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After all of that, I had the suspension doing exactly what I wanted, and then after starting to build, I had to redesign! I need to move the X axis for the upper rear mount on the a-arm back about 4". I plan to take measurements after the whole thing is built.

The reason for the move? The upright 1" SWG tube that would hold the a-arm mount was right in the way of the axle! Now, it is moved aft and the axle has a straight shot.

Of course this meant redoing all of the math, and the cool thing was that the Y and Z axis remained the same and the X axis changed rearward. I plugged all this in to see what nightmare I was in for, and the numbers are almost exactly the same as the stock Miata - in fact they are a little better and I have a little bit of anti-dive in now too.

Then I cut and tacked all the pieces in - seems to work well so far. I am getting a bunch of stuff from Kinetic hopefully next week and I'll plug everything in to check it.

I also did the side reinforcement bars. I was given some 1" DOM round (FREEBIE) so I used that instead of the 3/4". I also X-braced, which added about 3 pounds. I am waiting to do the forward reinforcement until I have the FU1 and FU2 bits in for the front suspension (my next big task).

Here are the photos:



Hopefully the whole project will remain this straight! It has so far...

This shows the upward angle of the rear - the rear lower mounts will go under this tube.

The upright tubes that will hold the rear upper mounts.


This is roughly where the diff will sit.




The uprights on the sides of the diff are visible too - I offset one to the passenger side, but still have room for the seat I want to use. I may use plate steel on one side of the tunnel to afford a bit more room.



And shots of the round tubing X bracing.

More to come!
Randy

OK, after a nightmare week that I will spare all of you from, I finally got some serious work done on the car. Pictures later, but for now a text description of what's gone on.

I received all of my parts from Kinetic (thanks Jack!), and they look really nice. That allowed me to play with the rear suspension, and I realized I needed to completely redo the rear (third time) - argh. I needed to have the upper mounting points level, as I am going to use heims instead of the bushings the Westfield uses.

I took the opportunity to change the geometry a little as well after playing with software more. I ended up lengthening the upper arms slightly, and lowering the mounting points a bit as well. That stabilized a lower roll center and gave me a bit more camber gain that more closely matches the front set up. I really think I have arrived at the final iteration. I also moved the upper mount point closer to the front on the upper member instead of having it mount on the upright, so it is very close to the stock Miata for mounting loaction in the X axis.

Once that was tackled, I started on the differential mount. For now, I just did the upper mounts - bolt in and easily removable but very stout. The pics will show it well. On the lower mount, I am still torn on how I want to do it. After looking at the way it mounts in the Miata and thinking over the design in most of the Locosts, I really think a PPF would be the best solution, even if it is a fabricated facsimile of the factory one that ties the lower diff mount to the gearbox through the center console. Not sure yet how I'm going to do it.

Once I download the pics, I'll post them here. I plan on working a bit tomorrow too to get the heim mounts for the suspension in place, then move on to the engine mounts.

Randy

Randy, for the PPF, take a look at mine.

Now you see why I used the Miata subframe--saves a lot of hassle trying to reinvent the wheel for a few pounds... the only major negative is you have to re-arrange the rear end a little, but parts are easier to come by.

Why not just measure the Miata subframe suspension points, and use that as reference? That way, you could save weight, while still being able to use the Miata rear suspension parts.

Randy, glad to see you back at it. Why not do as most Locosters do and let the frame be your PPF.

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He is a wise man who does not grieve for the things which he has not, but rejoices for those which he has.

I'm worried about the loads being transferred to the chassis, and the resulting possible welds cracking, metal fatigue, and vibration. Has anyone had any of these problems hard mounting the diff? I asked earlier and it led to a conversation, but no one actually said they had seen one failed by mounting it solid (and I know there have been examples, ie CMC, that have failed by not using a hard mount).

If there are no issues, I could just mount it solid to the rear chassis.

