??? and hopefully some answers to using the miata subframe.

[skou]

Matt,

I rendered out your model to visualize it a bit better.

My impression is that the upward sloping CB2 members may make locating the LCA brackets a bit tricky. Reference the second photo below of a Westfield Miata IRS with the LCA brackets mounted both above and below the equivalent of the CB2 members. There is a guy in the UK putting a Miata rear end into a Haynes chassis and he has gone to flat CB2 members (see third photo below). I believe I have also seen that layout on a couple of builds here.

The other concern is that CB2 and CB5 captivate the differential since it hangs from the 1/8" thick plates. The Westfield layout is a bit slicker IMHO in that it uses removable brackets to mount the differential above the equivalent CB5 members (see last photo below), allowing it to be removed from above.

A final comment is that the sketch has the UCA length noted as 9.8". The actual Miata UCA is closer to 8-7/16" in case you want to replicate the OEM geometry.

[chetcpo]

There's no question the westfield is a "slicker" design as you said, but the one rendered above isn't bad at all. It works good and is much easier to fabricate. The diff is not captive and can be removed for service, and the lower rear suspension pickup points aren't that challenging to integrate. Obviously the wider you make the lower A arm the harder of a time you will have.

A final comment is that the sketch has the UCA length noted as 9.8". The actual Miata UCA is closer to 8-7/16" in case you want to replicate the OEM geometry.

Well if you think the control arms in my sketch are too long you can forget about the Wesfield rear. Those UCAs appear to be at least 3" longer than mine.

[skou]

There's no question the westfield is a "slicker" design as you said, but the one rendered above isn't bad at all. It works good and is much easier to fabricate. The diff is not captive and can be removed for service...

Another shot of the UK guy's design showing that he's going to "hang" the differential in the same manner, leaving the challenge of getting it out for service.

Well if you think the control arms in my sketch are too long you can forget about the Wesfield rear. Those UCAs appear to be at least 3" longer than mine.

I didn't say they were too long. I was just pointing out that they are longer than the OEM length. Sorry if you took it that I was critiquing your design.

[chetcpo]

<snip>

Well if you think the control arms in my sketch are too long you can forget about the Wesfield rear. Those UCAs appear to be at least 3" longer than mine.

I didn't say they were too long. I was just pointing out that they are longer than the OEM length. Sorry if you took it that I was critiquing your design.

No, I didn't take offence, and I don't mind a critique. You pointed out that the OEM geometry wouldn't work with my sketch and then pointed out the superiority of the Westfield design. I was only stating that if OEM geometry is your goal then it wouldn't work with the Westfield design either. I agree though the Westfield design looks better. If you guys wanted to model it too there was a member here "Stein" who said they had drawings of the design. Attached are some shots of the frame.
He ended up selling the project uncomplete and it has passed through at least two other builder's hands, but hopefully he kept a copy of the plans. If not you can try to contact the other two owners that I know of. It went like this Stien-Winston-Titus-?

[dilbert]

You could have the two supports at the very bottom bolt into place instead of being welded. This way if you did need to take the diff out it would go out the bottom. At least in the sketch it doesn't have any other tubes in the way. Worst case scenario is that since you built the car from scratch, you cut these two tubes, replace the diff then re-weld them back in.

[skou]

If you guys wanted to model it too there was a member here "Stein" who said they had drawings of the design. Attached are some shots of the frame.

Yeah, I actually found Stein over on miata.net a few months ago as a Westfield-like IRS design is where my build is headed. I offered a couple of times to model up his design and post it for all to use McSorley style. Never heard back from him after the second PM.

[chetcpo]

You could have the two supports at the very bottom bolt into place instead of being welded. This way if you did need to take the diff out it would go out the bottom. At least in the sketch it doesn't have any other tubes in the way. Worst case scenario is that since you built the car from scratch, you cut these two tubes, replace the diff then re-weld them back in.

Like I said above, no need to cut anything the diff comes out fine.

[mattrogers]

Matt,

I rendered out your model to visualize it a bit better.

Slick, thanks! I always forget about perspective mode.

Well to be totally fair the "chetcpo" rear end you rendered for me is a copy of the CMC (now defunct) design. I've never measured or seen a CMC frame in person, but I've eyeballed the heck out of the one in K. Tanner's book and the design I drew was based on that.

