So lighter crank vs lightened flywheel? Does the lighter internal rotating mass have a different effect than a lightened flywheel?
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Originally Posted by BAHKACK
(Post 1469416)
So lighter crank vs lightened flywheel? Does the lighter internal rotating mass have a different effect than a lightened flywheel?
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By chance, do the cars that came with the lighter crank have a lower redline than a miata?
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Originally Posted by patsmx5
(Post 1469429)
The not fully counter weighted crank would increase main bearing loads.
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I believe the purpose of the counter weight is to provide a centrifugal force in the opposite direction of the rod/piston assembly, and reduce the load on the bearings. Think about holding a weight in on hand extended straight out, and spinning in a circle. Your core is having to work harder to keep you spinning on an axis. If you held another weight in the other and and extended it in the opposite direction, it would help balance. The smaller the weight, the less it would help. At least, that makes sense to me...
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Right, but you have less weight spinning as well. To my non-engineering brain, seems like a net zero in that regard.
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Yes, but the loading on the bearing is determined by the net force about the axis; when you have 2 equal centrifugal forces in opposite directions, there's zero net force on the bearing, in a purely mathematical sense. In real life there's gravity, and you can never get perfectly equal centrifugal forces.
You're right that the other piston assembly is going in the other direction, but it's separated by a moment arm, so to rely solely on that is introducing a bending force into the crank. To continue the analogy, it's like holding the weight in one hand, and another weight on a pole in the other. It would counter the weight in theory, assuming a perfectly rigid structure, but you're not a perfectly rigid structure, and neither is the crank. |
Originally Posted by concealer404
(Post 1469439)
Right, but you have less weight spinning as well. To my non-engineering brain, seems like a net zero in that regard.
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Bear with me,
So to me, reading this... the counterweights are only truly SUPER AWESOME if they weigh the same as what's attached to the crank, right? |
Originally Posted by concealer404
(Post 1469446)
Bear with me,
So to me, reading this... the counterweights are only truly SUPER AWESOME if they weigh the same as what's attached to the crank, right? |
Not necessarily weigh the same, but produce the same centrifugal force in the opposite direction. The rod and piston are not swinging freely on the other end. Only part of their weight is acting centrifugally on the crank.
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It does, but in the context of using either of the stock cranks in a not-stock scenario, seems like they're both compromises.
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Originally Posted by concealer404
(Post 1469449)
It does, but in the context of using either of the stock cranks in a not-stock scenario, seems like they're both compromises.
If you wanted to minimize main bearing wear, finely balancing the OEM parts is probably best, as mazda likely spec'd the miata crank to balance well with the miata rods/miata pistons. Yes there are compromises when changing components around. Ideally you would adjust the counterweights on the miata crank to suit the reciprocating assembly bolted to it, that's a lot of work, nobody(very few) would ever do that. |
Sure, anytime you use a highly engineered part in a context it was not designed for, it's not going to work as well. But if the 2 counterweight version is closer to the original design intent, then a single counterweight will be harder on the bearings.
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Originally Posted by patsmx5
(Post 1469451)
Ideally you would adjust the counterweights on the miata crank to suit the reciprocating assembly bolted to it, that's a lot of work, nobody(very few) would ever do that.
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Sounds like i'm going to make Nick come and hang out and drink beer while i go down this next rabbit hole. :lol:
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Originally Posted by concealer404
(Post 1469457)
Sounds like i'm going to make Nick come and hang out and drink beer while i go down this next rabbit hole. :lol:
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Originally Posted by concealer404
(Post 1469446)
Bear with me,
So to me, reading this... the counterweights are only truly SUPER AWESOME if they weigh the same as what's attached to the crank, right? The downside of counterweights is cost and MOI. The SUPER AWESOME thing is to make a counterweight that provides a lot of counterweighting without a large MOI. This means making it denser and closer to the crank centerline. Like bolted-on tungsten counterweights. Centripetal force of a counterweight (good) = mass * radius of CG * (angular velocity)^2 MOI of a counterweight goes up with the mass and the square of radius. Double the mass, cut the radius in half, and you get all the benefit with half the MOI increase. |
Originally Posted by afm
(Post 1469473)
Counterweights use centrifugal force to counteract the force (not centrifugal) from the inertia of reciprocating components without using a long lever of the crank that passes through the main bearing. A counterweight that's lighter still provides some of the benefit, just not as much. The benefit doesn't suddenly vanish when mismatched, since it still reduces crank bending, though not as much.
The downside of counterweights is cost and MOI. The SUPER AWESOME thing is to make a counterweight that provides a lot of counterweighting without a large MOI. This means making it denser and closer to the crank centerline. Like bolted-on tungsten counterweights. Centripetal force of a counterweight (good) = mass * radius of CG * (angular velocity)^2 MOI of a counterweight goes up with the mass and the square of radius. Double the mass, cut the radius in half, and you get all the benefit with half the MOI increase. The only way to increase mass AND reduce radius without interfering with the motion of the rod is to use a different material of a higher density. What material is 4 times the density of iron? Black hole matter? |
Originally Posted by Engi-ninja
(Post 1469475)
No, centripetal force is the force acting on the counter weight keeping it from flying off into space. Centrifugal force it what's acing on the crank bearing to counter the force of the rod/piston assembly.
By "not centrifugal," I mean exactly what I said. The force that counterweights counteract is predominantly the inertial force from the reciprocating masses. This is linear acceleration, not centripetal, centrifugal, or anything related to rotation. |
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