[FaRKle!]

Video Overview and Installation Guide

DIY HAS Writeup


So, I've wondered for a while if one could DIY a rear HAS setup at a reasonable cost. If possible, this would solve common issues like not getting the rear height/wheel gap/rake you want, not being able to choose your rear spring rate and flat ride ratio, and having to buy expensive "BMW fit" springs.

Spring Mounting Hardware
I noticed that aftermarket coilover systems typically had flat ground springs (like standard coilover springs) that seated against their ride height adjusters, and wondered if it was possible to obtain just the rear ride height hardware. As it turns out, both BC Racing (PN I-29-HLK) and Solowerks offer their rear perch/adjusters. I ended up buying a set of the BC Racing perches/adjusters for $100 shipped (you can select any F2x/F3x/F8x on the linked site's drop-down menus and it'll bring up the right PN).

One thing I bought that was completely unnecessary was Swift's Thrust Sheets. These are supposed to act like a radial bearing to allow the spring to spin so it doesn't bind during compression/decompression. You really don't need these.

Next, in order to use a standard coilover spring I'd need a lower spring perch/pad to rest it on. Since that side is also ground flat the perch/pad needed to have a flat base. I looked through Energy Suspension's Catalog and on page 14 found universal polyurethane spring pads. PN 9.6116 and 9.6121 looked like they'd fit over the protrusion/mount in the rear camber arm, so I bought the cheaper of the two (9.6116).

This part's flange/seat is too large to allow it to sit flush in the lower camber arm, but that was easily remedied by cutting it to the appropriate profile.

Lower spring pad not sitting flush


Modified lower spring pad


Picking Springs
The last piece of the puzzle was choosing which spring to use. Both the BC Racing and Solowerks perches can use 60mm ID springs, although you'll probably need to remove the plastic perch on the BC unit. With the perch you can use 2.5" (63.5mm) ID springs. 65mm ID springs also probably fit fine, and you can choose to use a 60mm to 65mm adapter if you like.

To pick my spring I looked through Hyperco, Eibach, and Swift's catalogs. For my particular application I'm targeting a rear ride freq around 1.8Hz, so a 900lb/in spring is what I was after. When looking through the catalogs and searching the PNs for the spring rates I wanted I was surprised at how cheap standard coilover springs are. I saw them ranging from $50 to about $80 each, and the ones I wanted (Hyperco 2.5" ID, 900lbs/in, 10" long), ended up only being $57 each shipped.

A 9" coilover spring is probably optimal for having the most adjustment range, however 8" and 10" springs are far more common. If you decide to go with a spring rate as high/higher than mine (or you want to drop your rear height really low), you'll probably want an 8" spring instead. If you're keeping things around OE rates or up to M3/4 (~650lbs/in) then a 10" spring will be fine. One thing that's important to note is that you want it so that when the camber arm is up and bolted to the rear knuckle the spring has some load on it so it can't pop out. This is a concern if you go with too short of a spring, and is why helper springs and couplers exist.

Rear HAS in Practice
Rear HAS installed


I ended up having to adjust my rear perch all the way to the top (minimum height) to achieve the 14" hub to fender measurement I was targeting. I might drop the rear a bit more, but will have to remove the locking ring to allow the height adjustment ring to get closer to the top. Many coilovers don't use a locking ring on the rear, like Bilstein, and KW, so I'm confident I don't need them.

So far I've been driving around on this setup about five days now and have been pleased with it. I'm also quite pleased this setup came in at <$250, which is less than it'd cost you to get a pair of M-Performance rear springs.

But what about the front?
I'm thinking about the front too, but haven't found anything yet that would be easy and cost effective...

[JaredG_F30]

Nice work!

[ali820]

Subscribed! Great work!

[cfm56d7b]

FaRKle!, I always appreciate your posts as suspension and brakes are my favorite areas of focus. Do you have your own shop?

[FaRKle!]

Originally Posted by cfm56d7b

FaRKle!, I always appreciate your posts as suspension and brakes are my favorite areas of focus. Do you have your own shop?

Thanks for the kind words. I find suspension very fascinating too, and it's entertaining to think of how our misconceptions/misunderstandings about it shape what we think "good," "sporty," ect. should be, versus what optimal really means.

I don't have an auto-shop, and my day job really has nothing to do with the auto industry (well unless you consider a great many orders up the supply and manufacturing chain).

