Anyone running Bilstein B8 (from B12 Pro Kit) with H&R Springs?

[99ron]

I should explain my question. Had my Z4MR for nearly 8 years now. It came with standard shocks but H&R springs on 19" Avant Garde M359s, currently with Michelin PS4S tyres. The car has clocked up 84K miles. The front shocks are I think original. The rears were changed around 54K. The H&R springs added around 64K miles. The only issue I had with the car's handling was that on really bumpy roads it struggled for traction and in extreme cases bounced like it trying to throw me out of the car, hanging on like a rodeo. Most of the time to be fair it was ok and there were known roads where I knew I'd have to back off a little. There was one particular dual-carriageway where if accelerating off a round-about and over a crest in the road the traction light would always activate. It's always been like that though.
So I took the plunge and decided to fit a Bilstein B12 Pro kit - B8 shocks with the Eibach springs figuring my shocks were due for a refresh and it might improve the traction and ride issues a little. Alignment was done supposedly to E46 M3CSL settings but looking at the numbers I think it's a severe interpretation of the CSL tollerances perhaps but seems to drive well on good surfaces. The car handling seems very much improved, not so much bouncing and traction seems great. Many others reporting an on-rails feel with this kit and I would not disagree with that.
The issue I have though, and I have since seen this same issue reported by others with this kit, is the way it now deals with impacts from things like sucken drain covers or particularly sharp pot holes or those 2-3 inch wide trenches that often get dug across our roads and then badly filled in and the tarmac sinks. The car now hits those type of things with an awful bang at the front, as if there is no suspension. Not sure if that's something bottoming out or the spring just being too hard to absorb the impacts. It's unbearable.

So at last the question - I have seen some threads here and other forums where some had re-installed the OEM springs at the front and it resolves that hash banging. I'm not sure how that works with the B8 as it's a shortened shock designed to work with lowered springs as far as I understand. In any case I don't have the OEM springs. I was thinking if fitting the H&R spring back in the front might improve things. Drops the front 25mm vs 20mm for the Eibach so not too different. So is there anyone out there running B8 shocks with the H&R springs or have any insight as to why this B12 Pro kit is soo bad as dealing with our terrible roads.
I know 19" wheels isn't helping me and I should probably have gone with the KWv3 but if there's a quick win by changing the front springs it's maybe worth a punt?

[mmm-five]

This is one of the reason I went for the B16s, as I wanted it as comfortable as possible on the road, with an easy adjustment for track (and for different tracks at that). I also didn't want it slammed, as there are too many speed bumps/cushions where I live.

Maybe the extra drop has exacerbated the short, sharp shocks. How about reverting to something closer to standard spring lengths or adjustable height shocks?

I also swapped away from 19" CSLs to OE 224Ms in the first year of ownership as the ride was very compromised on my regular weekend Welsh hoons, where a imperfectly placed offside tyre hitting a cats-eye would launch the car across the road...admittedly at quite high speeds

[PAC_Z4_F31_110Puma]

I've got the b12 fitted and have to say I don't have the same issues, it's firm but not that harsh that i'd want to revert, and Cambs/Suffolk isn't exactly free from road imperfections. Big difference is I've got 18's with PS4, not 19's with PS4S, so not only have I got more sidewall, but its also less stiff, going to account for a lot of your trouble. Not sure the e85 was ever designed with 19's in mind so you'd probably be harsher ride than any OEM or B12 setup is was intended for?

[maupineda]

there is no such a thing as B8 dampers for the Z4M, you get B6 in the B12 kit, so they are exactly the same as the OE ones length wise, if you lower the car, which you have, now you are riding the whole time well into the bump stops. so just put back the OE springs and cut the internal bump stop to 40mm, this returns you to the OE suspension travel values. if you want to lower the car, for any lowering, you need to shorten the stop further by the same amount. I would say I would not lower more than 10mm, but no aftermarket spring is that mild.

just the above will resolve your problem halfway, the issue with Bilstein is that they have just too much gas pressure and that adds to the spring rate of the entire system. the ideal solution will be to re-valve the dampers to your specifics.

