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I have Bilstein B8s on OEM sport springs, but I find the rear to be a bit harsh. It feels like it has very fast rebound that can make the ride feel quite jolty over potholes or bridge joints. My suspicions are it's riding on or very close to the bump stops. I'd like to improve that and was thinking of two solutions to add a little more shock travel:

- Dinan rear shock mounts
- Z4M bump stops

Has anyone replaced their rear Bilstein B8 bump stops with Z4M bump stops? Did it help with ride quality?

[bluewater328]

It doesn't have to be Dinan, but you should get the rear uppers that allow more travel. The both basically allow the shock to fully extend so you have more comfort as the shock isn't preloaded. I think turner does one too. That should aid with the ride.

Not sure if I understand the theory with the Z4M. Its basically shorter. So all you're really doing is allowing the shock to travel the distance farther without hitting the cushion that is the bumpstop. It isn't the bumpstop making it harsh, its the fact that you're fully compressed. I mean I get where its coming from with full free travel, but the bumpstop still compresses.

[bmw_solid]

Sorry to interrupt in but I would also like to know if changing the bump stops will make the rear more comfortable and if so, would it affect the car's handling/feel?

(Car: I have a 08 E92 330d SE with recent x4 OEM Sachs SE shocks fitted with all new mounts/ bump stops and dust covers.)

[Tambohamilton]

Originally Posted by bluewater328

It doesn't have to be Dinan, but you should get the rear uppers that allow more travel. The both basically allow the shock to fully extend so you have more comfort as the shock isn't preloaded. I think turner does one too. That should aid with the ride.

Not sure if I understand the theory with the Z4M. Its basically shorter. So all you're really doing is allowing the shock to travel the distance farther without hitting the cushion that is the bumpstop. It isn't the bumpstop making it harsh, its the fact that you're fully compressed. I mean I get where its coming from with full free travel, but the bumpstop still compresses.

This makes little sense. You either need to explain better, or rethink what you think you know.


OP, either of those bump stops (auxiliary springs) should help you. I'm not sure what the Dinan offering is exactly, but it's probably something similar to the Z4M; shorter and/or softer. The problem, from your description, is that the extra spring rate from the current long/firm bump stop is causing the suspension to extend too quickly (overpowering the shocks), and unsettling the car.

The only likely down side from this change is that you'll soften the rear roll rate, which is likely to increase understeer

[BSM n54iS]

You don't need Dinan rear shock mount kit...

https://www.e90post.com/forums/showthread.php?t=1145344

[Ozzie335i]

The Monroe are fine. And cheap.

Do rear shock mounts first.

If you weren't slamming into bump stocks before, then you shouldn't be now.

[rothwem]

Originally Posted by FCobra94

Do rear shock mounts first.

If you weren't slamming into bump stocks before, then you shouldn't be now.

I'm not sure if the B8s are like B6s, but the B6s come with a super short and hard bumpstop in the rear. I could see a softer bump improving the ride quality.

[Tambohamilton]

Originally Posted by FCobra94

Do rear shock mounts first.

This is a good point. Are your shock mounts in good shape?

Originally Posted by rothwem

I'm not sure if the B8s are like B6s, but the B6s come with a super short and hard bumpstop in the rear. I could see a softer bump improving the ride quality.

Pics? Pretty sure OEM parts are required no matter which brand/model shock is used.

Also, if you're ride quality is at all dependent on the durometer of your bump stops, you need to go back to the drawing board; that only points to the setup likely not being well thought out in the first place.

[rothwem]

Originally Posted by FCobra94

Pics? Pretty sure OEM parts are required no matter which brand/model shock is used.

Also, if you're ride quality is at all dependent on the durometer of your bump stops, you need to go back to the drawing board; that only points to the setup likely not being well thought out in the first place.

I literally just threw them out last week when I was cleaning my garage and workshop. I'll google and see if I can find a pic.

Anyways, BMW suspension is designed to be very dependent on the bumpstop. That's the reason the spring rates are so silly low. So it would make sense to me that the stiffness of the bumpstop, which is engaged basically any time you go over a bump more than an inch or so in height or go around any type of turn, would be important to the ride quality.

