Help! Pelvis turned into dust with BC Racing Coilovers

[LorenzeTorres]

Looking to get some advice/help regarding the ride quality on the rear of my BC's. These aren't my first set of coils, however it is my first time setting up suspension on my E30. I've never felt such bad ride quality before, which makes me wonder if something isn't set up right.

Application: 92 Vert
Issue: Ride in the rear feels really harsh and bouncy

Issue occurs when:
I feel the harsh drop when the car is coming down from a bump. It's as if the whole chassis is dropping down and there's no suspension travel.


The bouncy-ness comes from when the road is wavy and uneven.

What I've done so far:
Spring is dropped to lowest with both collars on
Raised the shock body to take pre-load off and add more travel?
-not sure is this is set right
-not sure if it made the situation worse or better

Front coils are set to 14 clicks from softest
Rear coils are set to 10 clicks from softest
It used to be 5 click from softest and the car was too bouncy
I'm thinking of taking the rear to 14 clicks from softest

I'm thinking of swapping the springs out, but not if it will help with the issues or which ones to get. The springs were the stock ones that came with the kit.

P.S. I've ran Billies and Koni's already and they didn't perform well for drifting, but they rode nice. I want to work on dialing in my current setup.

[Jb325is]

Is this a coilover rear setup? Any idea on the rateS?

[shootnjunky]

Why not call bc themselves?

[LorenzeTorres]

Why not call bc themselves?

I shall! But I want to see if anybody here has gone through all this already.

Is this a coilover rear setup? Any idea on the rateS?

This isn't a true coilover setup, where the shock and spring a separate. I'm guessing the rates are 8K rear from factory

[Mike36]

Curious about this too.

[Jb325is]

What are the front rates? 8k is pretty soft for an e30 rear, I would guess the shock isn't well matched to that

[ForcedFirebird]

When you soften rebound (shock is easier to pull OUT, not compress) and take all your preload away, you are creating space between the top of the spring and bottom of perch when at droop. So, when lowering rebound, the compression gets stiffer in comparison. Combine that with floating springs and you get that big hop on a dip in the road. Try increasing rebound and just a tiny bit of preload (just enough to keep spring wedged between adjuster and upper perch). Now instead of the chassis bouncing on the top of the spring, the wheel will want to come off the ground (keeping the wheel and chassis tighter together).

What are the front rates? 8k is pretty soft for an e30 rear, I would guess the shock isn't well matched to that

Not particularly unless the front to rear ratio is fubar. See the master e30 spring list below (8k is like 500lb-ish)...

Spring---------------------Front(lb/in.)------------------Rear(lb/in.)

Stock 325i*-----------------------106-------------------------245
Stock 325is*----------------------118-------------------------265
Stock 325ix*----------------------106-------------------------308
Stock M3*-------------------------114------------------------314
Stock M3--------------------------140------------------------250
Dinan Sport-----------------------172-------------------------300
H&R OE Sport---------------------180-------------------------380
H&R Sport-------------------------180------------------------380
H&R Sport (29664)---------------200--------------------------285
H&R Race-------------------------315------------------------ 570
H&R Coilovers***----------------340--------------------------380
Eibach Pro-Kit--------------------114 --------------------------280-326 progressive
Eibach Race----------------------160--------------------------445
IE SIII----------------------------315--------------------------570
Koni Coilovers-------------------342----------------------------407
Cosmo Coilovers----------------450-----------------------------560
Tokico----------------------------165--------------------------265
TMS J-Stock**-------------------680--------------------------1026
Alpina(est.)---------------------~200-------------------------~350
FK 60/60 cup kit-----------------379----------------------------379
Vogtland Standard***--------115/197---------------------------360
Vogtland CS***---------------137/200--------------------------400
Vogtland--------------------------225--------------------------360
Intrax-----------------------------126--------------------------250
King Springs "Low" -------- KDFL-01 220----------------KDRBL-14 440


*= Sourced from June, 1987 article of "Motor"
**= Alternate rates available
***= M3 Application

[LorenzeTorres]

What are the front rates? 8k is pretty soft for an e30 rear, I would guess the shock isn't well matched to that

8K fronts 8K rears. I’ve read that people have gone 10k-12k on rears

[e30davie]

The rear of e30 has a fairly large lever arm compared to the front so its not fair to compare front and rear springs wthout considering it. I thought the lever arm on the rear was like 0.67 where as the front is 0.9 or so.

[Jb325is]

8K fronts 8K rears. I’ve read that people have gone 10k-12k on rears

8k/8k is pretty dumb. The wheel rate conversion is like .90/.48 IIRC, so multiply those #'s by your front and rear spring rates and you'll see why that sucks. There are more factors to consider, such as desired wheel frequency and weight distribution for your setup...but your front/rear split is way wrong, so start there

[e30davie]

With the rates given above for the 325is of 118lb front, and 265lb rear, using the wheel rate conversion quoted above that makes them equal to 2.61kg/mm front, and 2.25kg/mm rear. Which makes the rear 85% of the front in oem.

With 8kg/8kg it makes it equivlent to 7.2kg front, and 3.84kg/mm rear. which i reckon -means the rear is not stiff enough if the goal is get similar ratios to oem just stiffer overall. With these rates i would expect the front to quite stiff, but the rear to be bouncy in comparison, which is what you describe?

8kg front and 12kg rear would be equivilent to 7.2kg front and 5.76kg rear. Making the rear ~80% of the front, which is pretty close to the same ratio as oem. The 12kg rears sound like a better option to try and keep the ratio of front to back similar to OEM.

Where does the choice of 8kg/8kg come from? I just googled BC coilovers and they are quoting 6kg/6kg which seems like an odd combination too.

[ForcedFirebird]

Yeah, those spring rates will make it bounce, also it will handle like a dump truck on turn-in entry.

[e30davie]

BC is a fairly reputable company. Why are they supplying such rates that dont seem to make any sense.

[ForcedFirebird]

BC is a fairly reputable company. Why are they supplying such rates that dont seem to make any sense.

BC is an OK company that provides inexpensive coil overs to a large client base. Doubt there's a tech in house that can relate to any specific mfgr.

[superj]

maybe the front and rear springs are swapped? I am not sure if coil overs look the same for front and rear but if they do, maybe you have the fronts out back and vice versa?