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After much experimentation, I have figured out that wishbone is only accurate for the front suspension.

Because there are different attachments for the arms at the upright, and there is no steering ball joint, the numbers are totally skewed.

I am still sticking with these numbers though, as they give me the stock Miata geometry with the ride height I want. I would like to be able to see it in bump for the camber change and roll center stability, oh well.

Randy

I haven't done it, but I think if you put the tie rod ends at the same point as an inboard control arm pivot (in Wishbone, not in the physical world, of course) you could make it simulate a rear end. I'm not real sure of where you'd need to put the outer tie rod end, but I think if you put it at the same Y and Z as the lower ball joint, it would work out.

Or not...?

-dave

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you can effectively ignore the toe changes induced by the steering ball joint, or you can make the length and attach points of the steering ball joints the same as your upper control arm and thus there is no toe induced upon bump/droop.

Like Dave said, just treat your rear attach bolts as 'ball joints's and the program works just fine. It's all just in 2-D. That's what you want anyway. Inducing toe from bump in the rear is bad.

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Miata based, custom chassis build in progress. First Drive!!! http://locostusa.com/forums/viewtopic.php?f=35&t=2269&start=375

I gave up on wishbone. As far as I can tell using a string computer, the roll center looks like it is about 3" and stable and camber gain is reasonable.

Here are the pictures of the rear subframe where I want it and how some of the measurements were taken:


Note how far forward the rear subframe intrudes into the cockpit.









Some wheel orientation shots (love those sweet wheels!) :




Here's how I oriented the rear subframe - raised from the build table 1" seems to be a perfect ride height position:



Rear bulkhead orientation:


Again, you can see how far into the cockpit the rear subframe intrudes with the wheelbase at book dimensions:


And now the beginning of the rear frame using the Miata subframe measurements and the Westfield type mounting:









Now I need to find the bushings for the round tube mounting of the diff points Westfield uses. I also need to fab up the rear a-arms (talking with Jack @ Kinetic on that).

I'll post more as I fit more tubes and get the mounts in.

Randy

So what's the problem with this? We are only concerned with the frontal plane here right?

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The numbers aren't the same for the stock Miata rear subframe though... am I correct with that?

Thanks for looking at it,
Randy

They should be really close. That's where I got them.

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Here are the "yellow" numbers for the inners as I measured them, and I was painstakeingly accurate as best as possible! Again, I used a ride height of 6.5".

Lower Rearward
X=6.5" behind the axle cL
Y=8.86" from the ground
Z=9.84" from chassis cL

Lower Forward
X=6.1" ahead of the axle cL
Y=8.94" from the ground
Z=9.84" from chassis cL

Upper Rearward
X=2.76" behind axle cL
Y=16.18" from the ground
Z=15.35" from chassis cL

Upper Forward
X=4.33" ahead of axle cL
Y=16.18" from ground
Z=15.35" from chassis cL

I'll plug them in and see how they look compared to the strings...

Here is what I come up with using the measurements and ride heights I took:



With roll, the RC stays very consistent, though it is a bit higher than I had calculated. I think it is still pretty good though.

Thoughts?

Thanks,
Randy

I think where your numbers and mine differ the most is that instead of measuring at any theoretical ride height, I set the inner lower pivot to the same height as the lower balljoint on a 23" tire. This levels the lower control arm. This is the position in which a unequal length double A arm system is designed to work.

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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.

So do you think I should raise the inner mount to match the outer lower ball joint height? I am sure it could be adjusted with the spring perch as well, but then I believe my ride height would be raised, and I don't want that. ***EDIT*** This doesn't make any sense, I think I was a little backwards with what I was seeing

I measured that height on the lower ball joint while it sat on the build table with the tire at my desired ride height, which means the arm would not have been exactly level as far as the mounting points go (though the arm itself looks level).

You can't see it perfect in this shot, but you can probably get the drift of what I am saying. The outer and inner mounts on the lower arm can be seen in this shot, and the inner was up on the 1" square to get the ride height where I thought it looked good and the arm was level.


Randy

I don't think it's the end of the world if you do it like you have it. People drive lowered Miatas and do fine with them. I was just explaining why your numbers were so different than mine. If you introduce droop (- bump) to you numbers, in an amount sufficient to raise the inner lower pivots level with the lower outer pivots our numbers should be closer to the same.

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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 building now though and haven't done the mount points for the uppers yet (or lowers really). If I raise the inners, I could get the correct geometry, or do you think it would matter? It would lower the roll center, but the stability and gains are the same pretty much. *****EDIT*****Again, this is backwards from what was actually going on - keep reading and I figure it out eventually

Thanks!
Randy

After playing a little with the wishbone settings, the roll center actually gets higher by moving the inner mounts higher from the ground and leaving the outers where they were. If I lower the inners, or bump the suspension (raise the outers), the roll center goes lower.

That doesn't make too much sense to me, am I missing something? I was missing something

On my measurements, the outers are a little bit lower (closer to the ground) than the inner mounts on both the upper and lower arms. It seems that is the way it should be. If I lowered the car (like a lowered Miata), wouldn't that raise the outers? If so, that would actually lower the roll center according to my playing.

I think I have managed to thoroughly confuse myself! I really think the original numbers are correct Keep reading to see the lightbulb go off above my sometimes thick head

Randy

OK, I think the issue is that with the outer ball joint locations where I had them on the build table, the suspension was already in droop a bit. Raising the outers puts the geometry in the right place, but my ride height will more closely be around 5.5" instead of 6.5". That works better for the front we discussed anyway, so that works great. The roll center is about where I had calculated on paper too, so it all makes me warm and fuzzy again.

The f6 function was what really helped me see it!

Thanks again for all of the help and input - this site is a great resource.

So to summarize, the numbers I have posted above are good for building the rear subframe, but when you are all finished, the outer numbers are going to be a bit different when the car is at ride height (but make sure you use the numbers posted to figure the a-arm dimensions).

Hope that helps!
Randy