Sway bars

[bastianshaw]

i dont get it...sway bars make u lose traction?...
what if i put IE 25/22mm?
ill fish tail all over the place?

[2002maniac]

Yes Trent, I was talking to you... were you joking about 1000lb+ spring rates in the rear in that other thread?

I think he was joking.

[whakiewes]

The problem with a stiff rear bar, is a little different than stiff springs. With a stiff sway bar, it turns the IRS into more of a solid type axle (not quite, but one can get the idea.) A stiffer spring in the rear can make up for roll, allowing full use of IRS.

Take a look around at the fast E30s...there is always a fine line to walk. And yes, I am running somewhat high rear rates (in comparison to the front) with no rear bar.

Its not turning the IRS into a solid axle. Its adding a pre-loaded spring. The point of this is to add a mulitplier to the outside wheel when going around a turn. I am not sure what shocks you are on, but to combat stiff springs you have to raise the rate of compression and rebound. For the suspension to be stable and now bouncy nor inoperable the shock is going to have to have enough rebound to counter act the spring, which provides the same effect as the sway bar. By not having a sway bar you are substituting the component with another effective tool, but in a different way. Either way they will have the same effect with the stiff spring/shock having a much worse ride quality and a less effective rear suspension. Essentially your shocks are working hard to provide the same work a sway bar could.

The E30 rear suspension is no different than almost any with a semi-trailing arm rear suspension. The wheels rates are off a bit of say a multi-link or something of that nature because it seperates the shock/strut to make a less effective setup. Thus you typically have to run a higher rate in the rear than in the front to counter act the 50/50 effect. Essentially if you had your car balanced at 50/50, then your suspension on both corners would be supporting the same load and the same weight. The front suspension is direct, probably 95% or so wheel rate, while the rear with its trailing arm mounted springs has probably closer to a 75-80% wheel rate. Sway bars are not accounted into this wheel rate, only springs. Once you weight in the sway bars it looks more like 100% and 75% because of the smaller sway. So say a rear sway bar at 19mm has a torsion rigidity of 150lbs/sq/in, front has an effective rate of of 220lbs/sq/in (all these numbers are merely hypothetical considering I know of know one with a machine capable of measuring these forces). This is going to be unbalanced because the front bar is roughly 10% more effective than the rear bar based on our 100/75% wheel rate calculations. So these drivers are going to have a front that is torsionally stiffer and induce more understeer. Now that we are passed how that works, back to spring rates. So when you remove your rear sway bar you have to make up for that added suspension component. So if you have 400lb springs in the front, with a sway bar on the rear you would need 500lb springs in the rear. This would give you an effective rate of 620lbs/in front and 650lbs rear, thus making the 22/19mm setup much more effective. What happens when you remove the rear sway bar. To make a balanced setup you would need a minimum of 775lb/in springs. Not sure how many people have ridden on linear springs of this rate, but its not fun, let alone on the street. On top of that, no over the counter shock is going to handle those kind of spring rates.

I have been toying with suspension, on both my BMW and my Porsche. I have used everything from Koni 2812's ($1100/each) to Ohlins 46 Series, Penskes and all. I have messed with all setups, adjustable both bolt type and blade type. I can tell you, the most effective route is not removing the rear sway bar. With a sway bar you are relying on a linear rate component. Without it you are relying on constantly adjusting (neither linear nor progressive) shocks and springs. Everything changes per inch, compression, rebound, spring rate, wheel rate, etc... The only linear part you have in your suspension are springs (if they are infact linear rate springs and not progressive J-stocks or HR Race, etc..) and sway bars. Why remove such a constant tool. Why didn't pro-race E30 M3's run sway barless in the rear, or 325i Touring cars. Its only in the past couple of years since the introduction of SpecE30 that people get these ideas. Proven professional racing pedigree clearly shows different than amateurs campaigning $10k race cars.

