[FaRKle!]

*Updated Info*
I recently stumbled upon ashiohsiao's FCM project thread in the 1-series forum where he went to Fat Cat Motorsports as well and got a set of custom tuned B6 based dampers for his RWD 118i. Based upon the data he posted of the RWD B6 damper dynos, I was able to compare it to my xDrive ones, and it looks like Bilstein is using the same damping for RWD models as well as xDrive. I guess that's not completely surprising given they use the same part number for all cars with a certain drive type (so one PN for all RWD and one for all AWD). Lazy...




*Original Post*
I recently received a set of Bilstein B6 front and rear shock/struts from Germany for a bespoke suspension project and dropped them off with my suspension builder this past weekend. He and I were interested to see how the B6s performed stock, so he put them on his shock dyno. Here's the plot:



What this shows is that that the rebound is much stronger than the compression in the front struts, which doesn't allow the strut to return to "normal" height after going over bumps, thus you lose ride height and "jack down." My suspension guru says that a bit more rebound over compression is desireable, but not the large ratios you see here. At the 2.5in/sec rate the rebound is 2.29x the compression force, and at 10in/sec it's still 1.94x higher. For the rears at the 2.5in/sec rate the rebound is 1.32x that of compression and 1.14x at 10in/sec.

Note that this graph doesn't zero out the nitrogen gas forces (42lbs front, 37lbs rear). These numbers are kind of high, and what that means is it takes a force of at least the pressure of the gas before the dampers will actually start moving and damping anything. This is typically why you feel more small/fine road textures with "sport" suspensions with high gas pressures vs a smoother ride. The dampers aren't actually doing anything/working in that regime!

The front PNs are 35-264583 (left) and 35-264590 (right) and the rears are 24-264570. I believe the rear shocks are available now in the USA, however the fronts never have been. These are the latest updated PNs (there was a recent revision) that are still pushed back to May for USA availability (and I'm fairly certain that date will get pushed back again). The front struts are what Bilstein recommends for all F30, 31, 32, 33, 34, and 36 xDrive cars, and the rear PNs are shared for all those chassis, and both sDrive and xDrive variants according to Bilstein's Euro catalog. It wouldn't surprise me if the sDrive front dampers have the same performance as these xDrive ones (given the rears are all the same across all models), and their PN differences are purely due to physical differences like the end link mounts on the struts.

[Tchao]

Interesting. Thanks for sharing!

What spring rates are you going to pair these dampeners with?
And more details on this bespoke project please :P

[sspade]

Wonder how much stiffer my B8's are...

[sspade]

Also:

B6, designed for use with stock height spring.

B8, designed for use with aftermarket lowering spring.

[FaRKle!]

Originally Posted by Tchao

Interesting. Thanks for sharing!

What spring rates are you going to pair these dampeners with?
And more details on this bespoke project please :P

I'm going to pair this with the Eibach 34N/mm (194lbf/in) front and 95N/mm (542lbf/in) rear springs on my car now. You'll have to be patient on the project details for now . I promise I'll do a full write up in my build log when it's finished.

Originally Posted by sspade

Wonder how much stiffer my B8's are...

If I had to guess, I'd think for the front struts Bilstein would want to make the compression force a higher on the B8 since there's less travel and/or put a stiffer but shorter internal bump stop in there. I don't think they would've changed anything with the excessive rebound though. The rears might've just been shortened. This is pure speculation though.

Originally Posted by sspade

Also:

B6, designed for use with stock height spring.

B8, designed for use with aftermarket lowering spring.

I would take the whole "designed for" thing with a grain of salt given they recommend the same front dampers regardless of weight in the front (4cyl and 6cyl engines) and the same rear dampers across every F3x (no matter if you have a light sedan rear on the back or heavy wagon/GT rear end). It probably has more to do with piston travel and maybe bumpstop length (the fronts have internal bump stops) than actual damping rates. BMW OE has individual PNs for 6cyl fronts, 4cyl fronts, xDrive, sDrive, and one for each of the rear styles too (so sedan doesn't share the same damper as wagon or GT).

Also there's no clear "line" what constitutes enough lowering that you should switch to B8. Is the Eibach/ACS very mild drop (pretty much brings you to sDrive m-sport height for xDrive cars) the line? Or significantly larger drop of H&R?

[sspade]

To add or further clarify the above...

The B8's are so short as a unit that they do not even accept bump stops up front. Im riding on a couple mm thick rubber donut LOL...

That being said, I have never bottomed out (crosses fingers and toes).

My thought is that the B6 and B8 are VERY VERY similar other than the shorter overall unit length of the B8.

