M3 Front sway bar with no rear sway bar? Thoughts?

[chris82]

I know there is a ton of info out there on the M3 FSB upgrade, I'd like to know if it is recommended on a car with no rear sway bar. I have the non-sport 128i, so there's nothing back there and I don't plan on putting one in anytime soon. If this was a street only car I wouldn't be concerned, but I might do a track day or two and I don't want some weird handling issues to arise. What do you think? How will it affect the balance of the car? Grip levels?

My car has the M3 RSFB and MPSS of course, other than that, the suspension is bone stock!

Thanks

[Ginger_Extract]

Meh, stock rear bar on these cars has such a marginal affect on handling. That said, the car will probably slalom better with the bigger front bar, but understeer more in big sweepers. YMMV.

[RyanDavies]

So, I generally dislike rear bars on cars for a number of reasons, and my first setup swings were trying to not use it. You will have to run absolutely absurd rear spring rates (1200s were not enough in the back) to correctly balance the car without a rear bar.

Now, that said, the OEM rear bar is a giant waste of time. 15lbs contributed at the wheel. That's effectively impossible to notice. So if the decision point is about how balance would be versus having no bar, versus the OE 12mm bar, I wouldn't worry about it at all.

[John_01]

To put that it into numbers with no rear bar and Stock 340lb/in springs.

Spring rate at wheel = (rear spring x motion ratio squared) + (effect of suspension bushings) = 340*0.316 + 36 = 143.4 lb/in

When adding the 12mm sway bar its 15lb per each wheel, but its 30lb effect in roll stiffness because the force is multiplied by the opposite wheel acting in the opposite direction.

The effective wheel rate in roll with 12mm sway bar is 143.4 +30 = 173.4lb/in.

The 12mm rear sway bar increases the roll stiffness at the rear by 21% over having no sway bar at all. IMO, yes this is a noticeable effect in some situations. Its about the same effect as increasing the rear spring rate by 100lb/in. Some people will say its not noticeable. If you are tracking the car, then it is likely to be noticeable if your car is already well setup.

[AlexR6]

I wouldn't do it... I tried and it was like the rear end was way slower than front end when changing direction! I did not like it at all and also the car was understeering a little bit more.

I then upgraded the rear bar too (or better installed since there was no rear bar) with the M3 one and now the car drives much better than stock!
Only note, I am on BC racing coilovers and not on standard suspension

[chris82]

Originally Posted by AlexR6

I wouldn't do it... I tried and it was like the rear end was way slower than front end when changing direction! I did not like it at all and also the car was understeering a little bit more.

I then upgraded the rear bar too (or better installed since there was no rear bar) with the M3 one and now the car drives much better than stock!
Only note, I am on BC racing coilovers and not on standard suspension

So even with the BC coilovers, you thought the rear still felt sluggish with just the M3 front sway bar installed?

[bNks334]

Originally Posted by chris82

So even with the BC coilovers, you thought the rear still felt sluggish with just the M3 front sway bar installed?

Not sure I agree with this, but to each his own. This depends heavily on what spring rates he chose from BC so take it with a grain of salt. I see some people are choosing rates like 8k/6k which I don't agree with at all...

[RyanDavies]

You'd need a metric fuckton of travel to make those rates work without being all over the bump stops constantly. 10Kish fronts use about 2-2.5in of bump travel, fwiw...

6K at the back would work if you had a 3in sway bar .

[AlexR6]

Originally Posted by bNks334

Not sure I agree with this, but to each his own. This depends heavily on what spring rates he chose from BC so take it with a grain of salt. I see some people are choosing rates like 8k/6k which I don't agree with at all...

My spring rates are 8 Kg/mm front and 12 Kg/mm rear.

But in any case my car didn't feel sluggish as when I had the stock RSFB but it was like not balanced. To make it more balanced I had to stiffen up rear dampers but then it was like not having rear suspensions

My favorite setup is bigger bars both front and rear otherwise stock setup! But I'm talking about driving on track, on public roads I drive like a granny

[bNks334]

Originally Posted by RyanDavies

You'd need a metric fuckton of travel to make those rates work without being all over the bump stops constantly. 10Kish fronts use about 2-2.5in of bump travel, fwiw...

6K at the back would work if you had a 3in sway bar .

Yeah it translates out to effective wheel rates of roughly 400/100 lb., a completely botched natural ride frequency, and a slightly softer effective rear wheel spring rate than the base suspension.

Admittedly I would've thought those numbers looked good too up until recently. I've been reading up on suspension so I am compelled to keep this thread going!

