Shock tech related thread

[Piledriver]

Decided to start a shock related thread.

Heres a compilation of notes on Bilstein valving and part#s//valving numbers, pulled from Bilsteins tech manuals and elsewhere online


Understanding Bilstein Valve Ratings (OLD STYLE)
Damping forces of Bilstein valvings for Off-Road are measured in Newtons at a velocity of 0.52 meters/seconds (approximately 20 inches/second). The ratings shown correspond to those measurements; rebound force is the first number, followed by compression force (rebound / compression). Conventionally, the ratings are written as one tenth the damping force in Newtons.
EXAMPLE: Valve rating: 275 / 78
Rebound force is 2750 Newtons at 0.52 m/s


Compression force is 780 Newtons at 0.52 m/s
Higher numbers mean higher (firmer) damping forces. For example, 360/80 has more control (is firmer) that 275/78, while 170/60 has less control (is softer) than 275/78.
For valving recommendations please refer to the Valving Guide.

S6G-1030 80/160 lbs converts to 355/711 [email protected]/s (~20in/second)

From eshocks.com
Recommendations for 7100 series T1 baja
Sport F 180/75 rear 360/180
prerun 180/75 550/240 (/2 shock rear equivalent)

Bilstein will machine the coil-over groove for $10 per shock: http://www.bilsteinus.com/services.php
1" from end of shock body, 0.060" WIDE, 0.025" DEEP.
...................
To help you better understand Bilstein's (newer) Numbering System, here is an example:
(I guess Bilstein gave up on SI units?)

S6G-1030=
"S" = Steel Body (grooved for snap ring)
"6" = Stroke in inches
"G" = 36mm (Small Body) Diameter
"10" = Rebound Dampening (Out)
"30" = Compression Dampening (In)

Note "90" is still only 2660 N so nowhere near the usual rebound force of common T1 rear off-road shocks.

Code: Select all

 
New code        rebound			compression
                NEW lb @20 ips     OLD N/10 @.52m/s (~20 ips)
10                     80                            36
20                   120                            53
30                   160                            72
35                   185                            82
40                   210                            93
45                   235                          105
50	               260	                       116
55	               290	                       129
60	               320	                       142
65	               350	                       156
70	               380	                       169
80	               480	                       214
90	               600	                       266
 

An interesting suspension calculator for determining damping needs, at 1ips anyway
http://farnorthracing.com/autocross_secrets16.html#

[Piledriver]

Inerters--- (AKA inertial dampers or "J-shocks" with units in "zogs" if you are McLaren)
The (no-longer) secret weapon of F1?
Probably coming to a production car under your ass soon, as well as a million industrial applications.

Both the older mechanical/flywheel and new hydraulic inerters are diagrammed in the following:
https://scarbsf1.wordpress.com/2011/11/ ... d-inerter/

As to the newer liquid inerters, I look at it and see it built as a bypass shock with various optional length/dia coils for the inerter mass flow tubing, thus easily tunable and possibly also is the shock. There's no reason you can't use hydraulic fluid if you can get sufficient mass, and a secondary nitrogen pressure package could only make it work better.
You could even use multiple bypasses/coil lengths and sizes for position sensitivity.

[PhillipM]

The trouble with home-tuning inerters - especially hydraulic units - is they are very sensitive to tyre stiffness and terrain - you can easily do more harm than good with one, I've been trying to get one to work on mine for years but it always ends up hurting more times than it helps. And they make a mess when they get out of phase with whatever terrain you're going over, to the point where bolts start snapping or your inerter tubing blows oil everywhere. My latest iteration has an overpressure valve but that leads to it doing some odd things on the threshold instead.

[ChadH]

I don't follow F1 that closely, so the inerter is new to me.

From what I gather, it's sort of a replacement to unsprung weight? But tune-able to counter the resonance of the tires and suspension bits?

[PhillipM]

It's basically the mechanical equivalent of a capacitor.

[raygreenwood]

HA!....HA!....(I will explain this response later)..... understand....after you wade through all of that (very good article by the way..thanks Piledriver!).....in this new fluid inerter there is nothing spinning (unless I lost it in there somewhere).

It is using a circuitous/spiral fluid path from one cell to the other....the length and drag of that fluid path and the spiral motion (which equals timing and time delay)....makes for a self tuning shock absorber with an infinitely variable valving system.

