Monday, January 27, 2014

Rear subframe




More photo documentation of the conversion of a Miata to a MiataV8.
The following pics are of a Miata rear subframe modified to accept an 8.8 IRS as found on '90's Thunderbirds and Lincoln MKVIII, as well as some early Mustang Cobras.


 On this version at the front I used 1/4" X 2" strap formed into pedestals to support the centersection with 3/16" gussets tying it all together. The formed pedestal mount design was originated by Rick Littlewood, I believe, who did a lot of original and inventive fab on his V8 conversion back in the day.

 The rear mount is more 1/4" strap made into angles to work with the same Energy Suspension part number 4.1126 mounts as the front. It's important to note that the rubber mounts and the steel tubing that runs thru them are designed to fit on the 1/2" thick mounting "ears" at the front of the differential, so using a different thickness material as shown means the length of the tube and the protrusions on the rubber mounts must be shortened accordingly to ensure the proper clamping load.  Any looseness at this point would be disastrous. The subframe crosspiece has a "nut bar" made of 3/8" thick stock welded to the crossmember with gussets similar to front. The gussets were cleaned and shiny bright before welding.







 Knowing I would be making more conversions, I made a simple wooden fixture to hang the diff in space while building the mounts.  

Critical dimensions are: Height of side support centerlines = 6.5", pinion support centerline = 8.0" as measured from the surface of the baseplate. The subframe is mounted on 1.5" blocks to allow clearance for the original axle mounts, which are left in place for the additional bracing.  

The diff is positioned so that the stub axle centers are the same as the original Miata axle centers, which locates the diff height and front to back position. These centers were determined by hanging the Miata rear end in place on the subframe in place on the baseplate. 

The body of the 8.8 diff is centered side to side with reference to the machined bosses surrounding the axle stub holes and positioned so the pinion flange is perpendicular to the mounting plane of the subframe. 

The subframe has six mounting holes, one of the front holes is the primary locator, very close to 12mm clearance, the other front hole is nominally 12mm, but oval, the rest are very generously clearanced, so I didn't try to locate from them other than to draw a rectangle using the centers of the 4 corner holes which became the reference for determining alignment.

It's harder to describe than it is to execute, since the thing is hanging in space, but my thinking is that it's easier to measure and lay out a simple fixture than it is to do a makeshift blocking and shimming of the irregularly shaped diff.  I've since moved on to a metal version, the new 8.8 as found on Explorers has different mounts that drove that decision.
















 

 
 The next few pics show another version using 2" X 2" square tubing for the front pedestals, pretty much the same rear mounts.  I used 1/4" thick plates on the mounting surface of the pedestals with lug nuts welded to the inside surface. Lug nuts work great as weld nuts. 1/2-18 bolts thru the rubber mounts. The next version uses some nice 3" X 5" extruded angle that has 3/8" thick legs. Cut to fit the insulators and the cantilevered rear boss on the cover. 2 1/2" wide I think. Need to find the pics.








The following pics are of the latest as of 1-26-2014 version using the 3" X 5" extruded angle rather than the bent 1/4" plate for the rear mounts. I wanted the thicker material to minimize the mods to the rubber mounts and their spacer, plus I think it looks better.















The chamfer on the corners seemed like a good idea, but they proved to be unnecessary as you can see in the pics of the welded in plate



 Rear mounts cut from extruded angle rather than bent from plate. The 3/8" thickness of the legs is more compatible with the rubber mounts, which are designed for the 1/2" ears on the front of the third member. I cut 1/8" off the metal spacer that runs thru the rubber mounts and sanded 1/16" off each of the rubber pieces where they fit into the big hole in the metal, which makes for a perfect fit on the 3/8" thickness. This step of modifying the bushings must be carried out, or they will not be properly preloaded and could lead to premature failure of the mount.








The bolts thru the cover casting mounting ear are metric, the bolts into the plate are 1/2-18 to mate to the
1/ 2-18 lug nuts that I used as weld nuts on the back side of the plate. The subframe crossmember has large clearance holes for the nuts.
You can see that the chamfers on the corners of the plate are unnecessary and in fact if the plate had corners, the weld would be continuous at the gussets.













This pic is of a previous build, I neglected to take a shot of the finished frame with the long gusset across the pedestals.  

EDIT 11-02-2021 I think it needs to be emphasized that these gussets across the front and rear portions of the subframe are very necessary. Recent instances of 8.8 converted subframe failures have been published on the internet; in some cases, the mounting scheme uses some of the ideas found here, but it doesn't appear that any attention was given to adding the gussets. 


Edit 11-4-2022. Some pics of the mounting scheme I chose for the Explorer 8.8. It appears to be a more robust version of the 8.8 case and fits really well into the Miata subframe. 

Not much detail in these pics but if there's any interest I can add some more.  Basically 2 big steel tubing bushings in the rear,  urethane mounting bushings and long thru bolts into the stock mounting holes on the rear cover.  I used the stock rubber mount on the front, similar to the front mount of the early T-Bird style 8.8.

I added quite a bit of  1/8 steel plating over and around the steel tubes, as they required a fairly large section of the subframe structure to be cut out, and I wanted to transfer the stresses to the outer extremities of the subframe, closer to where the subframe mounts to the unibody "frame".

 Also, the earlier version of this rear uses the single front mount as shown, the later ones use a double mount similar to the earlier T-Bird case. I'm not sure why the world of automotive engineers thinks its OK to use 3 mounting points instead of 4, but I see this 3 point design in other applications like the Getrag and BMW.  










