Roughly two months ago I ordered a set of re-valved shocks from Bernie at Stewart Development (SD). I decided upon the SD shocks because Bilsteins are a mono tube shock of excellent quality, because Bernie happily provides shock velocity/force charts with all of his shocks, they’re several hundred dollars less than other similar alternatives, I was curious about how his shock would compare with the Koni Sports, FCM re-valved Bilsteins, and GR-2s I have on the other family Miatas, and Bernie doesn’t add a Shraeder valve to fill the nitrogen reservoir below the fluid chamber.

Of those reasons only the lack of a Shraeder valve needs explanation. If there’s no Shraeder valve it can’t be knocked off and it can’t leak. The contrary argument is that lots of top shelf multi-adjustable shocks have Shraeder valves so that they can be quickly and easily disassembled for revalving and re-building; that’s a big plus if diving into the guts of your shocks is at all important to you. Because I’m not planning on revalving or rebuilding these shocks I’ve opted to embrace the KISS principal of engineering which follows the precept, “Keep it simple, stupid”.

I received the shocks on the day Bernie said I would, which cost him a bundle in FedEx shipping costs. The shocks came individually packaged in their original boxes and beyond the SD and placement location decals, the very slight tool marks where the shocks were clamped for disassembly, and the dyno graphs in the invoice envelope, there is no external evidence that they’re anything other than off-the-shelf NA HD Bilsteins.

Looking over the dyno graph the first thing that jumped to mind was, “dang; that’s a lot of rebound damping for stock springs”. Here’s one of the graphs Bernie provided:

-oops; can't figure out how to put the graph here, so it's down below.

Here’s another one of Bernie’s graphs showing my shock compared to a spec Miata shock: - down below too...

And here’s a graph I sent to Bernie showing his shock compared to other popular Miata shocks: - yep, down below.

With images of the shocks jacked down tight onto the bump stops in my mind I sent out an email to Bernie about my concerns and he responded that that’s how he sets up all the shocks with stock springs, they’ll be fine, and if I’m not completely satisfied he’d rework them. With those assurances, on the car they went.

 

A few words about the car:

It’s a ’95 with 28k miles, it has a Torsen, de-powered steering rack, Racing Beat 15/16” front bar, Mazdaspeed 14mm rear bar, stock springs all around, FCM tophats and bumpstops (36/46), 11 year old tires, and the ride height is 12.75” front, 13.5” rear.

After installing the shocks, and putting a few hundred miles on the car, here’s an updated version of what I wrote Bernie:

I’m surprised by how much the shocks have “broken in” over the past several hundred miles. They went from so stiff that control over rough roads was quite compromised, to a good to very good combination of handling and compliance. Impressions over the first few dozen miles are quite different from impressions garnered over the last hundred or so.

In many cases, such as roll control, transitional response, and feel at the limit, they’re absolutely brilliant. For most other driving, including droning down the highway or polking around the back roads, they’re reasonably good with room for improvement.

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.

 

Here are some additional notes I sent, and haven’t sent, to Bernie:

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. Not noticed on the FCM or Koni equipped cars.

The chassis doesn’t appear to jack down onto the bump stops except very rarely and only in corners with bumps that would be challenging to any shocks or chassis. In consequence the shocks aren’t harsh though the excessive pitching fore and aft is both tiring and detrimental to control.

The car is very predictable and easy to control at the limits. This is the shocks strongest point. In this regard it is better than the Koni Sports at either extreme of adjustment, and seemingly equal to the FCM Bilstein car.

Oversteer or understeer can be easily and confidently induced with steering or throttle inputs. I noticed no tendency to snap-oversteer despite attempts to induce oversteer by chopping the throttle in tight corners at the limit of traction.

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.

While I’m surprised that they shocks work as well as they do with as much rebound damping as they have, they may have just a bit too much. The pitching I’ve noticed (about 1.5 to 2 Hz.) can be duplicated on the Koni Sports equipped car by setting the rebound damping to its maximum setting. And while less rebound damping will reduce pitch motion, it should also improve grip on the rougher surfaces where the inside tires can’t drop to the road surface fast enough, and where the slow rebound causes the chassis to drop to meet the road a bit more than it should or than is optimum for either ride or overall traction.

 

Conclusion:

It would be easy enough to get the impression that the shocks don’t work. Actually, they work quite well and many who have them are reportedly very happy with them. In their present form they might be the hot setup for auto crossing or road racing on a smoother surface, I’ll leave that to others far better qualified to judge.

My criticisms are based on hammering the chassis on relatively rough back roads, combined with my certain knowledge that with regard to handling and comfort I can have my cake and eat it too.

On a rough back road comfort and handling aren’t mutually exclusive; they’re complementary as compliance is one of the key requirements for traction. Another key element is feel; and this the SD Bilsteins have in spades.

The trick is to improve the compliance without sacrificing too much of the feel. Bernie’s got some ideas, and in a few weeks we’ll see how they work.

 

Wow, excellent post. I wish I had that kind of experience to write such a good review.