Originally Posted by chris82

I know there is a ton of info out there on the M3 FSB upgrade, I'd like to know if it is recommended on a car with no rear sway bar. I have the non-sport 128i, so there's nothing back there and I don't plan on putting one in anytime soon. If this was a street only car I wouldn't be concerned, but I might do a track day or two and I don't want some weird handling issues to arise. What do you think? How will it affect the balance of the car? Grip levels?

My car has the M3 RSFB and MPSS of course, other than that, the suspension is bone stock!

Thanks

I'd read the following thread. It helps explain some of the trade-offs between tweaking sway bars vs. spring rates:

http://forums.nasioc.com/forums/show....php?t=2054712

Some food for thought:

The effective wheel rate and natural frequency of the base suspension is roughly 110 lbs/in. f/r and 1.2 hrz f/r, respectively.

The BMWPS I am using has an effective wheel rate of roughly 163 lbs/in. f/r and a natural frequency of roughly 1.45hz f/r (slightly softer than M3 ride quality).

If John_01 calculations are correct, you'll be close to the rear effective spring rate of the BMWPS, while cornering, with RSF Bushings and the aforementioned RSB. Doing both f/r bars would be better imo if you stay with the stock suspension. I know sway bars are cheap, but if you plan on going with a stiffer suspension in the future I'd hold off on modifying the sways. There are plenty of fast cars that run with no rear sway. If you do the sways now you may end up too stiff later.

As I am setup now (235 square, -1.7* camber, E92 M3 FSB, BMWPS), the car handles pretty neutral on track. I cant imagine going much stiffer in the rear though whether by sway bars or bushings. The car wants to over-steer as is. I like where it's at right now for a daily driven car. Not too harsh on the street and pretty forgiving when driven at the limit.

I'd like to think the softer rear helps hold grip? I could be wrong though and sloppy weight transfer could be the cause of the rear losing grip on my car? Aggressive front turn in followed by a slow to follow rear AlexR6 mentioned might cause it to snap out?

[chris82]

Originally Posted by bNks334

Yeah it translates out to effective wheel rates of roughly 400/100 lb., a completely botched natural ride frequency, and a slightly softer effective rear wheel spring rate than the base suspension.

Admittedly I would've thought those numbers looked good too up until recently. I've been reading up on suspension so I am compelled to keep this thread going!



I'd read the following thread. It helps explain some of the trade-offs between tweaking sway bars vs. spring rates:

http://forums.nasioc.com/forums/show....php?t=2054712

Some food for thought:

The effective wheel rate and natural frequency of the base suspension is roughly 110 lbs/in. f/r and 1.2 hrz f/r, respectively.

The BMWPS I am using has an effective wheel rate of roughly 163 lbs/in. f/r and a natural frequency of roughly 1.45hz f/r (slightly softer than M3 ride quality/harshness).

If John_01 calculations are correct, you'll be close to the effective spring rate of the BMWPS, while cornering, with RSF Bushings and the aforementioned RSB. Doing both f/r bars would be better imo if you stay with the stock suspension. I know sway bars are cheap, but if you plan on going with a stiffer suspension in the future I'd hold off on modifying the sways. There are plenty of fast cars that run with no rear sway. If you do the sways now you may end up too stiff later.

As I am setup now (235 square, -1.7* camber, E92 M3 FSB, BMWPS), the car handles pretty neutral on track. I cant imagine going much stiffer in the rear though whether by sway bars or bushings. The car wants to over-steer as is. I like where it's at right now for a daily driven car.

I'd like to think the softer rear helps hold grip? I could be wrong though and sloppy weight transfer could be the cause of the rear losing grip on my car? Aggressive front turn in followed by a slow to follow rear AlexR6 mentioned might cause it to snap out?

I think ride frequency is blown out of proportion. Besides, what are your options to balance it? I see 3 options: You can lower your front spring rate considerably, run a rear coilover suspension, or use a rear spring rate so high that god knows what will happen. The only reasonable option there is to lower your front spring rates, but then you are sacrificing front end grip. I'd like to know how much we are giving up by not having a balanced ride frequency...

[bNks334]

Originally Posted by chris82

I think ride frequency is blown out of proportion. Besides, what are your options to balance it? I see 3 options: You can lower your front spring rate considerably, run a rear coilover suspension, or use a rear spring rate so high that god knows what will happen. The only reasonable option there is to lower your front spring rates, but then you are sacrificing front end grip. I'd like to know how much we are giving up by not having a balanced ride frequency...

I commute 18k miles a year on NY roads. For sure I'm more concerned with preserving the natural frequency f/r ratio than someone who is building a track car. I was under the impression ride quality mattered since you'll mostly be using the car on the street.

