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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Sat, Feb-11-2017, 01:40:44 AM   #191
liam821
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

I also use a set of custom valved Bilstein mono-tube HD dampers in my Honda Civic eg race car. They're fantastic, especially when you consider the cost! I started with a brand new set for an Integra TypeR (I run a ITR style rear lower control arm) and sent them off to Bilstein. I told them what I wanted, the springs rates I run, and they did all the modifications for me. I'm an extremely happy customer.

Looking forward to seeing your results, OP, so far it looks awesome.
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Old Sun, Sep-16-2018, 04:26:32 AM   #192
ShaikhA
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

Shock dyno graphs illustrating the effect of nitrogen gas pressure. Why isn't gas pressure more talked-about??

Since Eric and I have previously shared shock dynos in this thread, I wanted to add a few more here. Although they're not specific to the E46 M3, the concept (especially the level of nitrogen charge pressure) applies to our BMWs and relates to observations people have made about ride quality and handling behaviors.

I've come to realize how critically important it is to measure and understand the effect of nitrogen gas pressure on a damper's real-world behavior. In this post I'll be more brief and link to two graphs showing damper force vs. velocity with a short discussion. In a subsequent post, I'll go into more detail about why we need nitrogen pressure inside the damper, and what happens if it's too high. In short, you need high-enough nitrogen pressure inside your damper to prevent any dissolved air from 'boiling' and 'foaming' (which reduces the damper's effectiveness) but also you need low-enough nitrogen pressure so you don't 'lock' the damper and prevent the damper from opening!

The graphs below were provided to me by a customer using Bilstein PSS10 on his F80 328 M Sport. He had someone near him perform dyno tests and he asked me to analyze the results and make suggestions on what damper settings and spring rates to use, etc. The first graph has the gas force subtracted, as is the common practice. The second includes the effect of the ~80 lb gas force, which raises the compression force and reduces the rebound force. This is an across-the-board change in the damper's real-world behavior. I've kept the scales the same for both graph so it's apple to apples comparison.



Same damper setting as above, but now gas force NOT subtracted (actual real-world behavior):



High nitrogen pressure and the resulting high gas force has an IMMEDIATE and CONTINUOUS impact on how the suspension tracks the ground. The gas force causes jerk, which is a sudden change in the acceleration of the vehicle. Jerk reduces grip as well as ride quality (grip and ride quality are really synonymous).

Keep in mind that gas pressure of 40 lb or more on our front strut suspensions will feel very stiff because of the high ~1:1 motion ratio of forces from the strut conducted to the wheel (and vice versa).



Bottom-line, if you find a monotube uncomfortable, high gas pressure and the resulting high gas force / rod force will often be the primary (hidden) cause. Make sure to find out the gas pressure in your front AND rear dampers, and notice if it's too high... You can use the 'bathroom scale' test as EricSMG did to get a sense of gas force!

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Old Sun, Sep-16-2018, 05:18:11 AM   #193
ShaikhA
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

This is the BMW E46 M3 'Ride Harmonizer' spreadsheet I've created with you folks in mind! Very proud to share it here for the first time!



FCM Elite E46 M3 Ride Harmonizer FRC and Ride Frequency spreadsheet




There are various inputs such vehicle weight, driver weight, weight distribution, tire / wheel weight, spring rates, sway bar dimensions, AND bump stop rates (you can approximate 200 lb/in for OE front, 100 lb/in for Bilstein black and OE rear, etc.).

I've created three sections which serve different purposes. This topic deserves its own thread but I figured to introduce here and then go into more detail later.

For those who want to understand the features in more depth, you can view this video and follow along. For a short summary, you can enter a few values for your car (such as car & driver weight) or leave those default. Then enter the spring rates and then examine ride frequencies that result. Our goal for better handling and better ride is to pursue 'Flat Ride', where the rear ride frequency is at least a few percent higher than the front. I recommend setting the resolution to at least 480p.