Ah ha, I see. Thanks for the provenance. I kind of like the fact that it's book-based on the outside, but then just has an IRS setup inside.

Don't start cutting yet though. I built a book frame for a friend last year and used those drawings and decided it would work better with the diff about 3/4" higher. I used a combination of pieces cut from 1/8" x 2" and 1/8" x 1" steel band to lift the diff up a bit without changing any of the rear frame tubes. I'll attach a pic below. (well, below the pic below, at the bottom) It would likely be easier just to raise the two cross tubes that carry the diff mounts, but I was already past that point when I decided to raise the diff on that particular frame.

If there's a good way to motivate me to do some math, it's stuff like this. I thought "about 3/4" higher: does he actually want it an inch higher? Or what's the actual number?" I measured the differential inputs and outputs and ear heights and something didn't add up on the differential mounting height. Then I saw that your car was designed around 20" tires (I realized I hadn't actually read your build log up until last night). That would make a big difference in differential mounting height. I ended up needing to add another 2.5" to the O3 mounting tube location along K3/4 to compensate for designing around 23" tires once you factor your change in.

Here's the quick numbers about the math:
Wheel size: 23"
Wheel radius: 11.5"
Frame clearance: 5"
Desired differential output height above bottom of frame: 11.5" - 5" = 6.5"
Differential ear height above differential output shafts: 6.175"
Differential ear height: 6.5" + 6.175" = 12.675"
O3 spacing from the top front corner of B1 along K4 = 11.5" (with some corrections, this is just dividing the height by cos(17°).

Thanks for the feedback.

[chetcpo]

Matt,

I rendered out your model to visualize it a bit better.

Slick, thanks! I always forget about perspective mode.

Well to be totally fair the "chetcpo" rear end you rendered for me is a copy of the CMC (now defunct) design. I've never measured or seen a CMC frame in person, but I've eyeballed the heck out of the one in K. Tanner's book and the design I drew was based on that.

Ah ha, I see. Thanks for the provenance. I kind of like the fact that it's book-based on the outside, but then just has an IRS setup inside.

Don't start cutting yet though. I built a book frame for a friend last year and used those drawings and decided it would work better with the diff about 3/4" higher. I used a combination of pieces cut from 1/8" x 2" and 1/8" x 1" steel band to lift the diff up a bit without changing any of the rear frame tubes. I'll attach a pic below. (well, below the pic below, at the bottom) It would likely be easier just to raise the two cross tubes that carry the diff mounts, but I was already past that point when I decided to raise the diff on that particular frame.

If there's a good way to motivate me to do some math, it's stuff like this. I thought "about 3/4" higher: does he actually want it an inch higher? Or what's the actual number?" I measured the differential inputs and outputs and ear heights and something didn't add up on the differential mounting height. Then I saw that your car was designed around 20" tires (I realized I hadn't actually read your build log up until last night). That would make a big difference in differential mounting height. I ended up needing to add another 2.5" to the O3 mounting tube location along K3/4 to compensate for designing around 23" tires once you factor your change in.

Here's the quick numbers about the math:
Wheel size: 23"
Wheel radius: 11.5"
Frame clearance: 5"
Desired differential output height above bottom of frame: 11.5" - 5" = 6.5"
Differential ear height above differential output shafts: 6.175"
Differential ear height: 6.5" + 6.175" = 12.675"
O3 spacing from the top front corner of B1 along K4 = 11.5" (with some corrections, this is just dividing the height by cos(17°).

Thanks for the feedback.

Here's some more for you to think about. The diff outputs can be moved around quite a bit without any ill effects due to the joints on both ends of the axles. The axles don't need to sit perfectly level at desired ride height or be perpendicular to the center line of the chassis. The reason I moved the diff up was to reduce the driveshaft angle from diff to transmission. Since the fuel tank sits above the diff carrier I could only raise it so far before it ran into the tank.

[RichardSIA]

Related ?

For my use I want to narrow the Miata rear about 5".
Seems it's not too difficult to mod the cradle.
I know this will affect the suspension geometry.
How badly?
I should go model it in my suspension program but am fighting the flu.
Bored out of my mind but unable to concentrate.
Only need a rough estimate at the moment, really significant or easily adjusted out?
Hope is to retain stock upper and lower arms for easy service.