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      11-07-2018, 08:23 PM   #1
ashiohsiao
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My F20 FCM Elite Project

Just a quick heads-up:
This thread is to share with you my currently in-progress FCM Elite project - Fat Cat Motorsports custom-revalved Bilstein dampers. I will try to be as detailed as possible, but the contents will of course be limited by what I am allowed by FCM to share.

For those of you, who are curious about FCM and can't wait for more info, there is actually a very interesting (but also very long) thread on an E43 M3 forum: http://www.m3forum.net/m3forum/showthread.php?t=534318

Now a bit of personal history... Please bear with me...

Similar to EricSMG from the above thread, I've been on a campaign for years to find the absolute best suspension setup and it had proved to be a total nightmare for me...

On my previous car, I had been through lots of different suspension setups, including K-Sport, Koni Yellow, H&R, Eibach, KYB New SR, RS-R, and a few other local brands in my country (Taiwan)... I had also tested some friends' cars with "high-end" OEM or aftermarket suspensions like AMG, Ohlins, and KW... while a few of those setups felt "okay" on certain roads, or under certain situations, there was ALWAYS some situations that those setups would come short... and people around me, whether friends or auto-shops, all told me the same thing: "Ride Quality & Handling CANNOT Co-Exist!"

But... I have this compulsive syndrome that simply refuses to accept that saying... so I kept trying to learn more about how suspensions work and came across Fat Cat Motorsports' YouTube channel: Suspension Truth https://www.youtube.com/user/SuspensionTruth/playlists

It took me more than a few years reading about FCM, and watching his videos (again and again) until I fully (well at least 90%) understand his concepts about how suspensions should work (also because he's been releasing new videos along the way over the past years).

There are a lot of videos, so I will summarize a few crucial points here:
1. Pretty much all "sport" or "racing" suspensions out there in the market have a bias towards rebound damping (vs. compression damping), which gives you an "illusion of control" or "illusion or handling", but in reality, gives you "JACKING-DOWN" - worse ride quality, worse grip, worse ride safety...
2. Bump stops are a super crucial factor in affecting ride quality, as well as handling
3. Ride quality and handling CAN co-exist, with ride quality exceeding an OE luxury sedan setup while handling & grip surpasses sports cars setups


I had also previously done some experiments based on FCM's videos and found that FCM's concepts are indeed correct (though contrary to what most believe). About that, I invite you to watch this video: https://www.youtube.com/watch?v=8NklC0Nn9Ys&t=1s

Right now I'd ordered a set of Bilstein B6 from Germany and have them sent to FCM for revalving (just arrived today), and so I will update as the project goes along.

I will also be consolidating and updating more info about FCM's concepts to share with you.
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      11-08-2018, 01:12 AM   #2
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Shaikh (Fat Cat Motorsports) did a bit of over-time work just now and got some initial dyno testing on the Bilstein B6, as well as measured the gas pressure.

Just in case someone doesn't know what the gas pressure is, it is the force needed to make the damper begin compressing.

So the gas pressure for the Bilstein B6 (F20 118i):
Front: 48lbs
Rear: 54lbs

These are both way too high, and cause discomfort. Let's assume I hit a bump and it generates a force of 45lbs, my dampers' gas pressure will NOT compress at all, causing the impact to pass into the car directly (also lifting the car).

Below are the damping graphs of the dampers, with the upper portion as the compression damping, and the lower portion as the rebound damping.

This is the damping graph of the front damper:
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This is the damping graph of the rear damper:
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When analyzing damping dyno, the usual practice is to remove the gas pressure from the graph, so both compression and rebound begin at 0. If you want to add in the gas pressure to see the net damping force, simply move the graph vertically upwards (both compression and rebound) by the gas pressure force.

