crazy, and i found out the frame had cracked. so pay attention if it suddenly develops.
if you have a vibration problem, don't be so quick to assume it's a bad motor and you need your crank balanced. it could be something much simpler.
