Manually adjusting shocks: F355, 550, 456, Mondial T

This likely applies mostly to the F355 crowd (due to popularity) but technically also applies to the 550, 456 and Mondial T as the shocks have the same active damping system....

I have a number of customers that have disabled the OE Ferrari/ Bilstein active damping system by at the least removing the red-top solenoid motors (and in some cases the computer, wiring, etc.) and manually adjusting the shocks by turning the little gear on the end of the shock rod. The following is my advice when choosing this tuning method:

• Don't do it

Why?

Primarily because of the nature of the system design, the fact that it's a little motor turning the damping control gear, that the system must be responsive in real-time or it's worthless, the gear rotation from full hard to full soft is less than 90°. To eyeball a setting and lock in place is then exceptionally difficult to match left and right side dampers.

The nature of the Bilstein design is that the gear attaches to a 4mm shaft concentric with the 14mm shock rod, turning a valve of sorts that controls "bleed" or hydraulic fluid bypassing the shim stack on the main shock piston. When set wide open a huge amount of flow is allowed past the main shim stack, hence the damper has a huge HUGE range of damping force available.

If you glance at the WSM, it has a simplified graphic display of damping logic, degrees rotation of the gear vs. road speed. The system also uses other inputs to determine gear position so ultimately the damping program logic graphically would look like a 3-D carpet of sorts akin to an ECU program. You'll notice that the gear never rotates to 90° and never to 0°.

I would say that due to the limiting factors of either hardware (sensors/ actuators) or software or both is the reason the damper is never full soft or full hard. Hence Ferrari specifies that Bilstein builds shocks that if run full hard (no bleed) the suspension is over-damped.

What does this mean?

If you run your suspension over damped it takes longer for the tire to recover from either a sudden input like a bump or a slower input like braking/ turning into a corner or accelerating out of a corner. If the tire's slow to recover simplistically stated it's spending too much time up in the air (literally or figuratively), you're missing out on maximizing contact patch time on the tarmac!

What to do?

Re-engage the active damping system. If you have a F355 Challenge make certain your shock ECU is the updated version distributed in 1997 iirc.

If you desire a change in damping this can be accomplished with a re-valve. Sure it's likely that the ECU can be altered but I don't know of anyone who's reverse engineered and reprogrammed....

If you want to just test what more damping will do then of course manually lock the adjuster and test, but know that under certain situations on track that damper setting may actually be slowing your lap times.

This is an exceptionally well thought out design, arguably state of the art in the 90's.

As an aside I'm working with Bilstein to adapt a manual control that will allow much greater resolution in the adjuster mechanism but it's a ways out before it will be available.

Best,
Rob

 

No, same size shock rod and tube, road car damper bolts right in place of Challenge and visa-versa. We recently had a F355C system in with one road car shock substituted likely during crash repair.

The internal valving is decidedly different.

The lower ride height on F355 Challenge is accomplished by a shorter spring package and location of the lower spring mount.

Technically yes this is true.

Good question. The fluid above vs. below only cares about the pressure differential, this is how a shock "creates" force, purely pressure difference above vs. below the main piston; where the piston is does not matter unless the shock has a physical application problem like the body or shaft is too long or something that would cause the piston to crash into the bottom of the damper tube. But this is also what the bump rubber is for, assuming the physical dimensions of the shock are chosen correctly.

Great questions, keep 'em coming!

Best,
Rob

 

Here's a force vs velocity screen shot, four dampers on full stiff, one set full soft.

Huge range and a great illustration of how powerful an effective bleed circuit can be.

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Best,
Rob

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Yes, it is simple but cannot be achieved with an unplugged actuator. You must lock the adjuster with a hose clamp or similar device; I used to use silicone RTV and let set overnight. Internal hydraulic flow will eventually move the adjuster to full soft.

The issue that by defeating the sophisticated active damping system is that you've defeated the sophisticated active damping system that Ferrari and Bilstein engineers spent a ton of development time on; plus active damping is actually outlawed in most racing rule books, so it is a desirable and arguably "unfair advantage" system.

Information I have found states response times of about 30-milliseconds.

And for those wondering about the Corvette vs Ferrari versions of this damping system, GM utilized a much simplified version to reduce cost and Ferrari went with the the whole enchilada using acceleration sensors in addition to just road speed input.

Best,
Rob

 

Thanks Greg!!

 

Hi Carl, good question; the quick answer is "it depends"...

• What's the application?
• Is this for competition?
• How is the car used?

For a street driven sporting car one can increase rates but I'll not say 50% higher or lower is okay in general. Once running the pertinent platform specifications through my calculator and having dyno data too then perhaps that large of an increase is just fine.

Here's something important to consider; damping philosophy has changed dramatically over the last thirty years, only within the last ten has there been a much more scientific data-driven approach to selecting spring and damping characteristics for both road cars and purpose built race cars.

Prior to about 2000 in my experience most sporting cars were over-damped; it feels great to the driver and often the engineers decided feel of the car outweighed other data. Many Ferraris are overdamped and undersprung compared to the way I specify springs and dampers today.

So for a Ferrari of the '90s you probably can "get away" with a big spring change and a Ferrari of the aughts you probably will degrade the ride quality with a large rate increase without touching the dampers.

You've asked a general question I've given a general answer; ultimately as you know one needs all the chassis data to make a proper selection or be willing to change out and test many different combinations (more expensive method).

For a competition car, yes, you must grind through the data first then tweak the dampers to fit the springs and any other mods like reduction in sprung and unsprung weight.

Best,
Rob

 

Did you get that vibration issue sorted?



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You described a severe vibration, above 100mph iirc, and I offered to test your Moton's as you thought perhaps it was suspension related....

 

Hi Stanislaus,

"How do they compare?"... For one they (Moton) will not change damping while you drive....

The advantage of changing from the OE Bilstein system to a Moton or other 2-way (independently adjusted bump and rebound) adjustable shock is

• Designed for user adjustment (vs. computer and solenoid actuator)
• User serviceable

That's the big advantage.

Similarities:

• Tunable/ revalvable

So why do guys switch? A shock designed for user serviceability is one reason, another frankly is cachet as dampers are very difficult to understand as to their workings, tuning, how-to, etc and new shinny parts must make the car faster.

There are other performance differences, machining tolerances can allow greater consistency shock-to-shock, detail differences in the design and functionality of the adjuster circuits, packaging, weight. Much of the detail differences effect properties like hysteresis (lag in response, especially while in transition from bump to rebound and visa-versa).

Biggest advantage is serviceability, the OE Bilsteins require a special machine to rebuild/ revalve or the installation of Schrader valves to restore nitrogen pressure.

The one caveat is that they must be valved correctly in the first place so that when making on-the-car adjustments, the change in damping characteristics actually changes the handling and or ride quality. Just ask Rob Lay about how he slowed down his Mazda racecar with shinny new expensive shocks.

Regarding why Ohlins doesn't offer a F355 application I'm sure they'd be happy to custom make a system for you. Most high-end damper manufacturers do just that. If you order Motons from me (or AST or Bilstein MDS) they'll be built to order. Nothing worse than inventory sitting for years until an order comes in...

Regarding Ohlins temperature compensating technology; you'll not benefit from it. A circle track guy will benefit most from that as there's such a fine line between running up front and falling back in the pack when it comes to chassis set up for the roundy-round crowd.

On one of your track days at TWS your shocks will go from ambient to 150°F in no more than two laps, likely less than one depending upon how many bumps you hit. Then it will stabilize at around 150 - 165°F during a 25 minute session as you have good airflow around the shocks.

Hope this helps.

Best,
Rob