[NA] Search for the Holy Grail (2024)

jimbonnie

Join Date: Apr 2008

Location: North Bay, California

Posts: 4,974

In this tale the Holy Grail is a superbly responsive, perfectly balanced, fully composed, and unreservedly comfortable road-going Miata roadster. Neither the autocross nor the racetrack are a part of this quest, so try to resist any temptation to think along those lines as you read along. With that in mind, I was fortunate enough to have stumbled across a completely stock and unmolested 14k mile ’95 red and tan with leather package car a year or so ago. From my experience and experimenting with two other ‘95’s I knew I wanted to fine tune and improve upon what I liked, and to tune out what I didn’t. I’m searching for the perfect back road Miata roadster, and nothing less will do.

How is it set up?

Front anti-sway bar is a Racing Beat 15/16” solid;
Rear anti-sway bar is stock or removed;
Front springs are 336 lbs/in (6 kg/mm) 8” long Swifts;
Rear springs are 224 lbs/in (4 kg/mm) 9” long Swifts;
Shocks are FCM re-valved Bilsteins with FCM modified NB tophats and coil over bottom perches;
Bump stops are FCM 36 mm front and rear.

Steering rack is de-powered with the rack piston removed and the pinion shaft welded.

Camber is -1.2 degrees at all four corners, front caster is 4.0 degrees, front toe is 0.0, rear toe is 1/32” in at each side - or 1/16” total in.

Front ride height is 320 mm (12.6”), rear ride height is 330 mm (13.0”) with a full tank. Tie rods ends are LE/R-package type.

Wheels are stock semi-hollow alloys, tires are 185/60R14 Riken Raptors. I didn’t spec the Rikens; they came new with the car when I purchased it.

How does it work?

Getting the obvious out of the way, the tires aren’t up to the task. To butcher a quote from Churchill, they’re, “a modest little tire, with much to be modest about.” The only thing good I can think to say about them is that they could be worse. Unfortunately they color much of the results - and not in a good way - but I promised Shaikh I’d give him some feedback on his shocks weeks ago, so onward.

Springs, swaybars, and roll couple

The Swift springs were chosen to reduce the stock body roll, to provide good compliance over rough roads, and to maintain ride comfort. More importantly, the spring rates were chosen to work with the sway bars to give nearly perfect front to rear grip that could be adjusted easily by the inclusion or deletion of the rear anti-sway bar.

The calculated front roll couple (FRC) is 58.5% with the rear anti-sway bar installed, and 61.0% with it removed. For comparison sake these values are quite close to stock. Traction fore and aft with the rear bar installed tends a bit towards oversteer. The degree of oversteer isn’t at all unsafe, but it is a bit more than necessary on tighter corners, and it may be a determent to best speed in the faster sweepers. In any case, it’s a tendency which should be easily and entirely mitigated by the removal of the rear bar. Ultimately I’m looking for near neutral handling with just a hint of oversteer, but I chose the FRC I did because I want to approach my goal from the over-steer side rather than the under-steer side of balance.

The calculated roll stiffness is just over twice that of the stock ’95 Miata, and the difference is obvious. At the limits of traction - and this is a significant caveat - body roll is essentially non-existent and difficult to differentiate from lean caused by road camber. What cornering roll will be like with high performance summer tires remains to be seen, but I’m positive it will not be an issue on a street driven car. In fact, a bit more roll in exchange for incremental increases in compliance, comfort, and traction would be a desirable compromise.

Shocks

Let’s get the easy part out of the way first. With the FCM NB tophats here is no discernable NVH from the shock mounts. The suspension is eerily quiet and fuss free in that regard and so this is end of the NVH story.

Concerning the FCM re-valved Bilsteins, even with double the spring rate of my other ’95, which is equipped with stock springs, Koni Sports, and FCM NB top hats and bump stops, wheel compliance over small and large bumps is significantly better and damping is far superior. Ride comfort over both small and large bumps is also noticeably better, but occasionally, and just occasionally, I encounter a kind of road with just the wrong undulation frequency that produces just a hint of fore and aft pitch. It’s more of a curiosity than an annoyance, but it’s there. I attribute this occasionally encountered pitch oscillation to the bounce frequency and the bounce frequency ratio of the springs, rather than to any deficiency of the shocks.

