[335m]

I just installed BC BR Coilovers as my only suspension upgrade and Bounce #2 is bothering me greatly. When I drive straight my car bounces up and down much too often, is this product the cheapest way to minimize bounce without doing full bushing replacements and stiffer springs that will ruin what I have left of ride quality?

[ajsalida]

There may be more than one reason why people are getting bounce #2.

a) Shocks are new and seals etc have not broken in, so there is stiction (sticky friction). Result is they do not move in response to bumps as they should, and if the existing shock mounts are softer than overall stiction in the shock, you get pogo-ing. Adding stiffer shock mounts can overcome the stiction and get shocks to move again and oppose motion like they should. Or let them break in a little.

b) Shocks are in good condition but are too soft for the springs, or springs are too firm for the shocks. Car still pogoes around but not because the shock mounts are too soft, it is because the shocks are just not damping the spring motion. Adding stiffer shock mounts may make this worse, or have no effect. Changing to stiffer shocks may also make it worse and now maybe shocks/springs are too firm for mounts.

c) Shocks are too stiff for the springs, too much damping, no stiction. Adding stiffer shock mounts will make this worse. Mainly bouncing off the tires here, which is made a lot worse if you have the run flats.

d) Shock mounts are installed improperly, nothing else wrong with anything. I see this often. For some reason the e9x mounts confound a lot of people, DIY types and even "pros", and it is easy to install the pieces in the wrong order or leave stuff out. One guy here had Dinan top-adjust rear konis, rear susp feels funny, he removes carpet etc, shock mount gone! All of it! nothing but a rod with not even the top nut on it. People leave out the lower washer/hat thing that the dust boot sits on often, too. That would cause problems because then nothing is clamping the lower half of the bushing to the underside of the unibody hole...easy to leave out the little metal sleeve inside the mount often too.

e) shock mounts (upper, lower or both) worn. Obvious.

f) shocks for whatever reason, at some key frequency, are stiffer than shock mounts. Rubber parts have resonances and those resonances interact. Changing the shock mount may move the freq where mounts are soft away from where shocks are firm and allow mounts to stay put while shock applies forces on the particular types of bumps or road surfaces the driver notices pogoing most.

g) lower shock nut not fully bolted on, shock just bounces around in the bottom mount. Everything else installed properly.

etc. etc.

If you've ever R&R'd the rear shocks yourself and looked at the mount, there is not much to it. A sandwich of two fairly thin hard foam clam shell type things, with a metal sleeve inside, that gets compressed by a washer underneath and on top over and under the hole in the unibody for the shock rod. I doubt it is possible to deflect it more than a tiny fraction of an inch under load when properly installed. If it is in fact installed properly, and you are feeling larger than fraction of an inch pogoing, something else is probably going on in the shock/spring/mount subsystem.

I should also mention that BMW models are notorious for tearing out rear upper shock mount points in unibody cars. I was amazed when I took apart my e9x car for the first time and there was no bracket/bolt reinforcement at all up there, just a hole that you clamp halves of the bushing over/under. I guess they know what they are doing.

[DrRobert]

Thanks for that, ajsalida. Not sure why I'm still having Bounce #2 after replacing the shocks, springs, bumpstops and SF bushings. Are the Koni FSDs too soft for the yellow BMWPS springs? Koni rep said they would be a good match. I've only had them a month or so, not sure if any more break-in time is needed. Lower rear shock mounts were replaced at FSD install as they looked worn, but upper rear mounts and front mounts looked fine and were not replaced. Car has 37K miles on it.

Thanks,
Robert

[andrey_gta]

People talking about bounce #2 over a bump should know Rear m3 arms help.

OEM arms in pre 2011 cars had the soft rear arms that do have some flex. 2011 cars fixed one of the arms with square bracketing. F30 cars fixed all the rear arms.

