Vibration - what is normal?
- Thread starter stouchton
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B_man_Al
Senior Member
Brent ...
Very interesting post you did on engine components! Raises more questions for me:
1- Do these numbers apply to a model B engine .. probably not I suspect?
2- Do the piston numbers include rings?
3- Any idea how much weight is added for oversize pistons, or are they compensated for by reduced material during casting/machining?
4-Do you know the weight of the original wrist pins?
Thanks in advance
Al
Very interesting post you did on engine components! Raises more questions for me:
1- Do these numbers apply to a model B engine .. probably not I suspect?
2- Do the piston numbers include rings?
3- Any idea how much weight is added for oversize pistons, or are they compensated for by reduced material during casting/machining?
4-Do you know the weight of the original wrist pins?
Thanks in advance
Al
marc hildebrant
Well-known member
Brent,
Thanks for the information regarding the tolerances on the engine/drive train.
If I had my engine at your shop, could you hold the same tolerances as when the Model A was built ?
Marc
Thanks for the information regarding the tolerances on the engine/drive train.
If I had my engine at your shop, could you hold the same tolerances as when the Model A was built ?
Marc
BruceUpstate NY
Senior Member
My lowest original mileage A (32,000 mi) had the smoothest engine I've ever had, including my two rebuilt engines balanced and counterbalanced by reputable builders in recent years. Any comments, rebuilders?
Kurt in NJ
Well-known member
A pistons are the same as B pistons, the original Ford pistons I have --about 30 ALL weight within 5 grams---no matter what size, and my original piston collection goes from std to .060, the set in the box was within 1 gram, I had 2 sets of B rods in the box, they weighed within 1/2 gram--including each end and total, I recently got a boxed set of V8 rods ---they all weighed the same, perhaps the scale has a 1/4 gram tolerance.
Through the years I have tried getting crankshafts ground at 4 places---in addition to the place that originally did my engine, several of the places had this for an answer when I questioned their work (.004-.008 off center)--- "It's just an old car, it won't matter"
as for wristpins and rings ---rings pretty much are the same weight, original ones that I have weighed weight the same for the same type, aftermarket ones can be anywhere
steve s
Well-known member
I didn't understand it all, but I agree with Brent about unfelt torsional vibrations. For many years I ran a vibration dampener on my A, and any felt difference had to be imagined. It was a repro of one designed by Ed Winfield and sold by the outfit in Rosemead, CA with NO claim that you would feel a difference. I always just hoped that my crankshaft would feel the difference.
Steve
Steve
Pete
BANNED
The best way to tell if a damper is working is to feel it after a prolonged drive at cruising speed. If it is warm it is working.
Mike K has the best graphic illustration of torsional vibration I have seen yet. It has been posted on here several times previously.
Purdy Swoft
Well-known member
As for mirror vibration, If the motor mounts are too tight it will increase vibration. Tightness of rear float-A-motor mounts can be experimented with. The float-A-motor rear rubber biscuits or pads harden with age and need to be replaced periodicly. The castilated nut under the front motor mount yoke only needs to be tightened enough to get the cotter pin in place. Most if not all model A's will have some mirror vibration, these mount adjustments can sometimes cause less vibration.
steve s
Well-known member
Pete,
Apparently, after a prolonged drive at cruising speed, EVERYTHING under my hood is "working."
Steve
tubadon40
Junior Member
Could I have "torsional Vibration"? My '30 phaeton runs smooth as silk under power. Then on deceleration it really gives me a foot massage. Up to 25 mph it is almost unnoticeable. Then at 30-35 it's bad but at 40 and up I really don't want to decelerate without disingaging the the clutch.
I purchased the engine from a Model A club guy that said it was put together by a local reputable rebuilder (now deceased).
None of the threads I've read discribes the bad "hum" type vibration on deceleration only. What do you think?
I purchased the engine from a Model A club guy that said it was put together by a local reputable rebuilder (now deceased).
None of the threads I've read discribes the bad "hum" type vibration on deceleration only. What do you think?
nkaminar
Well-known member
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.
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.
katy
Well-known member
Try retarding the spark when decelerating.
J Franklin
Well-known member
My car seems to do that but it is smooth and quiet running along. It was balanced when it was rebuilt to 1/2 gram and the clutch assembly was balanced at Edelbrock. It runs real nice and likes to cruise at 50. I have a Brumfield 6-1 head and figured it was compression and body harmonics so not worried at all.
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Pete
BANNED
By adding or subtracting weight to the crank assembly you can move the peak vibration frequency around but you can't get rid of it without redesigning the engine.
Remember, the amount of vibration is inversely proportional to the rigidity of the mounts.
If you had a 10000 lb. model A with rigid motor mounts, you probably would not feel any vibration.
Remember, the amount of vibration is inversely proportional to the rigidity of the mounts.
If you had a 10000 lb. model A with rigid motor mounts, you probably would not feel any vibration.