[charlie_m]

So I've been looking around for info on replacing my rear shocks on my stock 2012 E92 328IX, and decided on Bilstein. Since I wanted a little more damping than stock, next question was B6 or B8? I found lots of opinions and second or third-hand information and did a lot of reading. Bilstein guidance was to use the B6 (24-120395) for cars with stock spring heights, and to use B8 (24-120425) for cars with lowering springs or factory MSport springs. Since this car has the MSport package (S337), I ordered and installed a set of B8 shocks. Then I discovered that AWD cars are not equipped with MSport springs, and that I should have used the B6 shocks. In any case, now I have both in-hand and I thought I'd post a comparison of all three for future reference. So below is a short comparison of B6, B8 and OEM Sachs rear shock dimensions and damping.

Please refer to the pics for location info
* Point "A": bottom of the shock, trapezoidal support
* Point "B": Top of the shock body
* Point "C": Top of the bump stop
* Point "D": Top of the shock rod
* Point "E": Top of rod threads

Measurements

All measurements are in mm, taken at full shock extension.

                              Bilstein B8     Bilstein B6      Sachs OEM

Overall length     "A-D"         488             505              530
Rod Length         "B-D"         130             150              160
Bump Stop Length   "B-C"          60              60               80
Travel to bumpstop "C-D"          70              90               80
Rod Diameter         -            14              14               14
Body OD              -          52.3            52.3             48.2
Stud thread, bottom          M10-1.5         M10-1.5          M10-1.5
Stud thread, top             M10-1.0         M10-1.0          M10-1.0

Size Comparison

Bilstein bump stops are the same on the B6 and B8, and are 20mm shorter than Sachs OEM. Shock travel, until the bump stops come into play, is "Rod Length" - "Bump Stop Length", and is actually 10mm greater than OEM for the B6 and 10mm less than OEM for the B8 shock.

Damping Comparison

Damping between the two Bilstein units B6 and B8 is almost exactly the same, within manufacturing tolerances for a mass-produced shock; maybe 10lb difference in bump over about 4 in/sec. These are typical Bilstein digressive valving damping curves you see in their monotube designs. I did not dyno the Sachs OEM unit, as both of mine were blown (but only one was actively leaking). Please see the pic for the damping curves (both B6 and B8 are on one graph).

[Edit] I did disassemble the OEM shock and confirmed that it was a twin-tube design. Compared to the mono-tube design, twin-tube shocks usually have smaller diameter pistons, poorer damping control, shorter lifetimes and are subject to oil aeration (so further changing damping characteristics). Mono-tube shocks are able to keep the nitrogen gas separated from the oil, so there is no aeration.

If anyone is curious enough to send me another new rear shock (B4, or Sachs OEM?), I can test it as well (and return it of course).

Full Droop

Full droop is when the wheel assembly is at the bottom of it's travel. In my case (measuring from fender edge to center of the BMW roundel in the wheel cap), my full droop dimension went from 465/461 mm (L/R) with the Sachs to 440/440 mm with the B8. This tells me that I have about 25mm less droop travel than previously, and also that the shock is (now at least) the determining factor in droop travel.

Summary

I found out that the damping is the same between the B6 and B8 units. It was unclear from the Bilstein description (Performance vs Performance Plus) and that's what I wanted to know.

I've got to do some more measurements and decide if the shorter B8 will work without problems on my car. If not I will install the B6. I don't think that using the rear shocks as droop stops would be a problem (I seldom get much air ), just need to make sure installed bump travel is OK.

In either case, I'll have a set of rear shocks up for sale shortly, either B6 or B8.

[tlow98]

Love this. Great post!

[Hoody007]

Looking at this I wonder what the measurements of the stock M Sport struts are. I have the B6 on mine with OEM M Sport springs, as I was erroneously told by Turner that the B6 were the correct application, and the ride seems fine - obviously stiff, of course. The sport suspension is 10mm lowered from stock, and that seems to be the delta between the OEM non-sport Sachs and B6 anyway...

[mweisdorfer]

Originally Posted by Hoody007

Looking at this I wonder what the measurements of the stock M Sport struts are. I have the B6 on mine with OEM M Sport springs, as I was erroneously told by Turner that the B6 were the correct application, and the ride seems fine - obviously stiff, of course. The sport suspension is 10mm lowered from stock, and that seems to be the delta between the OEM non-sport Sachs and B6 anyway...

I believe B8's or the Bilstein B12 kit is about 1/2" lower than the m-sport suspension

[lowrydr310]

Don't the B6 and B8 dampers have internal bump stops? Or is that only on the struts (front)?

[charlie_m]

Originally Posted by lowrydr310

Don't the B6 and B8 dampers have internal bump stops? Or is that only on the struts (front)?

AFAIK there's no such thing as internal bump rubbers, ie a 'soft landing' inside the shock chamber. Maybe some people think of 'internal' to mean 'under the blue weather shield'?

Without the foam external bump rubber, the shock rod will travel all the way down until the top of the chrome rod is ready to go into the rod seal. IE, looking at the pics above, point 'D' will go all the way down to point 'B'.

Bump rubber hardness, shape and length, as well as length of travel until you hit the bump rubber, are important fine-tuning tools on serious track cars.

In fact I wonder how many poor outcomes of aftermarket suspension 'upgrades' is due to inadvertently running on the bump stops or having too little travel until hitting them.

[lnxguy]

Originally Posted by charlie_m

AFAIK there's no such thing as internal bump rubbers, ie a 'soft landing' inside the shock chamber. Maybe some people think of 'internal' to mean 'under the blue weather shield'?

Definitely such a thing as internal bump stops.. Pic of a Bilstein internal bump stop below.

http://www.brazeauracing.com/photos/bumpstop02.jpg

[charlie_m]

Ah, OK, internal to the strut but external from the pressurized oil / gas chamber.

Thanks, good to know the bump rubbers in front struts can be tuned as well.

[steifan]

Well done!

Here's my (relatively bad) take on a similar subject:
Bilstein B8 Sprint 24-120425 vs BMW 33528036026 / Sachs 814903001543

[charlie_m]

If you are just installing them now, you might want to mark (on the yellow) where the blue cover is when you are on the bumpstops. That way, once installed, you can tell how much (or how little) travel you have until the bumpstops make contact. I will be doing that next time.

[Andy198712]

Originally Posted by charlie_m

If you are just installing them now, you might want to mark (on the yellow) where the blue cover is when you are on the bumpstops. That way, once installed, you can tell how much (or how little) travel you have until the bumpstops make contact. I will be doing that next time.

That’s a good idea!
I’m looking into which bump stops to buy at the min and there’s mixed results!

[streetpro]

B8 has an internal bump stop for sure.
I have a bad experience with B8 on e39 platform and I was apparently not the only one, as one German colleague had the patience to get the replacement (after the fail like mine did within one year, zero hits, moderate driving, few kms) and the very same happened again - slope in the piston, clunking sounds, rust and moisture getting in up to the point that it freezes inside during the winter - harsh driving experience.

Once I switched to Sachs, it was 90% of the Bilsteins capability with zero hassle and more comfy ride.

[tjbuggy2b]

Thanks for this review/info!!! I am looking at mixing my MSS springs with B8s on my g30 540i but wasn't sure if the strut diameter would work. Obviously, the B8s for mine may be different than your shocks but it gave me enough confidence to give it a try.

[rackhenry]

Great review. had no idea that both B6 and B8 Have same damping charecterisitcs

[crashoverride]

This is such an amazing post with pics and all it should be a sticky pinned post for all to find easily