Stewart Development Re-valved NA Bilsteins – Revised
 

Intro

If you have the attention span of a lightning bolt, or have little interest in a very well balanced high quality shock for street driven NA Miatas on stock springs, I recommend you either bail now or skip to the end of these posts because this sucker is LONG. For the rest I hope the effort in reading is worth the reward.

Background

In an earlier thread I discussed a set of NA Bilstein shocks re-valved by Bernie Stewart, owner of Stewart Development, for my ’95 Miata. The good was this:

In many cases, such as roll control, transitional response, and feel at the limit, they’re absolutely brilliant.

The car is very predictable and easy to control at the limits.
Oversteer or understeer can be easily and confidently induced with steering or throttle inputs.

Roll control and transitional response are both excellent, and seemingly as good as or possibly better than the FCM Bilstein car which has roughly double the spring rates.

The bad was this:

The greatest opportunity for improvement is in ride quality. The chassis is in essentially constant motion over nearly all road surfaces, and over some particularly poor surfaces (Cataldo Road, Vacaville) this can be fatiguing and even punishing.

The chassis follows moderate to large road undulations too closely which causes the chassis to pitch in response. Pitching over rough large amplitude bumps can be so great that driver input (both throttle and steering) can be adversely affected to the degree that it becomes quite jerky. This is the shocks weakest point.

The inside front tire can be made to skip over road surface over tight bumpy corners. The inside wheel feels like it “hangs” above the road surface before re-contacting and regaining grip. The chassis and/or drivetrain sometimes feels like it “winds” up – an odd sensation not noticed before on the other cars and possibly attributed to jerky driver input resulting from large chassis pitch.

High-g cornering on moderately rough to rough corners can cause the steering wheel to saw back and forth as the inside tire gains and looses traction.

Occasionally lifts inside powered wheel when accelerating from a stop into a corner.

That’s the gist of it, but for those who want to read the entire thread, here it is:

https://www.miataturbo.net/suspension-brakes-drivetrain-49/stewart-development-re-valved-bilsteins-50874/

The shocks Bernie had sent me were pretty much a no-compromise autocross setup; brilliant on a smooth surface when sawing the steering wall left-right-left-right. I needed something different; something I could hammer on the rough wagon trails and goat paths that pass for back roads in Northern California. Certainly, and for my specific purposes, it seemed there was room for improvement. With Bernie’s encouragement, I sent the shocks back.

R&D

We talked several times, exchanged graphs, and came up with a plan. I send Bernie an email asking him to aim for specific damping rates at certain velocities, and to develop the rates up to those velocities as he thought best. In a nutshell, I asked for roughly double the bump damping and half the rebound damping of the original revalves. A couple weeks later Bernie wrote to tell me:

“I will be shipping your shocks tomorrow. I have put a lot of effort into these in hopes of as little compromise between street and track as possible. I am thinking that more and more people are going to being doing dual purpose with their cars in the future.

I think the shocks will have a lot of grip. The main part I was trying to achieve was to have the shocks "blow off" after the initial ramp. This will help keep the harshness out, but not give up body control.”

Along with the emails came a set of graphs from the newly revised shocks; the graphs are at the bottom of the post.

I went ahead a plotted the graphs for the front shocks on my spreadsheet for comparison with other commonly used Miata shocks. They’re a bit like the Koni’s Sports set at full hard early on, but they’re softer past 1.5 seconds and continue to be much softer further out. I found the Koni’s harsh at full hard, and I was hopeful that the early blow-off would address what I didn’t like about the Koni’s.

They’re also a bit stiffer than the NB OEM shocks – especially at low speeds – and this should be an improvement on criticisms I’ve heard about these as well. The NB OEM’s are regarded as quite comfortable and good overall, but I’ve also heard that they could use a bit more rebound damping. They’re geared more towards comfort than performance, but that’s not what I’m looking for – I’m after a better balance between the two.

Looking at the unloved NA HD Bilsteins, which are often felt to have too little bump and too much rebound damping, Bernie’s are “harder” up to 4 in/second and “softer” afterward. The NA HD’s are also not at all digressive on the rebound side, and only weakly digressive on the bump side. It doesn’t seem like anyone likes them very much, they’re reported to be very uncomfortable, and not outstanding with regard to performance either. It’s a mystery how Bilstein got the NA HD's so wrong, and an even bigger one that they haven’t corrected the problem.

Installation

The shocks arrived as expected and looked no different from when I’d sent them out weeks earlier. While Bernie was re-tuning them I took the opportunity to make a simple modification of Shaikh’s upper bushing to improve how it sits in the top of the upper shock mount, and to ensure it doesn’t become cocked sideways during either installation or afterwards. Most folks won't have a problem; me - I can screw anything up. The photos explain the mod.

With everything on hand, I reassembled the springs and perches over the shocks with a pair of suicide sticks, and put the assemblies on the car using the long-bolt method. I torqued the suspension hardware with all four wheels on the ground and got the same 12.75” front / 13.25” rear ride heights as earlier.