Here are three pictures to illustrate how to read these graphs:
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With gas pressure, I took some estimated readings:
Front:
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Rear:
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Some observations that I made myself (haven't discussed with Shaikh to confirm my understanding):
- Starting at 6 inch/sec velocity, the front dampers start to have more rebound damping force than compression damping, causing jacking-down.
- Rear damper damping force is increasing quite linearly, which will make the suspensions unable to effectively deal with uneven road surfaces at high velocity

Last edited by ashiohsiao; 11-12-2018 at 02:32 AM..
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      11-08-2018, 04:13 AM   #3
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The gas pressure has some benefits. If you keep you stock springs, and want a more sporty suspension setup, a Bilstein damper will effectively add to the spring rate with this pressure. It also prevents foaming and cavitation of the oil inside the damper.
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      11-08-2018, 07:10 PM   #4
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Quote:
Originally Posted by ovekvam View Post
The gas pressure has some benefits. If you keep you stock springs, and want a more sporty suspension setup, a Bilstein damper will effectively add to the spring rate with this pressure. It also prevents foaming and cavitation of the oil inside the damper.
Yes~ however, it seems that (according to Shaikh's findings), most monotubes use "too much" gas pressure.

Reading Shaikh's post on the M3 forum, he ended up lowering the gas pressure of the front from 30lbs to 20~23lbs, the rear from 37lbs to 21~24lbs~

Last edited by ashiohsiao; 11-11-2018 at 10:34 PM..
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      11-11-2018, 07:45 PM   #5
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Just had a detailed discussion with Shaikh on how I want the car to feel like on Friday, and while Shaikh is working on revalving the dampers, I figured it might be a good idea to share with you what I have done so far in working towards achieving my dream suspension setup.

For those of you who have driven the F20 with stock suspension, you might have noticed that the stock suspension, while quite comfortable in the aspect of soaking up big bumps and potholes, the handling feels loose with quite a lot of roll, and on freeway, it feels "floaty".

Before I applied for the actual FCM Elite revalving, I had a preliminary consultation with Shaikh, and basically I told him that I was saving up for the FCM Elite, but would like to do some preliminary setup on my car before-hand.

Shaikh had this spreadsheet that he made to calculate how different setups of springs, anti-roll bars, and bump stops would affect many of the aspects affecting how the car behaved and handled.

P.S. He's currently fine-tuning and finalizing the spreadsheet for the F20 and would release it to share with you when it's done.

So, based on my intended use of my car and my car specs, Shaikh recommended me to do the following:

- Upgrade my OE front anti-roll bar from 24mm to 28mm
- Upgrade my OE rear anti-roll bar from 13mm to 15mm
- Trim a portion of my front suspension bump stops off from the stiffer end

After the above initial tuning, my car currently:
- maintains "flat ride" (front spring frequency slightly slower than the rear spring frequency),
- has much less roll, and
- very neutral even when pushed hard

To keep the posts shorter, I will explain each of the three aspects in subsequent posts
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      11-11-2018, 08:08 PM   #6
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Flat Ride

"Flat Ride", in essence, refers to the rear springs having faster frequency than the front springs.

Basically, if the front springs frequency is faster than the rear's or the same, you get this:
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Without flat ride, the rear-end is unable to catch up with the front-end movement, whether in dealing bumps or potholes, or in cornering:
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With the rear springs having faster frequency, you get this:
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And now the car feels stable and smooth:
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Luckily, most BMWs have flat ride with stock suspensions, but if you have the chance to compare with a car that doesn't have flat ride, you will be surprised how big of a difference it is.

Shaikh has quite a few videos that talk about flat ride in depth, if you are interested:
https://www.youtube.com/watch?v=ZR0W0XyxN_Q
https://www.youtube.com/watch?v=Zpax2pMDGdw&t=2s

For my F20, I actually tried to contact BMW to get the exact frequency for my springs, but for some reasons, BMW did not have the data... so I used the bounce test to get a rough idea of the front and rear overall frequency:
https://www.youtube.com/watch?v=F-n60O2PmUg

Last edited by ashiohsiao; 11-12-2018 at 01:26 AM..
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      11-11-2018, 08:31 PM   #7
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Anti-Roll Bar

This video was one of the few videos that especially caught my attention when I was trying to learn more about suspension. This anti-roll bar concept, and the bump stops concept were what made me found that Shaikh really knew his stuff and that was why I started trying to get into understanding his other more profound concepts:
https://www.youtube.com/watch?v=WhWZ57baTOw&t=1s

The basic idea is, while anti-roll bar stabilizes the car and reduces rolling, it also reduces mechanical grip.

I actually tried to prove this concept by removing one of the link between the bar and the suspension of my car, causing the bar to be disabled, and to my surprise the grip really increased by an amazing margin!

