Cyclic Vibration

[PirateLabs]

This is going to be hard to describe.  I have had this, in one form or another, since I got this scoot about 3.5 years ago.  The best way to describe it is if you have ever been in an aircraft with twin engines, or a boat, and the engine rpm's are close, but not matched, you get this vibration that comes and goes in waves or beats, like when tuning a guitar.  Sometimes the beats speed up as the rpm/frequency gets farther apart, and sometimes the beats slow down as the rpms of the engines begin to match.

I get it at about 35 mph, usually.  I feel it in the seat of my pants and it is coming from the engine/drivetrain and not wheels or tires.  Having changed variators and weights many times on this scoot, and clutches and belts, it does not make this go away but it does move the speed at which it occurs and it also changes the span say from starting at 30 mph and going away at 35 mph to maybe a 10 or 12 mph span.

I just put on my other Koso (probably a fake) variator with my installed bronze bushing and a brand new clutch.  Riding to work now that vibration starts at 27 mph and gets worse and worse until you exceed 40 mph, then it dies down and goes away.  Good news is I hit 46 for the first time but getting there, the vibration was so bad, I stayed below 27 mph for my ride home from work as it felt like something was going to be destroyed if it kept up.  It has always done this but right now with this set-up, it is the worst it has been.  I remember one of Brent's videos where his variator was destroying itself and that is what I envision happening.  I am going to take it apart today again to inspect and maybe swap out my variator to my other one to see if it improves.  I used to think this was caused by the variator bushing I.D. getting worn out of round or just too big allowing the variator to flop about in a wobble.  But, if that were the case, the crank is turning at high rpm's long before I reach the magic speeds that start and end this vibration and I would think if it were the variator, it would do it from a certain rpm and stop at a higher rpm but, engine rpm does not seem to change it much.  Sliders are new and my clutch bearings on this new one (finally) feel and sound great.  Spinning the rear wheel on the stand shows the axle running very true as does the clutch bell.  No play in rear axle at all.  Crank is true as well.

Am I just hitting some sort of magic resonance that includes the gears, shafts and pulleys?  Do all 49cc Chinese 4T's do this?
Anyone had anything like this at all?  I could make a video but there is really no real noise associated with this just a beating, wowing vibration that indicates something not good is going on somewhere.  I should mention that whatever the speed zone is for a given set-up where this happens, staying out of that zone prevent ANY vibrations at all and the ride is smooth and all is well.

Thanks,

Bill

[190mech]

The ass end of the CVT is where the super high RPM's live,,Thats likely where the harmonics are,possibly clutch bell balance,tranny bearing or gear wear.When the CVT is in high gear,the back end is spinning waaay more that the crank,stuff like gear tooth count can cause it,odd count(hunting tooth)are always engaged in a different tooth each revolution(good!)..Sorry to cloud up the answer,just some things to ponder on...

[PirateLabs]

Thanks 190mech. So, it would be possible for my variator to be spinning at say "X" rpm and no issues. But, when I reach that "critical" speed on the rear gears/clutch, clutch bell, maybe a resonance is set up somehow between the variator and the gears, clutch, ect.? It may not be destructive but it sure feels like something is about to come apart. Instinctively, I knew not to stay in that range but it always confused me because before that speed (whatever it might be with whatever set-up) it is smooth as glass...and after that speed ...the same. Most of the time it is just a short bump on the way to a higher speed and I have not noticed anything destroyed yet. But with this variator/slider weight set-up it is very pronounced and it feels like something is about to come apart. That is why I took it very easy on my way home Friday because it was over 100 degrees F and I did NOT want to be pushing this thing home 7 miles.

In my machining days, when we would run into this, all we had to do was change either the mass of one of the rotating parts, or the rpm and it would go away and stay away. I have a nice drilled/vented clutch bell I have been using for about a year and I did not notice any difference in this issue between it and the stock bells. Maybe I can rig a balance up for the bell and remove/add some material to see if it changes it, and how it might change.

Anyway, thanks for the response, I was hoping someone might know what I was talking about. I was really hoping that like 20 people would say: "Yes, mine does that too." Ha ha.

Bill

[FrankenMech]

You are on the right track when you say; 'critical speed' and 'resonance'.

Changing your rear shock and engine mount bushings may help.

The fluid and piston valving in the rear shock is supposed to help dampen vibrations.

Adding 3 ounces of BBs to the rear tire may also help with a dynamic balance.

Check your frame to make sure there aren't any cracks or broken mounts.

[joeorange]

ive had this issue as well with my gy6 swap, but in my case it just rattles the fairings really bad when it happens. as 190mech described, it could be the drive train. but in my case, its probably from me welding the engine mount solid to the frame instead of having the second set of damper bushings from the mount instead of both engine and frame mount.

