The Ford Barn - View Single Post - Vibration

nkaminar · 08-22-2021, 06:43 AM

Warning: Technical discussion

No reciprocating engine can be perfectly balanced. I am not talking about making all the pistons and rods the same weights. When at TDC or BDC the pistons, rings, pin, and rod are all reversing direction and a counter weight on the crankshaft would have to counter the inertia of all these parts. When the crankshaft is at 90 degrees (approximately) from TDC or BDC the counter weight on the crankshaft only has to balance the bottom end of the rod. Two different counter weights. So the counter weights are a compromise and designed to balance 100% of the bottom end of the rod and about 70% of the rest of the parts. So the engine is never 100% balanced. As Brent talked about, lighter pistons and pins will help.

The original crankshaft on the Model A does not have any counter weights. This will set up a vibration at higher rpm in addition to what I described above. Crankshafts are available for the Model A that have the counter weights.

As Brent talked about, whether the crankshaft has counterweights or not, it is important to have all the parts weigh the same and have the crankshaft, flywheel, etc. dynamically balanced.

Actually, there is a way to have a reciprocating engine perfectly balanced and I know how to do that but it is not a practical solution.

08-22-2021, 06:43 AM	#32
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 3,897	Re: Vibration - what is normal? Warning: Technical discussion No reciprocating engine can be perfectly balanced. I am not talking about making all the pistons and rods the same weights. When at TDC or BDC the pistons, rings, pin, and rod are all reversing direction and a counter weight on the crankshaft would have to counter the inertia of all these parts. When the crankshaft is at 90 degrees (approximately) from TDC or BDC the counter weight on the crankshaft only has to balance the bottom end of the rod. Two different counter weights. So the counter weights are a compromise and designed to balance 100% of the bottom end of the rod and about 70% of the rest of the parts. So the engine is never 100% balanced. As Brent talked about, lighter pistons and pins will help. The original crankshaft on the Model A does not have any counter weights. This will set up a vibration at higher rpm in addition to what I described above. Crankshafts are available for the Model A that have the counter weights. As Brent talked about, whether the crankshaft has counterweights or not, it is important to have all the parts weigh the same and have the crankshaft, flywheel, etc. dynamically balanced. Actually, there is a way to have a reciprocating engine perfectly balanced and I know how to do that but it is not a practical solution. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.