Mathematical formula for balancing 69cc Chinese Bicycle motor

[Fabian]

here is a place that has the K16x21x10 but they charge a ridiculous low volume fee for orders outside of the U.S.

Even though every bearing supplier in my area (melbourne, Australia) says that this bearing is impossible to get, this website proves it is possible to order a K16x21x10

http://www.ebatmus.com/cart.epl?Buy=1&Quantity=1&ItemID=155241&Query=needle

 

[clay]

Gees, this is taking me back quite a few years now - a very long time indeed

In the early years of motorcycling my second bike was a Yamaha RZ250 with a set of modified carburettors and a set of race pipes.
It wasn't all that long before it blew the big end connecting rod bearing, after over rev chasing an early Suzuki GSXR 750.
Must have been about 115 miles an hour when it blew - certainly an intersting ride getting it to a stop.
After the thing was rebuilt with RZ350 cylinders, i left Melbourne and moved to Far North Queensland, a ride of some 3600 kilometers (2200 miles).
I loved that RZ250 with the 350cc engine capacity.
For a small engine, it really had some get-up and go.

Must have been up there for 2 years and i found a nice quiet straight road out the back of Cairns on the Atherton Tablelands.
I made another fatal engine mistake.
I maxed out the speed (loved it) speedo reading 220 kilometers (140 miles an hour) and it blew the big end connecting rod bearing
Made for another interesting stop from speed.
After the engine was rebuilt, someone told me that i should have used Yamaha TZ big end bearings as they were far more durable - if i only knew that before it was rebuilt, the first time.

From my experience, 2-strokes seem to have their achilles heel set in the big end bearing - they just don't tollerate over rev because of the limitation due to a needle roller bearing big end.

Fabian

fabian, what srdavo was saying and i was hesitant to repeat, was that we have never heard of a single big end rod bearing failure on a chinese motor.
everyone that i personally know who rides these bikes has had a small end bearing failure or a noisy one that was going to fail. my 40mph motor turned 8000 rpm and upon a recent inspection, (about two weeks ago) the only problem i found with the lower end was a couple thousandths of extra side play on the conn rod. as a matter of fact i threw the same bearing back on the crank pin after i balanced the assembly, which is what this thread was originally about right. we will all find out how well that worked out at the death race, so don't put too much faith in what i have to say.

 

[Fabian]

Hi Clay

It's really surprising how you manage to spin your engine to 8,000 rpm.
From my perspective that's an impossibility with the restricted orifice size of Chinese bicycle carburettors, having said that, over revving the motor down a steep hill to 8,000 rpm is possible but from my experice the big end bearing would destroy itself in the process.

When my engine is rebuilt with a professionally balanced crankshaft; high quality crankshaft bearings, clutch shaft bearings and big and small end connecting rod bearings (possibly even a replacement connecting rod sourced from another engine) i'll post a full photo tutorial on the whole process and procedure.

The karting engine specialist i'm talking to suggested lightening the crankshaft to reduce overall stresses it wasn't possible to remove enough material in the correct place on the crankshaft for optimised balance at the desired rpm.

When the failed big end connecting rod bearing is removed from the crankshaft pin, i'll also add those photos to the tutorial.

Fabian

 

[Fabian]

A very interesting response to a question posted on another site:

The bearings of choice for the large end of a two stroke connecting rod are caged needle bearings -- generally, the crankshaft rod journal serves as the inner race, so must be surface hardened to RC60, and the connecting rod large end serves as the outer race and must be similarly hardened. One of the weak points of these bearings is the cages -- as the rod moves through 360 degrees of rotation the surface speed of the bearing continuously changes speed, requiring the rollers to accelerate and decelerate.

This would not be much of a problem were it not for the mass of the cage holding the rollers in alignment: the normal copper coated steel cages weigh enough to push against the rollers and skid them across the races at high RPM, scarring the races and flattening the rollers. The highest end large end bearings use silver plated titanium cages which weigh as much as one third of their steel counterparts, and are therefore much less prone to skidding. Interestingly, the most common crank train failure is not the bearing itself, but the thrust washers centering the rod on the crank -- being copper or silver plated steel, they do not at all like high speed rubbing loads with the scanty lubrication in the crankcase, and they scrub and get hot enough to flash the oil, excluding lubrication from the bearing.