BUT, if you watched the video, and also read the comments (his reply to SG ///), you would notice that Shaikh also explained that running without an anti-roll bar may compromise flat ride, so he "tried his best to to keep at least some Flat Ride in the spring rate choice, then used as little sway bar as required".

In another video of his:
https://www.youtube.com/watch?v=LxMPhR_6ilg&t=155s

He also explained that for a RWD (rear-wheel drive), usually he would upgrade the front bar, but not the rear, or only upgrade the rear by a small margin.

Before I used the 15mm rear bar, I actually gave 19mm a try (note my stock is 13mm), and while the rolling was less, the rear tyres lost so much grip that even when driving under Comfort mode, going up-hill, my rear tyres kept slipping at corner exit...

Now with 15mm, I have reduced roll, but my rear tyres have enough grip to allow me to get on the throttle earlier on corner exit

Last edited by ashiohsiao; 11-12-2018 at 01:27 AM..
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      11-11-2018, 09:04 PM   #8
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Bump Stops

Many people only see the bump stop as a part of the suspension that protects the damper from bottoming out, but in my understanding of Shaik's concepts, using my own words, I see the bump stop as :
1. A part that protects the damper from bottoming out (duh...)
2. A second-stage suspension spring if bump stop only comes into contact when pushing hard
3. A secondary spring that adds to the actual spring rate if the suspension is designed to be riding on the bump stop

Most people know that front and rear spring rates affect how a car turns, and you can:
- Use stiffer rear springs to reduce understeer / increase oversteer
- Use softer rear springs to increase understeer / reduce oversteer
- Use stiffer front springs to increase understeer / reduce oversteer
- Use softer front springs to reduce understeer / increase oversteer


Let's talk about Point #3 first, if the car is designed to ride on the bump stops, your net spring rate is actually: coil spring rate + bump stop rate = net rate

Therefore, for a car riding on bump stops, you can achieve cornering behavior tuning not just by changing the actual springs, but also the bump stops.

Assuming your car currently understeers, you could potentially reduce understeer by changing the front bump stops to a set of softer bump stops, which would then give you less net spring rate, the same effect as using a set of softer front springs.

Point #2 is similar, but has two stages, which is what most BMWs have. Using the F20 as an example, the car is a bit tail-happy (slight oversteer tendency) most of the time, but when cornering very hard, you would notice that the car becomes quite understeer-y.

This is because when cornering very hard, the springs are compressed to the point that bump stops are being engaged, and since BMW uses a stiffer and longer bump stops in the front than the rear, your net spring rate for the front now becomes much higher than the rear, which then results in an increase in understeer tendency.

This is the reason why Shaikh recommended me to cut off a portion of the front bump stops, to delay the engagement of the front bump stops and to reduce the net spring rate when bump stops are engaged. And true enough, I haven't run into any understeering so far since I trimmed my bump stops.

However, there are two things to keep in mind when doing this:
1. You need to know how much of the bump stops you need to keep to allow the bump stops to still be able to protect the dampers from bottoming out
2. You should cut off the thicker (stiffer) section of bump stops, to allow the softer portion to cushion the impact when bump stops are engaged to prevent a sudden shift of resisting force during compression, which causes discomfort

Shaikh has a series of videos explaining about bump stops:
https://www.youtube.com/watch?v=iIYdjerfFPg&t=173s
https://www.youtube.com/watch?v=zMhK381UO80
https://www.youtube.com/watch?v=K4ieo6xoz4o
https://www.youtube.com/watch?v=AR_zt9rgN9Q
https://www.youtube.com/watch?v=FouX9qGbg3s
https://www.youtube.com/watch?v=6U4fPe48cJ8
https://www.youtube.com/watch?v=FKIVlqW_GH0

And there's also a video of him trimming off front bump stops and adding packers to the rear on a Mini Cooper:
https://www.youtube.com/watch?v=4VitRxorSrY

Last edited by ashiohsiao; 11-12-2018 at 01:27 AM..
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      11-12-2018, 06:03 AM   #9
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I have driven quite a few BMWs the last few decades, and none of them were balanced towards oversteer from the factory. They all have some understeer unless provoked. The older modes with semi trailing arms could very easily be thrown sideways with a throttle lift or slight braking during corner entry, particularly the E21, but also E36 Compact. After the introduction of the Z axle, the willingness to oversteer has been reduced.