[PirateLabs]

Jun 30, 2018 19:12:31 GMT -5 FrankenMech said:

You are on the right track when you say; 'critical speed' and 'resonance'.

Changing your rear shock and engine mount bushings may help.

The fluid and piston valving in the rear shock is supposed to help dampen vibrations.

Adding 3 ounces of BBs to the rear tire may also help with a dynamic balance.

Check your frame to make sure there aren't any cracks or broken mounts.

I never thought of the shock or a possible broken rubber isolation mount.  I swear it is coming from the cvt area but, those mounts are right under that area as well.  Excellent suggestions and I will look into both.

Thanks.

[greginisn]

Something else to consider and pointing to a rear end out of balance situation.  With a older car if the rear tires were badly out of balance the seat backs would quiver and shake which, in your case with the scooter, could well be the reason for your bum getting buzzed. So like the others said look for something back there.


Greg

[PirateLabs]

Jun 30, 2018 22:35:54 GMT -5 greginisn said:

Something else to consider and pointing to a rear end out of balance situation.  With a older car if the rear tires were badly out of balance the seat backs would quiver and shake which, in your case with the scooter, could well be the reason for your bum getting buzzed. So like the others said look for something back there.


Greg

Thanks for the reply.  I think it's more than that though because my ride is totally smooth up to a certain speed, and then also, smooth above the resonating speed range.  I hit 46 mph the other day and it was as smooth as glass but with this set-up, the resonating beats start around 28 mph and do not subside until I break 40.  It's so weird because you can feel the beats and, they change as the speed increases until they slow down and stop.  It feels like a bad U-joint on an old rear drive car.  When those went bad, you could really feel it at certain speeds.  I am liking the idea of the rubber mounts being involved here as that could transfer the beats between all of the rotating components at a certain point.  Might be just one is in bad shape.  I will know once I can tear it apart.  If they look good then...I will look at the shock...after that...who knows?

Thanks,

Bill

[90GTVert]

Jun 30, 2018 22:51:22 GMT -5 PirateLabs said:

I think it's more than that though because my ride is totally smooth up to a certain speed, and then also, smooth above the resonating speed range.  I hit 46 mph the other day and it was as smooth as glass but with this set-up, the resonating beats start around 28 mph and do not subside until I break 40.  It's so weird because you can feel the beats and, they change as the speed increases until they slow down and stop.

A tire out of balance can definitely do what you describe. I think every tire I've had out of balance on a 2 wheeler does just that. Seems fine till some range and a vibration starts gently with a low frequency then gets stronger with a higher frequency as speed increases and then fades out again as speed continues to increase. The only problem there is, the tire shouldn't be changing RPM based on any changes you make to the CVT or engine or gearbox. It still has to turn X RPM to go X MPH.


I had a bad vibe like this on my TaoTao. I ended up taking off parts and running it on the stand to see if I could get the vibration to stop. For example, if you remove the variator and run it on the stand and there's suddenly no vibration, then you have eliminated all of the CVT, gearbox, and wheel/tire as possible causes because none of them are spinning. Mine was a bent primary drive shaft. Bent plenty badly enough to see it, but somehow I did all sorts of more intricate tests and overlooked what was right in my face.

Try to sort it out ASAP as it can be bad for other components. Mine wore out the gearbox bearings prematurely thanks to the vibes.

[PirateLabs]

Jul 1, 2018 13:22:40 GMT -5 90GTVert said:

Jun 30, 2018 22:51:22 GMT -5 PirateLabs said:

I think it's more than that though because my ride is totally smooth up to a certain speed, and then also, smooth above the resonating speed range.  I hit 46 mph the other day and it was as smooth as glass but with this set-up, the resonating beats start around 28 mph and do not subside until I break 40.  It's so weird because you can feel the beats and, they change as the speed increases until they slow down and stop.

A tire out of balance can definitely do what you describe. I think every tire I've had out of balance on a 2 wheeler does just that. Seems fine till some range and a vibration starts gently with a low frequency then gets stronger with a higher frequency as speed increases and then fades out again as speed continues to increase. The only problem there is, the tire shouldn't be changing RPM based on any changes you make to the CVT or engine or gearbox. It still has to turn X RPM to go X MPH.


I had a bad vibe like this on my TaoTao. I ended up taking off parts and running it on the stand to see if I could get the vibration to stop. For example, if you remove the variator and run it on the stand and there's suddenly no vibration, then you have eliminated all of the CVT, gearbox, and wheel/tire as possible causes because none of them are spinning. Mine was a bent primary drive shaft. Bent plenty badly enough to see it, but somehow I did all sorts of more intricate tests and overlooked what was right in my face.