Therefore m3 arms ( the top two that fit) work well to remove flex of bounce #2 and maybe even #1. I did my rear arms first because of a deal that I found and I am thrilled that I did because it helped with bounce and made the car oversteer less than before

[ajsalida]

Originally Posted by DrRobert

Thanks for that, ajsalida. Not sure why I'm still having Bounce #2 after replacing the shocks, springs, bumpstops and SF bushings. Are the Koni FSDs too soft for the yellow BMWPS springs? Koni rep said they would be a good match. I've only had them a month or so, not sure if any more break-in time is needed. Lower rear shock mounts were replaced at FSD install as they looked worn, but upper rear mounts and front mounts looked fine and were not replaced. Car has 37K miles on it.

Thanks,
Robert

Oh yeah I bet they are too soft for the BMWPS yellow, if I am thinking of the right springs. As others have pointed out FSD are not intended for lowering or sportier/stiffer springs. You want Koni sports for that, the single adjustable kind, and then you'd likely be more towards the firm side of adjusting the Konis to get it all to feel right. Then you'll begin to start wondering about stiffness of all the other rubber bits, but at least you have a knob to turn to tune things.

I have never run BMWPS personally but have read they are stiffer and lower than stock ZSP (sport package) springs, by a good margin.

[DrRobert]

Originally Posted by ajsalida

Oh yeah I bet they are too soft for the BMWPS yellow, if I am thinking of the right springs. As others have pointed out FSD are not intended for lowering or sportier/stiffer springs. You want Koni sports for that, the single adjustable kind, and then you'd likely be more towards the firm side of adjusting the Konis to get it all to feel right.

I have never run BMWPS personally but have read they are stiffer and lower than stock ZSP (sport package) springs, by a good margin.

They are stiffer but only 1/2" lower than BMW sport springs.

[DrRobert]

Originally Posted by andrey_gta

People talking about bounce #2 over a bump should know Rear m3 arms help.

OEM arms in pre 2011 cars had the soft rear arms that do have some flex. 2011 cars fixed one of the arms with square bracketing. F30 cars fixed all the rear arms.

Therefore m3 arms ( the top two that fit) work well to remove flex of bounce #2 and maybe even #1. I did my rear arms first because of a deal that I found and I am thrilled that I did because it helped with bounce and made the car oversteer less than before

Can you be more specific which rear arms you are referring to?

E90 LCI rear suspension

[ajsalida]

Originally Posted by DrRobert

They are stiffer but only 1/2" lower than BMW sport springs.

OK good to know. It is the stiffer part that causes the excess bounce with a mismatched (under damped) shock, not the lowering.

As everyone recalls from first year ordinary differential equations and spring mass damper systems.

[DrRobert]

This is them:

BMW PERFORMANCE SPRINGS

[ajsalida]

Originally Posted by DrRobert

This is them:

BMW PERFORMANCE SPRINGS

That is quite a jump in stiffness. Some people say the BMWPS are too stiff, more like H&R rates but not as low.

As I understand them the Koni FSD are meant more for stock spring rates. Plus they have some sort of dual rate valve inside that supposedly combines firmer over low frequency (body control) and soft over high freq. Well stiff springs have higher nat freq and if the FSD decides to go soft there you are screwed.

They appear to employ blow off valves on the piston to do that, nothing new, but maybe they engage at lower piston speeds than normal (almost all shocks have blow off valves of one kind or another in addition to main valves, to prevent too high of forces inside the shock).

Damping forces by definition are proportional to velocity. So to get high forces at low piston speeds you need a restrictive valve for low speeds, then to get the soft feeling at high frequency you have a parallel blow off valve that opens at lower forces than normally would. That means you have almost no damping at higher piston speeds (flow goes thru there instead of the main valve).

But then if you put stiff springs on, the force from the stiffer spring is higher, opening the blow off valve earlier, making the system undamped earlier, if all that makes sense.

Anyway, you may wish to consider either going back to OEM ZSP, or ditching the FSD. The problem I would have with FSD is, that tech maybe works great at street speeds but get to a HPDE and all the input freq go up, now the FSD is not doing much there.

edit: so suppose you keep the BMWPS and go with stiffer shocks like Koni sports. Everything may be fine, no pogo but you find it unbearable on the street. Or you may like it but the next thing in the chain is too soft. Or it may be perfect.

[DrRobert]

OK thanks man. Gonna drive it for a while as is and think about it some more before doing anything else!