I'm hoping to produce more guides/tips/ideas content in the near future. The videos thing is pretty new for me, so I'm still working on a format I like, that is effective at giving people the relevant info they need without making it too long ("brevity is the soul of wit" and all).

[Logicoeur]

This is great stuff. I went the same route on a previous vehicle. The set of BCs, on those were designed with the rear spring seat sitting under the spring. It was a pain to adjust. Found another supplier who had designed the opposite, got swift springs, poly thrust sheets, and adjust able arms thoughout. I never got to power mods on that car, but it sure rode, and handled well. You're making it difficult not to go down the suspension rabbit whole as a first milestone.

[ktw]

FaRKle! Great work as always! Been lurking on-and-off, and your analysis/modifications are always well thought out.

How are the 900 lb/in springs working in practice? I'm considering making the same change for my F31 (petrol instead of diesel).

Background is I installed Bilstein B16 Komforts a while ago--wish I had seen some of your analysis of these earlier! Bilstein say the f/r main spring rates are 289/428 lb/in, which by my calculations puts the f/r natural frequencies at ~1.8/1.3 Hz. (Interestingly the B16 spring rates give much lower rear frequency than front for all applicable F3x models--still scratching my head over that.) A bounce test by hand seems to confirm the front at 1.8~1.9 Hz, and the rear at ~1.5 Hz, so I'm still refining the motion ratios and masses on my spreadsheet, but it seems clear a higher rear spring rate will be beneficial.

I'm curious if the 900 lb/in springs worked as expected to provide flat ride paired with the higher rate front springs you already have. I'm also wondering if there's a way to know whether the B16 dampers will work ok with that much higher spring rate. A custom damper tune may come in the future, but not right now.

[FaRKle!]

Originally Posted by ktw

FaRKle! Great work as always! Been lurking on-and-off, and your analysis/modifications are always well thought out.

How are the 900 lb/in springs working in practice? I'm considering making the same change for my F31 (petrol instead of diesel).

Background is I installed Bilstein B16 Komforts a while ago--wish I had seen some of your analysis of these earlier! Bilstein say the f/r main spring rates are 289/428 lb/in, which by my calculations puts the f/r natural frequencies at ~1.8/1.3 Hz. (Interestingly the B16 spring rates give much lower rear frequency than front for all applicable F3x models--still scratching my head over that.) A bounce test by hand seems to confirm the front at 1.8~1.9 Hz, and the rear at ~1.5 Hz, so I'm still refining the motion ratios and masses on my spreadsheet, but it seems clear a higher rear spring rate will be beneficial.

I'm curious if the 900 lb/in springs worked as expected to provide flat ride paired with the higher rate front springs you already have. I'm also wondering if there's a way to know whether the B16 dampers will work ok with that much higher spring rate. A custom damper tune may come in the future, but not right now.

I'm getting my rear dampers tuned for the 900lbs/in springs in Nov, however even running underdamped in the rear (they're tuned for 650lbs/in now) the ride is very smooth! There's a lot of construction and wavy/broken pavement around my office and I was actually surprised at how much smoother those features became. My rear frequency ratio is 1.25 (or 25%) of the front. The low speed underdamping makes the rear feel a little floaty, or take a bit longer to settle on turning/acceleration, but I can't complain about the ride smoothness.

Most coilovers I've seen (with exception of Ohlins R&T on 6cyl models) produce a higher front frequency. I believe they do this for a couple of reasons. The first is they want the user to have a "sporty" feel, meaning they want you to make you "feel connected to the road." If you've ever been in a really smooth riding car and looked down at the speedo and said "wow I didn't know I was going that fast!" (because it was so smooth), well that's what they don't want you to experience... They'd rather make slow "feel fast." The second reason I think they run really high front rates is to try and support the front of the car better due to the Macpherson struts. As the struts compress you lose camber, and thus cornering grip. Since they can't count on end users upgrading sway bars, they'd rather handle it themselves by stiffening the front springs to reduce roll. Lastly, higher front frequencies make the front more responsive due to shorter transients.

My final spring rates once I get the dampers retuned will be 245lbs/in front and 900lbs/in, in the rear.