This sounds like it explains why my car feels like it just wants to go faster and faster around corners. It feels so good as it approaches the limits.

Yet, my wife describes a section of bridge she crosses as "boingy" I admit my shock knowledge is minimal. Good job working with Bernie to help get us a nicely dialed revalve. If I increase spring rates, then I will definitely get them revalved.

 

The SD shocks do work better the faster you go; I think that's a reflection of Bernie's experience with competition cars and Penske shocks. Unfortunately, on the street, you can't always drive that way.

Boingy is a good description of how these shocks work on certain roads. If you increase spring rates you'll likely find the shocks work better and are less Boingy, as the springs will do a better job of extending the shocks to conform with the road surface.

Thanks for setting up the group buy, Djp; I wouldn't have this project if it weren't for your efforts.

Jim

 

Boingy. That's the perfect description of how I feel about my Tein RA's - the earlier monotube. Constant motion. I'll be watching this closely. I'm hoping Bernie can nail down dual use (street/track) shock valving. I'll be buying when he does.

 

Great stuff. Glad Bernie is still working on these, and glad someone is providing him with some really detailed feedback on how to improve the revalves for dual-use.

 

It seems highly re-bound damped performance shocks are the norm, not the exception. If I were to guess at a cause it's because the press, and then the customer, thinks he needs a "racing" shock because it sounds cooler than a "comfort" shock. The builder then gives us what we want; or at least what we think we want.

I also think it's a phenomena related to McPherson struts and their weird camber changes as the suspension moves. Anything to keep camber in control - be it springs, shocks, or sway bars - helps keep the handling from going to crap in a hard corner. In this case, the stiffer the part the better.

Fortunately for us Miatas have fantastic suspensions straight from the factory. If we don't screw them up by slamming, tucking, poking, stretching or otherwise undoing all the good the engineers at Mazda built into them, a good set of shocks that keep the tires firmly planted over a variety of challenging surfaces at speed is almost all they need. That's what I'm aiming for and I've got high hopes it will work out.

 

Thanks for taking the to provide this feedback. I think it is exactly what Bernie needed. I think most of us have such limited experience that we just aren't able to describe what's going on. My SD revalved bilsteins are sooo much better than the AGX shocks they replaced, that I'm thrilled with the revalves. I kept my FM springs because I wanted comfort around town.

 

I'm pretty sure I'm one of the least experienced drivers on this website, but I'm happy to give it a shot. Bernie's good to work with, and he's interested in what we all have to say, so we should end up with a great shock.

 

Thanks for this thread.

One of my projects after this season will be to get my suspension up to a higher level for autocross. I currently have FCMs with 550/350 springs. I need higher spring rates. I'll probably end up with something in the neighborhood of 750/425-475. So, I'll need a revalve.

My decision will revolve around where to send them off to get the revalve. If Bernie is around here, can he comment on whether he can provide a no-compromises revalve for those heavier spring rates? What would those dyno plots look like?

This is going to be an important upgrade, and I need it done right. I'll probably end up interviewing several parties to choose who will do the shocks.

 

seems to me the super high rebound is just a "Band-aid" way to increase spring rates when cornering...I spent a lot of time making my setup not "boingy" when I DD, nor does the rear feel like it's going to launch you from the car when you hit bumps.

 

I have Bilstein HD on stock springs now and its amazing, no more boing and very little airborne action. I think my daily handles better than my track car.

 

^+1 Brain, it is an autocross stock class trick to increase rebound rates to sky-high territory to jack down the suspension. For those of us that can use appropriate spring rates to begin with, this is not the way to go fast.

 

Do you mean "increase rebound damping" or "increase rebound speed"?

 

acutally, looking again, the low-speed looks to be inline with all the other shocks on the graph with the exception of increased bump. fwiw, I have koni sports with 400/250# springs and I leave them at full soft...

 

No-compromise race re-valves are what Bernie does; I think he's mostly a Penske ($$$$) tuner. This foray into compromised street Miatas is something relatively new for him, which is perhaps why we're sorting out bugs.

Certainly both Bernie and Shaikh are worth talking to, as are Emilio, Keith, and perhaps a few others. Hard to go wrong with that crowd.

 

If not the main reason, it's certainly one of the more important ones.

And it works too, but taken too far you loose grip over bumps.

 

Hustler, are we talking NA or NB Bilsteins?

On a back road I'm not surprised at all. Shaikh pretty much said the same thing, though I think he might have been talking about autocrossing:

"Surprisingly, the grip of the ’93LE with stock springs was higher than the ’91[450/350 springs]! The effect of spring rate reducing ultimate grip started to become apparent."

And here's some more Shakh wisdom:

"Three Key words: GRIP is KING! A fast suspension needs to have some compliance, otherwise grip is reduced. This means it also needs to be somewhat comfortable for the driver to have confidence. Thus, ultimate grip is tied to having a degree of compliance (ie comfort). Therefore, handling and comfort MUST co-exist!"