My two underpowered F20s have been very difficult to oversteer. I have to make a very aggressive corner entry during braking to make it happen. Fortunately it is also rather unwilling to end up in terminal understeer. Just a very slight understeer. With a square tyre setup, it is more balanced, but still understeers a bit. If I go too fast into a corner, the car ends up with a four wheel drift.

Based on my experience with chassis modification on my own BMWs, I would do the following to make the car handle better:

- Narrower rear tyres (same size in all corners)
- More negative camber up front (M4 front control arms?)
- 15 mm rear swaybar (mine also has 13 from the factory)
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      11-12-2018, 07:33 PM   #10
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Quote:
Originally Posted by ovekvam View Post
I have driven quite a few BMWs the last few decades, and none of them were balanced towards oversteer from the factory. They all have some understeer unless provoked. The older modes with semi trailing arms could very easily be thrown sideways with a throttle lift or slight braking during corner entry, particularly the E21, but also E36 Compact. After the introduction of the Z axle, the willingness to oversteer has been reduced.

My two underpowered F20s have been very difficult to oversteer. I have to make a very aggressive corner entry during braking to make it happen. Fortunately it is also rather unwilling to end up in terminal understeer. Just a very slight understeer. With a square tyre setup, it is more balanced, but still understeers a bit. If I go too fast into a corner, the car ends up with a four wheel drift.

Based on my experience with chassis modification on my own BMWs, I would do the following to make the car handle better:

- Narrower rear tyres (same size in all corners)
- More negative camber up front (M4 front control arms?)
- 15 mm rear swaybar (mine also has 13 from the factory)
Yup~ the "unless provoked" was what I meant about being slightly tail-happy. My previous couple of cars were all FWD, and so the more neutral turning and the tendency to be slightly tail-happy feels comparatively stronger for me.

I definitely agree with your three directions of reducing understeer, and I had done similar experiments in the past, including the bump stop trimming, and what I've found is that the most prominent change actually came from trimming the front bump stops, and the first time I tested a Touge time attack here in Taiwan after trimming the front bump stop, (with everything else the same), I actually shaved 76 seconds off my past record, and I simply could not get any understeer! (Not the F20)

On my F20, trimming the front stops does the same magic for me. There are these two big turns on my daily commute route, and before trimming, I could take the first turn at about 70km/hr and already I could feel the car was starting to have a bit of understeer, the same for the second turn at about 88km/hr. After trimming the front bump stops, so far I have test the first turn at about 82km/hr, no tyre squeak and no understeer, the second turn at 100km/hr, again no tire squeak and no understeer.

However, I do need to point out that trimming the front bump stops would also give you slightly more sway at low speed or slightly more initial roll when you begin a turn-in. Therefore, the ideal way may be changing the stiffness of the front and rear bump stops. The trimming method is just easier and more cost-effective.

I then told a few friends about this, and they also tried it on their cars, and they also were shocked at how much a different it made

Last edited by ashiohsiao; 11-12-2018 at 07:52 PM.. Reason: Add more information
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      11-12-2018, 08:03 PM   #11
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Quote:
Originally Posted by ovekvam View Post
I have driven quite a few BMWs the last few decades, and none of them were balanced towards oversteer from the factory. They all have some understeer unless provoked. The older modes with semi trailing arms could very easily be thrown sideways with a throttle lift or slight braking during corner entry, particularly the E21, but also E36 Compact. After the introduction of the Z axle, the willingness to oversteer has been reduced.

My two underpowered F20s have been very difficult to oversteer. I have to make a very aggressive corner entry during braking to make it happen. Fortunately it is also rather unwilling to end up in terminal understeer. Just a very slight understeer. With a square tyre setup, it is more balanced, but still understeers a bit. If I go too fast into a corner, the car ends up with a four wheel drift.