Has anyone compared B8 and B6 rear shocks visually?

[FaRKle!]

Originally Posted by sspade

To add or further clarify the above...

The B8's are so short as a unit that they do not even accept bump stops up front. Im riding on a couple mm thick rubber donut LOL...

That being said, I have never bottomed out (crosses fingers and toes).

My thought is that the B6 and B8 are VERY VERY similar other than the shorter overall unit length of the B8.

Has anyone compared B8 and B6 rear shocks visually?

B6 and B8 front struts have the bumpstops internal to the strut itself. You'll notice that the shaft coming out of the top of the strut is much too large in diameter to accommodate a BMW bump stop.

Here's a video (go to 6:18) showing it as the shock is pulled out of the strut.

One thing to note is that the bump stops are actually made to be ridden upon in many cases and they offer an additional spring rate giving you a "progressive" rate between being supported by the spring only, and spring and bump stop. Some bump stops have linear spring rates and others (conical, like the OE ones on our cars) have progressive rates. You can tune your bump stops to get the spring characteristics you want, including lengthening them so you ride on them earlier.

[sspade]

Originally Posted by FaRKle!

B6 and B8 front struts have the bumpstops internal to the strut itself. You'll notice that the shaft coming out of the top of the strut is much too large in diameter to accommodate a BMW bump stop.

Here's a video showing it as the shock is pulled out of the strut.

One thing to note is that the bump stops are actually made to be ridden upon in many cases and they offer an additional spring rate giving you a "progressive" rate between being supported by the spring only, and spring and bump stop. You can tune your bump stops to get the spring characteristics you want, including lengthening them so you ride on them earlier.

Umm...

My rear B8's used the rear OEM bump stop...

I installed them myself and I definitely remember the fronts not using stock bump stop, only a small rubber donut in its place.

Im about 99% sure that I installed my rear OEM bump stops with the B8.

Edit: I'm 200% sure that I installed my OEM rear bump stops with my upgraded Bilstein B8 shocks (and AC Schnitzer spring kit).

[sspade]

I think you will find that you're going to re use your OEM bump stops for the rear at least with the B6...

The B8 fronts are indeed larger diameter piston and their highest performing shock so I didnt have to use my front bump stops.

With the B6 - you may. Not sure.

[FaRKle!]

Originally Posted by sspade

Umm...

My rear B8's used the rear OEM bump stop...

I installed them myself and I definitely remember the fronts not using stock bump stop, only a small rubber donut in its place.

Im about 99% sure that I installed my rear OEM bump stops with the B8.

Edit: I'm 200% sure that I installed my OEM rear bump stops with my upgraded Bilstein B8 shocks (and AC Schnitzer spring kit).

Originally Posted by sspade

I think you will find that you're going to re use your OEM bump stops for the rear at least with the B6...

The B8 fronts are indeed larger diameter piston and their highest performing shock so I didnt have to use my front bump stops.

With the B6 - you may. Not sure.

I never said anything about the rear shocks' bumpstops . I was saying that you actually do have bump stops in the front, and that they're internal to the struts themselves. The rubber "donut" isn't a bump stop.

[sspade]

Originally Posted by FaRKle!

I never said anything about the rear shocks' bumpstops . I was saying that you actually do have bump stops in the front, and that they're internal to the struts themselves. The rubber "donut" isn't a bump stop.

Okay, so the rears are different... thats the slightly confusing part LOL

I figured the rubber donut wasnt doing shit, just saying thats all I remembered putting on

Here's a short video for other people reading this about the B6/B8 technology and differences:

[FaRKle!]

Bump with new info showing Bilstein tunes RWD dampers the same as AWD.

[skier_du]

Originally Posted by FaRKle!

Bump with new info showing Bilstein tunes RWD dampers the same as AWD.

This does not surprise me. I had the B8 paired with Swift Spec-R on my xDrive for a couple years. After my experience and what has been posted here, not sure I'll ever go back to Bilstein. The ride was overly firm on small stuff, and not firm enough to dampen out the big bumps. The internal bumpstop in the front disintegrated to little pieces. And I know the Swift springs are a high rate, which is probably why I never felt the jacking down a lot, but after a couple years (probably roughly 25k miles) they couldn't control the springs any more and the car felt very unstable. The last time I had the car on the track with the B8 it was very windy, and coming over a fast sweeper, creating over a hill onto the front straight, the wind got under the car a little and made it feel very light and a bit scary north of 110 there. The shocks were pretty much shot at that point. Although, when new, that setup felt pretty good on track.