Suspension frequency is more than just an arbitrary number. It kind of tells you how well a chosen spring rate is going to operate given a certain amount of corner weight that needs to be sprung. I made the comment about aggressive front spring rates because people run "out of sync" aftermarket suspensions on the street and then end up complaining the rear of the car is bouncy... Now consider hitting curbs on a track.

I presented the frequency numbers to draw a comparison between base suspension and BMWPS. What I was then trying to get at was that, based on the above calcs, the proposed rear bar and rsfb would result in an effective spring rate, while cornering, comparable to a 500 lb/in. rear spring (BMWPS rate). At that rate, my personal experience on track with the car is that I wouldn't want to be much stiffer in the rear without increasing grip somehow or $ for an LSD like so many have to do once modded.

I doubt you'll notice much of a difference at all with just an M3 front bar and base suspension. You have wider tires up front that will help offset any increase in under-steer from the FSB. You would still have to deal with excessive dive/squat and a cushy ride for a sport car on track.

[chris82]

Originally Posted by bNks334

I commute 18k miles a year on NY roads. For sure I'm more concerned with preserving the natural frequency f/r ratio than someone who is building a track car. I was under the impression ride quality mattered since you'll mostly be using the car on the street.

Suspension frequency is more than just an arbitrary number. It kind of tells you how well a chosen spring rate is going to operate given a certain amount of corner weight that needs to be sprung. I made the comment about aggressive front spring rates because people run "out of sync" aftermarket suspensions on the street and then end up complaining the rear of the car is bouncy... Now consider hitting curbs on a track.

I presented the frequency numbers to draw a comparison between base suspension and BMWPS. What I was then trying to get at was that, based on the above calcs, the proposed rear bar and rsfb would result in an effective spring rate, while cornering, comparable to a 500 lb/in. rear spring (BMWPS rate). At that rate, my personal experience on track with the car is that I wouldn't want to be much stiffer in the rear without increasing grip somehow or $ for an LSD like so many have to do once modded.

I doubt you'll notice much of a difference at all with just an M3 front bar and base suspension. You have wider tires up front that will help offset any increase in under-steer from the FSB. You would still have to deal with excessive dive/squat and a cushy ride for a sport car on track.

I interpreted your post as relating to ultimate grip levels, not comfort. For sure if you are on the road a lot you'll want to maintain a balanced frequency.

[Rob_BMW_PNW]

Not to jack this thread, but I have noticed everyone (or most) get the M3 front sway upgrade, is there any reason to NOT get the m3 vs a aftermarket like, Eibach or 'insert brand name'?

[RyanDavies]

If you do go the M3 route, get rid of the rubber bushings immediately. The bar (rear too) are epically bound up from the factory.

The Eibach 28mm bar also binds badly unless you use washers between the brackets and the mount points on the car.

[chris82]

Originally Posted by Rob_BMW_PNW

Not to jack this thread, but I have noticed everyone (or most) get the M3 front sway upgrade, is there any reason to NOT get the m3 vs a aftermarket like, Eibach or 'insert brand name'?

I personally go OE M3 if possible because BMW and my car is mainly street car as M3 was designed to be. If it's good enough for M3, good enough for my 128i. I also think it's some form of car OCD I have, I need everything to match lol

[Rob_BMW_PNW]

Originally Posted by chris82

I personally go OE M3 if possible because BMW and my car is mainly street car as M3 was designed to be. If it's good enough for M3, good enough for my 128i. I also think it's some form of car OCD I have, I need everything to match lol

No, I hear you. Do M series owners ever swap out their suspension pieces for 'better' ones?

Was thinking about doing the arms, and front sway from the M3 on my 135i too, we will be tracking it (hopefully a lot)

[chris82]

Originally Posted by Rob_BMW_PNW

No, I hear you. Do M series owners ever swap out their suspension pieces for 'better' ones?

Was thinking about doing the arms, and front sway from the M3 on my 135i too, we will be tracking it (hopefully a lot)

From my visits on M3post, yes, but it's not nearly as often as we do on here (they may replace a bushing here and there, minor stuff compared to the overhauls folks here do); the M3 is setup very well from the factory.

[bNks334]

Originally Posted by Rob_BMW_PNW

No, I hear you. Do M series owners ever swap out their suspension pieces for 'better' ones?

Was thinking about doing the arms, and front sway from the M3 on my 135i too, we will be tracking it (hopefully a lot)

The M3 bar has an added benefit of having stoppers to prevent the bushings from sliding on the bar. I greased the bushings up before installing my bar and didn't have any issues with it rotating freely.

As others have said, if the parts are aggressive enough for the M3 then they must be enough on a street driven car that is smaller, lighter, and produces less power. If you go too stiff you'll just end up reducing mechanical grip and the 1-series is pretty tire width limited.