Based on my experience with chassis modification on my own BMWs, I would do the following to make the car handle better:

- Narrower rear tyres (same size in all corners)
- More negative camber up front (M4 front control arms?)
- 15 mm rear swaybar (mine also has 13 from the factory)
Oh! Forgot to mention... Shaikh did recommended me to get camber plates for the front to get more negative camber should I want even better grip~
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      11-13-2018, 03:47 AM   #12
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Quote:
Originally Posted by ashiohsiao View Post
I then told a few friends about this, and they also tried it on their cars, and they also were shocked at how much a different it made
I agree that bumpstops are often overlooked during chassis tuning. Particularly monotube Bilstein shocks have very long internal bumpstops that make quite a lot of difference in handling.

My F20 seems rather neutral during turn in, since I am off the throttle and sometimes even braking. This is the situation where the outside front bumpstop is compressed. After I get back on the throttle, I get a slight understeer, and in this situation, the front bump stops should normally not be engaged.

My car has very good traction, so I would be willing so sacrifice a little bit of that for less understeer. That is why stiffer rear swaybar and narrower rear tyres seems like the way to go.
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      11-18-2018, 08:14 PM   #13
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Quote:
Originally Posted by ovekvam View Post
My car has very good traction, so I would be willing so sacrifice a little bit of that for less understeer. That is why stiffer rear swaybar and narrower rear tyres seems like the way to go.
Just got back from a trip

Yeah, upgrading sway bars to tune handling is definitely a good way, and I am happy to read that you are also using a 15mm for the rear~

At first I made a bad call upgrading the rear from 13mm to 19mm without consulting Shaikh (I also upgraded the front from 24mm to 28mm at the same time), and though the body roll was significantly reduced when cornering, the ride was actually quite a lot harsher!

Contrary to sales pitch saying stiffer sway bars reduce body roll without compromising comfort, I did feel the ride was compromised a LOT! Especially the rear... it was very jumpy and way too loose when I took it for some touge driving testing...

Later I learned about Shaikh's broad direction on sway bar tuning is:

For RWD:
- Upgrade the front sway bar is usually okay (on mine 24mm to 28mm)
- Upgrade the rear sway bar only by a very small amount, maybe the next thicker one (on mine 13m to 15mm)

For FWD/AWD/4WD:
- Using the OE front sway bar is usually more ideal
- Upgrade the rear sway bar is totally fine, and you can increase the thickness more

Then I tried to find what sway bars were available here in Taiwan for my F20, and for the front, there were KC Design, H&R, Eibach, and HardRace, and all of them were 28mm. For the rear, there were M135i/M140i OE 15mm, Eibach 16mm, HardRace 19mm, H&R 20mm, and KC 21mm. I also found a table someone made to compare the increase in stiffness in % (the last column), but I think not sure how accurate this table is.
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So I had my consultation with Shaikh, changed to the 15mm, and now the ride feels a lot more composed and stable, still maintaining like 70-80% of stock comfort, but less floaty, and the reduced in roll is not at all worse than the 19mm.

Before I trimmed the bump stops, I wanted to get a comparison reference, so I also did give this setup a touge driving test, and found that the handling was still more towards understeer-y when pushed hard. It was only after I trimmed one section off the front bump stops that the understeer was completely gone
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      11-19-2018, 02:18 AM   #14
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I am not using 15mm, it is just what I would prefer if I was to do modifications. Going from 13mm to 15mm sounds like a mild upgrade, but as the table indicates, it is a very significant change in stiffness. It does make the suspension less independent, and you can feel the difference in comfort if you hit a bump with the outside wheels in a corner.

The reason he recommends smaller changes to the rear swaybar on RWD cars, is that it affects traction. The 118i does however have a rather moderate power output, and lots of traction, so I don't see it as a problem to sacrifice some traction for more front end bite.

If you have higher power, it makes sense to install a limited slip differential when going for a stiffer rear swaybar.

On one of my previous cars, an E36 323ti, I installed Bilstein PSS coilovers. That made the suspension a lot stiffer, and reduced the body roll more than enough for my taste. There was however still some understeer. Since the car was rather stiff, I fixed the understeer issue by going to a softer (316i) front swaybar. After that, the balance was spot on.
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      01-07-2019, 08:04 PM   #15
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Just a quick update on the project~

The revalving is completed and I have the new vs old damping graphs (this time the gas pressure is included in the graphs).

