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      01-25-2023, 12:26 PM   #1
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Question about Arnott rear airbags, are they firmer or stiffer than OEM F85?

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Question about Arnott rear airbags. Are they any stiffer than oem F85 X5M rear bags? My stocks don't leak, only 50k on them, but I want to upgrade them. I'm a little lowered, so the less inflated airbag spring with lower psi is effectively softer and a lower spring rate. So I'm hoping the Arnott are firmer or higher equivalent spring rates..

Can anyone who made the switch on an F85 X5M or F86 X6M specifically chime in?

One big concern is that Arnott lists the same part number as compatible with ALL the E70 and F15 and F85. On BMW side, the OEM part numbers are all different, rightfully so. I'd have to assume that an OEM F85 rear airbag spring has different performance & handling characteristics and firmness compared to an E70 or F15 base, non sport and non EDC package car would have. I reached out to them to see which of all those theirs are equivalent to, and they were not able to answer that. They just said they're "manufactured to be the same as oem" and that they don't get any complaints about them. But same as which? That they wouldn't say. They do have a 30 day satisfaction and I can return if needed. But I'm hoping some of you can shed light before I put the time into that. Especially if it turns out they're even softer because they're trying to replicate a base comfort model.

Thanks.
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      01-25-2023, 12:32 PM   #2
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i wonder this same thing. 50i owner here
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      01-25-2023, 09:00 PM   #3
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F85 owner here, did the swap in a reputable independent shop, the tech told me he adjusted the PSI and height to the factory spec, and I don't feel any diff comparing to OEM
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      01-25-2023, 09:10 PM   #4
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That's unfortunate. I'm hoping to find an upgrade. Since lowering the car softens the rear when it already needs to be a bit firmer. Had hopes, but that's a bummer.

Anyone know of any bags that are a handling upgrade over stock? Firmer? Hope is that if at equal heights the aftermarket is stiffer, then the lowered aftermarket would at least be as good as stock ones at stock height and stock psi firmness.

Another thought is if Arnott bags intended for a different BMW are naturally shorter or stiffer, so that more psi in a smaller bag results in the same height but firmer. Haven't looked much at other models, but one I had in mind was maybe a rear bag from a 5 GT or 7 would have more air pressure in it to result in the same height as a lowered (less inflated) F85 bag.
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      01-26-2023, 02:04 AM   #5
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Did the swap myself on my F86. Didn't adjust psi or height and they feel and ride the same as the OEM ones they replaced.
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      01-26-2023, 06:39 AM   #6
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I've also discovered Bilstein makes replacement bags. That's a little more encouraging. I've had Bilstein products numerous times and it's usually at least some upgrade over stock. And the Arnott bag is marked "Max psi 7 bar" the Bilstein is marked 9 bar max. Leads me to believe the rubber could be thicker, which would be firmer. Might try those.

Anyone have experience with the Bilstein bags?
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      01-26-2023, 09:17 AM   #7
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The bag type is immaterial. The volume and pressure of air inside the bag is the "spring". The bcm adjusts to a target ride height based on ride height sensor and a table that defines the voltage of the sensor to a realtive pressure for the known volume of the bag. Lower ride height = lower presure.period, full stop, nothing you can do.

Your also thinking about this wrong. The dampers(shock) job is control and manage the compression and rebond impact of the spring. If you are worried about controlling "bouncing", you need to go with an adjustable shock ie KW.
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      01-26-2023, 09:48 AM   #8
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The pressure is a big part, yes, but I disagree that the bag material doesn't matter. If you compare two tires of equal size with the same psi, but one is a "touring" tire with a thin, flexible sidewall, and the other is a performance tire or RFT, one will obviously be drastically softer and the others will be stiffer. Same psi and same size. Same principle here. A thicker rubber will flex less than a thinner one, not really debatable.

And that's why the thought of a shorter bag, intended for a different but similar weight car ie 5 GT, could help. At least in theory. I don't know the measurements or psi of the oem bags. But a theoretical equation -

Say F85 bags have a measure of 5" deflated and 8" at 100psi (just for easy hypothetical math)

Say the F07 5 GT bags measure 4" deflated and 7" at 100psi. 1 inch shorter, same psi.

Again, just hypothetical numbers. But the logic and theory is there. And the reason I mentioned the 5 GT bags is because their weight is in the same range as X5s, but they obviously sit lower. And at least from comparing photos, the top and bottom fittings look the same. Where the E53 X5 bag looks very different.

I'm trying to think outside the box a bit, especially since options are very limited. And I get it, people like KW. Personally I do not, not for this car.

Then "deflating" the F85 bag to 7" might only take 80 psi but the 5 GT bag would be at 100 psi.
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      01-26-2023, 10:20 AM   #9
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Quote:
Originally Posted by ENDEE666 View Post
I've also discovered Bilstein makes replacement bags. That's a little more encouraging. I've had Bilstein products numerous times and it's usually at least some upgrade over stock. And the Arnott bag is marked "Max psi 7 bar" the Bilstein is marked 9 bar max. Leads me to believe the rubber could be thicker, which would be firmer. Might try those.

