Float-A-Motor bolt tightness

BillLee/Chandler TX

BillLee/Chandler TX

Senior Member
My car has Float-A-Motor mounts. Recent engine work gave me the opportunity to install new biscuits.

Question: how tight should the through-bolt be? Tightened all the way down with the biscuits compressed? Or left loose enough to cause little or no compression of the biscuits?

Thanks.

Bill Lee
 
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.

Your specific situation may vary, but for my ~75 Hp engine in a 160B the best is with the left two full turns past the point of inability to rotate the lower cast donut cover by hand, and ¾ turn on the right. This is, of course with the proper third element-the center cross member bracket and rubber holding the motor in proper height and angularity alignment with the frame.

If that’s enough of an answer, stop here. Otherwise be prepared for my two cents on the matter.:eek:
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 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 ****** 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.
WHY- 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 and WHY THEY DIDN’T CARRY OVER TO ‘32:
Applying more than 40 HP to stock mounts passes exponentially more vibration during acceleration. 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 some load to the firewall. That ‘32 ‘hanger’ assembly on the firewall is essentially what the third ****** float-a-motor mount does. It takes the weight load off the frame at the engine side mount points and allows the rotational pulses to ‘float’ on those mounts.
 
Well, I put float-a-motor mounts in, and what I did was tighten them just until the donuts started to compress.

I drove it a little and tried either tightening or loosening...whatever I didn't do the time before.

After about 5 drive and adjust events I'm happy....they are not as tight as you'd think they need to be...

I cannot turn the large top metal washer by hand, but I can make it creak a little......

I don't have the lower cast donut cover that Mike mentions...different style FAM?

Mine show about 5 threads on each side, and that's with a flat washer under each locknut and using the bolts supplied in the kit.
 
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Most 4 cylinder engines have INHERITANT vibrations.
I dealt with later DATSUNS/NISSANS/ETC & some gave us FITS, with VIBRATIONS/RESONANCES at certain RPMs.
Float-A-Motors are not a MIRACLE cure, but they do help.
Shakey Bill W.
 
Here are a couple of pictures...i don't know if they are of any use.....

Second picture is the passenger side....
 

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Perhaps Mike implies the lower rubber donut. Also, the donuts can slip out of the cast metal support lips during the snugging process. The bolts supplied with the kits are smaller dia. than the donut holes and not sure if one size larger bolt to fit the holes has any effect on the FAM's performance ?
 
1955cj5 said:
Well, I put float-a-motor mounts in, and what I did was tighten them just until the donuts started to compress.

I drove it a little and tried either tightening or loosening...whatever I didn't do the time before.

After about 5 drive and adjust events I'm happy....they are not as tight as you'd think they need to be...
Click to expand...

I agree.

They do not need to be very tight.

I changed the bolts from course to fine thread. I found it easier to just tighten a little.

Too loose and you will start to pick up some clutch chatter,

Too tight and you will not reduce the vibration as much.

It is a fine adjustment. It may take several tries to get it where you are happiest with the results.

Just my experience.

Chris W.
 
duke36 said:
Perhaps Mike implies the lower rubber donut. Also, the donuts can slip out of the cast metal support lips during the snugging process. The bolts supplied with the kits are smaller dia. than the donut holes and not sure if one size larger bolt to fit the holes has any effect on the FAM's performance ?
Click to expand...
The bolts are supposed to be SMALLER than the holes, so they don't transmit vibrations from one bracket to the other one.
Bill W.
 
Don't mean to belabor the FAM' issue, but with the new ductile iron FAM's, if tightening the bolts, it appears the top metal disk shouldn't be able to be moved when the upper donut is just beginning to be compressed. We're trying to get a feel for what the engine is supposed to be doing when "proper" set up is achieved compared to the stock set up.
 
I really, really like the detail presented by MikeK. He clearly has a handle on rotational input/loading and its effects on vibration and force induced by cyclical movement, and his correct concern for appropriate hardness/durometer of the rubber components.

Concerning the use of Floating motor mounts I have always been more concerned by two issues.

(one) is the one-time, in-line force produced when and if the car should be driven into something like those wheel chocks found in parking lots or the curbs at the streets, driving the fan into the radiator. The advantage of the stock motor mounts is the fact that when this happens the lack of the rubber components in the motor mounts prevents the motor from lunging forward as a result of the inertia created in the motor as a result of the sudden stop of the front wheels. Clearly, speed has an effect on the amount of inertia created. Of greater concern is what happens when a Model A is encounters either those wheel chocks or a curb with a rear wheel(s). When you think about it, the wheels are attached to the axle housing, attached to the torque tube, attached to the transmission , attached to the motor/water pump. At sufficient speed this force far outweighs the inertia created and drives this combination forward and puts the fan/nut into the radiator, as the sudden stop makes the radiator 'tip to the rear at the same time.

(two) If you look at the frame from a top view, it becomes pretty clear (I feel) that the combination of the clutch housing and the solid motor mounts were designed to be a chassis cross member to minimize torsional flex (as indicated by Mikes comments). By the fact that the combination of the solid motor mounts and the clutch housing has a large vertical element, it would seem to provide a greater contribution as a cross member than any of the others, and the addition of flexible motor mounts would seem to minimize the effect of having a cross member in this location. The fact that this location is near the center of the most heavily loaded section of the frame makes this even more puzzling to why the use of flexible mounts.

As always, this is merely the opinion of one old man.
 
Thank you Mike for your answer! And thank you for your two cents worth! We always learn from you.
Click to expand...
Huh? That's a 50 cent answer if I ever heard one. :D Good detailed info.

TerryO
 
Would anyone here including MikeK, talk about if the FAM contributes to low speed shimmy (AKA death wobble)? Never had it until they went on the car. Steering, king pins good as usual, Mel Gross F150 box adjusted fine and so on.

Never had it before. Ever until this. I hate the thought of using a steering stabilizer, but I don't want a shimmy ever, but love the smooth ride of the FAM.
 
There is something else wrong. Float a Motor mount would not cause this. Did you use the Float a Motor piece that holds up the transmission?

I used the front Float A Motor mount as well. The roaster has a nice firm feel but I'm not sure that it is worth the trouble of installing it.


Sent from my iPhone using Tapatalk
 
johnr6776@gmail.com said:
There is something else wrong. Float a Motor mount would not cause this. Did you use the Float a Motor piece that holds up the transmission?

I used the front Float A Motor mount as well. The roaster has a nice firm feel but I'm not sure that it is worth the trouble of installing it.


Sent from my iPhone using Tapatalk
Click to expand...

Yes, we did use the piece on the transmission.
 
When I installed the FAMs on my first A, the rubber block in the third mount (on the cross member) was too small. It fell out onto the road very soon after. I knew immediately when it went and the car would shudder unbelievably when I engaged the clutch in reverse.:eek:
Right from the very first sight of a FAM, I thought they were poorly designed because they give little or no "drive", as has been observed above. I've seen a better arrangement made by an owner that involved a piece of angle iron and "L" shaped pads with thinner rubber in the vertical plane to transfer the drive to the chassis.
 
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