BAHKACK

So lighter crank vs lightened flywheel? Does the lighter internal rotating mass have a different effect than a lightened flywheel?

patsmx5

iTrader: (16)

The not fully counter weighted crank would increase main bearing loads.

patsmx5

iTrader: (16)

By chance, do the cars that came with the lighter crank have a lower redline than a miata?

concealer404

iTrader: (3)

Why? Would the 4 cylinder crank not be balanced by the countering rod/piston combo going the other way?

Engi-ninja

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...

concealer404

iTrader: (3)

Right, but you have less weight spinning as well. To my non-engineering brain, seems like a net zero in that regard.

Engi-ninja

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.

patsmx5

iTrader: (16)

Spinning weight alone doesn't load the bearings, well, nothing more than gravity. The rod/pistons slinging back/forth puts thousands of pounds of force pulling on the crank throws up/down as it spins. The counter weights built into the crankshaft are designed to "counter the weight" of this force so that the bearing load is reduced and vibration is reduced. A fully counterweighted crank minimizes the loads on the mains by full counter balancing the reciprocating weight. As you can guess, a partially counter weighted crank only partially counters this weight.

concealer404

iTrader: (3)

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?

patsmx5

iTrader: (16)

It's a balance of force. You want to minimize the maximum force on that area of the crank. So find the CG of the two counterweights, and calculate the force that puts on the crank. Then calc the CG of the throw accross from it, and then calculate the force the reciprocating assembly puts on the throw. Countweight force - throw force - rod/piston force should be about zero to minimize loading on the mains. I'm sure google can explain it better if that doesn't make sense.

Engi-ninja

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.

concealer404

iTrader: (3)

It does, but in the context of using either of the stock cranks in a not-stock scenario, seems like they're both compromises.

patsmx5

iTrader: (16)

If you wanted to maximize main bearing wear, you could run the lightest counterweighted crank possible, with heavier-than-stock beefy rods and heavy forged pistons. That would be worst case.

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.

Engi-ninja

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.

Engi-ninja

And would probably result in projectile counterweights, unless someone really knows what they're doing and it's a bolt on weight.

concealer404

iTrader: (3)

Sounds like i'm going to make Nick come and hang out and drink beer while i go down this next rabbit hole. :lol:

Engi-ninja

Haha, sounds like a party! Engibeering...always a good idea.

afm

iTrader: (3)

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.

Engi-ninja

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.

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?

afm

iTrader: (3)

I think you're misreading what I'm saying. I'm not trying to open up a centripetal-vs-centrifugal discussion that adds nothing. I used the word centrifugal throughout my post, except in the mass-based calculation, where either is appropriate (the calculation is based on the mass of the counterweight, so it's kind of natural to examine the forces acting on it in that context).

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.