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PostPosted: December 12, 2007, 1:28 am 
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I'm working on the rear support for the tranny. I'm contemplating whether to mount it solid or include a little rubber.

I understand why that didn't work out for the differential nose mount, but I'm not sure the situation is the same for the engine.

I'm using the stock Miata motor mounts, which seem pretty well centered to support nearly all of the engine/transmission weight. The weight on the tranny mount is pretty low.

With the engine on rubber, it's going to move a little. It seems that hard-mounting the transmission would be asking for trouble, possibly leading to cracks in the mounts. This isn't a problem in the Miata (from what I understand) because the PPF has some flex, and is not hard-mounted to the frame anywhere.

I'm thinking of sandwiching maybe a 1/2" of hard rubber pad, about 2" x 4" or similar between the tranny and the frame. This would accommodate up/down vibrations, I think. I think this would require safety wiring those bolts, since you couldn't trust torque to keep them put.

However, I think this might put the bolts into pretty severe shear duty when the engine rotates under torque. The rubber won't cushion that blow...in fact it will be worse. Then again, those PPF bolts are pretty danged beefy.

Hmm...I think I've just about talked myself out of the rubber, but I'd like to hear opinions either way. What did you do, ye experienced types?

Here's the bracket I've got worked up. It will be welded to the frame once I decide on the rubber question and know how high to put them. I will most likely do the same thing on the bottom, except that bracket will bolt to the frame so I can still pull the engine.


Attachments:
File comment: This not only gives the tranny bracket a place to go, but helps triangulate the tunnel.
DSC03288.tranny.xbrace.JPG
DSC03288.tranny.xbrace.JPG [ 59.79 KiB | Viewed 9847 times ]
File comment: Gotta love that beautiful Mazda hardware! Never mind I grafted it to an old bed frame! ;-)
DSC03295.tranny.bracket.JPG
DSC03295.tranny.bracket.JPG [ 58.87 KiB | Viewed 9847 times ]
File comment: This bracket came together much more tidily than most of my efforts! :)
DSC03290.tranny.bracket.JPG
DSC03290.tranny.bracket.JPG [ 50.91 KiB | Viewed 9847 times ]
File comment: The bottom will get similar treatment, except removable to remove the engine.
DSC03299.tranny.lower.JPG
DSC03299.tranny.lower.JPG [ 40.9 KiB | Viewed 9848 times ]

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PostPosted: December 12, 2007, 2:54 am 
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FWIW, the PPF keeps the angles between the transmission and the diff in line (resists bending) but it does twist.

Does it help you?
Probably not, but you might understand the loads better..

Moti

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PostPosted: December 12, 2007, 8:40 am 
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I would design a steel ppf using dom and polygons/triangles, with steel brackets on the ends. If necessary, I would adjust my chassis framework for clearance.

I would also verify the stock angles from the slip yoke bearing cap and pinion flange coupler bearing cap with the ppf bolted up to the diff and trans. I would use these angles in the jigging process.

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PostPosted: December 12, 2007, 11:24 am 
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I'll get some pics of mine tonight. Basically it looks like the drawing below:

I took a big ol honkin poly bushing that was about an inch thick and had a 1/2" hole in through the center and cut it in half and sandwiched it with some washers. It allows the tranny to rock side to side with the engine on the mounts but securely locates it in the up and down and fore and aft planes.


Attachments:
trannymount.JPG
trannymount.JPG [ 15.88 KiB | Viewed 9803 times ]

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PostPosted: December 12, 2007, 11:24 am 
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I am in the same boat. I don't know if I should mount my tranny "HARD", or build myself some sort of rubber insert.

I was thinking of just cutting a old tire and using just a very thing section of that, so that there was some cushion added, but not much......just enough to take some of the vibration out of the shifter.

What are you thoughts?

J. R.

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PostPosted: December 12, 2007, 1:57 pm 
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I used some 1/4" thick (or something around that) conveyor belting from Tractor Supply. It's really about 1/8, but I put two pieces on top of each other, and since it is rather hard rubber with reinforcement in the middle, I'm not too worried about a lot of slop and/or movement, but with the rubber engine mounts in the front, I won't have to worry too much about bending the frame either.

Nate
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PostPosted: December 12, 2007, 5:02 pm 
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What does everyone think about cutting 8" or so off of each end of the stock PPF and welding up a new one? It would eliminate trying to jig something up to get a good fit to the diff and tranny? With the existing shape it wouldn't intrude too much on tunnel width and there should be enough height to work with.


