"To BMW or Arnott To BMW?" Finally time for airbags!

[FieroSS]

Oh and in terms of ride height. Ride height didn't change after installing the new Arnotts. With the old sport/non-sport set I had adjustable links set the same on both sides which gave me a level, lowered ride height. I haven't changed those links with the new bags and the height is still the same and level.

-Tim

[adm750]

There's been a lot of convo on the length differences between the sport bags and the standard bags...and a general lack of hard data to compare the two.

I could see the sportay bags being a touch shorter...simply for the application. But that's almost somewhat irrelevant to the concept. I am no airbag scientist..(as a matter of fact, I am no scientist at all....) but I would think (in my pea brain...) that the difference between the two airbags would be the spring rate....which would be controlled by the durometer of the material used or maybe by thicker layers of bag material used, ( possibly even some variation of internal valving ??? ) in an effort to change the rate of said bag.

Maybe with airbags, (as compared to coil sprang-a-langs) simply having a shorter bag changes the effective spring rate of the bag. Again...there's a level of scientistrification involved that I am unfamiliar with...
Really the change in ride height does not a 'sport' car make. It's a shorter spring, yes, but the spring rate differs....otherwise everyone would simply cut coils on their standard springs, slap some style 5's on, and deem their ride a sport pack car...haha

[dannyzabolotny]

There's been a lot of convo on the length differences between the sport bags and the standard bags...and a general lack of hard data to compare the two.

I could see the sportay bags being a touch shorter...simply for the application. But that's almost somewhat irrelevant to the concept. I am no airbag scientist..(as a matter of fact, I am no scientist at all....) but I would think (in my pea brain...) that the difference between the two airbags would be the spring rate....which would be controlled by the durometer of the material used or maybe by thicker layers of bag material used, ( possibly even some variation of internal valving ??? ) in an effort to change the rate of said bag.

Maybe with airbags, (as compared to coil sprang-a-langs) simply having a shorter bag changes the effective spring rate of the bag. Again...there's a level of scientistrification involved that I am unfamiliar with...
Really the change in ride height does not a 'sport' car make. It's a shorter spring, yes, but the spring rate differs....otherwise everyone would simply cut coils on their standard springs, slap some style 5's on, and deem their ride a sport pack car...haha

Good points! Though I think a lot of that becomes less relevant once you start messing with the rear ride height with either INPA or lowering links.

I remember one time I bought a really crappy 98 540i (sold pretty soon after) that had the springs compressed with bolt-in spring compressors. Somebody actually drove the car like that, yikes!

[geargrinder]

I read up on air springs at one point, and the bag flex is not the driver of spring rate and the bag is designed aspiring for no "balloon" effect at all IIRC. Of course I'm sure there's some real world stretch but it's not what sets spring rate, I bet it's in the sub percentage point range. What they use to set spring rate is overall bag dimension / displacement vs the reservoir displacement. If you want to tune the system and increase the rate, you'd focus on reducing the size of the rezzies I think.

Honestly I suspect the bag difference is more Just mechanical design oriented to adjust and parallel / optimize where the bag "folds" at normal ride height. Perhaps theres some slight spring rate difference if you compared a normal bag vs a sport bag at the same sport ride height side by side but I bet it's minimally noticeable in the real world.

An interesting pointless science project would be to have the two bags side by side and jig them to fix both to the same "sport ride height", then fill both with water and see if there's a notable volume difference between them. If you were worried about them not being at pressure and not fully expanded, instead you could pressurize them to some nominal pressure w a compressor while jigged, and then drop them in a bucket and measure the water displacement... To fully model the system you'd also need the reservoir volumes including hoses...

[jp5Touring]

[Thrifty S50]

....

I arnott'd

[adm750]

jp5....that's exactly the scientistrification I was talking about....

yeah...GG...uuuhhhh....we're gonna need a few more beers than I currently got in the 'fridge for that project brother... ;0)

[jp5Touring]

jp5....that's exactly the scientistrification I was talking about....

yeah...GG...uuuhhhh....we're gonna need a few more beers than I currently got in the 'fridge for that project brother... ;0)

German engineering

[dannyzabolotny]

Well the difference must not be very big since Arnott offers a one-size-fits-all bag for both sport and non-sport tourings.

[geargrinder]

Came across some other stuff on airsprings talking about the shape of the piston under the bladder being an important driver to spring rate, i.e. if the 'piston' (aka the plug at the bottom) is wedge shaped, the spring rate is more progressive, because it physically ramps down the area of the spring as it compresses faster, v.s. a pure cylinder shape. So that's another thing that could be different between the 2 springs. Unless we have 2 dead springs sport/regular to cut open it'll be hard to know tho'. (anybody got a dead base-suspension spring they want to send me? I'll donate the dead sport spring for research purposes...)

[FieroSS]

I've got (1) of each; i'll cut them open

-Tim

[geargrinder]

Cool!

