Terrible vibration

Discussion in '2-Stroke Engines' started by KR4WM, Jun 26, 2009.

  1. KR4WM

    KR4WM New Member

    I just finished my first motored bike build- a "Black Stallion" 66/80cc from Kings Motor Bikes. It fired right up (thanks to all here for your excellent tips in many different posts!) and ran well. For my first ride, I kept it on the down-low to help break in the motor. I probably did not exceed 15MPH, and rode around our block a couple of times (about two miles total). One poster recommended two slow and easy 15 minute rides, followed by complete cooling off before riding again. Nothing has blown up, so I guess he was on the money.

    One thing I have to say, if you guys are putting these things together in 3 to 6 hours, you must really have it down to an art! It took me one 4-hour evening session and an 8 hour day session to finish this one off! But then, I'm a perfectionist, didn't have all my tools at-hand, and had continuing interruptions with phone calls, TV show breaks, meals, etc. I still need to install the chain guard, but that'll have to wait until cooler weather. And I didn't coat the inside of the gas tank. It's downright U-G-L-Y, and I plan to replace it with something nicer looking.

    I could not restrain myself at the end of yesterdays ride (the second of the 15 minute rides) and cranked it wide open to see what it could do. As it built RPMs, it passed a certain range and the entire bike began vibrating like the engine was totally mis-balanced! The gas tank loosened and shifted, and I had trouble keeping my hands on the handlebars! I dropped RPMs, and the vibration went completely away. I tried doing this several times, and each time I crossed that certain RPM barrier, the bike would go into wild vibration!

    I'm thinking this is -NOT- normal. I'm also wondering if it could possibly be one of the internal needle bearings being either out of tolerance or worse? Or is this normal for these engines to do this? If not, is there a fix? I guess I can restrain myself from cranking down on the throttle- it certainly moves fast enough before it hits this rough throttle range. But the vibration is so bad, it does concern me. It concerns me a LOT!

    My build consists of the Black Stallion and a Wal-Mart "Cranbrook" $84.00 bicycle. Real el-cheapo transportation for around the neighborhood! It fits in my shed next to my Harley Ultra quite well! (And to think I wanted a golf cart!)

    -Web
     

  2. Tinker1980

    Tinker1980 Guest

    I have a BGF 66cc motor on a Kulana moon dog, and like your build, once I get to about 28 MPH it starts to really vibrate bad. I normally just stay below that speed, since the motor seems to hit a "sweet spot" around 24-25 MPH. Also, running a bicycle with no suspension at 30 MPH on west Tulsa streets is a sure fire way to get thrown into the pavement.

    Like you, I spent a bit of time on my first build. Took me a good week, working on it every day after work, but once I had it going it was great! The first machine didn't have the greatest brakes or tires, so I ended up putting that motor on my moon dog which was practically screaming "motorize me". I managed to do that install in about 3 hours, and that was without full use of my right hand. It gets easier after the first time... and MB's are kinda like potato chips - can't just build one!

    -Mark
     
  3. KR4WM

    KR4WM New Member

    Thanks for the quick reply!

    OK, that answers my question. I was under the impression (from reading here) that with a 44 tooth sprocket, I could expect about 40+ MPH. I purchased a speedometer, but have not yet installed it. I'll repost in this same thread after I get it up and running to let others know what kind of speed I'm getting.

    I have an old Tillotson racing carb from 20+ years ago that was never modified for methanol. I was thinking about using it with this motor, but wanted to get it broken in really well before trying that. But if it's going to "act up" and go into oscillation (vibration) at 28~30MPH, I don't see any reason to waste a $100.00 carburetor on it. It seems to be doing just fine with the one that came with it anyhow!

    Thanks! -Web
     
  4. DJEEPER

    DJEEPER Member

    Make sure your chain is tight tight tight! otherwise it will pick up a resonance frequency and vibrate the bike so hard your hands will sting.
     