Disclaimer to my calculations - for one they are all hypothetical numbers. There are thousands of other calculations. I have never measure the rear suspension on the E30, so the 75% is a rough calculation. Shocks also have valving rates in force per inch that weren't accounted for, although shocks generally do nothing for ride quality, its the springs that you feel. Sway bar links also make a difference, but be clear that they change the effectiveness of a sway bar, NOT the torsional rigidity. With strut mounted links, it reacts faster, not stiffer. I would venture to say that majority of drivers who run without a rear sway bar have many other problems in the suspension and they are using the anti-rear sway bar mod as a band-aid.

In one track event in my Porsche we did play around with removing the rear bar, then the front bar. It does have an opposite effect than one would imagine though. It makes the rear end so sloppy that it reacts with oversteer, thus many drifters use this setup. Its clear how sloppy and unstable this setup was as soon as we tried to run a hot lap NOT sideways. Added nearly 5 seconds to my time on a 1.30 sec lap. Front bar was even worse, giving so much understeer I was scrubbing down to half of my normal speed through several turns.

Wes

[matt]

You aren't right about a lot of that stuff.

The sway bar isn't preloaded.

Sway bar links also make a difference, but be clear that they change the effectiveness of a sway bar, NOT the torsional rigidity. With strut mounted links, it reacts faster, not stiffer. I would venture to say that majority of drivers who run without a rear sway bar have many other problems in the suspension and they are using the anti-rear sway bar mod as a band-aid.

That's straight up wrong. When you connect the front sway to the strut rather than the control arm, you change the leverage ratio of the sway bar... less wheel movement per bar movement.

No rear sway bar is a great setup for e30s, if you can deal with the high rear spring rates it takes to make it work. Sway bars are a great tuning tool, but they cost you grip. The e30 front suspension's MASSIVE roll moment (once lowered) needs a big front sway to keep body roll (and camber change) in check, but the rear end stays planted a lot better with no sway bar to help pick up the inside rear tire.

[e30 gangsta]

so then with that 25/22 ie kit id be in heaven. All the non body roll goodness i want. With a bit of less grip up front. And all the oversteer i could ever want correct?

[whakiewes]

You aren't right about a lot of that stuff.

The sway bar isn't preloaded.


That's straight up wrong. When you connect the front sway to the strut rather than the control arm, you change the leverage ratio of the sway bar... less wheel movement per bar movement.

No rear sway bar is a great setup for e30s, if you can deal with the high rear spring rates it takes to make it work. Sway bars are a great tuning tool, but they cost you grip. The e30 front suspension's MASSIVE roll moment (once lowered) needs a big front sway to keep body roll (and camber change) in check, but the rear end stays planted a lot better with no sway bar to help pick up the inside rear tire.

How is the bar not preloaded - using the other strut as a method of controlling the movement, it always has force applied to it. Second, you said exactly what I said about about the strut mounted sway. It changes the effectiveness of a sway bar, but in no way does it increase the torsional rigidity of a sway bar. Nothing but a larger or thicker bar will do that. You are correct, without a sway bar and stiff enough springs you are creating the same force, but whats the point? In a properly setup suspension which I will gladly say 80% or more E30's aren't, you should have roll to the point of wheel lift, and even more so with 225's you will never have the grip to encounter that much physical inertia. So you are taking out a critical component of a car, adding in another very inconsistant factor, and saying its better. I just don't see the logic. I have driven cars without rear sway bars, and they are nothing but sloppy. You'd have to run an entirely unrealistic spring/shock rate to counter act what a sway bar does. Rather than buying quality shocks and top rate springs, then working with shocks and springs to settle out the car, your using botched together components, experiencing roll to the point where your inside wheels are lifting, and removing the sway bar as a band-aid. If you took the time to TUNE the suspension, correctly valve shocks per the application, shorten stroke to match your lowering specifications so that you are experiencing the full travel of the shock, and then FINE TUNE the entire setup with a sway bar, you would have a different opinion, guaranteed.