I've found that for our BMWs we NEED to pick a rear spring rate that creates Flat Ride, and that often means we need a larger front bar with a stock rear bar. Most 'matched' sway bar kits (where you add an aftermarket front AND rear bar) end up with TOO MUCH rear roll stiffness which forces you to use a TOO SOFT rear spring and that causes pitches (which messes up the ride, handling, grip, and your brain - literally!). Pitching is literally a cause for physical and psychological dis-ease and the US Navy studied it extensively back in the 1960s.



Bump stop behavior is essential to understand and account for, and damper behavior will contribute to overall stiffness, plus can bias the suspension toward jacking down (which makes the bump stops engage more often). I'll create another thread and cover more details, but feel free to make use of it, ask some questions here. It'll help you select spring rates so Flat Ride can work for you!

Shaikh
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Old Tue, Nov-06-2018, 06:59:36 PM   #194
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

A couple BMW E46 M3 front Ohlins DFV shock dynos. There's an artifact in the '6 clicks from full stiff' test so don't panic about the offset at 0 inches per sec.



The gas force measured on the E46 M3 Ohlins DFV front was 54 lb. That's quite high for a strut and assuming a 14mm shaft would equate to about 225 psi fill pressure.



===

Now, see the difference with what we did on Eric_SMG's front FCM Elite Bilstein Stage 2 setup. First, even at FULL SOFT once you're outside the low-speed shock velocity range, the Ohlins is ALWAYS JACKING DOWN on any bump or dip! And who runs their Ohlins at full soft?! Not many I'd bet.



The FCM Elite Stage 2 setup has a bit more mid speed compression, lower high speed compression slope, and less mid speed rebound as well as less high speed rebound. This combination gives both better bottoming resistance on medium bumps and better compliance on larger bumps, plus the suspension is 'resetting' back to a neutral point since it has very little jacking down happening at very high velocities. It wants to be driven hard, and will be forgiving on rougher / broken surfaces especially driving at the limit. And yes, that means our setup will be FASTER than Ohlins on track as you aren't being launching into the air or jacked into the bump stops if you try and use the berms / curbing.



For gas force, there's a big difference as well. Eric's FCM Stage 2 front struts are about 23 lb gas force while the Ohlins are 54 lb. These are important differences you'll definitely feel with the 20 lb lower gas force and lower high speed compression slope plus MUCH less rebound high speed slope giving FAR better large bump compliance and small feature, rough-surface tracking with FCM vs. Ohlins!

I couldn't test the Ohlins higher than 5 in/sec because I'd need to disassemble the entire strut (and adjuster mechanism) which I didn't want to do. Just note the overall curve shape and the ratio of compression to rebound damping. The Ohlins compression shape looks pretty good, though could be a bit firmer in the low and mid speed range. But yeah, that rebound is quite stiff. With Flat Ride spring rates present you don't need as much damping anyway so more reason to soften the damping and enjoy the grip benefits but the high speed rebound isn't blowing off very well...

Knowing / seeing all this, if you have Ohlins or decide to get them, I'd definitely ask if they can lower the gas pressure to 150-175 psi-g *IF* that won't compromise the damping performance. When I back-calculate Eric's front gas fill pressure ended up about 150 psi-g. They might not want to or be able to use that lower fill pressure.

For the FCM setups, we're able to get lower gas force although the Bilstein shafts are 11mm while the Ohlins seems to be 14mm so no matter what you did to lower fill pressure, the Ohlins will ALWAYS have higher gas force than I can get with the Bilstein. Plus, as a reminder, we can incorporate the Ripple Reducer modification with a few drilled holes (and makes the damper track marbled/micro-rough surfaces better than the Ohlins).

Relevant video where I point out the difference in high speed compression slope with 8mm (non-adjustable Bilstein) post vs 12mm (adjustable / Ohlins DFV or Bilstein PSS9) post:


I normally like larger shaft diameter if you're trying to build a lot of compression, but with the nearly 1:1 front motion ratio on our BMWs, we don't NEED a lot of compression for the car to work properly - our dampers can actually use lower forces than a factory Miata (with its lower 0.6:1 front motion ratio). So an 11mm shaft delivers PLENTY of oil displacement for our needs! A larger shaft becomes a liability when you're aiming for maximum traction and lower gas pressure. This is a reason a lot of 'high end' suspensions for BMW, Porsche, etc. end up with VERY high gas pressures - the excessively large strut / shock shafts diameter create very high gas force the cause jerk and harshness. This includes Moton, AST, MCS, etc. that use non-inverted design and 19mm or 22mm shafts! Holy high gas force, Batman!