I deal with fluid rheology and shear every day in my business. The choice of Mercury as a working fluid is primarily because....like say...solder pastes (about 90-95% solids) and say a silver conductive ink made just for solar cell printing (about 85% solids, 5% solvent and 10% binder)....which exhibit virtually no shear thinning or thixotropy...meaning their fluid dynamics are nearly 1:1 when pressure is added......having a liquid metal that does not get thinner or thicker as pressure/stress increases or decreases is key to getting exact tuning.....by only altering length or diameter (or both) of the fluid snake between chambers.

Actually...they could have probably used bismuth or indium alloy for far less toxicity risk.

BUT....HA!....this exact same methodology...is actually used to a narrow extent...inside of the original shocks/dampers used on the oil bath style VW type 4 (411/412) and super beetle struts.

I did a fluid flow path analysis of them about 15 years ago and made some cheesy but passable diagrams I will dig out. The fluid flow inside of those original 1968 strut units...was nothing like what is inside of most modern strut units.
They used a standard poppit/flap valve with orifice and spring om the piston head...and the similarity ended there. The strut chamber was divided into two chambers as most are....inside the shock body....but the THIRD chamber that all other struts and shocks do not have...was the strut housing itself and the oil bath reservoir inside.

While the piston valving...like normal..regulates how fast the fluid transfers from one side of the piston...from one chamber...to the other side of the piston...to the other chamber.....the top chamber where fluid went to during the compression stroke had a circuitous orifice style relief valve body that returned fluid to the outer cavity or resorvoir when it reached a certain volume....which limited the speed of pressure loading on the compression stroke....and acted as a moderator and speed brake on the rebound stroke...and acted along with a second orifice set on the other end to moderate the replenishment rate of the compression chamber on the rebound stroke.

Very complex and very long lasting...and used about 90% of all of the fluid dynamics of that fluid inerter....except for the metal fluid....which is a big game changer. Ray

[Piledriver]

I got from the article that the fluid mass flow through the tubing gives a response like an inertial load.
The high specific gravity mercury requires far less volume to get the job done, but it was pretty clear most anything could work to some extent.(water and hydraulic fluid were mentioned IIRC)

It also sounds like Phillips units use oil as the working fluid.
(sounds like he has access to a MUCH better quality "used parts" bin than most, perhaps out back of the McLaren shops )

I don't know if its critical to be in a helical passage to work, or just to be compact, they mentioned something about initially discovering the effect while working on the long F/R anti dive/squat coupling tubing setup in use by some (all?) teams.

[Piledriver]

Exquisite thread/article on pirate4x4 on suspension design.
Is not just about rock crawling.
http://www.pirate4x4.com/tech/billavist ... rs/Part_1/

[PhillipM]

Exactly Piledriver - it's nothing to do with a twin tube shocks external res. and control valve, that's normal tech, it's using the inertia of the fluid column to absorb and release energy, and the reason for using mercury is to get the largest effect in the smallest volume, it's all about the density - the helically coiled tube is for the same reasoning - you can package it in the same place you'd put a normal heave damper and spring.

Mine use Pro RSF 2.5wt because obviously mercury is not what you want to be using at home - plus I'd get a policeman at the door after order a couple of litres from ebay - but the result is you need a larger and longer tubing for the same effect and then you get compliance issues creeping in - mind, suspension installation stiffness is still a much bigger factor at the minute.
One of the more interesting possible uses for us off road is you can potentially run softer valving and springing, let the car hit the bumpstops and use the inerter to prevent any kickoff, and also use it to absorb energy from a large landing and have it released over a longer timespan so your softer damping rates can still cope with it - it seems a little more promising route than trying to tune it conventionally for tyre contact patch optimisation, although I'm going to keep trying on that front too, as it requires far lower loadings, but as per earlier, comes with it's own resonance issues.
I'll keep breaking things until I get there

[raygreenwood]

Exactly Piledriver - it's nothing to do with a twin tube shocks external res. and control valve, that's normal tech, it's using the inertia of the fluid column to absorb and release energy, and the reason for using mercury is to get the largest effect in the smallest volume, it's all about the density - the helically coiled tube is for the same reasoning - you can package it in the same place you'd put a normal heave damper and spring.

Mine use Pro RSF 2.5wt because obviously mercury is not what you want to be using at home - plus I'd get a policeman at the door after order a couple of litres from ebay - but the result is you need a larger and longer tubing for the same effect and then you get compliance issues creeping in - mind, suspension installation stiffness is still a much bigger factor at the minute.
One of the more interesting possible uses for us off road is you can potentially run softer valving and springing, let the car hit the bumpstops and use the inerter to prevent any kickoff, and also use it to absorb energy from a large landing and have it released over a longer timespan so your softer damping rates can still cope with it - it seems a little more promising route than trying to tune it conventionally for tyre contact patch optimisation, although I'm going to keep trying on that front too, as it requires far lower loadings, but as per earlier, comes with it's own resonance issues.
I'll keep breaking things until I get there

Yes...the mercury definitely has higher mass and max density....and the rotational spin through the spiral...useful inertial effect....and as you noted....the spiral is the easiest way to increase the mass being worked in the smallest space.