The best cure for this, done by Walter Kaaden first for MZ, and then Suzuki, was to eliminate the thrust washers altogether and center the rod with washers on the small end inside the piston, improving RPM potential by as much as 500 mean FPM, or 1,500 RPM for the 50 and 125 multi cylinder engines.

On the small end, the cage part of the needle bearings used is again the weak link, but for a different reason. The bearing load is almost exclusively linear (in line with the cylinder) and the cage doesn't really rotate very much at all, so the decelerative / accelerative forces at TDC and BDC flex the cage, eventually causing fatigue cracks quickly followed by outright failure which sends the cage pieces and rollers showering down into the crankcase, with disastrous consequences to all those expensive parts. The most reliable needle bearings are simply the needles crowded together with no cage at all. This, of course, is a real bear to assemble, but almost absolutely insures the small end bearing is quite reliable

 

[Fabian]

So looking for a solution, one would choose the highest quality caged needle roller bearing available for the small end and big end bearings, using a specification exceeding the load limits produced by the engine at it's maximum usable rpm.

 

[Fabian]

Finally, finally, i've found a place in Australia specialising in small sized bearings.

Brilliant, i can get both the small end and big end connecting rod bearings in the sizes used in Happy Time style Chinese motorised bicycle engines, along with the crankshaft bearings/clutch shaft bearings.

It's a one stop shop for our bearing requirements.
Scroll down to the second last bearing pictured on the page - the K16x21x10


http://www.minibearings.com.au/stor...OLLERCAGETYPE/needlerollerbearings/all/121/3/

Fabian

 

[clay]

Hi Clay

It's really surprising how you manage to spin your engine to 8,000 rpm.
From my perspective that's an impossibility with the restricted orifice size of Chinese bicycle carburettors, having said that, over revving the motor down a steep hill to 8,000 rpm is possible but from my experice the big end bearing would destroy itself in the process.

When my engine is rebuilt with a professionally balanced crankshaft; high quality crankshaft bearings, clutch shaft bearings and big and small end connecting rod bearings (possibly even a replacement connecting rod sourced from another engine) i'll post a full photo tutorial on the whole process and procedure.

The karting engine specialist i'm talking to suggested lightening the crankshaft to reduce overall stresses it wasn't possible to remove enough material in the correct place on the crankshaft for optimised balance at the desired rpm.

When the failed big end connecting rod bearing is removed from the crankshaft pin, i'll also add those photos to the tutorial.

Fabian

i have yet to see anything remotely close to a tutorial. your karting specialist is right though, as you could plainly see from my previous photos, i had to make the removeable "balance pads" from the stock crank and make them out of aluminum. that was the only way to add enough weight to the side opposite the crank pin. as for your proffesional opinion about my carburetion situation and 8000rpm redline, maybe these pics will help you understand.
i'm a little curious about your plans for your exhaust though.

 

[Fabian]

Hi Clay

It's fairly obvious from the photos of your dual carb setup that you'll easilly have enough intake flow to supply 8,000 rpm.
Typically, a stock setup with single carb wouldn't have any chance of going to those rpms (let alone for an unbalanced crankshaft shaking things to pieces) - the engine in my bike blew the bottom end bearing at 5,500 rpm, zooming down a steep hill. I suspect the bike was going faster than the engine's ability to rev and keep up with road speed.

Fabian

 

[Fabian]

Before i can even post a photo tutorial, i need to obtain a non Chinese K16x21x10 needle roller bearing - this is proving to be a task of impossible proportions.

I am going insane with frustration trying to get this part.
There are a few websites that list it as a stock item, yet after contact, they advise they have 'no stock' and want a volume order before stocking the bearing.
Asking for just a 'single' bearing has the sound of someone laughing loudly on the other end of the telephone - if you order 100 bearings, no problem at all.

Fabian

 

[Fabian]

Been busy researching crankshaft balancing methods.

http://www.brufnut.de/SS100/ENGINE/CRANK_BALANCE/crank_balance.htm