Also, the shape of the bumpstop makes a big impact as well since that goes along with the durometer. Smaller cross section would mean less area for the shock force, so less resistance/lower rate. The Bilstein bumps I'm mentioning are like little cylinders with no progressiveness, so I would think they'd engage more abruptly than the OEM cone shaped ones.

Edit: check this out here, I circled the provided Bilstein bumpstop.

[Tambohamilton]

Originally Posted by rothwem

Anyways, BMW suspension is designed to be very dependent on the bumpstop. That's the reason the spring rates are so silly low. So it would make sense to me that the stiffness of the bumpstop, which is engaged basically any time you go over a bump more than an inch or so in height or go around any type of turn, would be important to the ride quality.

Also, the shape of the bumpstop makes a big impact as well since that goes along with the durometer. Smaller cross section would mean less area for the shock force, so less resistance/lower rate.

+1. BMW actually call them 'auxiliary springs', not bump stops. They're designed to be very progressive; almost dual rate, so they have a spring portion of their stroke, and a bump stop portion. Even with stock suspension (sport or non-sport), they will be engaged very slightly at ride height.

[bmw_solid]

Originally Posted by Tambohamilton

+1. BMW actually call them 'auxiliary springs', not bump stops. They're designed to be very progressive; almost dual rate, so they have a spring portion of their stroke, and a bump stop portion. Even with stock suspension (sport or non-sport), they will be engaged very slightly at ride height.

Interesting, I never knew they would be engaged even at ride height.

[bmw_solid]

So what about the front OEM bump stops on the E9x models? Are they also not very good like the rear OEM stops or do they do their job right?

Also, what is the best rear bump stop for a SE suspension, where ride comfort will be increased but not much affect on handling? Bilsteins?

[Tambohamilton]

Same story with the fronts; they're intended to work pretty much exactly the same as the rears. In fact, they probably drive the oversteer/understeer equation as much as any other component. But basically if you lower the car at all, the suspension gets very close the the very firm portion of the aux spring, which results in harshness and overpowering the rebound damping of the shocks.

The best? How long is a piece of string?

Originally Posted by rothwem

I'll google and see if I can find a pic.

Ahh, yup...that makes sense. Hadn't had the need to mess with the rear suspension in a while, but you are correct on the Bilstein supplied part.

Originally Posted by rothwem

Anyways, BMW suspension is designed to be very dependent on the bumpstop. That's the reason the spring rates are so silly low. So it would make sense to me that the stiffness of the bumpstop, which is engaged basically any time you go over a bump more than an inch or so in height or go around any type of turn, would be important to the ride quality.

Also, the shape of the bumpstop makes a big impact as well since that goes along with the durometer. Smaller cross section would mean less area for the shock force, so less resistance/lower rate. The Bilstein bumps I'm mentioning are like little cylinders with no progressiveness, so I would think they'd engage more abruptly than the OEM cone shaped ones.

Yup agreed....all that applies to STOCK suspension; the bump stops have much less of a degree of intervention when you start changing parts and modifying spring rates, etc. though.

Of course stock bump stops are going to come into play much more often when comparing stock dampers to aftermarket; that's exactly what happens when the factory attempts to tune a car with an under-damped suspension and also needs to dial in heavily favored understeer on top of that as well. That's the exact reason why the shape/height/stiffness of the stock bump stop is different...it's meant to be progressive in nature as you continue to load it up with weight; especially laterally.

Throw in a properly calibrated damper though and they go from contributing to the spring rate to mostily protecting the damper from damage; hence the change in shape, height, and durometer of the material itself. If that weren't the case then adding in Dinan upper shock mounts, etc. which increase travel length, would be a heavy detriment to ride/handling, since by definition, the contact ratio would be radically different, all else being equal.

