Tranny mount and spacers?

[rodo]

Removed my transmission mount two nights ago. Had a shop light so there were shadows under the car. Driveshaft was out and I was doing the guibo/transmission mount dance. Got the rubbery mount bent, this part off, that part off, pushing pulling guibo. Anyway, the mount comes off and lots of washers fall down under the car. Today I'm under getting ready to reinstall and discover there are about ten wee spacers, similar to the thin metal shims that go in the a-arms to adjust for alignment, on the ground. So...er...they are too small for the guibo bolts, and that would not make sense anyway; maybe someone used them for the transmission mount, somewhere? But why? What the heck was being adjusted? I popped the new mount in for a dry fit and it sure seems okay. Can't imagine what gaps someone was trying to sort out.

Anyone heard of these for the tranny mount?

[RRoller123]

I posted a long series on this. It involves driveline alignment, to remove any differential angle between transmission output angle and pumpkin input angle. Few people realize that in a system of universal joints, each universal joint INDUCES a vibration, in the form of a cyclical change in rotational speed on the joint output side, even when input side rotational speed is constant. The solution is to have that variation that is the OUTPUT of joint 1, be the INPUT of joint 2, and the input and output shaft angles MATCH closely. I know it is complex to explain, but if you search on the forum for driveline angle alignment or adjustment, or something like that, it will come up, with some videos. I was amazed how smoothly the rear end performed after aligning the driveshafts properly on my car. Noticeable difference. It doesn't take much. A couple of degrees makes a big difference.

[18Fiatsandcounting]

This is puzzling, and it's possible that someone tried to do a driveshaft alignment as RRoller suggests. I've never seen spacers on anything that you mentioned.

It sounds like these washers (spacers) fell out when you removed the rear transmission mount. There are only 6 nuts on this mount: 2 that attach the top bracket to the rear transmission casing, 2 that attach the mount itself to the studs on the underside of the car, and 2 (along with 2 bolts and 2 washers) that attach the top (rubber) part of the mount to the bottom bracket of the mount.

Most of these things are pretty fixed in space. If you put spacers on the two studs/nut for the top of the transmission mount, all you would do is move the mount rearwards and then it might not line up with the studs on the underside of the body. You can't move the transmission upwards because the brackets can't go any higher than the underside of the body allows. You could put spacers in to lower the transmission mount, and maybe this was an attempt to get at what RRoller was suggesting. There is no side-to-side adjustment that I can think of.

Try just putting it back together without any spacers and see how it lines up and drives. If it seems OK, I'd toss those spacers or throw them in the scrap parts bin for some other use.

-Bryan

[SteinOnkel]

I think I had a spacer or two in there as well. And some tired and cracked rubber thingies too. I zipped it all back together with my impact pretty much willy nilly. I haven't noticed a difference in vibration, but I drive with the top down 99% of the time. I'd take RRoller's advice if you want to do it properly, though.

Also, check this out:

https://www.youtube.com/watch?v=BM0CpaVQhpw

Seems to be a thing. Didn't know that, I'm more of a FWD guy.

[18Fiatsandcounting]

Also, check this out:

https://www.youtube.com/watch?v=BM0CpaVQhpw

Interesting, but how would you change the angle on a Fiat spider differential, as the angle would seem to be locked in place by the upper and lower locating rods which are of fixed length? So, unless you cut those rods and either added metal or removed metal (and rewelded), I'm not sure how to change the differential angle on a spider.

Where's RRoller when you need him?

-Bryan

[RRoller123]

On our little cars, we don't need to change the angle of the diff, it starts out pretty close to the transmission, such that the adjustments can be made up front at the drive shafts. It would be very difficult to tilt the rear axle, involves changing either length or mounting location of the rods. The hot rodders and racers all do this of course, with adjustable trailing arms, threaded so length can be changed for axle alignment, tilt etc.

[RRoller123]

Here is the original post series on the topic:

http://www.fiatspider.com/f15/viewtopic.php?f=5&t=39335

It morphed into my next topic of interest, cam lobe timing effects, so one needs to scroll down a ways to get to the original driveshaft alignment study, but it is all there.

Pete

[rodo]

Fascinating stuff, and I'm sure now the spacers came from the pillow block bracket/mount. Read through the other links and great, want to balance things out. I have no idea how one measures the angles! With a digital angle thingy (from Harbor Freight)? And you put it...where? On the shaft? Then on the...er...splined shaft from the transmission?

And...er...if I'm following things correctly: great, so it works better if the joints are in the right angles, cuts down on vibration. Okay, i get that, but one of the issues is that when diving (ie: moving over irregularities in the road surface) the rear end is moving up and down giving the rear ujoint and the front ujoint extra movement, and the guibo is taking up some of the shock of the whole drive train moving. Right? So does all that movement when the car is traveling not negate some of the perfection in the driveshaft/joint train?