I also noted fairly limited droop travel on the Ohlins which means the spring needs to be preloaded quite significantly for a street-friendly ride height. A shorter spring would help this and would be an option for someone who wanted to a more track-focused setup. But the damper bias toward rebound and fairly high gas force at the strut are definitely disadvantages of the setup.

These graphs pertain to the E46 M3 front. I'll measure the Ohlins DFV rear when I can do that as well.
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Take our short Ride Harmony survey and earn a credit on FCM Elite Bilstein damper services

Last edited by ShaikhA; Tue, Nov-06-2018 at 07:05:55 PM.
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Old Tue, Nov-06-2018, 09:01:57 PM   #195
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

that's an interesting finding!
MCA suspension uses a 12mm shaft for the front of their e46 M3 kit, and sure enough when i tried to order top hats, i was told it was way too thin; so i had to make adapter sleeves to fit the GC street camber plates.
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Old Tue, Nov-06-2018, 10:03:59 PM   #196
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

I just talked to the guy I was going to have rebuild my ohlins for the new spring rates— he said 150-175 psi is no problem (and that that’s what he uses for his race setups) and that on his shock dyno they’re good down to ~90-100 psi.
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Old Wed, Nov-07-2018, 01:12:59 AM   #197
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

Quote:
Originally Posted by ShaikhA View Post
For gas force, there's a big difference as well. Eric's FCM Stage 2 front struts are about 23 lb gas force while the Ohlins are 54 lb. These are important differences you'll definitely feel with the 20 lb lower gas force and lower high speed compression slope plus MUCH less rebound high speed slope giving FAR better large bump compliance and small feature, rough-surface tracking with FCM vs. Ohlins!
Hey Shaikh!

I much prefer the Stage 1 valving all around. Front/rear, comp/rebound. For Stage 2 we went too soft and the car got a little floaty.

For reference, you may want to use the Stage 1 dynos for comparison. You'll remember that Stage 1 has a good bit more low speed rebound (and compression) and this gives the car a very tight/locked down/responsive feeling to steering inputs and crests in the road at speed. Combine this with your ripple reducer and lower gas pressure and Stage 1 was the ultimate sweet spot - maximum control without the harshness... but firm. In my quest for perfection I had you go too far to reduce the 'firmness' but... we went too far (Stage 2).

Oh, also - we went too short on the front bumpstops with Stage 2. Again, Stage 1 bumpstop length (longer) w/3mm packer was perfect.
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Old Wed, Nov-07-2018, 08:27:29 PM   #198
ShaikhA
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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

Quote:
Originally Posted by EricSMG View Post
Hey Shaikh!

I much prefer the Stage 1 valving all around. Front/rear, comp/rebound. For Stage 2 we went too soft and the car got a little floaty.

For reference, you may want to use the Stage 1 dynos for comparison. You'll remember that Stage 1 has a good bit more low speed rebound (and compression) and this gives the car a very tight/locked down/responsive feeling to steering inputs and crests in the road at speed.
Eric! That's interesting to hear, although I am looking at the dynos and the Ver 1 fronts actually had LESS low speed compression and rebound than Ver 2, but MORE mid and high speed rebound than you have now on Ver 2! This suggests to me you were noticing / appreciating the original valving I did with more high speed damping and also slightly more bleed / Ripple Reducer. As you observed, the original front damping was more compliant on smaller features while having more control on larger amplitude movements (crests/dips). The Ver 2 fronts have more 'jerk' via the added low speed damping which ironically fights against the sway bar's ability to articulate with the suspension.

I noticed a similar result when I went to stiffer springs AND the GC Race front bar. During that setup change, I decided to actually SOFTEN the front damping (by drilling another Ripple Reducer hole!) instead of STIFFENING the front as 'any sane person would do.' I wanted to see what happens when I had a lot of roll stiffness via springs and sways but didn't emphasize damping to 'control it' (because know now that Flat Ride does so much of that 'control!').

