Topic: Hard vibration at specific rpm

[jakec]

My 550 vibrates hard at about 5,900 - 6,200 RPM. Only in that range and the vibration is really strong. Like you can hear it. I always try to jump through that range as quickly as possible to get on the other side.

It's not all stock, the front end is different. But it's hard to figure out where the vibration is coming from while i'm riding. I thought about setting the bike up on the stand and looking around but I don't want to get it too hot and wake up the neighbors.

[Bankerdanny]

That is odd. I would be double checking the motor mounts.

[jakec]

What would make them vibrate? Too tight? I added new bolts and retorqued the front engine hanger bolts because I removed a crash bar that was on the bike.

The vibration can be heard aloud and can be felt all the way to the grips, but only in that very small RPM range

[spotty]

its a resonant vibration caused by that sissy bar

[dave500]

is it in any gear?you specify an rpm.

[1976cb750f836]

Has bike always done this? Has engine benn into? Does it do it in neutral setting still?

[dave500]

only thing I can think of is loose alternator rotor?nothing else will cause a vibration these engines spin like tops!

[jakec]

It's coming from the front of the bike, I can feel that. It has always done it but I've only been riding it about a month. And the engine is very dry and never opened at about 6,000 miles. It does it in every gear, yes.

Last night on the highway I tried grabbing various parts of the bike to see if I could calm it down and grabbing the headlight seemed to work a little. So I may try and add some padding to islolate it, see if that helps.

[TwoTired]

Sure it isn't motion speed related?

Wheel balanced?

Tire properly seated on the rim? 

Modern tire installers don't even check this, cause tubeless rims self-seat if they pop on.  Tubeless tires on tube type rims won't self seat and need to be "massaged" on.   There is usually a molded line on the tire near the rim that must be equidistant to the rim all the way around on both sides. 

Cheers,

[PeWe]

Too high for "typical inlet fours vibrations" which is in the 4700- 5100 rpm area. I found that on a Kawasaki forum when searching on vibrations in that rpm range my CB750 K6 had. I was worried that cam did not get oil. Reason: loose  exhaust spigot that made the entire exhaust to sing in a bad way mechanically rotating dry without oil.

I have however, heard a higher pitch vibrations from my K2 that must be cooling flanges in head and cylinder to sing. Those start just at or after 5000 rpm to 5500. The rubber dampers between flanges does not reduce all. All vibrations only heard not sensed by hands on handle bar.

If you can feel vibrations via your feet, tighten the engine bolts and verify that drive chain is not too tight.

[jakec]

I know this is probably useless but this is where it "feels" like its coming from shown in the attached:

Also hard to diagnose because the bike is not stock:



Tires were professionally installed by a vintage shop so I think that's fine, also it's a very high frequency vibration (I have ridden on an un-balanced front tire and this doesn't feel like that).

I have just had the exhaust off last week in order to re-tighten chain (brand new chain). So should be able to confirm new exhaust gaskets, tightened flanges, and correct chain slack.

Two things I can think of - the headlight is on a stainless mount that is resting against the lower triple clamp and secured with zip ties. I know it's vibrating a little but not sure if it's the main culprit. Also steering head spanner nut is a little on the looser side (but still tightened "enough." I did this because any tighter and it was binding a bit.

[dave500]

put the stock forks back on and it will go away.

[jakec]

y

[spotty]

Chain might be the culprit, could have developed a tight spot ? I had one do that years ago and it shook the bike something chronic but only at certain speeds ( not sure why it would do it at certain revs like yours ?)
Wouldn't think the steering head but would do it,  that would be more likely to cause a weave or shake
The headlight wouldn't cause the whole bike to vibrate, it would just rattle away to itself
For it to only happen at certain revs it must be something directly attached to the thing creating those revs is, the engine
So if it's not the chain check everything else that attaches the engine to the frame like all the bolts etc
Or.......it's the backrest

[dave500]

that rake is #$%*ing stupid man, no #$%*!itll throw all the handling to the #$%* house,it might be cool looks but it makes an otherwise pretty good handling for an old bike into a piece of #$%* handler,dont tell anyone it makes it better,its a disaster on #$%*ing wheels and pretty obvious,i think your just #$%*ing with us?

[spotty]

that rake is #$%*ing stupid man, no #$%*!itll throw all the handling to the #$%* house,it might be cool looks but it makes an otherwise pretty good handling for an old bike into a piece of #$%* handler,dont tell anyone it makes it better,its a disaster on #$%*ing wheels and pretty obvious,i think your just #$%*ing with us?

Jesus , did someone forget to take their angry pills this morning ?

We're supposed to be here to support and encourage each other regardless of individual idiosyncrasies which may lead us down some dark passageways, yes those forks and rake may make for some interesting handling characteristics but if that's what floats his boat then leave him to it. FFS, some people think restoration and originality is what everyone should be made to do, ' preserve a classic' sort of bullsh1t, which is another way of saying ' replicating yesterday's mistakes today'......