  5. Happy Valley

    Happy Valley Active Member

    There are a lot of things you can search for in these pages that has been said about cutting down the vibration but most of it stems from the fact that the crankshafts are unbalanced in these Ht engines, unless you get lucky and find the odd one that is the exception.

    And you're right, there is a lot of speed expectation written into these things, too much.
    But ask yourself and not trying to be a wise guy here, but do you really want to do 40+ mph on an $84 bicycle? That's what the Harley is for. Ride safe.
     
    Last edited: Jun 26, 2009
  6. bluegoatwoods

    bluegoatwoods Well-Known Member

    Happy Valley's point is a good one.

    Personally, I don't recommend anything over about 20 mph. I do think I'm in a minority on that one, though.

    But I'd be scared to death to take a cheap bike up to 40 and I wouldn't feel very good about it on a top-of-the-line bike.

    Though I have wondered about a very good bike with suspension and "weighted" wheels.
    Getting the balance right would be the hard part. But with the mass of the wheels increased by some significant amount, things might become safer.
     
  7. Hawaii_Ed

    Hawaii_Ed Member

    No way you'll see 40 with that setup :) I was in the 30's with a 36T on my beach cruiser. It would get a good vibration at 28 or so, then smooth out over 30. of course that motor started running like **** at 1500 miles LOL. My new zoombicycles slant is silky smooth in comparison to that motor. I also put a new basic engine on the beach cruiser, and it is pretty smooth all the way up to 26, but have not topped it out yet. Did you mount with rubber? I found mine is smoother without it, everything tightened up better, less jumping around.
     
  8. Bike916

    Bike916 New Member

    I have a old GT outpost mountain bike that I just mounted a PK80 to. It has a definite vibration that is felt at higher speeds. I was just thinking its normal, but it can be smoothend out...which is good to hear.
     
  9. Mountainman

    Mountainman Active Member

    probably not the problem -- but that break in ???

    probably not the problem -- but that break in period don't sound proper
    two miles isn't anything
    if I was using straight miliage and no other known break in factors
    ((at least)) 50 miles -- closer to 100 would maybe even be better

    when we used to buy small Hondas and Yamahas -- different beast
    they called for 500 miles

    ride that thing
     
  10. KR4WM

    KR4WM New Member

    By all means, I don't think he meant that two 15 minute rides was a proper break-in, I think that was just to "stretch things out" a little, and ensure proper running during the break-in period. I would think two heat/cool cycles would show up any loose bolts or gaskets. BTW, the instruction sheet recommends checking head bolt torque during EVERY PRE-RIDE CHECK! I think that's a little excessive, and is just asking to have a head bolt broken off! I think 500 miles is much more realistic, although a single-gear setup is bound to rotate more times in a shorter period of time.

    Also- I understand about the Harley being for high-speed runs, and you're right, it's probably not safe to ride a bicycle designed for 10 to 15 miles an hour at 40 miles an hour without some SERIOUS modifications! Probably need high-speed bearings in the wheel hubs, which would lead to extra reinforced hubs, larger spokes, tire balancing, and some frame bracing (especially at MY weight!). I just thought it would be cool to blow the doors off the neighborhood golf carts with my bicycle!!! :grin5:

    I'll keep it turned down... don't worry! Thanks for everyone's ideas/tips/concern! -Web
     
  11. KR4WM

    KR4WM New Member

    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
     
  12. 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.
     
  13. KR4WM

    KR4WM New Member

    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
     
  14. Molotov256

    Molotov256 Member

    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... :punk:
     
    Last edited: Jun 27, 2009
  15. kiwirider

    kiwirider New Member

    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....!
     
  16. Hawaii_Ed

    Hawaii_Ed Member

    I agree, I did my last two builds bolted right to the frame with no rubber, and they are much smoother.
     
  17. 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?...
     
  18. Scotchmo

    Scotchmo Member

    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.
     
    Last edited: Jun 29, 2009
  19. Bryan Smith

    Bryan Smith Member

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

    MikeJ Member

    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
     
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