full circle vs half circle crank

[mac90]

info posted my greeny

greeny



half circle v full circle crank shaft

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To clear up your debate regarding the peak rpm achieved by the use of a full circle crank or half circle crank we must remember certain aspects of a two stroke engine.

As the piston moves towards top-dead-centre the reed valve opens and draws mixture into the crank case, this flow is not influenced by the volume of the crank shaft, as it is simply the volume of the piston within its stroke and its velocity that decides how much mixture enters the crank case.

However on the transfer stroke of the two-stroke crank shaft volume does count as the crank cases are sealed by the reeds. Gas does not 'flow' in a two stroke crank case, but is forced through a path of least resistance. This is similar to a garden hose. If you let the water flow freely from the hose it will flow slowly. If you put your thumb on the end of the hose, the velocity of the water will increase, as the volume has been reduced. Similarly, if a full circle crank is used, the crank case volume is reduced and the crankcase compression rises, therefore increasing gas velocities. However this in itself will not determine peak rpm, just as adjusting normal compression will not determine maximum rpm by itself.

For example, port volume differential will influence gas flow velocity. This is the area of the base of the transfer ports compared to the cylinder area of the port. For example, if the base area is twice that of the cylinder area theoretically gas velocity will have doubled (excluding boundary layer losses). In a properly tuned engine the cyclinder ports should be around 85% of the base area, allowing the gas to accelerate as it is 'funnelled' into the cylinder.

Exhaust pulse timing will dictate more than anything your peak revs. This is determined by the relative height of your exhaust port and the dimensions of your exhaust chamber. A short fat chamber will work better at high revs as the exhaust pulse will be reflected earlier, thus reaching the exhaust port in a shorter period of time to drag the next pulse out. A shorter chamber should be combined with a relatively high exhaust port and shorter gearing and a later clutch cut in. This will of course sacrifice bottom end 'grunt'. The opposites will also apply.

In short go with a full circle crank, they are good enough for 125cc GP machines producing nearly 60hp. Polini, bizeta and blata use them in engines producing 18hp. A full circle crank will give you greater flywheel effect making the bike easier to ride, smoother and easier to start.

Hope that helped

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