Terrible vibration

[KR4WM]

Almost forgot- the engine chain was ALMOST long enough to fit (within a few thousandths) without the tensioner. I had to leave an extra link in, and because of that, had to install the (optional?) tensioner wheel. By my calculations (and experience) after a few miles, the engine chain should stretch enough to allow me to remove another link, which will allow me to get rid of the motor chain tensioner. It -could- be that it is the cause of at least part of the vibration. It sure is loose and floppy! Whatever kind of bearing they used there is by no means precision! I'd say there is at least 1/16" of side-to-side slop in that pulley!

Thanks, -Web

 

[cabinfever1977]

i have a 44t and i can hit 35mph. i am using rubber under my engine mounts and it really helps to cut down on vibration. my tank is on rear rack. but i dont recommend anyone to ride at high speeds since i had a chain master link break on me at 35mph,bike and me ok,but who knows next time. i fixed it with another master chain link. i prefer to ride at 20-25mph. you know you can use a spacer on engine mount or rotate engine slighty to adjust your chain and you wont have to use tensioner.

 

[KR4WM]

Thanks for the info

I installed a speedometer, and took extremely critical measurements to calibrate it as accurately as possible. I hit 25MPH before vibration set in. At 25.3MPH, the vibration begins. I'll just ride along around 23MPH or so and won't have the problem.

Didn't think about using spacers- the engine is sitting at the bottom of the "V" and tight on the frame. The front is installed with an adapter plate (included in my kit, but I made my own out of aircraft grade 1/4" aluminum). There is no play side-to-side. If I loosen the engine mount bolts, when I tighten them, the motor goes back to the same position. Oh- I might get a few thousandths, but that's about it. In any event, what's holding me back from making this adjustment is the hot weather! 105F heat index today!!!

Thanks, -Web

 

[Molotov256]

Engine spacers is a good idea... I just started messing with that whole idea on a recent build, and I like it better than the tensioner, overall. I ended up pulling out the stock mount rods and putting some longer ones in so I had space to stack some flat washers to space out the motor. Still haven't got it down perfect, but it helps. The tensioner is a lot easier to work with, though.

If your tubes are small enough, I learned from another post that leather makes a great motor mount bushing material. I used an old belt I had in my closet to fatten up the tubes on an old ten speed, and it REALLY cut down on vibration.

The other bike I have running used to have painfully bad vibrations at higher RPMs, but had great torque at low RPMs. I swapped the 44T sprocket for a 32T, and it runs smooth and fast now.

I guess overall, weigh out your situation, the characteristics of your motor, and the size of your tubes, and see if one of those paths fits you. I know lots of folks on here encourage slow, responsible riding, but hey, if you wanna go 30+, that's your prerogative! hehehe...

 

[kiwirider]

I had a bad vibration with my HT engine also. I was thinking that the engine was going to come apart at higher RPM it was that bad. I was also unhappy with the front engine mount. The kit I got didn't come with any special mount, just a couple of bits of mild steel with 4 holes so you can clamp the frame. I made a mount at the time I put it all together and used some rubber to add some damping effect. I thought it was going to be OK but the motor would slip to one side and the chain would come loose so after some time and looking at options I found here, I decided to make one from a bit of alloy and an auto muffler clamp. The motor is good and tight now with no movement side to side at all...and zero vibration. I am sure that the vibration was due to the fact that the engine was given some room to move via the rubber. The motor mount was never designed to have any damping added, it doesn't need it when everything is good and tight to the bike frame. So, I have fixed one issue i have had (vibration) now all I have is the problem of no power over 20mph....!

 

[Hawaii_Ed]

I agree, I did my last two builds bolted right to the frame with no rubber, and they are much smoother.

 

[cabinfever1977]

almost all engines are designed to have rubber engine mounts,everyone go look at a motorcycle and a car and tell me what you find for engine mounts.you will find steel and hard rubber mounts. if you dont have that you will have vibrations and a broken frame sooner or later.

if the rubber dont work for you,then your engine wasnt tight enough and you probally did it wrong.

btw i have 5 engine mounts on my bike,how many do you have?...