He didn't ask " why does my bike drift wide on tight corners and require a football field to do a U turn" , if he had then ,yes, slagging off that aspect of the bike would be appropriate but even then, a little more light hearted wouldn't hurt

Personally I still think it's the backrest.....

[Medyo Bastos]

sounds like harmonics, try taking the sissy bar off. bike looks great, ride it like you stole it!


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[dave500]

that rake is #$%*ing stupid man, no #$%*!itll throw all the handling to the #$%* house,it might be cool looks but it makes an otherwise pretty good handling for an old bike into a piece of #$%* handler,dont tell anyone it makes it better,its a disaster on #$%*ing wheels and pretty obvious,i think your just #$%*ing with us?

Jesus , did someone forget to take their angry pills this morning ?

We're supposed to be here to support and encourage each other regardless of individual idiosyncrasies which may lead us down some dark passageways, yes those forks and rake may make for some interesting handling characteristics but if that's what floats his boat then leave him to it. FFS, some people think restoration and originality is what everyone should be made to do, ' preserve a classic' sort of bullsh1t, which is another way of saying ' replicating yesterday's mistakes today'......

He didn't ask " why does my bike drift wide on tight corners and require a football field to do a U turn" , if he had then ,yes, slagging off that aspect of the bike would be appropriate but even then, a little more light hearted wouldn't hurt

Personally I still think it's the backrest.....

come on spotty im only stirring it up a bit!

[jakec]

Sissy bar is attached to the seat which is isolated from the frame by rubber feet which I installed. The headlight turned out to be the "loud" part of the equation. I did about 100 mile ride todat and the vibration feeling was noticeably diminished once I added some damper between the headlight mount and where it's attached to the lower triple. I don't think the source of the vibration is solved yet but I did solve the worst part.

Regarding the the build, I found the bike this way sitting in a barn since the 80s. Based on Dave's photo I care more for originality on these bikes than he does. Unfortunately I'm not sure what he's trying to say in his second post so I can't refute that.


the handling is the same at speed but bad at low speeds. But the girls love the way it looks.

[PeWe]

Replace the fork tubes with shorter. Nothing between what you have and stock?

[jakec]

What do you mean by the second part of that, nothing between?

[PeWe]

Lenght

My CB750 had 6" (15cm) longer tubes back in 1980. That's the style back then.
Your bike has same angle as next size, 8" (20 cm longer) or even longer.

My bike had no high frequency vibrations, only very low, horrible wobble that occurred at 100mph and lower in corners leaning bike and WOT.
I was sometimes close to abandon ship in a controlled way instead of flying off the road into the nearby forest.

[Tracksnblades1]

Can you duplicate the vibration by holding revs between 5900-6200 in neutral sitting still...?

I would try to eliminate engine vibrations  from chassis, wheel, chain vibrations....🤓

[Romhog]

I came across this post and was trying to figure out this exact situation. At certain RPM's while in neutral sitting still I can feel vibration though the seat / my butt. I just rebuilt the engine and am concerned I messed something up. What do you think it would be if I can feel it while sitting still in neutral?

Things going though my head are. 1 cylinder getting more fuel from messing up needle position on one carb? My counter to my own thought on that is I did a temp reading on each exhaust and they are about 6-7 degrees of each other. None of them stand out at a different temp.

Only other thing is maybe a loose generator rotor?? I'm thinking I got that thing tightened but I can check that easy enough I guess.

Just wanted someone's thoughts on what I may have done.

Thanks in advance!

Can you duplicate the vibration by holding revs between 5900-6200 in neutral sitting still...?

I would try to eliminate engine vibrations  from chassis, wheel, chain vibrations....🤓

[exponent]

that rake is #$%*ing stupid man, no #$%*!itll throw all the handling to the #$%* house,it might be cool looks but it makes an otherwise pretty good handling for an old bike into a piece of #$%* handler,dont tell anyone it makes it better,its a disaster on #$%*ing wheels and pretty obvious,i think your just #$%*ing with us?

Jesus , did someone forget to take their angry pills this morning ?

We're supposed to be here to support and encourage each other regardless of individual idiosyncrasies which may lead us down some dark passageways, yes those forks and rake may make for some interesting handling characteristics but if that's what floats his boat then leave him to it. FFS, some people think restoration and originality is what everyone should be made to do, ' preserve a classic' sort of bullsh1t, which is another way of saying ' replicating yesterday's mistakes today'......

He didn't ask " why does my bike drift wide on tight corners and require a football field to do a U turn" , if he had then ,yes, slagging off that aspect of the bike would be appropriate but even then, a little more light hearted wouldn't hurt

Personally I still think it's the backrest.....

Seriously, sounds like ravings if a drunk red faced mf sitting at the end of the bar. While the problem is difficult to diagnose it's defo not the choice of forks