 

PNG files of the fabricated pieces involved in the conversion. Open in new tab.








Link to a handy list of door codes for identifying the rear end  installed at the factory for Tbirds and Cougars. Probably good for just about any Ford, actually.

http://www.tccoa.com/articles/gears/gears.html

The important info:


Under the "AX" on the door jam sticker:
ConventionalLockRear Axle Ratio
GH2.26
BC2.47
8M2.73
7-3.07
YZ3.08
4D3.42
FR3.45
5(W5)E(XE)3.27
6W3.73
2K3.55
A-3.63
J-3.85


20 comments:

  1. Hey Mike would you put a link on your page to subscribe by email please? It's an option in the lay out page. I really appreciate you putting this out here for us!!!

    ReplyDelete
  2. What part number are those Energy Suspension bushings? I searched their catalog, but didn't find any that exactly look like that. 9.4102 seemed similar, but they call for 1/4" thick plate.

    ReplyDelete
    Replies
    1. Energy Suspension 4.1126, differential carrier bushing for Thunderbird 8.8 carrier. Available in red or black. 2 sets required for this approach.

      Delete
    2. Thank you for the reply. Thanks for all the great info, too. I'm about to order all the parts to build up the rear end.

      Delete
  3. Mike,

    I have been looking at your posts but did not see the answer. What are you doing for axles from the center section to the hubs? I have found some at the driveshaft shop web site 8.8 to miata hubs. But I thought I saw you mention factory five axles somewhere.

    ReplyDelete
    Replies
    1. Wow, you're absolutely right!! I've got the section on 5-lug hubs and this part on the rear subframe/8.8, but no real mention of the axles. Basically, Factory Five Roadsters uses the 8.8 in their IRS option for their Cobra kit car. It happens to be the same length as required for the Miata, so, with either modified Ford hubs to get the 5 lugs, or broached Miata hubs to retain the 4 lugs, those axles complete the rear driveline. As of this writing, I have purchased 3 sets, others that I know of, another 4 sets, without issue. Call them and ask for the parts dept. You want the "standard" length, not the ones for the pin drive option. You will be pleasantly surprised at the price.

      Delete
    2. Update as of May 2016, FFR no longer sells these axles. They have changed their design to the later Ford IRS and use DSS axles. Too bad, they were a real bargain amongst axles.

      Delete
  4. Mike

    Is the diff centered left to right in the subframe?

    Thanks

    ReplyDelete
    Replies
    1. I answered this offline, but to document it here, yes, the diff is centered to allow equal length axles like the ones from Factory Five. This puts the pinion off center, just like it is in the T-Bird, Cougar, and Lincoln. Doesn't matter, that's why there are u-joints in the system.

      Delete
  5. Hey Mike do u offer these pieces for sale, all the kits I see I don't like but all 3 designs here are beautiful and look the best

    ReplyDelete
  6. quick question, how did you determine the pinon angle and what did you reference it to?

    ReplyDelete
  7. As far as offering for sale, I'm really not in a position to do that at this time, too many irons in the fire.

    The pinion angle should be checked against the engine/trans installation angle. Most everybody recommends 1 1/2 to 2 degrees down angle for the engine, I prefer to just set it level relative to the pinch weld at the bottom of the frame rails, then adjust the diff to the same angle after installation. The little fixture that you can see for holding the diff is set up for level, so it's usually pretty close. 1/2" body washers can be used to tweak the diff one way or the other.

    The measurement is done by putting an angle gauge against the front crankshaft pulley, noting the angle, then making sure the same angle exists at the front of the ujoint flange on the diff. It's really simple, there are several phone apps that measure inclination that work great. The car doesn't have to be level, you are measuring the engine relative to the diff. If major adjustments need to be made, either the engine/trans or the diff can be tweaked.

    ReplyDelete
  8. I should also mention that the body of the diff must be centered in the subframe, not the pinion flange. It actually doesn't matter as far as functionality, but the FFR axles that this thing is designed around are the same length. If the pinion is centered, the axles must be different lengths. So the measurement should be made at the flat surface adjacent to the seals where the cv joints plug in.

    Again, the fixture takes care of all the alignment issues. The fixture came about because of the difficulty of measuring things and supporting the diff in space while adding the mounts.

    ReplyDelete
  9. Great pics and details on this design, I think I'm going to go with something just like this.

    How is the exhaust clearance with the diff and a dual exhaust? Is there room to go a bit higher with the diff for extra clearance? Doesn't seem like it.

    Any reason to keep the original diff mounting brackets? Not sure they provide alot of extra strength - seems like you seam welded the rear subframe.

    ReplyDelete
    Replies
    1. Exhaust clearance is adequate for 2.5" tubing, a little tight for 3". One side is slightly closer than the other, due to the shape of the housing.

      The top of the diff ends up about 3/8"-1/2" from the body shell, with the axles pretty much at the same height as stock so everything lines up as it should.

      Use of the original mounting mounting scheme is kind of a personal matter, I prefer this approach, it is by no means the only solution. I am looking at the mounting requirements for the late model Explorer, so there's another variation.

      Yes, the subframe is seam welded where necessary. There are also wide gussets that go from side to side.

      Delete
  10. Hello, I was wondering if maybe youd be able to make it on my subframe if I sent it to you?

    ReplyDelete
  11. Back in 1982 I did a V8 swap putting a Chevy SB into a Chevy Vega. I ended up building two of them and did a pretty decent job of it. I wish I had someone like Mike to offer this kind of help like you guys get doing the swap on the Miata. Can take a lot of head scratching to figure this stuff out.

    ReplyDelete