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Yo, Andrew-
Looks really good! Complex, but good!
I like the "cradle" for the diff and such in the small tubing.

Thanks JD. Its funny - the camera makes it look way more complicated than it actually is. I guess the dark colored tubes and poor lighting don't help.

Yo, Andrew-
I noticed the same thing when we turned the frame on its side and I took pictures. Funny how a different perspective and/or a picture makes the frame look totally different.

The rear end design of yours looks like the newer Caterhams, with the triangulated cradle done in smaller tubing. I like it!


JD

Its hard to keep that whole rear suspension "box" simple - I've tried my best by not adding unnecessary tubes.

The important parts are slowly finding their home.


I finally broke down and built a press brake. I should have done this years ago before I even cut a single tube. It would have saved me endless hours of banging on steel with a hammer in an attempt to make a clean bend (which, no matter what, still looked like I wacked it with a hammer). Since sheet and plate don't like sharp corners, I radiused the outer corner of the upper angle using the bench grinder.


A good test for this new tool was the differential and shock mounts.



The 0.040" for the shock mounts was effortless and it bent the 1/8" plate for the differential mounts with ease. I still have to cap the shock mounts and add some doublers around the bolt holes since 0.040" sheet wont have enough bearing strength by its self.

I'm dubbing it a successful tool. Now to see what else I can bend.....

Wow, it's great to see this coming together. I've missed a couple of updates, I shouldn't rely on "new posts" so much. It's exciting when projects get to this stage. I think the small tubing looks very Lotus or 60's...

I like what you are doing with the new chassis. That said I am a bit concerned about what would happen in a hit from the rear. You upper suspension tubes seem to provide a bulk of your resistance to the back folding up but are not well triangulated to distribute an impact into the roll structure. Furthermore as the rear collapses moving forward those upper rules have no where to go but into the driver/passenger's back. Just something to consider. I know you said you have a few tubes left and perhaps they will address threse concerns.

Looks great overall though. Nice to see some minimalist round tube work going on.

Are you sure that itty bitty tube can handle the torques put out by the long diff ears?

Thanks guys.

Nocones - that has always been one of my dislikes about using a unequal length dual arm IRS on these cars. There seems to be no way around having "that" tube that is conveniently placed at about the same position as the passengers. I wanted to get the suspension mounted since I had two ideas for reinforcing but I wasn't sure which one would work.



Right now I will be adding a few 0.75"x0.049" tubes where the red and the green lines are. As a bonus the differential mount should help to absorb some energy in the event of a rear impact. Even though it is 1/8" plate, it is still weaker than the tubes around it in compression.

Carguy - you would be amazed at what little tubes can transmit. I cringe a little when I see the 1/4" armor plating come out for these cars. There is always the chance it will totally fail in some spectacular fashion but the math looks hopeful.

There are two torques acting on the differential. The torque from the driveshaft tries to rotate the differential left or right. At the worst it will be the engine torque x first gear ratio x some shock load value. The second torque tries to rotate the diff's snout up and down. It is the previous torque times the rear end ratio - by default this will be much worse and is the reason people who have hard mounted the snouts of their Miata diffs have had the mounts tear out.

The torque you were questioning will only be about 150 ft-lbs x 3.14 x 1 (we'll assume no shock for now) or about 470 ft-lbs. The long "ears" actually help in this case since they provide a longer moment arm for the two mounts to react the load. In the end each mount will see about 235 lbs vertically.

When you factor in the 4.1 rear end ratio, this torque increases significantly to 1,930 ft-lbs - the snout mount isn't so lucky. Still though the snout mount only sees 1,000-2,000 lbs in the vertical direction and the ears see at most the same in the horizontal direction. The 3/4" tubes the ears attach to can withstand approximately 5,400 lbs axially and the smaller tubes can carry about 3,500 lbs.

The killer for the diff is really the shock loads. As long as there is enough rubber to isolate everything and all of the mounts are the similar stiffnesses, it seems like you are typically safe. Most production cars are rubber mount differentials to even weaker structures than we are and they do fine. The key is to not create a hard mount which will carry a disproportional amount of load.

Now watch my diff tear out of the frame on the first launch.....

I like the minimalistic look of smaller round tubes. 1.5"x0.090" cage tubing isn't exactly great at the whole "add lightness" thing but I figure it can be offset with some lighter material where appropriate.

On that note if I ever modify the front uprights, the front Miata rotors are going to the shelf and I'm replacing them with a set of 1.8L rear rotors. That front clip is heavy and I'm blaming the rotors and calipers for a reasonable chunk.

Two months to the day after the official retrofit started, the car was back on four wheels.






Most of the welding is done aside from a few seams around the engine. I only have a few more tabs, two driveshaft hoops, the rear end curved tubes, a bunch of sheet metal, some paint, and some wiring and I'll be back in business!

Nocones - They're kind of hard to see but I was able to get one set of tubes in place to reinforce the back a little more (the ones that were green in the picture a few posts up.)

Looks Good!

Excuse the dumb question, but what are the hex-shaped members above the rear lower control arms? My guess is that they are used to control toe-in and toe-out at the rear wheels. I can't see what the outer ends are connected to from the photos.

Cheers,

Lonnie

You guessed right - its the rear toe-link. I don't have a better picture but you can get the idea from this shot:

Im curious did you make the adjusters for the inter rod ends on the rear upper control arms or did you buy them? If you made them how did you do it? I was thinking that one could turn the treads off of a larger bolt and then drill and tap the center of the bolt for the rod end.

You pretty much summed up my process. I used the drawings on McMaster's site to find a grade 5 bolt with enough shoulder length to give me at least 3 diameters of thread engagement.

Just add "use a good H3 tap" somewhere on the list and you'll be set.