Speed & insane vibrations

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dylanr26

New Member
Yesterday on the way home from work I hit 32 (using gps so it was correct) on a completely stock china girl or whatever (44t sprocket) the vibrations were insane. Was it because it was revving at high rpms or because its a cheap huffy cranbrook beach cruiser?

I recently ordered a low profile air filter and a banana looking expansion chamber. Im thinking about a 36t sprocket (how fast will I go then?) But it feels like my bike will fall apart going faster then 32. Any bikes out there that handle speed well? I might just switch the engine onto another bike. Thanks.shft.
 
dylanr26 said:
Yesterday on the way home from work I hit 32 (using gps so it was correct) on a completely stock china girl or whatever (44t sprocket) the vibrations were insane. Was it because it was revving at high rpms or because its a cheap huffy cranbrook beach cruiser?

I recently ordered a low profile air filter and a banana looking expansion chamber. Im thinking about a 36t sprocket (how fast will I go then?) But it feels like my bike will fall apart going faster then 32. Any bikes out there that handle speed well? I might just switch the engine onto another bike. Thanks.shft.
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32 MPH with a 44T is pretty good, the vibes will be there no matter what bike you put that engine on, I have one bike that has an older engine I bought from an ebay vendor a few years back, it runs great and has good power but vibrates like crazy at above 32-34 MPH, I have a 36T sprocket on the bike and it works good for me on it.

If I had a 44T on that bike I'd be having the same results you are, a better flowing exhaust makes a noticeable difference on these little engines and if you go with a 36T on it you'll probably get 32-34 MPH fairly smooth and then the vibes will probably kick in above that speed if it does like mine.

I have another older bike build that has a 41T and it vibes pretty bad above 28 MPH that one will get an engine swap soon...

I have two other bikes that both have the Gen IV engines from thatsdax, these engines run the smoothest I have had, both bikes will hit 43-45 MPH and they dont have the severe vibe issue that the older ebay bought engines have.

most if not all of your vibration problem is because of an untrue and badly balanced crankshaft which is common for most of the china girl engines, Neil aka (motorbicycleracing) & Thats Dax sells a better balanced engine that doesn't have the serious vibration issue that many or most of the other engines have, I bought three lowers from thats dax and have two of them built up and running, working on the third soon, these engines will give you great speed and good power without the horrible vibes if you ever decide you want an engine that runs smoother, my bikes cruise at 40+ MPH with 36T tooth sprocket.

Map
 
Another facor that contributes to extreme vibration is an improperly mounted engine. It MUST be securely mounted to the frame, no rubber in the engine mounts and the mount fasteners torqued tight. The mount saddles must also conform to the frame with little to no gaps.

Other areas to look at is the exhaust. Is it secured in such a way that it isn't hanging out there in the breeze and vibrating? It should be secured to the frame as well as to the engine.

Knobby tires don't help either. They can add their own rough ride.

There are pages of discussion about vibrations here to read if you just type in 'vibration' in the search feature.

Balancing, even a perfectly balanced cranshaft, will not eliminate the inherent pulsations that come from a single cylinder engine, especially a 2 stroke. It isn't vibration but what you feel is the result of the combustion, the shock of it, being transfered into the engine and ultimately the frame and then into you. You can't take that away unless you shut down the engine.

A good example of this is high quality, precision made 2 stroke, single cylinder model airplane engines. Even with a high dollar engine and balanced propellers you still need to cushion the delicate radio components against the 'vibration' from the engine.

Tom
 
Tom is dead on right here, there are other factors involved beside just the crank balance, my dax Gen IV engines set up the same as my other older ebay engine have about half or less vibration though, they don't even buzz my hands, and turn way more rpms and actually get smoother the higher they rev rather than the opposite which is how my other engines are which get insane vibes at higher rpms as if they are gonna fly apart at any time....LOL! and those kind of vibes limit power and rpm if the engine is that out of balance.

Great points Tom, all of those things need to be done right not only for dealing with vibes that may come from things not being mounted correctly, but having all that stuff right is gonna add better reliability to the bike also.

Map
dnut
 
these often smooth out quite a bit after run-in, but loosening of mounts could be a problem & for some frames, just moving one mount up 1/2 inch & other mount down 1/2 inch can make a big diff
 
Go for the 36 sprocket- your cruising speeds will be at far less revs-
and that really cuts vibration

unless you are very heavy or riding a lot of very long very steep hills- go for a smaller sprock- I ride a 34 in flat florida on my 66cc- 39 on the 50cc but may change that to 36 someday

Also beach tires/ knobby tires contribute a lot to vibration- smooth tread helps if you want speed- and narrow doesn't hurt either- you could get some 1.75 or 1.50 tires and see a tremendous difference and rolll smoother
 
I can agree with the mounting, when I welded tabs to my frame and bolted the engine on much more solid mine became very smooth, for the aft mount, I'm using 2 of the aft mount saddles to sandwich the seat tube which is also way better than using that steel strap and it also made a big difference once the rpms go up.
I thought my engine had a bad balance issue until I solid mounted it.
 
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