Try to sort it out ASAP as it can be bad for other components. Mine wore out the gearbox bearings prematurely thanks to the vibes.

Thanks Brent.  This scoot has always done this since I got it but, depending what clutch and variator I was using it was hardly noticeable most of the time and was still in the 30-35 mph range.  I put on a new rear tire not too long ago and there was no change at all.  I took everything apart today and, my bronze variator bushing is holding up well and there is no play at all on the slide bushing I am using. (Thanks again for showing us how to do that)  No play in the crank and the axle turns true and has 0 end play.  After I put it back together I spun the rear wheel on the stand and the clutch bell turns true as well with no run out.  Now, one thing I did notice that may be normal but, I have changed clutches like 8 times since I had this scoot for various reasons and I do not remember if all did this.  If I pull on the rear pulley to check end play, it slides out away from the casting toward the clutch bell about .050" total movement.  I guess it needs some clearance between it and the bell otherwise it would make the clutch bell spin all of the time.  That is the only play I could find anywhere in the cvt system.  I had to replace a broken muffler flange and when I got that done I went for a test ride and...it is still doing it although maybe not as pronounced except this time I really felt it in the handlebars along with my seat which I do not remember from before.  It still really flies though, ever since I used that Koso vari and the longest slide bushing I had which was easily .100" longer than any of my others.

This is going to sound stupid but, I have never spun my front wheel/tire on the stand to check the bearings on it.  I know I said it changes with my cvt swaps, and it does, but I wonder if it is just because with some set-ups, I stay in that critical range longer, or shorter?  (One set-up I ran 35 was the top end) I told another fellow on here that it reminded me of a u-joint going bad in an old rear wheel drive car...so maybe a front wheel bearing vibrating at a certain frequency/speed?  It is 105 heat index here today but I am going back out to check my front wheel and tire.  The tire is the original one that was on the scoot when I got it so, it is pretty old.

Oh, checked all of the engine mount rubber bushings and all "looked" good but I did not take any out to examine them in detail.  I do need to get to the bottom of this as it feels rotational and it feels like something bad might happen if it is allowed to continue...just like a bad bearing can feel.

Thank you very much,

Bill

[PirateLabs]

FrankenMech mentioned the rear shock so I looked at that too and I did notice that even though there is a rubber isolation bushing in the bottom mount location, the way the shock is bolted to it has the back side bent and pulled in so far it has metal to metal contact with the swing arm casting. Sort of defeats the purpose of the isolation rubber mount. I don't know if that is the cause of this or not but, I am going to replace the shock on general principles and make sure there is no metal to metal contact when I install it.

Bill

[PirateLabs]

Well, spun the front wheel on the stand and it has a little drag from the disk brake, but not too much. The scoot coasts free and my top speed is actually up a few mph. Might be because it has been over 100 F here the past few days and with the sun shinning on that black master cylinder I figure it might be expanding the brake fluid a little. Anyway, bearings make no noise and there is no play side to side at all. This morning on the way to work, I could feel the vibrations in my feet, butt and hands on the handlebars. I spent a lot of my 8 mile ride seeing if I could locate where it was the most pronounced and I think it is coming from more toward the front of the scoots as in like in the battery area which might mean that rubber insert has failed. Is that one hard to replace? That is the only thing in that area I could think of but, I might also be wrong in the location. It would explain why I can feel it in the handlebars so much. I have been riding it very easy and mostly staying out of that critical region and my ride was uneventful. I will get to the bottom of this eventually...I hope.

Bill

[AtariGuy]

Try pushing, twisting, shoving, and hauling that whole engine around in any possible direction. If you have a faulted bushing, you'd know. With the bike slowly gaining more and more top speed, check your exhaust studs, your intake studs, and the seal around your carb. While you're at it, make sure your head is still properly to torque too. Dont want the vibes loosening important parts on you

[Moat]

It's a "beat frequency" caused by imperfect balance/trueness of the front and rear spinning drive components (variator and rear pulley/clutch assy) as they fall in and out of positional sync with each other. The only way to reduce or eliminate the issue would be to balance the assemblies - difficult, at best, but maybe worth a shot in your case, considering the severity. Both of my scooters do it to some degree - but not badly enough to warrant messing with it.

Multi-engine propeller aircraft often have prop synchronizers to specifically deal with the "beating", as it's not only obnoxious to crew and passengers, but can cause eventual fatigue failures to engine mounts and other associated structures (although you can often hear them beat during takeoff and initial climb, as a synchronizer failure then could be disastrous - so they're switched off until they've gained some altitude).

Bob

[tom1k4]

Hy.I have the same problem. Did you manage to find a solution?