Before you do - maybe measure how much they each hold in water while being held to same height? Maybe eyeball how compressed a 'normal static ride height' bag is on your car, then squeeze each one to generally more or less that height and fill 'em up? Could make a little "H" shaped jig with some 2x4 scrap ends or something to emulate the height, slide the bag into it, fill, pour out and measure, repeat...

[adm750]

Came across some other stuff on airsprings talking about the shape of the piston under the bladder being an important driver to spring rate...

hmmm.....that somewhat goes along with my half-assed "internal valving" (sp?) theory.... That wedge shaped piston actually makes sense...

There is a difference....it's just dialing in on what the difference is... Neat thread all...

- - - Updated - - -

I've got (1) of each; i'll cut them open

-Tim

holy cow, that's right...you were the guy that had a sport bag and a non-sport bag in your car right ?? Thanks for doing this

[FieroSS]

Yes, I bought the car with (1) of each in there. I also made a comment about ride height which I've realized is useless. Of course ride height didn't change; its controlled by the sensors. The difference would have been ride quality or a spring rate difference from side to side and I didn't notice any difference on that front. I autocrossed the car a few times with the miss-matched bags and with the new Arnott's but its apples to oranges. The miss-matched bags had suspension components that were blown out and very old factory shocks. The Arnott's had all new arms, bushings, shocks and tires so it was like driving a completely different car.

Next week I'll do some fact finding with my son (he loves the experiment stuff) and we'll figure out volume of each and see what the insides look like.

-Tim

[Thrifty S50]

Quick update. Put the arnott bags in on friday, took maybe an hour. Went pretty smooth, didnt break any of the original retaining clips (somehow), but when all was buttoned up the bags did not seem to fill up, or rather, only filled up to where they were before the bag swap?

I had went with what GG said, lowering the car only slightly lower than stock (just about where tire meets fender) and turned it on (only removed neg battery cable for procedure) and verified pump came on. I let it "pump" for a bit then dropped it completely at which point it was back at the "sagging" level (tucking tire), at which point the SLS light came on as pump continued to.... pump.

Left it at my buddies shop so he could figure it out. Monday he said drivers side leveling sensor was faulty so he replaced. Not sure if that fixed it since it will now be thursday... can anyone walk me through ista+ and how to reset it to factory height? Wasnt throwing any "codes" when we hooked it up right after.

The bags definately arent empty as i putt it around the block and theres no rubbing or scraping of anykind... Also verified there were no leaks, had car lifted again as pump was working, couldnt hear any leaking type noises coming from the wheel wells or tanks, hopped in the backand no leaking from the fittings.

Frustrating!

[dannyzabolotny]

You can tune the rear ride height with INPA, but it's a rather fiddly process so I switched to using physical lowering links on the sensors. Much easier to dial everything in that way.

[Thrifty S50]

Not looking to lower, looking to tell the system to fill back up to stock height. Thanks though!

And unfortunately i only have access to ista+, not inpa, but regardless, i tried messing with the SLS functions and it wasnt too intuative. I made it to some height calibration page but failed at that too as i couldn't really get it to do anything even after entering my measured height in MM.

Kinda at a loss, just wanna cruise da wago!

[geargrinder]

My strong suggesto is don't mess with the ride height yet! If it was right before it should be right and exactly the same now! Messing with that is 1000% uneccessary after a bag swap! It'd be like having a problem after putting new spark plugs then deciding to re-do your cam timing... Nothing should have needed changing in the settings at ALL!!!

The first thing I'd do is use the live data functions to read what the controller is seeing and try and diagnose whats going on. You should be able to see the exact ride height sensor readings and find out what it thinks the deal is. How to do that in ISTA I dunno as I have it but haven't really used it at all ever. In INPA its pretty easy.

Are you sure the top caps are on fully? Note when I did mine I thought one was snapped all the way on but it was not. Hopefully you didn't put the forward cargo floor cover back on yet. I'd spray those bag-tops with soapy water and then get the pump to run and see if you have bubbles, either at the big rez-tank or small pump-fill lines.

BTW, you might start your own thread on this... since that's what people should do...

[Thrifty S50]

Why start my own thread when i can just hi-jack yours? JP! Naw i dont think its thread worthy, ill figure it out. I was just wanted to ask a quick Q.

You can have your thread back now GG

[wagons ho]

There's been a lot of convo on the length differences between the sport bags and the standard bags...and a general lack of hard data to compare the two.

I could see the sportay bags being a touch shorter...simply for the application. But that's almost somewhat irrelevant to the concept. I am no airbag scientist..(as a matter of fact, I am no scientist at all....) but I would think (in my pea brain...) that the difference between the two airbags would be the spring rate....which would be controlled by the durometer of the material used or maybe by thicker layers of bag material used, ( possibly even some variation of internal valving ??? ) in an effort to change the rate of said bag.

