Crankshaft Comparison

biknut · Oct 27, 2011

For ease of conversation, lets refer to the crankshaft on the left as, original, and the one on the right as, new style.

My main question is do they interchange? If not, what's the new style supposed to fit in? 2 of the original style cranks I have came out of wide front mount blocks. 50mm width between the front mounting studs. I'm guessing maybe the new style crank fits in the motor blocks with the narrow, 40mm or so, front mounting stud width. The original crank has bolt on fly weights, while the new style has one piece wheels.

These are the dimensions

original crank assy, 1574g total weight
105mm total width
90mm flywheel dia

New style crank

1392g total weight
101mm total width
89mm flywheel dia.

Smiling Brick · Oct 29, 2011

Having just tried to replace a screw-on crank with a solid crank, I can say that in my case, they are not interchangeable. The solid crank has a slightly shorter conrod and will cause the piston to hit the flywheel, snapping pieces off of the piston skirt.

biknut · Oct 30, 2011

Smiling Brick said:
Having just tried to replace a screw-on crank with a solid crank, I can say that in my case, they are not interchangeable. The solid crank has a slightly shorter conrod and will cause the piston to hit the flywheel, snapping pieces off of the piston skirt.

After reading your post I tried the one piece crank, and mine doesn't hit the piston. I'm not sure what crank you might have.

This one was advertised as a crank for an 80cc gen II motor whatever that is.

biknut · Oct 30, 2011

I've now bought 4 cranks from 3 different major suppliers and all of them are different degrees of crap.

rustycase · Oct 30, 2011

Is there any difference in stroke between original and new style?

From what you've posted, the new one should rev quicker because the mass is less, and there's a bit less potential for vibration???

Tnx for the thread!
rc

biknut · Oct 30, 2011

One of the things I learned so far is, if you put crank on a truing stand with just a rod, gravity will pull the crankshaft into a crank pin up position.

3 of the 4 cranks I have all do this, but I just took a brand new motor completely apart to test the crank and it hangs crank pin down WTF. It's a good thing I didn't slap that motor in a bike, it would have probably vibrated my fillings out.

biknut · Oct 30, 2011

rustycase said:
Is there any difference in stroke between original and new style?

From what you've posted, the new one should rev quicker because the mass is less, and there's a bit less potential for vibration???

Tnx for the thread!
rc

As far as I can tell there's no difference in the stroke. The main difference I see is with the dimensions of the flywheels.

If I were to swap a new style crank into cases formally containing a original crank, assuming it would fit, there would be more empty space in the crankcase around the flywheels. Both the diameter and width is a little smaller.

I've read where people said that would lower the compression, but I don't quite understand how that works.

As far as vibration, I don't think overall weight matters much, only how well the crank is balanced.

I'm thinking a heavy flywheel would rev a little slower, but would be a little better getting off the line, and going up hills.

maniac57 · Oct 31, 2011

Think about the difference between a motocrosser and an enduro. The motocrosser has a light flywheel for explosive acceleration and max top end. The enduro has a heavy crank for low end smoothness and tractability. Hope this helps.

killercanuck · Oct 31, 2011

I don't know about the "low compression" claim, but you'd think with more airspace in the case you'd have a better charge going upstairs, eh?

biknut · Oct 31, 2011

killercanuck said:
I don't know about the "low compression" claim, but you'd think with more airspace in the case you'd have a better charge going upstairs, eh?

I was kind of thinking the same thing. More space in the crankcase would seem like more room for charge.

I wish someone would explain why it's supposed to be better for the crankcase to be small.

Tohri · Nov 1, 2011

Remember, you're displacing less than the motor's displacement in terms of fresh fuel air charge. The piston comes down X mms, and displaces Y CCs on it's way. Some of that chuffs back out the intake port. The rest is compressed to a certain PSI, and blows up through the transfers.

The volume of the crankcase determines the peak pressure of the Fuel air charge pre transfer. The less volume, the higher that peak pressure. Which can mean a lot of things.

Generally, you want that peak pressure to be high. More pressure means more charge gets through the transfers quicker.

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Crankshaft Comparison

biknut

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Smiling Brick

New Member

biknut

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biknut

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rustycase

Gutter Rider

biknut

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biknut

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maniac57

Old, Fat, and still faster than you

killercanuck

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biknut

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Tohri

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