NC1: search for comfort on bad roads

[matthew-m]

Andrew,

Yes, FCM helped fix some problems, but their only suggestion for fixing this particular problem was change either the front or rear springs to improve the "frequency". I still don't understand it, but as soon as I have some extra cash I may try 2 new springs... The problem is when will I have an extra $600 for springs and installation cost! It has only been 2 1/2 years...

I bought the car new at the end of 2006 and didn't replace the springs/shocks until June 2011. I don't remember when I decided the shocks were 'bad'; at the time of the coil-over install the car had 54000 miles.

The stock ride was merely OK, and I asked FCM to improve: comfort on California's broken freeways/rural roads; less dive/squat; less bottoming out on dips/bumps; and less body roll. There is less of everything on my list... I suspect new shocks and sway bars would have been a better place to start.

I have ridden in other modified Miatas and I know its possible to have the rear not beating me up on every bump. I just don't know how to fix it easily.

Sorry I hi-jacked your thread. You described the same symptoms I have been experiencing for the last 3+ years and I was hoping the forum had some (easy) suggestions. I look forward to your comments on the new parts.

[skeeler]

I have FCM coil-overs with 300 lb/in front and 224 lb/in rear springs. According to Shaikh, the front damping is at 0.58, 0.65 and 0.41 at 2/5/10. Rear is 0.48, 0.44 0.31. Tires are (very used) Michelin Pilot Super Sport 215/45/17 on Enkei 17x8 wheels.

I see that you have bounce frequencies of 1.79 and 1.72 Hz, a F/R ratio of 0.96. Many people, including Shaikh at Fat Cat would tell you that for best comfort, you want a "flat ride" setup with the rear a little higher than the front. That should help a bit. You can go to 250/224 springs for a ratio of 1.07 or 300/280 for a ratio of 1.07. That way, you only have to change one pair of springs.

You can get good springs for 50-75 dollars each, and installation isn't that bad if you have a jack, stands, a torque wrench, and a socket sets. Are you running the Tein 90- to 70-mm tapered spring in the rear? Those are starting to get hard to come by.

Before you start changing springs, let's consider your current setup. What sway bars are you running? Also, have you had the car aligned? What specs?

Given your power oversteer, I wonder if your "very used" tires also very old so that they aren't providing good grip. Can you borrow someone's wheels with new tires for a test drive?

[matthew-m]

Power over-steer occurs whether the tires are new or old, hot or cold. I've tried extreme summer tires and had over-steer. I've had 2 sets of tires since adding the coil-overs and all have exhibited over-steer.

I do not have the desire to change springs myself. I was under the impression I have Eibach springs, but I have no idea. I only planned to change one set of springs, though I never decided whether to go down on the front rate or up on the rear.

Sway bars are stock and the car was aligned and corner balanced. Specs are 1.5 neg all around and toe is 1/16" in front, 3/32" in rear, or they were 2 years and 25,000 miles ago.

[jboemler]

Before you go replacing things, you might try adding a half-degree of camber in the rear...

[monty11ez]

Adding more camber isn't going to fix the frequency. Skeeler's suggestion is right on point. You will either need more spring in front or less in the rear. Also I hear you can use the 100-70mm tapered springs as well. Brian also sells adapters for regular springs.

[skeeler]

I was under the impression I have Eibach springs, but I have no idea.

Just FYI, you most likely have Eibach 65-mm, 8-inch main springs in front and Tein 90- to 70-mm, 8-inch tapered springs main in the rear, You probably also have Hyperco helper springs front and rear.

I used Brian's delrin adaptors for straight springs, but I've recently switched to using offset adaptors 3D printed by auto-X-er to provide additional damper-to-spring clearance.

[skeeler]

Sway bars are stock and the car was aligned and corner balanced.

You might also need to adjust your sway bars. With your springs and sways, I estimate your front roll couple at 61% or 62%. For comparison, find my car pretty balanced at 71% using the same spreadsheet. So, that means you have 10 percentage points more of your roll stiffness coming from the rear than I do. That should give you some decent oversteer. The easiest thing to try is to remove the rear sway bar. That takes literally 10 minutes, and you can do it with 1 wrench and 1 ratchet without jacking up the car. (Running without the rear sway is not unusual.) That my solve or at least improve your situation. If it makes it somewhat better, then we know we are on the right track; you can then add a stiffer front sway and/or stiffer front springs.

Given that your bounce frequencies are a little out of whack, I'd suggest some combination of much stiffer front sway bar and either softer front spring or stiffer rear springs.

[skeeler]

Given that your bounce frequencies are a little out of whack, I'd suggest some combination of much stiffer front sway bar and either softer front spring or stiffer rear springs.

OK, having played with the numbers a bit, here are my suggestions:

If you want to go slightly softer than your current setup, then go to 250/224 springs and red dot/green dot sway bars.
If you want to go slightly stiffer, then go 300/280 and red dot front sway with no rear sway.

Does your car have an LSD? In other words, is the rear sway bar green dot or blue dot?

But like I said, try just pulling off the rear sway bar first, to see if that helps.

[matthew-m]

Skeeler,

Thank you for the suggestions. I will probably go to a softer front spring when I replace the rotors and pads. I'll give Brian a call when it is time to place an order...

I have no idea what you are describing in terms of the color dots on the sways. Yes, my car has the LSD that was included with the sport suspension option. I will go ahead and try dis-connecting the rear sway when I get new tires.

Matthew

[skeeler]

Mazda's OEM anti-roll (sway) bars have a spot of color painted on them to identify their rate, since it's hard to tell just by looking. RX-8 sway bars fit the NC, since it's built on an RX-8 platform. Here are the colors for the two cars:

Green front: NC soft top
Green rear: NC w/o LSD

Blue front: NC PRHT
Blue rear: NC w/LSD

Red: RX-8 with normal suspension
Yellow: RX-8 with Sport supsension
Orange: RX-8 R3