[ajsalida]

Sure. Good plan, mull it over, see if you can feel what is going on while you drive it more under different conditions and investigate options.

[andrey_gta]

Originally Posted by DrRobert

Can you be more specific which rear arms you are referring to?

E90 LCI rear suspension

The M3 arms that ones that replace #18 and #19 in the rear. TRW M3 arm kits are available for about $400

I think the LCI's and 335is had #18 that was different - boxed sheet metal style arm. I don't recommend doing this arm as a retrofit because its cost is the same as TRW M3 rear arm

[spxxx]

do these effect ride height?

[alexx03]

so if im understanding this correctly this can be used with coilovers?
we just need to order the 12mm kit?

[mlifxs]

Originally Posted by alexx03

so if im understanding this correctly this can be used with coilovers?
we just need to order the 12mm kit?

if it were me, I would check regarding my coilovers. I don't think all coilovers with externally adjustable rears are 12mm. If in doubt, ask the company that made your coilovers.

are you using the Genuine BMW upper shock mount now, with your coilovers?

[alexx03]

Originally Posted by mlifxs

if it were me, I would check regarding my coilovers. I don't think all coilovers with externally adjustable rears are 12mm. If in doubt, ask the company that made your coilovers.

are you using the Genuine BMW upper shock mount now, with your coilovers?

i have BC coilovers and i am using the stock shock mounts,so i would say yes genuine BMW.
i notice both of these bumps everyone is talking about,the side to side and the pogo up and down.
the BC coilovers and MFactory LSD are my only suspension mod,ordered M3 front and rear control arm kit last week.
how would i be able to find out if they are 10mm or 12mm?

[mlifxs]

Originally Posted by alexx03

i have BC coilovers and i am using the stock shock mounts,so i would say yes genuine BMW.
i notice both of these bumps everyone is talking about,the side to side and the pogo up and down.
the BC coilovers and MFactory LSD are my only suspension mod,ordered M3 front and rear control arm kit last week.
how would i be able to find out if they are 10mm or 12mm?

If you're using the stock rear mounts, then I strongly suspect they are 10mm, thus my question and comment.

fyi the stock mount I took off my car and replaced with Dinan 10mm is as follows: 33506771738
http://www.realoem.com/bmw/partxref.do?part=33506771738

I could be wrong but I think stock is always 10mm. But since you have BC coilovers, why not ask BC "what is shaft size of my rear shocks, 10mm or 12mm?"

[alexx03]

Originally Posted by mlifxs

If you're using the stock rear mounts, then I strongly suspect they are 10mm, thus my question and comment.

fyi the stock mount I took off my car and replaced with Dinan 10mm is as follows: 33506771738
http://www.realoem.com/bmw/partxref.do?part=33506771738

I could be wrong but I think stock is always 10mm. But since you have BC coilovers, why not ask BC "what is shaft size of my rear shocks, 10mm or 12mm?"

"what is shaft size of my rear shocks, 10mm or 12mm?"
thats what i was wondering,i didnt know what that top part would be called sorry.
im pretty sure i ripped the yellow bushing off my stock washer and then used the supplied BC washer and nut.
thanks for help

[TheSt|G]

As someone who has both the Dinan RSMs w/M3 camberlinks and the M3 RSFBs, the latter is significantly more dramatic of a change than the former. That being said, they really don't address the same issue...

[DrRobert]

Originally Posted by andrey_gta

The M3 arms that ones that replace #18 and #19 in the rear. TRW M3 arm kits are available for about $400

I think the LCI's and 335is had #18 that was different - boxed sheet metal style arm. I don't recommend doing this arm as a retrofit because its cost is the same as TRW M3 rear arm

I replaced part #19 in the diagram (rear guide rods) but not #18 (wishbones). Would replacing the OEM wishbones with M3 provide additional rear stability? Any other rear suspension bits that will decrease bounce or sway?

[ajsalida]

Originally Posted by DrRobert

I replaced part #19 in the diagram (rear guide rods) but not #18 (wishbones). Would replacing the OEM wishbones with M3 provide additional rear stability? Any other rear suspension bits that will decrease bounce or sway?

Going to PM you something...