For anyone interested, Fat Cat Motorsports does a great job of diving way deeper into this topic (especially as it relates to packers, etc.)...the fundamental content was created a while back, but the overarching logic still applies:
https://www.youtube.com/user/SuspensionTruth/videos

[Tambohamilton]

Originally Posted by FCobra94

Yup agreed....all that applies to STOCK suspension; the bump stops have much less of a degree of intervention when you start changing parts and modifying spring rates, etc. though.

Yes...but not really. FCM's softest bump stop in that video was 200lb/in *initial* rate...BMW performance front springs for e9x aren't even that stiff (~190lb/in). So when you get into the aux spring/bump stop, the total wheel rate doubles almost instantly. Very firm aftermarket front springs for e9x may be somewhere around 400lb/in...so add the 200lb/in initial rate of the bump stop and it's a sudden jump to 1.5x the wheel rate from the spring alone.

And that all assumes that your ride height results in the same static aux spring engagement, which is often not the case, since aftermarket springs tend to lower the car (increase aux engagement) too.

The Eibach Pro Kit springs are a prime example; they are barely any stiffer than the stock springs, but they have the car sit 1/2" (?) further into its travel. That's 1/2" deeper into the aux spring, and closer to the extremely progressive area.

My point is; don't underestimate the influence of stock or near-stock bump stops on ride and handling. Their effect is *massive*, even with firm aftermarket springs.

Originally Posted by Tambohamilton

The Eibach Pro Kit springs are a prime example; they are barely any stiffer than the stock springs, but they have the car sit 1/2" (?) further into its travel. That's 1/2" deeper into the aux spring, and closer to the extremely progressive area.

Exactly...if that wasn't the case then there wouldn't be 1,000+ threads titled "which bumpstops should I run with XYZ lowering springs" dated back to ~2006. There's a reason why people have to scrounge through OEM parts bins to try and piece together shorter/stiffer/softer/etc. bumpstops as an afterthought when choosing such poorly devised springs in the first place...mostly due to the exact reasons you just posted.

Also, the whole point of going with stiffer springs in the first place is to rely less on the exponentially inconsistent spring rate provided by the bumpstops and more on the consistent progressive nature that the springs provide as they are fed higher loads. Paired with an appropriate damper, a proper setup will have both: sufficient length of travel so that straight line capability is managed without bottoming out every time you run over a pebble and also predictable weight transfer is accounted for when nearing handling limits.

If that weren't the case then keeping the stock junk and just shimming each damper, front and rear, to achieve desired under/oversteer results via preloading the bumpstops would be the most logical choice, but that's obviously a poor decision in most every instance.

Again, bumpstops are the last piece of the puzzle; it's not one size fits all, especially when springs, dampers, sway bars, etc. are all being swapped out at the same time and no thought is being invested into all those pieces and how they'll correlate/interact once combined as a whole.

[rothwem]

Since this topic is way off the rail already and we're talking about bump stops, anyone ever tried these?—

https://eibach.com/us/c-112-products...g-systems.html

Basically tiny baby springs that fit over the shock shaft

[Welcome to NBA Jam]

Thanks for the detailed discussion all. It sounds like first I should start with rear upper shock mounts to give a little more travel. I have noticed that the E90 in general tends to sit on the rear bump stops. Why they made this design decision is unknown to me - however, I'm not an engineer, so I'm sure they have their reason.

I'm hoping this will help out with the harsh pogostick-like feel of hitting larger bumps in the road. The front feels normal and acceptable, but the rear feels like it has very little travel. It's a harsh compression and rebound.

[Tambohamilton]

The front should also rest on the bump stops; about 0-5mm preload on them at all 4 corners. The reason is that the bump stops ("auxiliary springs ") are there to make the wheel rate ramp up fast when the suspension is compressed further than the static level. This allows immediate and improved body control (reduced roll/wallow), but still decent initial compliance for comfort.

(Probably) all street cars have a higher ride frequency (stiffer suspension) at the rear than the front - this gives a flatter ride (less porpoising) after hitting a bump; oscillations of the rear end happen faster than those at the front, which hopefully contacted the bump first ,and therefore the back end gets in sync with the front quickly. So the rear end is going to feel harsher than the front, with stock springs.