 

[Scotchmo]

Rubber mounting is rarely used on an engine this small. The amplitude of the vibration is not that great. A proper solid mount is all that is needed and is much easier to engineer than a proper isolated mount.

There are two ways to try to handle the engine vibration. Reduce the vibration or isolate it. A solid mount reduces it so the motor shakes less. A proper rubber mount allows the motor to shake more but isolates it from the rider and frame. The easiest method for this size motor, is to mount it as rigid as possible. You want the mass of the frame and bike to become a solid part of the engine. A heavy frame mounted solid to the engine will go a long way toward reducing vibration. The front mount is the shortest, most rigid attachment available on these engines. The downtube is the stiffest support on a bike frame. A short, stiff, solid connection between the engine and front downtube will reduce resonant vibration, and keep the mounting studs from fatiguing. The back mount should also be as solid as possible, though it is not as rigid as the front mount.

I am still working on making my motorbike legal, but I have test driven it at least 20 miles. Vibration is not a problem. I cut the entire downtube out of a Schwinn Delmar frame. I bent up a new downtube out of 1-1/4x.09 tubing and welded it in place. The new downtube runs at a 70 degree angle to the seat tube where the engine mounts. The motor now seats perfectly and securely into the frame. I’ll post pictures later

I see so many long, spindly, front mounts. Often they are made of relatively thin plates or a long bolt through the frame. Then when the user experiences excessive vibration and resonance, they put rubber between the mounts and frame in an attempt to isolate the engine. That can help isolate the vibrations some, but it will cause other problems unless properly done. Simple rubber shims aren’t the answer. A well engineered isolated mount can be the smoothest riding option. But these bikes by there nature should be simple. That engineered rubber mount is going to be more complex than a solid mount. In my opinion, a 66cc engine shouldn’t even need rubber mounts.

 

[Bryan Smith]

I used strips of plastic from a thick plastic bottle on my mounts. Very rigid, but easy on the paint job.

 

[MikeJ]

Hi Scotchmo -

I see you are new; welcome to this board. I recently took out my first bike build using a 66 cc engine. I am concerned about the vibration. I'm going to call my bike "Boneshaker" until I can get vibration under control. You seem to view vibrations from an engineering point.

I took a second engine apart for unrelated reasons (stripped one bottom end stud thread; that will be repaired later). My thoughts are the engines produce vibration axially through the cylinder because of power pulses AND because the rotating assembly is out of balance by a little bit. (Face it, these cheap engines are not dynamically balanced before they are shipped out.) In the engine I took apart,the 82-tooth reduction gear had a side-to-side wobble as it rotated; maybe one millimeter, but easily noticeable. (I can explain my thought on why that is in a later message). With that wobble, it would set up a side-to-side shake (a 6,150 crankshaft rpm would produce 1,500 wobbles per minute, or a 25 Hz side-to-side wobble, and combine that with the 102.5 Hz front-back-up-down vibrations caused by the out-of-balance rotating mass. Some days you have to wonder how these engines hold together.

The amount of vibration is a function of balance of moving parts. For my question, assume vibration gets worse with higher engine rpm. If the engine is solidly anchored to the bike frame, all that vibration is transferred to the frame. My thoughts: use dense mass external and internal to the bike frame tubes to dampen vibrations.

Some ideas:
Epoxy a solid steel or lead rod to inside the tubes, very near the engine mounts.
Epoxy steel or lead shot inside the tubes, very near the engine mounts.
Bolt external lead mass to the outside of the tubes, not sure of best location.

The external mass (assuming lead) can be melted down at home and poured into a mold like a muffin tin to let cool. It is easy to machine and still relatively rigid for bolting.

I am thinking about bolting one or more lead masses to the tubes, using rubber or dense foam insulation between the tube and the mass. And hope frame vibrations are absorbed by the masses and compressible insulation.

Any thought about this approach?

Thanks,
MikeJ