crank mod, removing material

JerboaJohn

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Jul 29, 2018
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what would happen after I've balanced the crank to the piston and removed some material on the lathe?. it'd be equal all the way around, that's what the lathe does in circles. I'd probably do the sides so I could fill in with JB not to increase the case volume. but just wondering, how much weight does it really need? lighter is faster, right?
 


C

coolki

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what would happen after I've balanced the crank to the piston and removed some material on the lathe?. it'd be equal all the way around, that's what the lathe does in circles. I'd probably do the sides so I could fill in with JB not to increase the case volume. but just wondering, how much weight does it really need? lighter is faster, right?
depends on how much materail is being removed, were talking mere grams.
it would probalby spin up faster as theres less wieght on the crankshaft.
honestly the best thing to do is balence it, if your talking about the 66cc engines.
 

dparenti13

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I'm kinda just guessing here, but physics tells me less weight = less rotational inertia to overcome, so it might rev easier? Kinda like a lightened flywheel? I could be wrong...
 

Street Ryderz

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Oct 14, 2016
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I'm kinda just guessing here, but physics tells me less weight = less rotational inertia to overcome, so it might rev easier? Kinda like a lightened flywheel? I could be wrong...
Less rotational mass will spin up faster with no load but under load that inertia is needed to over come the compression and driven load,It will also turn less top rpm due to the loss of inertia/mass.I've balanced a crank that completly off set the reciprocating mass and it was a dog,at 50% there is some loss but very smooth 30-40% works best IMO and puts the sweet spot up high.Matching the sweet spot to the pipe staging allows safer high rpm use and usualy yeilds good top speed
 

JerboaJohn

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Jul 29, 2018
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ah ok,, TY,,
thought it might be something like that. then probly not oughtta take anything off but to balance it I guess.
I got the idea bcs I saw 2 stroke stuffing's vid on truing a crank, and he had one that was beveled edges. so I got thinking
 

dparenti13

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Sep 24, 2016
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Less rotational mass will spin up faster with no load but under load that inertia is needed to over come the compression and driven load,It will also turn less top rpm due to the loss of inertia/mass.I've balanced a crank that completly off set the reciprocating mass and it was a dog,at 50% there is some loss but very smooth 30-40% works best IMO and puts the sweet spot up high.Matching the sweet spot to the pipe staging allows safer high rpm use and usualy yeilds good top speed
That makes a lot more sense, thanks
 

CrazyDan

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Jul 9, 2016
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what would happen after I've balanced the crank to the piston and removed some material on the lathe?. it'd be equal all the way around, that's what the lathe does in circles. I'd probably do the sides so I could fill in with JB not to increase the case volume. but just wondering, how much weight does it really need? lighter is faster, right?
Lighter can be faster, but you also lose power faster in random headwinds and uphill sections. You need to find the happy medium between momentum and spinning up to rpm faster. If you want a f***-ton of momentum, make a 1 ton flywheel that takes 5 miles of flat ground to spin up, and can take you up a mile long hill stretch gaining 500' altitude, coasting... or you can get a flywheel that can barely keep your engine spinning, but revs up easier than blowing a feather up another foot as it falls.
All of the rotational mass in an engine works together. Lighter revs faster, but loses rpm much easier when heavier loads are presented. Heavier is harder to rev, but also harder to lose rpms under heavier loads. Only problem is, a heavier rotating mass normally can't achieve the same rpm as a lighter mass: hence the happy medium. How do you want your engine to work?
 
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