However there will be no opportunity for "stored energy" at all in that diagram...unless its loaded against something that stores energy...i.e. a hysdrostatic space (pressure head of air/nitrogen etc.) a spring or even the rubber bumpers. Elasticity at some level is required to store energy...unless this unit is vertical and using gravity and the weight/mass of the mercury to create pressure. But that would be slow and require huge weight.

This is a tuned damper...moving fluid from one chamber to another...through aan orifice that is using....an uncommon tuning method. The orifice does not use diameter...alone. It does not use length...alone.

Where inertia comes in....from what I can see.....using a fluid with "0" shear thinning or thickening characteristics.....Mercury...which is an almost perfect Newtonian fluid.....is that BECAUSE of the spiral shape/path of the orifice....the fluid as it progresses through the spiral and picks up speed.....will attempt to spread outward....rdially.... due to centrifugal force.
With a normal ...slightly non-Newtonian fluid, this would cause fluid shear (somewhat) heating and some expansion. Not with Mercury or bismuth/indium alloy.

With the mercury the harder the pressure that is applied to it...the faster it moves through the spiral. The faster it moves....the faster the spin rate through the spiral....the faster it moves in a spiral...the higher the centrifugal force. The centrifugal increases the wall pressure of the Mercury on the outward side of the spiral bore.....that increases drag...greatly....acting as a damper to decrease speed of flow and decrease draining of fluid from the side under pressure.

High speed fluctuation from side to side caused by the rod actuated central mounting they show increases operating leverage greatly....meaning that this system works at a really high efficiency. The heavy mass of the mercury fluid means that that as pressure is arrested in one direction and applied in the other by this cylinder design....the change in direction of the mass of fluid is an operative part of the tuning. Large energy waves crashing into each other from opposite directions is a big exchange of energy.

By the way....the strut damper I described in my last post...is NOT a standard twin tube design with a reservoir and control valve... and has little in common with anything in use. Its a single tube with an integral reservoir...yes ...has a lower control orifice like a normal unit, and a standard poppit valve...but uses a separate fluid timing circuit/valve...as the main control.... based around fluid shear and drag...not orifice diameters or spring tension (which made it almost infinitely variable within the middle stroke range)....and NOT controlled by flap valves and springs....which are normal damper technology.

It was also hard to control long term. Any minuscule loss in fluid volume changed the tuning. Using anything but the exact fluid it was tuned for.....would either produce a totally different valving range (at best) or literally destroy the assembly within just a few miles from over pressure. This is why all the way back in 1968....the dealers would not repair, refill or service these units....only replace as is...a complete unit that came with its own new oil fill.

If you disassembled it.....and got the fluid snake valve or seal bushing out of position my even .005"....it would destroy itself in short order.
Amazingly designed and built units for just something as mundane as strut damper. Kind of stupidly expensive as well.

The valve head worked using much of the same timing/drag principles as the unit we are speaking of here except for the inertial spin....and thats because it uses a Non-Newtonian fluid.

The inertial spin they are describing in this article ...through a long snake under pressure...can only really work with a Newtonian fluid....in snakes that short. You can do something similar is you have a really long snake and a lot of fluid inside.....and that also depends on the rheology of the fluid. Ray

[raygreenwood]

Oh....man....I just caught it. I was thinking about the spiral so much as a fluid path.....I totally missed the shape.

It IS inertial.
So lets call left side chamber #1 and right side chamber #2. Yes.....the spriral passage is a method of providing length, drag and delay......but the mass is so high on this material....that once say
....the mass of mercury in chamber 1 starts moving to the right.....the spiral passage slows it down (a little like a standard orifice in function) and keeps this movement......that has inertia.... moving longer.........if any counter force is added to the other side, on the right..... it must overcome the directional enertia of the mercury coming from the left.

Not stored energy. ....but delayed energy that has inertia to overcome in order to allow the mercury to move the other direction. As was noted.....the spiral is just a good efficent use of space.

I understand the inertial force......though the radial drag and compression I mentioned earlier would also come into play. The math has got to be ugly. Ray

[PhillipM]

Bingo, just like a capacitor. Absorbs the peak energy and dissipates it when the input drops, using the inertia of the fluid column.

