1994 Mazda Miata: Suspension Walkaround
June 25, 2010
A lot of car people really admire the original Mazda Miata because it's such a simple and well-executed design. But there are also those who'll never get it. I heard one myself last week while I was driving our '94 Miata with the top down: "Look at that. That car is so bleeping blay."
Whatever, dude. I call I can say is you're missing out. He probably assumes it's front-wheel drive, too.
Let's move along. I'll point out Jay's bolt-on mods along the way.
The secret of the Miata's success is the fact that it is, in fact, a lightweight rear-drive roadster with double wishbone suspension.
A heavier L-shaped lower arm (yellow) handles the bulk of the suspension loads, and a lighter A-shaped upper arm (green) helps define the steering axis and the caster and camber angles.
Because the engine drives the rear wheels, there's room for a front-mounted steering rack that's mounted ahead (white) of the front axle centerline. Steering precision benefits as a result.
The original springs and shocks were coil-overs too, but the lower spring perch was welded in place and the springs and shocks were considerably softer. I measured the springs on my 1990 Miata once, and they came out at 130 lb/in. These springs are considerably stiffer at 425 lb/in. Part of the increase provides a sportier ride via a higher ride frequency. But there is a practical aspect, too. If you lower a car, the springs must be stiffer in order to absorb the same amount of energy over a shorter distance.
The yellow Bilstein shocks are monotube units, whereas the original dampers were a twin-tube design. As the name implies, monotube shocks are made of a single tube, and that allows any internally-generated heat to wick out easily. Twin tube shocks have a tube within a tube, and that makes them store heat, like a thermos bottle. It also makes for a smaller diameter valve because it runs in the innermost tube. Both of these factors, taken together, limits the amount of damping they can produce without fade.
Here's another view of the upper and lower arms.
The outer ball joint (black) is easily replaceable because it is bolted on to the end of the lower arm.
Here's another view of the L-shaped lower arm. Both inner pivots (yellow) are made up of eccentric cams.
Good news: camber and caster are both adjustable.
Bad news: it's a pain in the butt to adjust because each affects the other. Loosely speaking, the front one primarily affects camber and the rear one primarily affects caster. Also, the pivot bolts aren't particularly huge. If you run over an apex curb on a track day, things can move and your alignment may readjust itself to a setting you'd rather not have. Same goes for potholes.
Here you can see one of the potential pitfalls of aftermarket coil-over springs with adjustable spring perches. Springs flex a little to the side as they compress, and the outside diameter of the threaded collar can be somewhat close to the inside diameter of the spring.
These rub marks (green) tell the tale. You can hear it, too. Some aftermarket spring sellers (Ground Control comes to mind) offer barrel-shaped springs that provide clearance in this critical area. The rubbing and the noise isn't a huge deal for a race car (my Miata rally car squeaked like you wouldn't believe), but it's less than ideal for street coil-over applications like this.
A much larger Racing Beat stabilizer bar has been fitted, but it bolts into the stock location. A larger bar requires a larger bushing to pivot in, and the red color of this one indicates that it's urethane, not rubber.
This stab bar is hollow to save weight, but that doesn't affect its torsional stiffness as much as you'd think because the outer extremes of the cross-section is where most of the work is done. The torsional stiffness of a stab bar is proportional to its outside diameter to the FOURTH power (OD^4). The torsional stiffness of a hollow bar is proportional to OD^4-ID^4
Our old stock front stab bar was 19 mm in diameter -- about 0.75 inches. This racing beat bar is 1.125 inches in diameter on the outsdie and 0.875 inches in diameter on the inside. Do the math using the fourth power of those diameters and you'll see that this stabilizer bar is 3.25 times stiffer than the stock unit.
You can actually see that it's a hollow bar by the way it looks at the end where it's squished flat. For the record, Racing Beat says it weighs 5.5 pounds.
The original fixed-length stabilizer bar links have been replaced here by adjustable ones. But you don't really need to adjust something like this.
The idea behind them is that fixed-length links might lead to a little stabilizer bar preload. That's no big deal on the street, but it can matter on the track. So what you'd do is get the car set up and aligned, get the driver and the fuel installed and then have a friend or crewmember adjust the length of these links to remove the preload. One of the heim ends is left-hand thread and the other is right hand thread, so you don't need to remove them for adjustment. Simply loosen the thing jamb nuts and then twirl the black center section to the desired length.
If the word "crewmember" doesn't apply to you then the word "adjustable" probably doesn't either.
However, the fact that these links have spherical heim ends instead of rubber bushings is something a street enthusiast might appreciate.
No, "R" isn't for rear. There are the stock right-front brake calipers on our Miata. They're single-piston sliding calipers and cast-iron ventilated rotors. Simple, but very effective. Easily upgraded for track use with aftermarket pads.
Our back, the first-gen Miata has another pair of upper and lower wishbones. And we've got another colorful coil-over replacement, too.
Like the front, the lower arm is the higher-stressed one that does most of the heavy lifting. By the look of it, the spring/shock arm ratio might be 0.80, but its lean angle takes some of that away. The tiny paperclip of a rear stabilizer bar (yellow) operates at something like a 0.60 motion ratio. We haven't upsized this one yet because the car feels balanced as it sits.
But it looks like we could trim the ends off those stab link bolts to save some weight. :)
There's nothing much to see here but 177,000 miles of baked-on road grime.
Another lower control arm, another pair of alignment adjustment bolts. Move these to adjust camber. Make sure they're tight.
My 1990 and 1991 Miatas did not come with these braces, but our '94 came with them stock. I think they were added whan the engine size was upped from 1.6 liters to 1.8 liters. Whatever, they're a good upgrade to consider.
The lower arms connect to horns on the rear sub-frame that hang down. The longitudinal braces (yellow) tie them together and to the unibody, and the later one (green) stiffens things up side-to-side. Since most of the lateral suspension loads go through the lower control arms to these horns, these stiffening braces are potentially huge if you add lots of horsepower and lots of tire, like we plan to do.
Braking duties are carried out by single-piston sliding calipers and solid cast iron rotors. The calipers and pads are the same as my 1.6-liter car, but the rotor diameter is larger, and the larger moment arm that represents allows them to apply more torque.
This is probably the lightest wheel/tire combo you're likely to see on stock wheels, and the low unsprung weight they represent is another secret to the Miata's success. That's what you get with 185/60R14 tires on light 14" factory alloys. The larger 949-Racing 9x15 wheels we've put on since are supposed to be no heavier than the stock wheels here. With tires, who knows? We'll see.
Dan Edmunds, Director of Vehicle Testing @ 177,362 miles