So you buy HR Race/Koni or Bilstein. Koni's are going to be valved either B13 or B23, good for at most 500lbs/in springs in rebound and compression. Thats at full stiff. Bilsteins are going to be valved something along the lines of 150/100 or so, good for 350/400lb springs tops. Bilstein Sports are going to be 1" or so shorter to accomodate 1" of lowering, while most springs are 2" or more. Now 1" of your high speed dampening is used of the total 3" of high speed. What does it take to get a tuned suspension; measuring your max length at your desired wheel travel, then max length at desired ride height, computing compression and rebound from you desired spring rates then measuring wheel rates, loads, tire/wheel weights, inverted or standard, deciding if monotube or twin-tube is what you need, buying quality Eibach ERS or Hyperco Springs, then testing and tuning. The easiest way to tune this is by buying double (independent rebound and compression dampening) or triple adjustable (independent high and low speed compression, and rebound dampening) and spending 12hrs at a track solidifying you setup. If you want to go Bilstein expect to have 4-6 week delays and $400 every time you need an adjustment. You start at full soft and work each adjustment up to its desired level of perfection, then once everything is settled you adjust or leave you sway bar. This is how suspension is done; not buy botching together over the counter parts and removing the rear sway bar as a band-aid for solving much more critical suspension issues. Go through $15k in shocks and springs in one year and you will learn a little more about suspension tuning.

Thanks!

[trent]

Considering I am not using a single over the counter part, and have worked in conjunction with an engineer at Bilstein (using Bilstein ASN series shocks, along with Eibach ERS) to build my full rear coilover setup front and rear, I understand your points, and you took what I said a little too literally. I built my setup around being able to not run a rear sway bar, or a very little one. I have not done timed testings yet, but I will very soon. I will be able to tune the car with either rear spring, or the additional of a small sway bar.

Wes, I appreciate your lengthy comments, but I believe you are reading into this all a little too much. Most of us do not have unlimited budgets to work with, but morely, try to make something out of nothing.

[matt]

Keep in mind, neither Trent nor I are running anything close to off the shelf spring rates or shock valving.

When you're looking at a car that's exclusively for autocrossing, where the corners are tight and slow and acceleration is key to getting around faster... a setup with no rear bar makes a lot of sense. This applies doubly to cars with a lot of power... turbos and such. :pimp:

The sway bars are not supposed to be preloaded at all when you're driving straight. They pivot in their mounts, you know.

Sorry for the miscommunication about the leverage ratios... you will definitely make the sway bar contribute more to the roll resistance of the car by changing the mounting points to the strut.

And regarding the "proper" way to setup cars... We don't all have massive amounts of money to throw at our cars. In fact, the one I'm working on now has an even bigger problem... my whole suspension tuning budget is $200. You'd be surprised how much improvement is possible for cheap when you're crafty. And know how shocks and springs work at the most basic level.

[whakiewes]

Probably you are right, I am reading into it too much. Simply though I just don't think that things such as that should be told as 'good' mods. Johnny rocket is going to take his rear sway bar off of his bone stock IS and end up in a ditch. If your on a custom built suspension then that is one thing, but those that are going to try it because its what people say, such as in this thread, makes it dangerous. The guy that posted in this thread wasn't looking to spend $2000 on custom Koni's or Bilsteins, but find a good sway bar setup for a street suspension. The 22/19 or 20/16 would be a great setup. 25/22 may be a bit large, but on my Porsche 968 I was running factory 30/19mm setup with no problems. I am not putting that kind of setup on my 325i, but I just don't think its good to preach it. Mainly the reason is also a local friend was considering doing this on his complete stock suspension versus buying sway bars.

I had Bilstein Escort Cups on my Porsche before they failed. I spent about $1000 on rebuild fees on adjustments so when it was time to upgrade I went with the balls to the wall Koni 2812 setup, inverted and all. I was much happier with the Koni's over the Bilsteins bar none!

Wes