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Thread: Converting from Coilovers - Understanding the Physics!

  1. #1
    Join Date
    Nov 2012
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    UK
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    390

    Default Converting from Coilovers - Understanding the Physics!

    Good day Stancework peeps!

    So I currently run HSD monopros on my E46 M3 with custom 16kg 125mm front springs & helpers and BC 18kg 120mm rear springs. The ride is very firm which I like 'most' of the time and was quite surprised by actually how comfortable going that stiff actually is as long as the surface is not stupid! Compared to my standard BC set up on my E60, it seems to handle the bumps better and with more grace, unless those bumps get too big! The reason for going so stiff initially was to help with clearance for the 18x9.5 et10s up front and 18x11 et8 in the rear without having to run silly stretch (I run 225/40 and 265/35 respectively and around 4.5* deg camber all round). But having had it for over 2 years now, I do actually quite prefer the firmer ride, although I have not yet pushed it crazy hard in fear of rubbing at the back. I've still had fun with it however with some great touring trips to the likes of southern Italy and Scottish Highlands.

    The car:






    Now, as per pretty much anyone running low static, you do get fed up with the constant battle with speed humps, driveways, bumps in the road, will I get on the ferry, into the hotel car park etc. The recent Highland trip for example, due to a couple of suitcases and supplies, the extra weight meant that the smallest bump caused the rear arches to contact the tyres. My arches got f*cked up over the 4 days and the outside of the tyre is somewhat buggered. I wasn't then able to push the cars performance to where I would of liked. So I started looking at solutions using air...

    I don't really want to go any lower and as cool as airing out is I think I can live without it. I want the handling characteristics I have now at this ride height with the ease of flexibility to go up for clearance on the front and load compensation rear. At first I was thinking of running air cups on the front and going full spring/damper coilover on the rear with an airbag in the OEM spring location. I then get the same coil ride I like but with clearance up front with the cups as needed and the bags rear to compensate for load or bumpy roads to stop the arches hitting the tyres. Unfortunately tho, my HSDs being a fat inverted damper, cannot accept an air cup. So as I would need to spend out on either new front coilovers or even a full kit. I believe I would also potentially need to run two separate management systems, one to air the cups on a switch and one to manually put pressure in as required in the rears? With that cost, I may as well then consider a full air set up. But thats where I get stuck with some of the physics...

    From what I have grasped and understood, air springs are typically progressive in nature, especially in most of the kits on the market with the double bellow or tapered versions. They have an optimum psi for comfort and ideally you should adjust your setup to be at this psi for everyday driving and adjust the height on the damper thread to suit the look/stance you want at this stiffness. As the bag is progressive, it will give a comfortable ride in normal conditions but firmer, less roll under weight changes/load (ie cornering), compared to a linear coil spring at the same stiffness under load. Got that bit.

    Now, that's fine. If I want to achieve what I had aimed for with the coils & cups set up as detailed earlier, this is my low setting and I can even run it a little firmer for the ride and clearance I need and in theory should actually be more comfortable than my linear spring set up - right? And I can then still raise it up for clearance all round and adjust for weight changes or bumpier roads in the back. Fair enough. So it seems ideal, but thoughts on my logic?

    Furthermore, I have read that with a lot of people set ups, the run 4 levels: high, comfort everyday, firmer possibly track mode or highway and then aired out. Assuming the 2nd level of comfort is at the designed bag psi, when dropping down the pressure for the sportier mode does it not lose the stiffness? How do people cope with creating any clearance issues? Or do they ensure their set up tucks so clearance is not an issue? And finally, surely this effectively softer setting is worse for more enthusiastic driving?

    Thanks for listening.

  2. #2
    Join Date
    Nov 2012
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    UK
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    390

    Default

    No thoughts anyone?

  3. #3
    Join Date
    Jul 2018
    Location
    Maine
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    478

    Default

    Honestly dude I would just install rubber bump-stops so the car literally CAN'T rub. But they won't stand up to that abuse forever, that being said. I (cheap bastard) would either do that or increase the rear spring rate somehow. Bags are good but a franken-bag-coil setup sounds more trouble than it's worth imo
    You can daily a monster truck, it's still not practical.
    Dailylow.

    Intro;
    http://www.stanceworks.com/forums/sh...19#post1598619

  4. #4
    Join Date
    Nov 2012
    Location
    UK
    Posts
    390

    Default

    Bigger bump stops is not a long term fix.

    The "franken-bag-coil" set up is what a lot of estate cars and trucks use to level out loads and is the easiest way to compensate for loads running coilovers without having to physically adjust the height each time on the collars.

    Am I correct in thinking with regarding to air ride setups, that if you are running it at a lower psi than the bags 'design psi' that it will be softer and potentially bouncier?

  5. #5
    Join Date
    Jul 2018
    Location
    Maine
    Posts
    478

    Default

    I mentioned that. I know trucks use it but I haven't seen it on a lowered car yet. Not saying it's impossible just saying itd be easier to do one or the other rather than both.

    Yes I would assume you're correct until you bottomed out
    You can daily a monster truck, it's still not practical.
    Dailylow.

    Intro;
    http://www.stanceworks.com/forums/sh...19#post1598619

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