Rear 'Float-A-Motor' Engine mount - Necessary ?

[DougVieyra]

Yesterday, while looking for some obscure Model A part, I stumbled upon an old set (never installed) of 'Float-A-Motor' rear engine mounts. The kind with the heavy rubber donuts, etc., which are supposed to reduce engine vibration to the car.

Thirty-five years ago I installed a set like these on my '30 Coupe. In the early days (1950's-1960's) they seemed like a useful tool to help dampen the excessive engine vibration of the old stock 4 banger. And they did work.

Now however, with today's often heavily modified / modernized engines (balanced 'B' crank & rods, etc. - 'Touring Engines') I am wondering if there would be a need. The engine in my '31 Coupe has had everything done to it that has been recommended to bring it into the 2014 driving world. It seems to run smoothly to my feel. So even though I have a 'Float-A-Motor' kit for it, I am not sure it needs it.

How do the rest of you feel about these rear engine mounts? Besides the rote of the past, do any of you feel a real need for these 'vibration reducers' on today's balanced and well-tuned engines ?

[BRENT in 10-uh-C]

Doug, in the past this has been discussed extensively, ...and even more so in the old Fordbarn forum. I will comment two things regarding F-A-Ms.

1) As been discussed many times, these are a 'bandaid' to cover up a problem. As you mentioned, it does not even require modern or modified engine pieces to make an engine operate smoothly. It just takes the reciprocating parts already there to be balanced and the vibrations are almost all eliminated.

2a) The bigger issue I think that makes F-A-M use detrimental is the stock flywheel housing and motor mounts are effectively a "4th crossmember" for the frame. Using F-A-M mounts effectively removes 25% of the frame's crossmember.

2b) Also, when you study how a Model-A was engineered, you realize the motion of the vehicle is transferred from the tires thru the wheels which push the rear axle. That motion is transferred to the rest of the vehicle thru the rear axle torque tube into the rear of the transmission. The transmission is connected to the flywheel housing and it is the rear motor mounts that effective push/pull the entire vehicle. When F-A-M mounts are substituted for the rigid blocks, it totally changes the engineering of the vehicle. It is worth noting this was so important in the initial engineering of the 'A' that Henry actually riveted those blocks to the frame rails for rigidity. Afterwards his team devised a way where these would mount in rubber yet still be rigid. From my experience with F-A-M mounts, if the rubbers are tightened enough to eliminate any movement, then they are so tight they are not effective at their initial purpose of reducing engine vibration. Therefore why even have them installed?



Just my thoughts....

.

[Jim Brierley]

F-a-M's were in my tudor when i got it. the car felt "a little loose" so i replaced them with stock mounts. i like it much better this way, and it is smooth.

[Purdy Swoft]

I agree with Brent on this one, word for word. A friend talked me into installing F-A-Ms in our 31 coupe back in 1980 . My coupe had the original balanced rotating assembly at the time and was already about as smooth as a four cylinder can get. As a matter of fact, the coupe was so smooth on long drives my daughter would often go to sleep. My friend Wood Rives, now passed spoke of how the F-A-Ms would remove the vibration in the rear view mirror that model A's have at certain RPMs . Probably for woods' sake, because he mentioned it every time that we talked, I went ahead and installed the F-A-Ms. They seemed to work good at first and did remove the vibration in the mirror. After a while I began to experience chatter when taking off. This is a common symptom along with others that are caused by F-A-Ms . I don't recommend them and about the only good thing that I can say about F-A-Ms is that they make engine removal and replacement much easier. This post isn't meant to step on others toes that like them. This is just my finding, others should do as they please.