Evaluation

Late that evening I took my first test drive over my 75 mile test loop in the dark. Doing so relieved me of nearly all road traffic, forced me to keep speeds moderate, and left me to focus on the suspension instead of the scenery. In a couple of hours the loop took me over a dozen or so miles of recently paved freeway, forty or fifty miles of decent but nicely tight windy back roads, and the remaining fifteen or twenty miles were on broken up, busted up, and pretty rough but very windy one-and-a-half lane wide roads over several narrow passes. The pace was relaxed to brisk, and seven or eight raccoons scattered throughout the back road portion of my ride kept me alert.

As for the shocks, they were transformed. What had been harsh and unforgiving was now comfortable and controlled. Even Cataldo Road, which I had characterized with the original re-valves as “fatiguing” and “punishing”, was perfectly enjoyable and relaxed. The revised shocks are more comfortable than my Koni Sports set at full soft, but far better controlled as well. In short, the newly re-valved Bilsteins were comfortable enough for 500 mile days of brisk back road driving over nearly any type of road surface you’re likely to encounter. So far so good, but I had to wait for daylight to see how they’d perform at faster speeds.

The next morning I headed out for the same loop, only in the opposite direction. My impressions from the previous evening were confirmed, but I was also pleased that higher speed performance was nearly as sharp as the original set up. As before the car is very predictable and easy to control at the limits, and oversteer or understeer could be easily induced with steering or throttle inputs, but the tires skipping over tight bumpy corners and the steering wheel to sawing back and forth as the inside tire gained and lost traction were gone. So was the feeling that the inside wheel felt like it was “hanging” above the road surface before re-contacting and regaining grip, as well as the sensation that the chassis and/or drivetrain was “winding” up. Instead, the limit of traction was indicated by a fine-sandpaper-over-plastic hiss from the tires but absent the former dramatic and unwanted harsh feedback. If I gave up 10% on the response side of the equation over the original re-valves, I gained 50% on the traction side and 500% on the comfort side; a compromise very much to my liking.

Final Analysis and Comparison

As a sanity check I drove my other ‘95 with Shaikh’s 6k/k re-valved Bilstein coilovers over much of the same course the following day. I commute with a another ’94 Miata on Koni Sports – the same car I recently took on a 2,400 mile tour of the southwest deserts - and my wife’s ’95 turbo Miata has GR-2s.

Shaikh’s suspension is brilliant, but on a poor road there’s no escaping that it has double the stock spring rate. That it remains reasonably to quite comfortable on poor roads, and very comfortable on all others, is a testament to the performance potential of Bilsteins in general and Shaikh’s years of development in particular.

The Koni’s are perfectly adequate, but no more. They have the advantage that the rebound damping can be adjusted to accommodate different spring rates, but I find them unremarkable as far as comfort, control, and handling are concerned on stock springs. They might last forever, but I don’t look forward to driving on them for how they feel or perform.

The GR-2s are also adequate, and quite a bit cheaper than the Koni’s, but durability is a question. Certainly they show their limits earlier than the Koni’s, especially when the damping on the Koni’s are turned up a notch or two. I view these as a commuter shock for someone on a tight budget, and given those parameters the buyer will probably be satisfied. They’re not bad, they’re just not great.

I’d rate the revised Stewart Development Bilsteins for stock springs in the same sphere as Shaikh’s re-valved Bilsteins. Every non-adjustable shock is a compromise - Bernie’s, Shaikh’s, Bilstein’s - but both Bernie and Shaikh have gotten it completely right. They’re both excellent products, superbly balanced for comfort and performance, and I’d unhesitatingly recommend either. As they use the excellent and precise Bilstein mono-shock body, both of these shocks are also quite a step up from the Koni’s, GR-2s, and most every other popular shock in their price range. They’re also reported to be bullet proof, as well as easily and affordably rebuildable; another set of major pluses.

For a street driver on stock springs who’s contemplating replacing a pair of worn out shocks, who doesn’t want to spend the time, research, money, or comfort penalty of alternative spring rates, who loves their car and expects to keep it for a number of years, and who appreciates that Mazda got it very right with their original concept – but who wants the sporting side of that concept to be a bit more pronounced, these are the shock’s I’d recommend.

You can spend less, and you’ll get much less. You can spend more, but it will have to be a lot more (Ohlins or some other very high quality adjustable shock) if you want to exceed the performance of these. For performance they’re excellent; for value, right now I can’t see their equal.

Do you think the FCM's with double the spring rates have more grip in sweepers?
Have less grip in the rough stuff?

Very nice review. Thanks

I can't speak about high speed sweepers, Jason, because the roads around here are tight and narrow. Out by Lake Berryessa there are some lower speed sweepers (60 - 80 mph) and I don't think there would be a great deal of difference between the two as neither will be on the bump stops at sane road speeds and bigger camber changes on the stock springs will be largely made up by potentially greater compliance over bumps and potholes. On the race track, things will be very different as the stock springs won't keep the suspension off of the bump stops as well, and camber changes will be greater at any given cornering force, so there the advantage is to the stiffer sprung setup - but that's no big surprise.