Yes, the math is ugly, so much of it you have to assume to be true or 'close enough' because the compliance and compressability effects - not to mention the harmonics of a sudden reversal - and I'm still not getting close enough to use it yet. Although I can improve where I want to now, but I can't dial out the harmonics or the occasional resonance that breaks things (as in snapping M10 12.9 bolts in double shear kind of breaking things, the forces if it gets out of step are nasty!)

[Piledriver]

This weekend, I'm hoping to directly measure the front and rear motion ratios of my squareback with stock front end and 944 turbo rear suspension in order to make a possibly reasonable guess about spring rate and damping settings using the calculators from earlier in the thread.

...Will also measure unsprung weight before the bars go in, both stock and 944T, and same for front stock incl steel wheel/tire and the 16x6 Fuchs setup with the Wilwood brakes, and the very popular (but apparently too big for T1/t3 weight distribution) 944T/928 front setup as well... as I scored some nice new directional Meyle rotors for $11 ea shipped.
(965(964T) rears, but the offset/width work out, all else fails for $11 I'll use them as lamp bases)

The front I just need to get the shock and arm angle for, the rear is perhaps a bit more interesting.

The supposed motion ratio of a 944T rear shock or coil over suspension is .42, but that's in a 944.
In a T1 or a squareback, the rear shock or coil over is (from memory) closer to vertical...
...but there is a HUGE shock<>arm angle variation depending on extension, making it quite progressive.

My plan is to measure both shock angle/travel and arm angles F/R +/- 2" from target ride height, and use that info to select springs and damping.
I expect the rear ratio to be a lot closer to the arms native .80ish ratio vs. .42 as the arm is just about level/square to shock at actual ride height, as are the fronts. (will measure all and post)

Sound like a reasonable methodology?

The only thing that will prevent it is more rain, scheduled a couple days off into next weekend in case it doesn't dry out.

[Piledriver]

Rear shock/coil over motion ratio and unsprung weights:

28mm rear bars in a 73 square, translates to 254 lb/in rate based on published data for 944.

944 turbo rear arms (early) in a type 3 (T1 should be same location)
shock movement as % of axle, measured +/- 1" (at axle) of springplate level, level is ride height

motion ratio is 0.60 as measured +/- 1" of ride height.(2" stroke at axle)
repeated direct measurement using good digital calipers, high confidence of accuracy.

Alleged "known" ratio in a 944T is .42, so the shocks at a different angle, or .42 is an average.
I'll use .6 for spring/damping calculations as ride height is where it runs.

So a given shock or coil over on a T1/t3 with that suspension will be ~50% higher rate than in a 944.
...or the "specs" for a 944 are ~50% off typical due to the averaged motion ratio.

Weights:
Measured with my CPS Compute-A-Charge refrigerant scale, resolution to 1g/.25 oz, and seems to be very accurate/repeatable.

Total rear unsprung weight with tire
92 lbs/side

Rear unsprung weight, TA, spring late, 944T rotor and caliper, no bars, no shock etc no axle
34 lbs. (was still slowly creeping down due to a tiny bit of bushing stiction, but seems to be about right)
Axle weighed iirc 14 12oz so lets add 8 lbs and call it
42 lbs w/o tire.

245/50-16 Cooper Zeon RS3-A on a Fuch 944T 16x8" forged manhole cover
50.00lb (stock diameter+ 25.8")

Stock steel rim and 165/80 15
35lb 10 oz
Same size/brand tire on a 5.5x15 914 mahle
30lb 12 oz

[Piledriver]

Bingo, just like a capacitor. Absorbs the peak energy and dissipates it when the input drops, using the inertia of the fluid column.

Yes, the math is ugly, so much of it you have to assume to be true or 'close enough' because the compliance and compressability effects - not to mention the harmonics of a sudden reversal - and I'm still not getting close enough to use it yet. Although I can improve where I want to now, but I can't dial out the harmonics or the occasional resonance that breaks things (as in snapping M10 12.9 bolts in double shear kind of breaking things, the forces if it gets out of step are nasty!)

Keeping with the "capacitor" anology, the suspension as a whole is a tuned circuit, including the mass of the car.

If you want to widen the bandwidth and reduce the peak forces of a tuned electronic circuit, you reduce its "q", or quality by basically shorting it out somewhat with a resistance. It will respond over a wider range, but have less force.
This can be variable, even electronically controlled.

That's essentially what something like a ~conventionally valved shock piston would provide, digressive valving would for example greatly reduce the inerter coils effect at higher velocities while providing relatively more at low velocities.
(assuming that is what you want)