If the B16 dampers' dampening range encompasses the regular B6's behavior, then there should be a setting to accommodate the 900lbs/in springs. Heck, you may even want to go higher than 900lbs/in (with a shorter spring and helper spring) if that's the case. I can look through the data I've built up to plug the numbers for ride frequency and damping ratios with the B6, and check PNs to get a better idea if that's true, but I usually charge a small consulting fee for that.

[ktw]

Glad to hear it's working well, and I'm certainly looking forward to your review of the damper tune later.

Even higher than 900 lb/in might be a good idea like you mentioned to get a good frequency ratio. I haven't seen any application of main+helper spring in a non coil-over-damper setup, but I also don't see any reason it wouldn't work.

My original thinking was to do 900 lb/in rear and then later decrease the front spring rate slightly. For that I need to find an upper spring perch to adapt the stock roller bearing to a standard spring size (perhaps the perch supplied with Ohlins R&T coilovers). That's a project for another time though, and changing rear springs is so cheap/easy, maybe I should start with a higher rate and decide later on decreasing both front and rear. Back to the spreadsheets and catalogues to do some optimizing...

You make some good points about why manufacturers might choose the spring rates they do for coilovers. I don't have much experience with various cars/suspension setups, but I do think the B16s make some tradeoffs for "feeling fast" over actually maximizing grip, hence I'm here thinking about more modifications! Also the 428 lb/in rear springs are not very happy when there's a bike rack and two mountain bikes hanging off the back--sometimes there are practical considerations too.

[FaRKle!]

Originally Posted by ktw

Even higher than 900 lb/in might be a good idea like you mentioned to get a good frequency ratio. I haven't seen any application of main+helper spring in a non coil-over-damper setup, but I also don't see any reason it wouldn't work.

The helper spring is to keep the main spring loaded during droop events. With a really stiff spring the compression isn't all that much, so on droop, it can become completely unloaded and loose on its perches. The helper spring prevents that.

Your existing springs have a similar function. Bilstein uses progressive springs with two spring rates. If you look at your spring at static height you'll see all the initial rate coils already bound! So in normal driving you're only using the final rate coils. If the suspension becomes unloaded, that's when the softer rate comes into play to keep the overall spring loaded and in place.

Originally Posted by ktw

My original thinking was to do 900 lb/in rear and then later decrease the front spring rate slightly. For that I need to find an upper spring perch to adapt the stock roller bearing to a standard spring size (perhaps the perch supplied with Ohlins R&T coilovers). That's a project for another time though, and changing rear springs is so cheap/easy, maybe I should start with a higher rate and decide later on decreasing both front and rear. Back to the spreadsheets and catalogues to do some optimizing...

I thought about upper adapters, but the ones I saw were all for 60mm springs. Bilstein uses 70mm ID springs in the front IIRC. Machining an adapter seemed expensive. It's definitely more straight forward to go stiffer in the rear.

Originally Posted by ktw

You make some good points about why manufacturers might choose the spring rates they do for coilovers. I don't have much experience with various cars/suspension setups, but I do think the B16s make some tradeoffs for "feeling fast" over actually maximizing grip, hence I'm here thinking about more modifications! Also the 428 lb/in rear springs are not very happy when there's a bike rack and two mountain bikes hanging off the back--sometimes there are practical considerations too.

428lbs/in is softer than the OE F31 spring (485lbs/in). Heck it's softer than F30 M-sport OE rear springs. Terrible choice by Bilstein IMO.

[johnung]

Originally Posted by FaRKle!

[SIZE="4"]Video Overview and Installation Guide[/SIZE]

[SIZE="4"]DIY HAS Writeup[/SIZE]


So, I've wondered for a while if one could DIY a rear HAS setup at a reasonable cost. If possible, this would solve common issues like not getting the rear height/wheel gap/rake you want, not being able to choose your rear spring rate and flat ride ratio, and having to buy expensive "BMW fit" springs.

[SIZE="3"]Spring Mounting Hardware[/SIZE]
I noticed that aftermarket coilover systems typically had flat ground springs (like standard coilover springs) that seated against their ride height adjusters, and wondered if it was possible to obtain just the rear ride height hardware. As it turns out, both BC Racing (PN I-29-HLK) and Solowerks offer their rear perch/adjusters. I ended up buying a set of the BC Racing perches/adjusters for $100 shipped (you can select any F2x/F3x/F8x on the linked site's drop-down menus and it'll bring up the right PN).