Maybe with airbags, (as compared to coil sprang-a-langs) simply having a shorter bag changes the effective spring rate of the bag. Again...there's a level of scientistrification involved that I am unfamiliar with...
Really the change in ride height does not a 'sport' car make. It's a shorter spring, yes, but the spring rate differs....otherwise everyone would simply cut coils on their standard springs, slap some style 5's on, and deem their ride a sport pack car...haha

Joining into the fray, late as i had no need for bags until now.
You hit on it there adm750. There is an effective spring rate for each length of bag.
So how much are we compromising in spring rate on the one size fits all bag.

There is a difference or the factory would not have made two.

[geargrinder]

Joining into the fray, late as i had no need for bags until now.
You hit on it there adm750. There is an effective spring rate for each length of bag.
So how much are we compromising in spring rate on the one size fits all bag.

Ho'man I don't think you're following the full technical details. Unless the interior of the 'piston' plug is shaped differently, or there's a really notable difference in diameter or otherwise interior volume at the same height, its unlikely the springs have any significantly different rate.

I mean there may be a difference but I'm pretty sure its extremely small (unless we get the cut-apart analysis that's been promised and find really something different inside..).

The drivers of different spring rate would be:
1. Different 'piston' plug shape inside the bottom
2. Different reservoir volumes (this would be huge)
3. Significantly different internal volume at the exact same ride height

So far I think the real differences above are negligible. Plug is an unknown, waiting to hear about that tho'.
Pretty sure the rezzies are not different (and those nobody is changing for purposes of this discussion anyway...).
Pretty sure the volume won't be notably different when set to same ride height. (Obviously the non-sport can hold more when fully extended).

There is a difference or the factory would not have made two.

Honestly my current belief/theory is it has to do mainly with optimal bag shape at ride height. In otherwords the non-sport bag, when lowered, will have more rubber rolledup/folded up at the bottom, which might be less ideal for wear on the rubber bag as you can imagine, the rubber will likely fatigue more quickly over the years if its rolled/folded over 'all the time' vs only on bumps.

But when somebody can do a side by side volume measurement (jigged to exact same 'estimated sport' ride height please!) and look inside at the plug... we're all just guessing.

[FieroSS]

Alright, after some investigation and surgery I've found the difference between a sport and non-sport BMW airbag. And before I dive into the details; it looks like Meghan Trainor was right!

So if some of you missed my previous post; i bought my car and discovered it had (1) bag with a white base (sport #37121095081) and (1) bag with a black base (non-sport #37121094614). Sitting next to each other I couldn't see any meaningful difference between the 2. They were exactly the same height and appeared to be IDENTICAL on the outside at least.

20180109_175502_resized.jpg20180109_175314_resized.jpg

After some more discussion on here I decided to cut them open and really try to find a difference. I tried what GG has been asking about and made a little rig to hold the top at the same distance but filling them with water through a 3/8" hole on the top was a huge pain. And my most precise measuring devices are some kitchen measuring cups so I figured my data would have a large margin of error so I cleaned up the mess and grabbed a knife.

I cut them open and found: absolutely nothing different. The steel cup on the bottom is the same depth and diameter between the (two)

The rubber is the same thickness (.120") with (2) layers of fibers. The rubber is also the same durometer measured on the inside (74-76 shore A). The top caps are identical and have the same overall diameter (Ø4.73") and same orifice (Ø3/8"). The bottom sealing ring where the rubber is secured is the same diameter on both (Ø3.21")

20180120_113827_resized.jpg20180120_113848_resized.jpg

But then I found the difference; the diameter of the bottom plastic base. I measured the very top where it meets the metal sealing ring and then (3) more times at 1" increments moving towards the bottom of the base. In the picture; you can see that directly below the sealing ring, the white base is a larger diameter right away.

Here are the measurements and the side by side:

White (sport):
Ø3.41"
Ø3.56"
Ø3.60"
Ø3.64"

Black (non-sport):
Ø3.21"
Ø3.39"
Ø3.45"
Ø3.53"

20180120_113644_resized.jpg

I don't know much about airbag engineering and design but the BMW folks found that a large diameter base (average Ø.143" larger) changed the characteristics enough to consider it a "sport" airbag. My Arnotts are in the car so I can't measure them. Perhaps they design theirs to be in the middle of these (2) so you won't really notice a difference whichever style you are replacing? I'm not sure

So like I was saying; Meghan Trainor was correct! When it comes to E39 airbags, apparently its all about that BASE.

-Tim

[wagons ho]

Fantastic info!! Thanks fiero ss.

[geargrinder]

AGREED! Ok so I've read enough tech on airbags to know that WOULD make a difference! The "buttplug" on the bottom acts as a piston and it's taper and shape will affect the linearity (aka non-linearity) for sure. The sport initial plug size should increase spring rate. However the non-sport has a greater delta (.32" vs .23") which would maybe mean the non-sport is more progressive vs the sport being more linear. Might make sense as sport setups often prefer linear springs vs oem sports which are often progressive to allow softer ride for early deflections. Sport starts stiffer and is more linear, NS starts softer but ramps faster.

Awesome. Thanks brother.

PS who gunna cut open a new Arnott so we can match that up!?

[adm750]

Guys, this is really neat. Thanks for the autopsy Tim..!

Great thread...