[18Fiatsandcounting]

Fascinating stuff, and I'm sure now the spacers came from the pillow block bracket/mount.

Just to make sure I'm following you here. Do you mean the rear transmission mount, or the center driveshaft support (with the bearing) between the front (short) and rear (long) sections of the driveshaft? If it's the rear driveshaft support, I could see how shims would either raise or lower that section of the driveshaft depending on where you put the shims.

-Bryan

[rodo]

I did actually mean, weirdly, the transmission mount, not the bearing mount. Was under the car and I think I see what the PO or some wrench along the years, did. Not only did ten of the shims fall out but so did three washers. When I was trying to test fit the transmission mount bracket, I noticed that the passenger side would hit the body before the driver's side. Appears to me that, some time in the car's mythical past, it bottomed out on something and pressed up the floor slightly, enough to throw the trans mount off. So someone stuffed shims and washers in there to level things out. That's what it appears to be. I'm sure they are not from the bearing mount as I had that dangling loose, nuts off and the rubber utterly shot, and the shims/washers did not fall out then.

[18Fiatsandcounting]

I noticed that the passenger side would hit the body before the driver's side. Appears to me that, some time in the car's mythical past, it bottomed out on something and pressed up the floor slightly, enough to throw the trans mount off.

Or, the PO routinely carried his entire set of gym weights on the passenger floorboard...!

Sounds like you found issue, Rodo, and you can probably just shim it up so that the bracket for the rear transmission mount is level with the underside of the body. Unless you can figure out a way to push the floorboard down. Maybe contact the PO and see if he wants to sell those weights, and put them on the driver's side?!

-Bryan

[rodo]

Hm, the PO is incredibly ripped...

That was funny, Bryan, thanks. My plan now it button it all up, get things as level as I can, and then...drive it. See what is what. If I'm shaking and shimmying all over the road, I'll know one place to look.

[18Fiatsandcounting]

Hm, the PO is incredibly ripped...

Well, there you go! Mystery solved with a totally buff PO.

But seriously, I'm guessing you'll be just fine once you put it back together as level as you can, and take it for a spin. For what it's worth, Fiat spider driveline vibration issues seem to happen around 40 mph and multiples of that, but I can't explain why. Mine has a very slight vibration at 39, a more noticeable throbbing at 78 mph, and at 156 mph, well, that remains to be seen... Just something I need to address some day (I mean fixing the vibration, not reaching 156 mph to test for vibration).

-Bryan

[RRoller123]

Yes, the Harbor Freight digital angle gauge is small enough to be very useful, and has a magnetic base that holds right onto the driveshaft. Just be sure that it is aligned along the length of the shaft as closely as visibly possible, and not rotated to either side. It is not, ultimately, the accuracy of the reading, but the differential between readings off the sections of the drive shaft that is important. Place them on the shafts, and for the diff, place it on the mounting surface and account for the 90 degree difference. For the engine, you go off the crank pulley as the datum for block tilt. this is the starting point for the entire system. Gives Crankshaft angle.

Fascinating stuff, and I'm sure now the spacers came from the pillow block bracket/mount. Read through the other links and great, want to balance things out. I have no idea how one measures the angles! With a digital angle thingy (from Harbor Freight)? And you put it...where? On the shaft? Then on the...er...splined shaft from the transmission?

And...er...if I'm following things correctly: great, so it works better if the joints are in the right angles, cuts down on vibration. Okay, i get that, but one of the issues is that when diving (ie: moving over irregularities in the road surface) the rear end is moving up and down giving the rear ujoint and the front ujoint extra movement, and the guibo is taking up some of the shock of the whole drive train moving. Right? So does all that movement when the car is traveling not negate some of the perfection in the driveshaft/joint train?

The changes in angle while driving are of course unavoidable. But at least we know when starting from a well adjusted datum, that the induced vibrations are minimized as the rear axle moves up and down. Given the length of the trailing arm versus the relatively small magnitude of motion, the change in angle is very low. I.E. Even though the rear axle moves up and down in an arc, it is a very small arc movement, the pumpkin angle effectively changes by next to nothing. You can test this yourself by measuring the angle with the car artificially jacked up a bit, and then again with a couple of helpers standing or sitting on the bumper for the down position. And these are exaggerated positions anyway, as well. The suspension spends very little time off the datum point anyway, just for bumps and acceleration.

The hot rodders with adjustable rear end suspensions actually will account for the rear end sag when they set up their systems, so that under full off the line acceleration, the car's rear end will will drop into the proper driveshaft alignment position off the line. I.E. they deliberately miscalibrate the driveline angle, make it a little bit biased, to accommodate this. But unless one travels every mile in 1/4 or 1/8 mile increments, this makes no sense for us. This rear axle up and down motion is is actually the only real non trivial degree of freedom in the system under operation.

[rodo]

Dang, RRoller, that is helpful and unexpected. Thanks for the info and the intel on the angle gauge. I will work to square things away!