[Brian T]

Hello Doug,
The mounts that you found are they these type that Vince Falter has on his web site.?
http://www.fordgarage.com/pages/erafloatamotor.htm

[BILL WILLIAMSON]

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Personally, I feel that Float-A Motors are a great IMPROVEMENT, not just a BANDAID. The word, "FLOAT" scares some folks! They should have called them, Smooth-A Motors!
I once "stole" a Jaguar sedan that went through 4 owners, because of a BAD ROAR, throughout the car! The L/F motor mount bolt was TOO LONG & went completely through the motor mount, to the uni-body frame! QUICK FIX & I sold the car for GREAT PROFIT$$$$$$$.
Bill W.

[JohnLaVoy]

Not a fan of the float a motor mounts I agree with Brent on this it is a band aid and the original mounts work well, non of our cars has these mounts.

[MikeK]

Yes, I believe the rear piece is necessary with float-a-motors, and I respectfully disagree with the contentions that the frame will suffer and nothing further is to be gained from them if your engine is assembled and balanced to factory specs. I have float-a-motors in my 160B, stock mounts in my 40B. To decide you need to understand a little background stuff-

Inline 4 cylinder Otto-cycle engine running vibration is attributable to three sources:

1) Primary dynamic imbalance of moving parts. THIS MAKES THE ENGINE WANT TO GYRATE AROUND THE CRANK CENTER. Ford did a good job of keeping this below the noticeable threshold, something many rebuilds do not achieve. If you have a dynamically balanced rotating assembly comprised of weight-matched parts in your engine this is not an issue. Contrary to layman belief, crankshaft counterweights have nothing to do with this running balance. Henry’s un-countered cranks were in very close dynamic as well as static balance. Adding counterweights does nothing to make an already dynamically balanced assembly ‘smoother’. The counterweights do two things: a/reduce crankshaft bending force against the main bearings, and b/add mass forward of the flywheel, reducing crank wind-up or twist by storing and slowly releasing the pulsed piston power. This second part, b/, DOES reduce overall vibration, but that fits into category 3) below. The primary dynamic vibration frequency is 1X (equal to) RPM.

2) Second order harmonic imbalance. THIS MAKES THE ENTIRE ENGINE VIBRATE UP/DOWN. This is caused by non-linear and mis-matched piston acceleration/deceleration profiles between pistons traveling up and those traveling down. Four design factors contribute to this: a/connecting rod length (infinity is best but impossible). b/ Ratio of bore to stroke. (Over-square is best, A’s are under-square). c/ Different crankshaft torsional load during the four Otto cycle strokes, causing opposing stroke pistons (two go up as two go down) to impart mismatched cancellations. (less horsepower is better, negative cylinder line offset is better). Model A’s that are ‘modernized’ to give more HP deliver more second order vibration. A’s were designed with positive (+0.125”) cylinder offset to produce more useable torque at a lower rpm. That added second order vibration. The only way to effectively reduce second order imbalance in an I-4 is with an opposing weighted countershaft geared to the crankshaft. Even then, the I-4 design is never perfect at all speeds and power output levels. The second-order vibration frequency is 2X RPM.

3) Torsional pulsation about the crankshaft axis. THIS MAKES ONE SIDE MOUNT PULSE UP WHILE THE OTHER SIDE PULSES DOWN. If you had an infinite number of cylinders there would be none. Adding rotational mass to the crankshaft and flywheel reduces this but a compromise must be made. More mass = sluggish engine acceleration. A four cylinder engine delivers a torque pulse every 180 degrees of crank rotation. The torsional vibration frequency is 2X RPM.

ARITHMETIC: Remember physics? You need to sum all the vectors, both in magnitude and time. That’s what your butt actually feels! Adding #2 & 3 above, the left side engine mount will always transmit more ‘up’ force and the right side more ‘down’ force, in pulses twice the engine RPM. Parts of #1 will add to the 2&3 up/down/ and parts of #1 will force the engine sideways, back and forth between the frame rails.

