by Nick Jenkins
NA-VIN Miatas (1990-1997) have very little wheel travel in back. On a relatively smooth road, this has little effect on ride and handling—the springs and shocks easily soak up small bumps and road imperfections. On a rough road, however, an early Miata can skitter across the pavement and ride like a buckboard when the suspension fully compresses, or "bottoms". In a stock Miata, the suspension bottoms when the top of the shock body hits a hard rubber cylinder, called a bump stop, that fits between the shock and the floor of the trunk (it’s integral to the dust boot that fits over the shock). When the bump stop is engaged, the suspension can't compress any farther. Although the rubber bump stop is slightly compressible, with a spring rate of about a zillion, for all intents and purposes it’s solid.
What happens when your Miata’s suspension bottoms? Let's use a big 2" pothole as an example. As the wheel crosses over the hole, the first thing that happens is the spring quickly extends, pushing the wheel to the bottom of the hole, much faster than it would fall on its own. This is good, as it keeps the tire in contact with the road. If you're in a turn, you don't want your tires losing contact with the road. The lighter the wheel, brakes, and other suspension bits (called collectively "unsprung weight"), the faster the spring will be able to push the wheel down, and the better the contact will be with the road.
The next thing that happens, now that the spring is extended and not supporting that corner of the car as much, is that the car starts to fall into the hole. In this case it falls more slowly than it would fall on its own, because the spring still has some tension in it, and the shock also slows the rate of compression. From a ride standpoint, this is exactly what we want. The corner of the car gently dips, the shock and spring cushioning the blow so you barely feel it. Aren't shocks and springs great?
Unfortunately, the story doesn't end there. Left on its own, the car would settle into the pothole, back onto its springs, in a second or two. But long before that happens, the tire reaches the forward edge of the pothole. Now things don't go so well. If the unsprung weight is light, the edge of the pothole will push the wheel up into the wheel well, compressing the spring before the car feels much of anything. But soon the extra compression in the spring will start pushing that corner of the car up—fast. The shock, which is designed to resist spring movement, will add to the bump by not allowing the spring to compress fast enough.
What's worse, because of the Miata's limited wheel travel, a 2" pothole will cause the suspension to bottom (fully compress) before the wheel has climbed out of the hole. The car will suddenly be forced to move upward at the same speed as the wheel, i.e. almost instantaneously, until the wheel clears the pothole. The bump becomes a sharp jolt, absorbed only partly by the seat cushions and a little bit by the tire itself. The car's upward momentum, now accelerated wildly, can send that corner of the car into the air, taking with it any tire contact and corresponding traction.
When worse comes to worst, a single pothole is not your biggest problem. On a rough road, the compressions and extensions come too fast for the springs and shocks to keep up. In the middle of an extension, for example, you can hit a bump
Flyin’ Miata shock mounts
that immediately wants to compress the spring. And instead of a single wheel and one corner of the car, it's happening all over the place. Time to slow down, which is never fun.
What can you do about it? The best thing to do is increase wheel travel. Since the problem is almostexclusively in the back of the Miata (bottoming rarely occurs up front), suspension changes can be limited to just two wheels. Flyin' Miata sells custom-built shock mounts for the back end of an early Miata that add about an inch of wheel travel, and this is all you really need on anything that passes for a paved surface. They're not cheap at around $250 for the pair, but if your regular commute includes some rough patches, they may be worth it.
Another common solution is a switch to softer bump stops. Overall travel is increased because the shock can now compress the bump stop, pushing closer to the floor of the trunk, and at the same time
MCU bump stop fitted to a Koni shock.
your spring rate increases as the bump stop acts as a secondary spring. Soft bump stops, generally made of multi-cellular urethane (i.e. foam), are typically much longer than the stock hard rubber stops, which means they engage the shock sooner, further cushioning the blow. MCU bump stops are available from many vendors, including Flyin' Miata, Ground Control, and FatCat Motorsports.