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PostPosted: December 12, 2007, 5:24 pm 
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Stein wrote:
What does everyone think about cutting 8" or so off of each end of the stock PPF and welding up a new one? It would eliminate trying to jig something up to get a good fit to the diff and tranny? With the existing shape it wouldn't intrude too much on tunnel width and there should be enough height to work with.


It's been done, but it makes for a very wide tunnel.

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PostPosted: December 12, 2007, 6:01 pm 
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The PPF slopes downward toward the diff, then the flanges bend upward to set the rear joint angle. If the center is cut out and the PPF ends are aligned, the rear joint angle would be way off. The PPF is very hard aluminum. There is little or no twisting since repeated bending of aluminum alloys results in failure. Steel can be bent back and forth repeatedly and never fail, depending on the specifics.

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PostPosted: December 12, 2007, 6:22 pm 
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Miatav8,MstrASE,A&P,F wrote:
The PPF is very hard Aluminium.


This begs the question, do the PPF's have any value on the resale market or just for aluminum scrap?


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PostPosted: December 12, 2007, 9:55 pm 
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If you have engine mounts with rubber, you need a tranny mount with rubber also.

I haven't much experience with sport cars (C. Shelby's term), but lots with Jeeps and such. If one part can flex and the other can't, things break. Mount them both solid or, both with rubber or poly. I've seen more than one broken tranny because of poor design.

Maybe with a Locost there will be less stress, but my car will have flexible mounts on engine and tranny.

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PostPosted: December 12, 2007, 10:15 pm 
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Here's the pics of mine as it sits now. I'll likely put a washer between the tranny and the upper bumper too. I still haven't decided whether or not I should put another one right beside it in the other hole.[/img]


Attachments:
transmount2.jpg
transmount2.jpg [ 38.65 KiB | Viewed 9695 times ]
transmount1.jpg
transmount1.jpg [ 134.88 KiB | Viewed 9696 times ]

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PostPosted: December 13, 2007, 11:32 am 
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chetcpo wrote:
Here's the pics of mine as it sits now. I'll likely put a washer between the tranny and the upper bumper too. I still haven't decided whether or not I should put another one right beside it in the other hole.[/img]


Is that a piece of 1" tube welded onto the frame? That's what it looks like from above, but I'm confused by the shot from below.

Regarding using the second hole...why not use it?

I'm assuming the panel in the foot well will be removable for accessing the nut? Or maybe an inspection panel? Have you considered a captive nut above, and drill the head of the safety bolt for safety wire? Might save some headaches later on.

Okay...stupid question time. <ignorance="exposed">Where to do you buy poly bushings? AutoZone? Are they different grades/densities/etc., and how do you pick which one you need? Or does it just not matter that much?</ignorance>

Thanks for the pics, Chet. I like the look of what you're doing.

Howlin Mad wrote:
If you have engine mounts with rubber, you need a tranny mount with rubber also. ... Mount them both solid or, both with rubber or poly.


Thanks, Mad Man. Between you and Chet, I'm set on putting some cushion back there.

When you say, "both with rubber or poly," does that mean they need to be the same at both ends? Or are the rubber/poly intermixable? I wouldn't think it would make that much different that far apart, with such limited range of motion.

I'm assuming the stock Miata motor mounts are rubber...can anyone confirm that?

-dave

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PostPosted: December 13, 2007, 11:32 am 
I don't see the benefit of a PPF in a Locost. It makes sense in a production car because it allows the NVH from the drivetrain to be nicely isolated from the chassis while keeping the drivetrain rigid, but in a Locost I would prefer to see the extra weight used to make the entire frame stiffer. People get hung up on using the PPF because Mazda used it in the Miata, but there's nothing in the Miata drivetrain that makes it required any more than there is an any other IRS design.

If you're using rubber motor mounts, you'll need to let the tail of the transmission move a bit. My setup is similar to Nate's, a small bit of rubber and only one bolt through the tail of the transmission. From what I recall, the Westfield is pretty much the same. The majority of the forces go through the motor mounts, the transmission mount is just there to keep the transmission from dragging on the road really. Mine has worked out fairly well over the past few years.


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PostPosted: December 13, 2007, 11:36 am 
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Yessir Dave, that's just another piece of 1" tube welded onto the inside of the frame rail, with a hole drilled through it. (well actually, two holes)

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