Since I don't actually own a Miata, I hope one of you experst has the answer at hand.
Are the front and rear a-arm inner pivots the same distance from the ground plane?
I can find information that confirms my rear ground clearance to the inner pivot as being 3.75" minus the tire radius. This assumes that the a-arm is parallel to the ground. Does the same formula apply to the front?
Warren is your man on this stuff. I'm pretty sure they're mounted level, but he's spent more time trying to duplicate the geometry than most.
I'm assuming you are referring to everything as viewed from the side of the car.
Short answer: No, the front control arms are aimed down (they would make downforce if they were wings) from the side view. For that matter, the rear control arms are actually a little tilted up (they would make lift if they were wings) on the Miatas I've measured. A millimeter of measuring error makes a big difference with that particular part of vehicle geometry.
Long answer:
Depending on the depth you want to go, this stuff is pretty hard to quantify and analyze on an existing, real-world car.
Emilio (from 949 Racing) and Shaikh (from Fat Cat Motorsports) are guys that you'll want to search on Google with the terms "miata suspension geometry". Information is pretty scattered on the internet, and there really isn't a good source that I've found for the whole picture on Miata suspension geometry. There are lots of tidbits to find, especially on the Locost forums. This stuff is pretty sensitive. Your static alignment, fuel level, your personal weight, and ride heights all affect your suspension geometry in measurable ways.
I can tell you those measurements in that picture are not precise enough for accurate geometric analysis. The real numbers are considerably more metric. Anti-dive is so sensitive that a millimeter error in measurement could affect the handling noticeably.
I think you're asking if the lower control arms are parallel to the ground plane in a side view of the car. If that is the case, the rear LCAs are almost parallel. From the stock NBs that I've measured (both standard and HardS) there is a tiny bit of anti-squat, contrary to what the internet will tell you. This may just be an artifact of the ride height of the particular cars that were measured (all stock). The front geometry is much easier to see; it has a significant amount of anti-dive built in.
One thing to keep in mind is that the subframes themselves are almost never parallel to the ground in a Miata.
Unfortunately, measuring this stuff on a car is quite difficult. Are you building a car with Miata components? If so, it might be easier to measure the components individually and tweak the geometry to your purposes. Just remember that measurement errors stack up, so positive measurement techniques help a lot. A pair of calipers, a very flat piece of steel or aluminum, and a height gauge make things a lot more straightforward. The equipment doesn't need to be expensive; cheap digital calipers are fine to a couple hundredths as long as you treat them like they're made of glass.
Just for curiosity's sake, suspension kinematics are even more complex that simple points and geometry, especially when bushings and control arm deflection are involved. On the OEM level, the only way to truly analyze a car's geometry is on a special piece of equipment, usually called a "7-post shaker rig". Here are some videos of an NC Miata on such a device: http://www.youtube.com/watch?v=lp0NUm8vyec http://www.youtube.com/watch?v=6x8_BZwCPbM http://www.youtube.com/watch?v=ZBgxoMBu2lA
Warren, I've used 7 post rigs before, but it was more for durability testing. Cool toys for testing your welds!
Moving on...
Without any way to determine front and rear lower control angles for squat and anti-dive, I have a choice of making up a number from thin air or eliminating the feature.Neither one is ideal, but this wont be a racecar either, just a street rod. Under the heading of "what is safest? "I will go ahead and install the suspension assemblies with the LCAs parallel to the ground both front and rear. This seems to be the path most followed on the Locost forum from what I can determine. The suspension will be adjustable with shims if I should ever find the magic numbers and or decide to experiment.
Going back to my original question then, if a line were projected through the front LCA pivots and the rear LCA pivots, would they both be the same distance from the ground plane? The answer seems to be that due to anti squat, it is a moot point.
Going to have to get a Miata up on a chassis plate with a height indicator to answer this one! I tried to measure one on a garage floor with a tape measure and I could not find any noticable difference front or rear. So we must be talking on the order of a couple of mm at most. Not easy to measure with a standard tape measure where you are estimating the center of the bolt while cramed under a Miata sitting on the ground.
You'll be safe with the LCAs parallel to the ground on a street rod. If it's going to be more than 2k lbs, I would set up the front subframe tilted slightly down (maybe 1°). That would be more for bumpsteer than anti-dive.
Here's some info for you. All measurements are from the Mazda underbody dimensions document and refernce distance from some unspecified ground plane.
Front subframe
front mounting point 290mm high, 812mm wide
rear mounting point (upper one) 163mm, 600mm
rear mounting point (lower one) 79mm, 600mm
Rear subframe
Front mounting point 309mm, 800mm
Rear mounting point 319mm, 800mm
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