One thing I bought that was completely unnecessary was Swift's Thrust Sheets. These are supposed to act like a radial bearing to allow the spring to spin so it doesn't bind during compression/decompression. You really don't need these.

Next, in order to use a standard coilover spring I'd need a lower spring perch/pad to rest it on. Since that side is also ground flat the perch/pad needed to have a flat base. I looked through Energy Suspension's Catalog and on page 14 found universal polyurethane spring pads. PN 9.6116 and 9.6121 looked like they'd fit over the protrusion/mount in the rear camber arm, so I bought the cheaper of the two (9.6116).

This part's flange/seat is too large to allow it to sit flush in the lower camber arm, but that was easily remedied by cutting it to the appropriate profile.

Lower spring pad not sitting flush


Modified lower spring pad


[SIZE="3"]Picking Springs[/SIZE]
The last piece of the puzzle was choosing which spring to use. Both the BC Racing and Solowerks perches can use 60mm ID springs, although you'll probably need to remove the plastic perch on the BC unit. With the perch you can use 2.5" (63.5mm) ID springs. 65mm ID springs also probably fit fine, and you can choose to use a 60mm to 65mm adapter if you like.

To pick my spring I looked through Hyperco, Eibach, and Swift's catalogs. For my particular application I'm targeting a rear ride freq around 1.8Hz, so a 900lb/in spring is what I was after. When looking through the catalogs and searching the PNs for the spring rates I wanted I was surprised at how cheap standard coilover springs are. I saw them ranging from $50 to about $80 each, and the ones I wanted (Hyperco 2.5" ID, 900lbs/in, 10" long), ended up only being $57 each shipped.

A 9" coilover spring is probably optimal for having the most adjustment range, however 8" and 10" springs are far more common. If you decide to go with a spring rate as high/higher than mine (or you want to drop your rear height really low), you'll probably want an 8" spring instead. If you're keeping things around OE rates or up to M3/4 (~650lbs/in) then a 10" spring will be fine. One thing that's important to note is that you want it so that when the camber arm is up and bolted to the rear knuckle the spring has some load on it so it can't pop out. This is a concern if you go with too short of a spring, and is why helper springs and couplers exist.

[SIZE="3"]Rear HAS in Practice[/SIZE]
Rear HAS installed


I ended up having to adjust my rear perch all the way to the top (minimum height) to achieve the 14" hub to fender measurement I was targeting. I might drop the rear a bit more, but will have to remove the locking ring to allow the height adjustment ring to get closer to the top. Many coilovers don't use a locking ring on the rear, like Bilstein, and KW, so I'm confident I don't need them.

So far I've been driving around on this setup about five days now and have been pleased with it. I'm also quite pleased this setup came in at <$250, which is less than it'd cost you to get a pair of M-Performance rear springs.

But what about the front?
I'm thinking about the front too, but haven't found anything yet that would be easy and cost effective...

Nice write up! Great stuff! Thanks for your efforts!

I'm wondering if your method may work for my issue. I'm finished my suspension mods but I still have a slight reverse rake. Wondering if it's possible to adjust the rear slightly higher?

Here's my setup: 2015 335i xdrive non-adaptive suspension. Koni Special Active Shocks, Eibach Spring Kit (-06 part number with mild 0.8"/0.6" drop). H&R sway bars and a front strut brace. Tires are summer Michelin PS4S 245/40-18, winter Michelin X-Ice il3 225/45-18

What parts would I need to slightly raise the rear while still using my rear Eibachs? What would the adjustment range be?

Thanks in advance!

[blumagic]

nice work, i wonder if the ground control f80 coilover sleeves would work for the front of an f30. I've read f32 springs have been put on an f80 so i'm assuming it could work.

They have a place holder in their shop for f30 coilover conversion but the link is empty. Might be something there in the future.

[FaRKle!]

Originally Posted by blumagic

nice work, i wonder if the ground control f80 coilover sleeves would work for the front of an f30. I've read f32 springs have been put on an f80 so i'm assuming it could work.

They have a place holder in their shop for f30 coilover conversion but the link is empty. Might be something there in the future.

F8x front struts have a pressed on piece that contains the spring perch and end link mount. Those parts are welded onto F2x/F3x struts, so in order to use a coilover sleeve kit you'd need to get an angle grinder to cut off those parts.