The front end dug into turns even better, despite being numerically 'under-damped.' I feel a secret of optimizing front grip is to minimize low-speed damping and prevent jacking down. The softer Ver 1 low speed damping decreased the side-to-side coupling of the two front tires and the ratcheting that results as the sway bar flexes and the damping are resisting that flex. Recall that we now know there's a sizeable roll stiffness contribution from the front sway bar (~58% of total OE roll stiffness!) which means anything that causes more 'jerk' causes less compliance and grip on the road or in corners. It was revealing to me in my experiment with softer low speed despite more spring and sway bar stiffness that a stiffer setup was actually faster with slightly less damping than a conventionally-minded mechanical engineer would think if they're not actually noting the effect on grip.

Quote:
Combine this with your ripple reducer and lower gas pressure and Stage 1 was the ultimate sweet spot - maximum control without the harshness... but firm. In my quest for perfection I had you go too far to reduce the 'firmness' but... we went too far (Stage 2).
Excellent - we're in sync with our description, although up front at least, it's softer low speed with firmer mid & high-speed damping that you liked. Compliance and bottoming resistance.

===

On the rears, the whole picture makes even more sense. In reviewing the differences between Ver 1 and Ver 2 rear, I noticed something interesting that I've been paying attention to more in the past few years... the rebound-to-compression ratio. On the original Ver 1, I went for a slight amount of jacking down which you found to help settle the car faster than the Ver 2 (current) which is softer overall damping and no jacking down.

It's much easier to get the rears out to play with and much of my experimentation after our initial visit has been with rear rebound-to-compression bias. As you noticed the added high speed rebound of the original Ver 1 helps improve settling response. You don't need much and I could do a Version 3 which would be slightly less rebound bias than Ver 1 but with more overall damping than Ver 2.

I also like the faster settling response on larger bumps with my own Ver 1-style damping, with front at neutral-jacking and rear slightly rebound-biased. No matter how much bias I use, it's FAR LESS than OE or aftermarket (like Koni, Bilstein, Ohlins, etc.) so we're still in the 'orders of magnitude' improvement over anything else out there! Those are all WAY over-damped on rear rebound and a reason they uniformly ride more harshly and have worse grip compared to what I'm doing with Bilstein digressive pistons.

BTW, your observations tie into my thoughts about the rear damping being critical in setting up the overall vehicle character. I'm finding that a slight rebound bias at the rear, with neutral jacking at the front, gives the most neutral handling, minimal understeer, and fast settling that doesn't cause undue harshness on rough roads. The best of everything, with just a little compromise on pure comfort.

For someone who wants a 'full rally' or 'pure comfort' setup I would make sure there's no jacking down at all and increase overall damping ratios to provide more total damper force. That's like your current Version 2. If I added more overall rear compression and rebound damping to your current setup you'd definitely find it less floaty. Various options available for getting the sweet spot you want.

Quote:
Oh, also - we went too short on the front bumpstops with Stage 2. Again, Stage 1 bumpstop length (longer) w/3mm packer was perfect.
I read you on the bump stop length, and I have that noted With softer springs requiring more suspension travel, you'd want the bump stop to engage over a longer distance than you would with stiffer springs. The new VD-KBO (velocity-dependent KBO) piston I've designed will help add some very-high-speed compression damping to help decelerate hard bottoming events, like a rally car would.

You could either just switch back to the longer stop, or add a 20mm segment of a 'black' (soft density) Bilstein stop to the shortened original white (medium density) your Bilstein HDs came with. That'd be an easy drop-in for you - just remove the strut insert and add the new bump stop segment. It's also a two-stage bump stop - initially soft, then firming up with more compression / bottoming.

Thanks for your feedback! Let's do a project on your F80! Or find me someone up here to work with!
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Last edited by ShaikhA; Wed, Nov-07-2018 at 08:32:26 PM.
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Discussing Shock dynos, Fat Cat Motorsports and custom valved Bilstiens in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)