As for the bump stops, I’m certain I must be encountering them intermittently, but I sure can’t remember having ever felt them, and it’s not through a lack of trying. This is probably the most obvious difference between the Bilsteins and the Koni Sports. It’s not that the Koni’s are poor in this regard; it’s just that the re-valved Bilsteins are outstanding. The sensation of bottoming-out is for all practical purposes a thing of the past; and good riddance.

Total traction remains in question because of the truly mediocre Riken Raptors. There is that occasional bump in the occasional corner in which I think I can feel the inside front tire break traction before the others - and perhaps a bit too early - but again, what can I reasonably expect from the present tires? I’m confident the FCM Bilsteins are doing their job and that with lighter wheels and good rubber traction should be outstanding; but I just can’t report so at this time.

De-powered Steering and alignment

Coupled with zero front toe and minimal rear toe (1/16” total), response is accurate, instantaneous, and entirely free of any discernable play. Even the smallest possible steering inputs result in a commensurate response from the front end, and larger inputs place the nose of the car just where you’d want it, right now. You might not want this much liveliness out of a commuter, or need it in central Nebraska, but it’s perfect for back road scratching here in northern California.

Steering effort with the de-powered rack at 4.0 degrees camber and the medium traction tires is lighter than expected. Quite a bit lighter in fact, and a bit more effort and feedback would be desirable. Driving moderately tight corners with one hand on the wheel and the other on the shifter is quite comfortable, though I do grip with both hands through the tighter sections where fast corrections might be necessary. Stickier tires should ultimately give me the feedback I’d like, but for now I’m enjoying the near power-steering ease.

The search for the Holy Grail

Isn’t quite over. Most of what I’m seeking will probably be found in a set of 15x7 +41mm offset Enkei RPF-1 wheels and UHP rubber, but there’s still more to be had beyond that.

Steering response with the de-powered rack and the existing alignment seems perfect, but I did think once or twice a few weeks ago that I may have detected just a tiny hint of bump steer. It’s probably a fabrication entirely in my head because I’m conscious my ride height is slightly lower than the LE/R tie rods ends are designed to accommodate, and I should therefore have a bit of bump steer, but real or imagined I’ll probably go 5 mm to 10 mm higher at all four corners just to be sure it’s not an issue, and to gain that extra added bit of available bump travel. Besides, with the present fashion to go lower I feel it’s almost my duty to go higher.

With body roll more than under control I also believe I can get away with slightly softer spring rates to help address the occasional fore and aft pitch. I’m thinking that the MazdaSpeed springs (215 lbs/in front, 158 lbs/in rear) might be a step in the right direction. A bit more traction and compliance on truly awful road surfaces might be an additional added benefit of the slightly softer springs.

As for the shocks, and despite not having been able to give the FCM re-valved Bilsteins a truly fair and thorough evaluation because of the limited tires, I am 100% completely satisfied with both the shocks and my dealings with Shaikh. The re-valved Bilstein’s perform brilliantly and they do everything Shaikh said they would. There’s no downside; no, “I wish they were softer/stiffer/more comfortable/more controlled.” Yes, I could have paid less, but I also could have gotten less. While some others may be completely satisfied with re-valved Bilsteins from Bilstein or others, a significant number seem to feel they’ve not gotten everything they had hoped for, either in performance or service. I’ve never heard this sentiment expressed about FMC Bilsteins, and I’ve none to report myself.

As for Shaikh, he’s a gentleman and a scholar and deserving of all the praise others have given him. I have no reservations whatsoever about asking him for help with my next project and he’ll likely hear from me sooner than he’d expect.

In the interests of full disclosure I’ve not driven or ridden in any Miata’s equipped with Keith’s (Flying Miata) AFCO’s or Emilio’s (949Racing) XIDA’s shocks; nor have I experienced Ohlins or Penske shocks; but I’d sure love to try them all. I’ve no doubt Keith’s and Emilio’s products are universally excellent, and I would certainly and unhesitatingly include them in consideration for any future products.

I’ll update this thread as I’ve put more miles on the car, and after I’ve chucked it into a couple of hairpins with the rear sway bar removed, but for now, this is it.

Happy New Years to each and every one of you.

Jim