I've also included my own analysis of the changes from the revalving.
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      02-17-2019, 11:09 PM   #16
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Have finally received the suspensions and had them installed last week. Will provide an in-depth update of how the ride feels and handles after some mileage~

Last edited by ashiohsiao; 02-22-2019 at 02:54 AM..
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      02-21-2019, 08:52 PM   #17
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Just had my first week of daily commute driving, and am excited to provide how I am finding the dampers so far:
  • The first thing I noticed is that they definitely ride more comfortably than my BMW stock (Non-M) dampers. It is interesting that they provide me with much more road condition feedback (I sense more of how the road conditions are), yet I feel less impact. It's like: "I sense it, but my body doesn't feel it."
  • There is much more support (less initial roll) at turn-in, giving me more confident turn-in steering feel, but the turning handling is totally different from the rigid stiff feeling you get from most aftermarket sport suspensions. It feels like when I turn in slow, it feels comfortable almost like the stock feeling, but better handling; while when I turn in fast, the quick response feels even faster than any of the sport suspensions I have tried before, yet still maintains a casual & comfortable feel.
  • For the front 1:1 compression and damping ratio, which provides a non-jacking setup, I can tell that there is pretty much no nose-dive or any of those "dipping feel" when you suddenly drive over a deep potholes, or a downward slope. This is even more prominent when doing a fast cornering, and running over road joints or very uneven surface mid-corner; the car feels so composed from turn-in to exit, and tires feel planted and undisturbed running through those surfaces.
  • They give me a false sense of speed from these dampers. I would go faster and faster because my body just doesn't feel that the car is going that fast, so now I need to be careful checking my speed before speed cameras
  • Got the chance to test a little bit of these two big turns on my daily commute (when there is no other car), and for the bigger turn, my previous stock dampers (with upgraded stabilizer bars) allowed me to take that turn at about 100km/hr, and just the other day I was pushing through that turn and was at 120km/hr at corner-exit, yet I could tell that the car could go faster still (just need to slowly get used to how fast my car can take corners now)
  • So far I have only driven to work and back, with one detour to my parents', so I also got to test the ripple reducer (their basement parking has all the slopes made like washboards... and like I described in the first point: The car still got the high-frequency jerking, and I could "sense" the overall movement, but my body didn't get the impact from that jerking at all.

Haven't got a chance to do touge testing, will update in the future~
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      02-22-2019, 12:52 AM   #18
ovekvam
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I read somewhere that it was an aim for BMW R&D engineers to reduce the speed feeling in new cars. BMWs should be fast without feeling fast. Sounds like you have acheived the same thing. I look for the opposite in cars. I want to sense the speed even when going slowly, which is what I do most of the time in traffic. If you go from an F20 to something like an E30, going at the same speed feels a lot faster.
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      02-26-2019, 08:43 PM   #19
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During our consultation session, we also agreed on including three pairs of packers for the rear dampers, should I want to fine-tune the cornering behavior (making it more prone to understeer or oversteer).

So far I haven't used any packers, and will be adding one pair of packers this week for testing~

Will keep it posted here~
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      02-26-2019, 11:58 PM   #20
ovekvam
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Wouldn't the use of bump stops make the handling less consistant and predictable? I think it would be better to tune the cornering balance by modifying the rear swaybar.
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      02-27-2019, 01:22 AM   #21
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Quote:
Originally Posted by ovekvam View Post
Wouldn't the use of bump stops make the handling less consistant and predictable? I think it would be better to tune the cornering balance by modifying the rear swaybar.
From my previous experiment with trying different bump stops (length and hardness), they do change the overall behavior and tendency, but do not cause the behavior to be inconsistent or unpredictable.

According to Shaikh, the combined choice of coil spring frequency (flat ride), and sway bars is the first step, which I had done previously before installing the FCM-tuned dampers. Then the FCM dampers is the second step. The packers for the rear dampers are for the third step fine-tuning. Adding packers to make the rear bump stops engage earlier will shift the behavior more towards oversteer.

Right now, without any packers, when pushed to the very limit, I find the behavior to be slightly more towards the understeer side, that's why I will be adding a pair of packers at a time (from nothing to 3 pairs, so 4 fine-tuning settings), to see which I like better~
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      02-27-2019, 05:23 AM   #22
ovekvam
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I think I would have gone for stiffer rear swaybar and less bump stops to improve turn in and make the car more stable at the limit.
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