Thanks for the photos Nate - I have certainly studied your build and think it is a great job! Is that 1/8" plate steel? I am glad I went through the trouble of building my own IRS, as it is not only lighter, but has a lower roll center and the camber gain suits the front design better, which will make the car feel much more stable than if there were differing roll centers and camber gains front to rear. It is certainly a pain though! This thing better handle like a formula car now.

I'll see if I can post the pics up.

Randy

A test for photos a different way. This one's a shot of the pretty new Kinetic pieces .

A shot of the car - if you squint hard, it looks almost finished (OK, so you have to squint really hard).


And the upper diff mounts begin, using 1/8" plate for the mounts.


And the holes are drilled with the mounting bolts in place.


A wider shot of the same thing that also shows the updated rear design for the heims:


The aft anchors, and you can see the angle used for the bolt flanges - I still need to drill all of the holes yet:


The angle of the diff - this requires the mounts on the top to be angled down slightly (which is how it sits in the original Miata subframe as well).


The side anchors - this shot also shows the location center of the rear upper a-arm mount. It is a small black sharpie mark.


Another shot of the mount anchors:


And another...


One more - I always appreciate multiple shots of the same thing in people's build logs, so I figured I would do the same.


The mounting leg - the angle is facing the wrong way on the upper member. If you mounted it this way, you wouldn't be able to remove the mount once everything is welded. I started with this idea because it keeps the rear bulkhead aluminum sheet clean, but I can notch it cleanly I think.


Final iteration of the rear subframe all welded up, and the mounts in place all welded up. I still need to drill the bolt holes once I put the diff back in place.


So it is now all caught up - I will get to work on more this afternoon. I figure I can get the bolt holes drilled and tack in the heim mounts for the a-arms. Once I get those in, I plan to drop the engine/gearbox in to do the engine mounts. Are you guys mounting the gearbox to the center tunnel since the PPF is not mounting to it anymore?

Thanks!
Randy

Is there a way to upload photos at the full 256kb, but not have them huge on the post itself unless you click it?

Thanks,
Randy

No sorry, I don't think there is a way for bigger pics. You could always host them on Flickr or somewhere like that and just provide a link.

The stock PPF on the miata isn't as substantial as a Locost frame is it? I've never heard of one of those failing. Flawed logic perhaps, but it makes sense to me. I think as long as you don't place all of the load on one concentrated area of the frame (IE support the nose of the diff from the top and the bottom) you will be fine.

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He is a wise man who does not grieve for the things which he has not, but rejoices for those which he has.

It's not flawed, except that the PPF distributes the load with moment arm. The force on the mount when it is bolted right to the frame at the rear is more instantaneous, whereas when you have a long arm going to the front of the car, it isn't.

Also, the load for the PPF on the Miata is handled by the front engine mounts and the rear diff mounts - there is no load on the frame itself other than where the mounts are for the engine and gearbox.

Maybe mine is flawed!

Randy

Strange... I know exactly what the PPF is from the context of the discussion, but not a clue what PPF stands for.

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http://www.SportsCarDesigner.com .... You want to design your own car... so go ahead. Sports Car Designer is the answer.

If you want a less instantaneous reaction to torque to simulate the gradual loading and progressive resistance provided by the PPF perhaps you could mount it to the frame with poly busings.

*edit* SCD, PPF= PowerPlant Frame.

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He is a wise man who does not grieve for the things which he has not, but rejoices for those which he has.

Now where to find some poly bushings that I can use... not everyone has old Neon parts laying around . I do like how yours turned out though Chet. I was trying to come up with something I could do without having to find or buy more parts.

My only concern there is the bushings failing like they did (albeit they had a much different durometer) on the CMC chassis.

Randy

Also, what do you guys think of the diff mounts above? I was thinking of gusseting the anchors and legs to the plate - necessary?

Randy

In addition to the gussets, also consider a large diameter washer or doubler under that bolt head to help distribute the load over a larger area of that plate.

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Jack Collins