Either of these improvements can be completely and immediately eradicated by the simple and common expedient of replacing the stock springs with "better" aftermarket springs. Many people do this simply because they think the stock car sits too high, and they like the look of a lowered Miata. Others do it for the improved handling. As long as the owners are aware of the associated ride penalty, either excuse is acceptable. In any case, lowering the car much more than an inch will reduce wheel travel to the point where anything but the smoothest road surface will cause unacceptable handling problems as the suspension constantly bottoms out.
One additional problem, hinted at on various Miata forums and vendor sites but only recently exposed on a BAMD driving event as a real threat to lowered Miatas, is coil bind. It's not common, and so far only seems to occur with the combination of Koni Sport shocks, Flyin' Miata springs, and Flyin' Miata tophats, or other tophats designed to increase wheel travel. What is coil
Flyin’ Miata spring under static load.
bind? Imagine climbing under your Miata and precisely measuring the distance between each coil in the spring. Then add up all of those measurements. Coil bind can occur if the sum is less than the amount of wheel travel you have.
When all of the spaces between the coils are used up—the coils are stacked one on topof another—the spring is no longer a spring but is instead a solid steel cylinder. If this happens in a bump, all compression stops. Instantly, huge forces are imparted to the upper and lower spring mounts. The damage to either or both can be severe. On a Koni Sport shock (for the NA), the lower spring mount ("perch") is supported by an impossibly tiny metal ring. Surprisingly, this ring can take thousands of pounds of pressure without deforming and coming loose. It can't, however, take forces imparted by a hard coil bind.
The Koni spring perch sits a mere 1/4"-1/2” above the spinning axle shaft. Despite this proximity, the relative
motion of the two components ensures that this distance never decreases, so ordinarily it's not a problem. If the spring perch
Severed axle shaft (above), and the “tool” used to cut it in half (below).
collapses, however, the distance reduces to zero, and the perch (and the spring, and the weight of that corner of the car) is now supported entirely by the spinning axle shaft. Again very surprisingly, the car will actually run in this mode, with only a little added noise from the back end as the spring perch starts to cut through the axle like a blunt lathe tool.
How much time you have until the spring perch completely severs the axle shaft varies depending on your speed and thenumber of stops you make (which allows the smoking shaft to cool a bit), however 50 miles is the current average for all known examples (1). This will allow you to finish a BAMD run and get within a mile or two of home. When the shaft does divide in half, the noise is not much louder than, say, running over a large bolt. If you're quick you'll catch a glimpse of axle parts flying out the rear. If you have an open differential you'll immediately coast to a stop. A limited-slip might allow you to push on for a few more miles, one corner of the car completely springless and resting its weight on the dead shock body.
If you're more practical than that, or you simply don't like the idea of using an axle shaft to support the weight of the car, you'll park your Miata the moment you suspect you've bound the coils. Damage may then be limited to a single shock. A quick under-car inspection will show virtually nothing wrong, so you have to make a point of checking the spring
Although the perch looks bad, it managed to support the car on the axle for fifty miles.
perch-to-axle shaft distance. A bright, polished ring on the shaft in the vicinity of the perch will also be a good clue.
Should you be concerned about coil bind? First of all, it's not possible on a stock Miata—only a lowered car with aftermarket tophats is susceptible. In any case the Miata is a well-built vehicle with a solid suspension design, and as long as the lug nuts are torqued, the wheels will keep rolling in almost any situation. Also, there’s nothing inherently wrong with Flyin’ Miata springs—when installed in a stock NA, or in combination with Tockico Illuminas or other brands of shocks, the springs perform well. Konis can be a problem because they have a body that's 3/8" shorter than other rear shocks on the market. That extra 3/8" of compression, along with the additional travel provided by aftermarket tophats, can put the springs into bind.
The Miata in the pictures fell victim not to a pothole, but to a three-inch drop-off on the apex of a turn. The car is back on the road again, with an intact axle and a brand new set of Konis. Longer MCU bump stops were fitted so that (in theory) the spring rate will climb to a zillion before the coils can bind. We’ll see how that works out in practice.