MODEL A ENGINE MOUNTS:
Stock design permits very limited up/down movement with very little frame loading if the rubber has not age-hardened and the assembly is not over-tightened. Side thrusts can compress the large flat rubber sheet very little and flex the frame alternately left, then right. Part of the forward driving force of the vehicle is delivered to the frame through the top of the rear spring to the rear crossmember, part is delivered through the two side engine mounts. The design was an adequacy compromise for slightly less than 40 horsepower. As soon as Henry upped it to 50 horsepower (the Model B) engine mounting changed. The range of movement was changed and a third element was added in '32 to transfer rotational load to the firewall. Applying more than 40 HP to stock mounts passes exponentially more vibration during acceleration.

Float-a-motors permit more movement in both vertical and side directions than stock A mounts if the rubber donuts are not too hard or over-tightened. That’s a big ‘IF’, but achievable. To facilitate dampening up/down pulses a third element, the rear tranny support with a rubber pad, is added. Failure to use it will shift the paradigm. Float-a-motors will always damp more side thrust toward the frame rails than stock mounts, but at a cost. There is less driving force delivered to the frame at the attachment point, as it flexes slightly forward, increasing the driving load on the rear spring/crossmember. Setup is critical. The donuts may need to be shimmed or trimmed to achieve the proper engine height and tilt. Proper loading on the third transmission support element needs to be present. The mount donuts need to be unequally tightened left/right to achieve maximum dampening during acceleration. The instructions do not explain how to judge, measure, or do any of this.

WILL YOUR FRAME BEND IF YOU USE FLOAT-A-MOTORS?
All 80 year old A frames bend from the pounding force of the engine and transmission mass at the side mounts. The relatively tight stock mounts make both frame rails react in tandem. Float-a-motors uncouple this but the loading is actually reduced at the original frame mount points if the transmission third support rubber pad is properly loaded. The force is moved to a single down point on the center cross-member. Without the trans mount (often omitted) the total loading is the same but uncoupled and directed at the original mount points, permitting the right frame rail to see more total downward load when the car bounds over a bump than with stock mounts. A’s originally hammered over unpaved roads. That degree of accumulated repetitive stress is no longer the norm.

[Smog Tech]

What does the Float-A-Motor Trans mount look like?

[Purdy Swoft]

There is a good picture of the float-A-motor mount , showing all parts in the left upper of page 32 in Brattons catalog. Everybody needs this book for quick reference. It beats the mechanics handbook by a long shot with far less mistakes!!!

[DougVieyra]

I really want to thank everyone who took the time to post their thoughts on the subject. Some pros - mostly cons. When all is said and done - at least with me - I will fall back on my original thought. The 'Touring Engine' in my '31 Coupe gives off no noticeable 'vibration' - so I think I will forego the hassle and effort needed to install these Float-A-Motor rear engine mounts.
Thanks to all for your input.

I think many other Ford Barn members have also gotten an education from all your experience and reasoning. I really LOVE this site. It is so helpful, as well as entertaining, and a place of 'common ground'. I was happy to see that at the National Meet this past week, that there was a FORD BARN get-together where many of the 'Barners' had a chance to meet face-to-face.

Doug Vieyra, Eureka, Calif - Sunny and 63 degrees at 3 pm

[Mike V. Florida]

Originally Posted by DougVieyra

I really want to thank everyone who took the time to post their thoughts on the subject. Some pros - mostly cons. When all is said and done - at least with me - I will fall back on my original thought. The 'Touring Engine' in my '31 Coupe gives off no noticeable 'vibration' - so I think I will forego the hassle and effort needed to install these Float-A-Motor rear engine mounts.
Thanks to all for your input.

I think many other Ford Barn members have also gotten an education from all your experience and reasoning. I really LOVE this site. It is so helpful, as well as entertaining, and a place of 'common ground'. I was happy to see that at the National Meet this past week, that there was a FORD BARN get-together where many of the 'Barners' had a chance to meet face-to-face.

Doug Vieyra, Eureka, Calif - Sunny and 63 degrees at 3 pm

Smart man! Someone here has on the bottom of all his posts something like "Fix it until it's broken" glad you did not do this.