Terrible vibration

KR4WM

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Joined
Jun 18, 2009
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11
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
 


T

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
 

KR4WM

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Jun 18, 2009
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11
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
 

DJEEPER

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Jun 21, 2009
Messages
147
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.
 

Happy Valley

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Jul 13, 2008
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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.
 
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bluegoatwoods

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Mar 23, 2008
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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.
 

Hawaii_Ed

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Mar 9, 2009
Messages
698
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.
 

Bike916

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Jun 24, 2009
Messages
15
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.
 

Mountainman

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Jun 15, 2008
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3,578
probably not the problem -- but that break in ???

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.

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
 

KR4WM

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Joined
Jun 18, 2009
Messages
11
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

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!!! :D

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