[FaRKle!]

Originally Posted by johnung

I'm wondering if your method may work for my issue. I'm finished my suspension mods but I still have a slight reverse rake. Wondering if it's possible to adjust the rear slightly higher?

What parts would I need to slightly raise the rear while still using my rear Eibachs? What would the adjustment range be?

Thanks in advance!

For OE-style springs and perches you can use spacers/shims placed in/on top of the spring cup as I had in the pic below. I used 0.1" thick kydex to make them. In the pic below I have three shims in the cup (which is why the rubber looks like it's sticking out of the cup some) and two on top of the cup.


If you need a set I can make some fairly cheaply.

[johnung]

Originally Posted by FaRKle!

For OE-style springs and perches you can use spacers/shims placed in/on top of the spring cup as I had in the pic below. I used 0.1" thick kydex to make them. In the pic below I have three shims in the cup (which is why the rubber looks like it's sticking out of the cup some) and two on top of the cup.


If you need a set I can make some fairly cheaply.

Thanks! That's a pretty cool solution. I googled Kydex and it's even orderable through Walmart in many different thicknesses at <$10.

[ktw]

Just a couple FYIs to add to this:

• For Bilstein coilovers (I have 48-245463) the upper perch spring seat has an OD of 59.3 mm, so a 60 mm ID spring fits perfectly.
• I used Energy Suspension 96103 for the lower spring pad. Fits the control arm without trimming, and covers the bottom of a 60 mm ID spring. It has a smaller OD on the pad area than 96116. Should cover the bottom of most 60 mm springs, but larger springs may overhang quite a bit.
• I ended up using a 9", 160 Nm spring (as Farkle noted would work). The perch is adjusted about 31 mm below the highest point with about 8 mm of room to raise the ride height, and hub-to-fender is just over 14". Of course, the correct spring length will depend on your chosen spring rate and desired ride height, but figured I'd add this data point.

If anyone is interested I can comment on how the B16 dampers work with the much higher spring rate after I have some time on the setup.

[Logicoeur]

Originally Posted by ktw

Just a couple FYIs to add to this:

• For Bilstein coilovers (I have 48-245463) the upper perch spring seat has an OD of 59.3 mm, so a 60 mm ID spring fits perfectly.
• I used Energy Suspension 96103 for the lower spring pad. Fits the control arm without trimming, and covers the bottom of a 60 mm ID spring. It has a smaller OD on the pad area than 96116. Should cover the bottom of most 60 mm springs, but larger springs may overhang quite a bit.
• I ended up using a 9", 160 Nm spring (as Farkle noted would work). The perch is adjusted about 31 mm below the highest point with about 8 mm of room to raise the ride height, and hub-to-fender is just over 14". Of course, the correct spring length will depend on your chosen spring rate and desired ride height, but figured I'd add this data point.

If anyone is interested I can comment on how the B16 dampers work with the much higher spring rate after I have some time on the setup.

How have they been?

[ktw]

The increased rear spring rate has certainly been an improvement. I feel like the car settles faster after bumps and the front and rear feel more in sync in turns. Before it felt like the rear was transferring weight much more slowly, so the car would sort of twist on the longitudinal axis at turn entry. I can't say I can feel the rear being particularly underdamped (rear adjuster set to 5 now, FWIW), but perhaps a more sensitive driver would notice. I wouldn't say the higher spring rate made the rear much (or any?) harsher.

[blumagic]

Farkle, you know if I should target 650lbs/12k with off the shelf b8s? I ordered the BC hardware for the rear and and going to order swifts.

Watched your vid that the dampers will raise the ride height and can’t have that lol

[FaRKle!]

Originally Posted by blumagic

Farkle, you know if I should target 650lbs/12k with off the shelf b8s? I ordered the BC hardware for the rear and and going to order swifts.

Watched your vid that the dampers will raise the ride height and can’t have that lol

B8s can handle a 650lbs/in rear springs no problem. You can even go higher in spring rate if you want.

[blumagic]

Originally Posted by FaRKle!

B8s can handle a 650lbs/in rear springs no problem. You can even go higher in spring rate if you want.

Nice thanks, maybe i'll try 14k. I was thinking of trying the swift bump springs as well instead of bump stops.

[Logicoeur]

Have you figured anything out for the front?