As for the normal rough stuff, practically speaking there's not that great a difference as both sets of shocks do a terrific job of keeping the tires in contact with the road. It's likely that on really rough ground - say a rippled dirt road - the softer sprung vehicle will probably have an an advantage because it will tend to skip over the surface less.

I guess what I'm saying, and I'm a bit surprised at it myself, is that there is less difference between the two setups than I would have expected given the markedly different spring rates. The softer sprung setup has very similar levels of control, and the stiffer sprung set up has very similar levels of comfort. Not the same, exactly, but very similar; and in both cases both control and comfort are excellent. For driving really fast I prefer the stiffer setup; for driving pretty fast and really far, the softer; but either setup will do either job nearly as well.

Great writeup. I'm sending mine back for re-revalving when it gets cold. I have experienced some of the things that you mentioned. The most problematic was an undulating section of I65 through Birmingham. The car followed the dips so quickly that my ass was coming out of the seat. As I was coming down into the seat the car was coming back up. It was very unpleasant collision. I don't need to recap everything, your post did a great job, and I've experienced a lot of the same things you have. It's sounds like you guys found a way to make a good thing better. GOOD JOB!

When you send them back keep in mind that your FM springs will change things somewhat in comparison to my stock springs, and you'll probably want more rebound damping than I have. Let us know how it works out for you.

Jim

I would be more than happy to go over set-ups with anyone, my customer or not. What everyone needs to keep in mind, like Jim said, is the intent with the car. At first, just about everyone wants a race car for the street, with very few understanding what that means. Stiffer springs doesn't always mean a faster car. You can achieve much more with a softer rate, an adjustable sway bar and the right shocks, with little compromise.

I was going to do another revalve, but after my track session this weekend i am going to keep them the way they are. They are nice for daily driving, and i only had an issue when going over major road undulations on the highway. They performed extremely well this weekend. Bernie, you may be getting a call from my instructor Mark Burke. He was very impressed with the way my car felt. He's a spec miata that is running a little higher HP than spec now. I rode in his car and nearly crapped my pants. He has a stock miata that he is wants to improve the suspension.

Sounds like Bernie's getting things dialed in. Here's a quote from over at the other website:

"I spoke with Bernie a couple of different times concerning my application and desires. I'm running 500# front and 300# rear at 13"F and 13.5"R, 1.125" tubular front bar 11mm rear bar. I autox 5-10 weekends a year running r-comps and drive a minimum of 100 miles to each event towing a tire trailer. I wanted a reasonably comfortable street car while still being relatively crisp and flat while autoxing. I've been very happy with the results. I've set FTD at one event, second at another, and won my class or been at the top the majority events attended. I went up on spring rates with Bernie's re-valves and the car rides better on the street than running the 450/250 combo I had before. Broken pavement and pot holes are very noticeable but not harsh at all. My wife was impressed at the improvement in ride over rough rail crossings with Bernie's Bilsteins. Needless to say I was surprised.

The big thing that you have to remember is that our cars are a compromise. My car is a relatively fast street car that handles pretty well, not a race car. Overall I'm very happy."

Since the FCM custom adjustables have been mentioned in this thread, I'll pipe up - turning the damping down (we usually ship NA/NB Variant 2 setups at about 3 turns from full stiff) improves ride and grip on rougher roads. I run about 5 turns out on my '94R (currently 425/350 springs with 1.25" RB front bar). I like the stiffer damper settings (1-2 turns) for autocross to improve transitional response. On track, it depends upon what your preferences are but I'd suggest experimenting in the 3-5 T range.

Shaikh, nice to have your input and thanks for posting. We in the Miata world are very fortunate to have two custom shock tuners able to make shocks capable of winning not only races, but championships.

Jim

The settings on my adjustable FCM's were so spot on as delivered I've never felt the need to adjust it, either for autox (where it was noticeably easier to drive than the Tein Flex with the roughly the same 6f/5r kg spring rates), or for bumpy backroads. Granted I never experimented... FWIW

My pleasure to help, Jim!

I remember Jason's comments to me when he got his new FCMs installed. 'Spot on' is high praise indeed ;) He's helped product development so much as have many engineers and racers I've known, learned from and worked with. Jason, you may consider adding damping for autocross, and going softer on Mt. Hamilton or bumpier roads.

The spring issue is a constant source of debate. One of these days soon I've got to calculate how much spring is required to keep the shocks off of the bump stops at 1.3 g's with a little bit more for road flaws.

The moment from lateral acceleration doesn't all act on the springs and sways. Some of the forces go through the A arms and not the springs/sways. i.e. its moment arm isn't from the CG to the ground, it's from the CG to the roll center (smaller). The more the car is lowered, the greater this moment arm is.

Thank's Jason. That's the correct way to do it but it means effort taken from a different problem. What do you think about the very conservative assumption that at maximum lateral acceleration all of the vehicle weight is on the outer two wheels. Yeah, I know; it's wildly conservative - but it sure simplifies the calculations.