[99ron]

Thanks for the responses.
@mmm-five - Yes thinking I should have spent more and gone to coilovers. Just put new PS4S tyres on the 19s so really don't want to ditch them unless I have to. And the ride is improved over the less severe bumps compared to the OEM Shock + H&R spring setup I had before and handling soo much so. If I can get rid of the crash over larger bumps then I'd be happy.

@PAC_Z4_F31_110Puma - I think the B12 kit for 30si and Z4M is maybe different so perhaps works better, even taking in account wheel sizes.

@maupineda - What you're saying makes absolute sense. Yes your are correct. B6. I read B8 off the evolve site. Makes sense then why those reverting back to the OEM front spring lose the harsh banging even without cutting the bump stops, because it isn't hitting the bump stops anymore anyway. Just unbelievable then that Bilstein sell the Pro kit with the 20mm lowered front springs that the shock isn't really optimised for. I thought that was the whole point of the kit but I guess not.

So my options are:
1 - Buy some OEM springs for the front and optionally modify the bump stops, although if I'm getting the springs changed may as well do the bump stops anyway.
2 - Just modify the bumps and allow a little extra for the 20mm lowered Eibach.
3 - Do buy some KWv3 and hope the handling feels as good as it does now with the benefit of improved ride.

I didn't go coilovers because I don't track the car and don't need the adjustability and thought the B12 Pro kit would give me that perfect fast ride setup with a little improvement in the ride. It's a weekend toy doing 2K miles a year so If I can lose that awful banging off the front bump stops then I think maybe I could be happy with that. I love the Z, never owned any car this long before, so it has to be worth a try, cheaper than another 2K+ for coilovers.

Thanks everyone for the insights. Really appreciated. Need to think it over and have a chat to Ross @ RBM to see it he's up for modifying bump-stops.

cheers

[maupineda]

You can install Eibachs and cut the stop to 25mm. I would not got shorter than this, this way you only loose 5mm on free travel compared to stock after lowering.

After that it just gets expensive.

The issue with Bilstein HD is they have too much gas and that makes them harsh. I read they should only have 40in/lb, but they have close to 60 so you need at least 60lb to make them move so small but sharp bumps cause that very effect you complain. Cutting the stumps will improve, but you will still have some of it

As me how I know!

[Matt.13]

My car has the same problem after fitting the b12 kit . Can i ask what the bump stops look like and can you cut them while fitted to the car ? .
Failing this i am thinking of just putting some oem springs back on the car. Which is a shame as i really like the stance i have now

[maupineda]

You must take the strut out. Stop are internal to the strut. There is a nut at the bottom that you remove and then the shock cartridge comes out with the stop

[99ron]

https://youtu.be/y7vt5Q6sPvY for the strut disassembly.

https://youtu.be/ff_09MxcR3k
Eibach example of trimming the bump stops
I'll be paying my excellent local indie

[Matt.13]

Thanks for the info . I will try the bump stop mod

[99ron]

Great. Let us know how you get on. Mine is booked in but 2 weeks away. Will report back. Bit nervous of cutting bump stops as there are conflicting opinions but I can't see how the current setup with this kit is correct. Nothing to lose I guess as currently I don't want to drive it with that horrible impact over bumps.

@maupineda how did you trim the bump stops, as in which end? Assume its progressive so harder one end than the other? So maybe a bit off each end?

[Matt.13]

My car is booked in for the 9th august so i will update then. I think i will just cut the bump stops in half and go from there.
I will update once the car is back

[maupineda]

Great. Let us know how you get on. Mine is booked in but 2 weeks away. Will report back. Bit nervous of cutting bump stops as there are conflicting opinions but I can't see how the current setup with this kit is correct. Nothing to lose I guess as currently I don't want to drive it with that horrible impact over bumps.

@maupineda how did you trim the bump stops, as in which end? Assume its progressive so harder one end than the other? So maybe a bit off each end?

Here are some photos, I basically just cut them from 70 to 45mm, I kept the soft part but did not go down to 40mm to prevent actual bottom out that may damage the shocks.

[Matt.13]

Thanks for the pictures - very usefull

[PDJ]

A quick google and it looks like they are not much more than £15 for a bump stop

Did not find the exact one but they all looked the same