NVH Noise Vibration Harshness Abatement

NVH (Noise Vibration Harshness) is really the key delineator over the years of Miata production. Having crawled around these cars for 25 years, most of the chassis updates to a given generation have been geared towards reducing NVH or emissions compliance. A 30k mile NA6 stills feels shakier than a 100k mile NB2. Add rough road wear and tear, autocross/track use, chassis loads from far grippier tires than OEM, extra power, stiffer springs and sway bars and the resonant frequency of the chassis drops as the car literally “wears out”. These little cars turn into rattle buckets with years of use. This is especially relevant to us as designers of high performance suspension systems for each generation of Miata. The owner of a beat to heck 200k mile NA6 will expect a magic carpet after buying a set of Xidas regardless of the condition of their chassis. A rattle free ride might not happen if everything else is loosey goosey. The NB chassis gained significant torsional and bending stiffness over the NA. Add to that bolted on components (engine, accessories, drivetrain) that generated less NVH and the NB’s are just a better place to start if low NVH is the goal. So what can be done to your rattly NA?

Bolt check
One of the things we do to our race cars before every major race weekend is a top to bottom bolt check. It goes up on the hoist. We go under, inside the interior, under the hood. Check every fastener you can reach. See signs of something loose or wobbly, fix it. In a typical high mileage NA interior there are many sources of squeaks and rattles. Glove box door, roof latches, door cards, center console, steering column covers, etc.

Whats the frequency Kenneth?
The NA6 was a wiggly, flexy little chassis when it rolled off the production line. From there it just gets worse. OEM’s often use resonant frequency as a measure of overall chassis rigidity. Excite (hit a bump) the bare tub or complete car and measure the frequency that it vibrates at. A few really stiff coupes can measure almost 60hz. That’s 60 oscillations a second. How that translates to what the driver feels is a tiny, imperceptible buzz that is quickly damped. The BMW E46, for example, is 30hz. C5 Corvette coupe is 17hz. I don’t know what the 90 NA6 was but I’d guess something closer to 13hz new and probably well under 10hz when they have some miles on the tub. That’s low enough for you to see and feel each wave peak, the windshield header or dash moving back and forth a few times a second. Awesome suspension, tires etc are not going to mask the fact that we are driving noodles.

Stiffer please
In simple terms, there are two ways to raise the natural, resonant frequency of the tub. Damp the oscillations and make it stiffer. I’m not talking about damping with shocks. I’m talking more about damping the sprung mass of the chassis that doesn’t have hinges. Meaningful improvements to damping the tub will be difficult without some serious engineering and mods. Thankfully, the alternate option of simply making the tub stiffer is within our reach and cost effective.

Below are the mods we have found to be the most effective, in order of NVH improvement. Not everyone will agree but at least this list is a good place to start.

1. Full 8 pt weld in race cage. It will feel like a different car. Utterly impractical and not safe with OEM passive restraints on the street but the way to go for dedicated track cars.

2. Seam welding. Impractical for a street build as it involves stripping your car to the bare tub then stripping most of the paint. Ugh. It’s a ton of work but we do this to every race car we build if the rules allow it.

3. Door bars. Specifically the Hard Dog bolt in door bars. Bang for the buck these do more to reduce chassis “jiggle” over bumps than any other mod. The downside is they are only safe with one piece racing seats that have side bolsters between you and the bar. It’s unsafe to have a big steel tube next your hip in a side impact unless there is an FIA certified race seat in between.  Door bars also require swinging your leg over the bar which make ingress/egress more work. Door bars affect torsional rigidity enough to require retuning chassis balance after their installation. More than once we turned a tight (understeer) car in to a loose (oversteer) one just by adding door bars.

4. Roll bar bolted to bulkhead and rear shock mounts. We like the Hard Dog roll bars but any good quality bar with a one piece main hoop that bolts directly to the rear bulkhead will do the trick. Despite what most people think, most of the torsional flex in a Miata chassis under cornering loads is behind the firewall.

5. Frame rails. There are several good quality bolt on frame rail stiffeners on the market. They all work.

6. Door bushings. Surprisingly, just changing the soft rubber OEM door bushings to harder Delrin / Acetal POM nylon or high durometer PU (polyurethane) make a significant improvement. They allow the doors themselves to act as door bars which boxes the open central tub structure.

7. Fender braces. Again, several brands on the market and they all seem to help reduce cowl shake. No real change in performance or handling balance that we have been able to detect but they do raise the resonant frequency of the chassis.

8. Rear chassis braces in trunk. Very small gains there.

9. Front shock tower brace (STB). These will reduce cowl shake a bit but not affect handling.

10. Subframe chassis braces. Looks cool, doesn’t affect handling much. Helps reduce NVH.

Of these mods, only 1-5 seem to affect torsional rigidity enough to require retuning suspension afterwards. Typically that’s just adjusting one sway bar or tweaking rear ride height a bit to get the pre-mod handling balance back. If you ever wonder why CSP autocross cars run chassis setups different than everyone else, they’re sustaining 1.7G cornering loads with zero reinforcements of any kind. Massive chassis twisting so they compensate (band-aid) with sway bars, spring rates, ride height and alignment.

Much Excite
The two components to the shakes in any car are lack of chassis stiffness (low resonant frequency) and something to start a vibration (or in engineering speak, “excite”). Your Miata doesn’t shake at all when it’s parked.  Here is a partial list of situations that can excite a vibration and cause a bad shake to start up. In order of how common the cause is:

1.  Tires out of round. No tire or wheel is perfectly round. Generally, higher quality tires have greater concentricity (roundness). New tires have a red (lightest point) and yellow (heaviest point) dot sticker on them. For a typical single valve wheel, it’s a good idea align the valve with the red dot. For 180° opposed dual valve wheels like the 6UL, this alignment can be skipped. However the high and low spots of any tire are not marked. New high quality wheels generally have concentricity under .030 or less than 1mm. In general, new wheels are far more balanced and round than new tires. If you have a wheel/tire that is causing a shake, it’s going to be caused by the tire 99% of the time. Road force balancing allows the tire installer to rotate the tire on the rim to find the position that causes the least vibration. A wheel  & tire can be terribly out of round but actually balance perfectly. Do not confuse balanced with round. When diagnosing a wheel tire combo, it’s vital to simply spin the combo slowly by hand on the balancer and look for lack of roundness. Look at the rim bead compared to tread. Again, 99% of the time, the wheel is round but the tire isn’t.

2. PPF height not set correctly. The PPF is a brace that connects the transmission to differential. Most folks do not know that the specific angle of the driveshaft relative to the trans and diff is critical to avoid driveshaft vibrations. These two angles should each be within 1-3° and within 1/2° of each other. Correct PPF adjustment achieves this. The diff pinion angle to driveshaft is fixed and not adjustable. The trans angle is, however. If the PPF height is not set to FSM (Factory Service Manual) specifications, the resulting angle mis-match can cause a severe driveshaft vibration.

3. Bent wheels, uneven tire wear.  It should go without saying that if your wheels are bent (track, autox) and tires worn unevenly (flat spots from spins or skids), you are going to find it impossible to get rid of the shakes.

4. Worn, poorly designed or incorrectly adjusted shocks. The high speed (high shaft speed, not road speed) portion of the shock valving must be within a certain window to damp high frequency oscillations. Too much high speed compression or rebound will make the car feel “jiggly” and also greatly reduce traction.  Too little can cause a particularly heavy wheel/tire combo to start oscillating uncontrollably.  Shocks generally don’t cause shakes though, so this is a last resort in diagnostics, not the first thing you check.