Anyone have experience with the Bilstein bags?
Do you have a link to the Bilstein bags?
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      01-26-2023, 11:08 AM   #10
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40-221595 is the Bilstein part number.
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      01-26-2023, 02:43 PM   #11
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ENDEE666 I've been running them for a good while ever since OEM bags gave out. They look to be solid so far and yes, the material does seem more rigid. I recall it being a tad more expensive than Arnott but nothing close to the point of breaking the bank...

Firmness-wise, I can't see why there is a need to make the rear firmer when you go lower with springs. My F16 is on VTF and the rear seems so stiff already...
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      01-26-2023, 02:58 PM   #12
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Quote:
Originally Posted by lht5059 View Post
ENDEE666 I've been running them for a good while ever since OEM bags gave out. They look to be solid so far and yes, the material does seem more rigid. I recall it being a tad more expensive than Arnott but nothing close to the point of breaking the bank...

Firmness-wise, I can't see why there is a need to make the rear firmer when you go lower with springs. My F16 is on VTF and the rear seems so stiff already...
You're running the Bilstein air bags? If so, that might be why you don't think it should be firmer, if the Bilstein are in fact firmer. Especially since you're lowered as well.

And since going lower, the rear feels softer. Because it is. I notice it handling wise and I get more squat during accel. It's not terrible, I just want it to be better if I can.

It's not bouncy like most people assume or complain about. I'm not that slammed, and I also still have full shock travel because I bought Vettepro's one off setup. I love it. Just wanted to firm up the rear.
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      01-26-2023, 04:03 PM   #13
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Put KW adjustable springs in the rear and code out the air pump. That's what I was going to do.
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      01-26-2023, 04:11 PM   #14
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Quote:
Originally Posted by VETTEPRO View Post
Put KW adjustable springs in the rear and code out the air pump. That's what I was going to do.
My suspension hero! I agree, I think the solution with certainty is definitely the springs and perch and no more air. For me, that's probably fine since I don't tow.

I am going to try out the Bilstein first though. Happy to be the test case. Not looking for a dramatic difference, especially if it allows me to keep the air. If it at least feels like stock firmness at the lower height, I'd be plenty happy. Also saves the considerable amount of time adjusting a coilover. I've had them on several cars, and that's always annoying. If the Bilstein don't seem any different from stock, then a rear spring and threaded perch will be next. I know BC sells the springs and perches by themselves. Not sure if KW does. But BC also has numerous spring rates to choose from.
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      01-26-2023, 05:32 PM   #15
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Quote:
Originally Posted by VETTEPRO View Post
Put KW adjustable springs in the rear and code out the air pump. That's what I was going to do.
Are you able to share/do a quick write up on how to code the pump out. I know allot of us would be very interested in this.

Have seen a number of people say that its "theoretically possible", but no info on how.
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      01-26-2023, 05:35 PM   #16
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Bimmercode is one option. I've also seen options that just remove a fuse for the pump. There are write ups and even YouTube videos on this.
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      01-26-2023, 05:54 PM   #17
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Quote:
Originally Posted by ENDEE666 View Post
My suspension hero! I agree, I think the solution with certainty is definitely the springs and perch and no more air. For me, that's probably fine since I don't tow.

I am going to try out the Bilstein first though. Happy to be the test case. Not looking for a dramatic difference, especially if it allows me to keep the air. If it at least feels like stock firmness at the lower height, I'd be plenty happy. Also saves the considerable amount of time adjusting a coilover. I've had them on several cars, and that's always annoying. If the Bilstein don't seem any different from stock, then a rear spring and threaded perch will be next. I know BC sells the springs and perches by themselves. Not sure if KW does. But BC also has numerous spring rates to choose from.

I don't think the Bilstein OE will be any stiffer tbh.
Wish we can find a solution which works great with the VTF kit.
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      01-26-2023, 06:01 PM   #18
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Sounds like lht5059 has them and says the material is more rigid. He's also lowered on VTF. Based on them being rated 2 Bar higher psi, it seems likely. I may also try out bags from F07 5 GT. If they're intended to be shorter, they might be a higher psi at smaller height.
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      01-26-2023, 06:11 PM   #19
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Quote:
Originally Posted by ENDEE666 View Post
Bimmercode is one option. I've also seen options that just remove a fuse for the pump. There are write ups and even YouTube videos on this.
Where in bimmercode is the option? Can you please link to these write ups or vids?
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      01-27-2023, 06:42 AM   #20
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Ordered the Bilstein yesterday and they shipped today. I'll report back.
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      01-27-2023, 11:03 AM   #21
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Quote:
Originally Posted by ENDEE666 View Post
Ordered the Bilstein yesterday and they shipped today. I'll report back.
Very excited for the outcome!

Where have you ordered yours?
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      01-27-2023, 02:48 PM   #22
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I’m sure bags are bags when it comes to stiffness as it’s the air that’s supporting you not the rubber, surely that just holds in the air. An empty bag that is just rubber won’t hold up anything.

I’ve fitted bilstein and arnott and thought the bilstein was ok Arnott are the ones I prefer. Both are far better than the eBay £30 ones. Pretty sure Arnott are linked to continental tyres too which is no bad thing as there a premium brand in the tyre world which is rubber.
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