Z4 Rear Sway Bars

Okay, I want to post in here real quick before you all get yourselves in a situation that is hard to handle. BMW designed these cars with understeer to keep the rear end from coming around for a reason, it is easier to control. With that said, if you overcompensate, you can end up with a phenomenon known a "snap oversteer", in other words you can turn 180' in an instant with NO control. Wanting to take out some of the understeer is fine by me, but please test your settings in a safe environment before turning it loose on the streets :thumbsup:
 
I agree completely, ovrkll. I just finished installation of my rear bar today, and will be posting the how-to I experienced shortly.

Due to the local traffic, I could not test it safely on the road. With my setting in the middle position on each side of the H&R, the rear end definitely felt more buttoned-together and tighter. I will not have a chance to test it in more dramatice cornering situations safely until this week (in the parking lot after work hours). I will also be curious to learn how the new rear bar affects the stabillity and traction control intervention, particularly when we get some rain.

But you're absolutely right. Safety first. :driving:
 
I used the softest setting for general around town cruising on my old car. If I had done some track events, I probably would have gone with the mid setting, the stiffest setting is usually reserved for a track-only car.
 
If it's any help I found that fully firm rear and one notch up from soft on the front was the perfect balance for my needs. You can adjust these bars using different holes on either side to give you a setting midway between standard settings (ie the same hole on both sides).
 
I have to question the "wisdom" of using swaybars to cure understeer/oversteer problems on a car as a first step, when there are a ton of simple, free changes that would go a lot more to cure which end will lose grip first.

The only thing swaybars are good for, is to eliminate body-roll. On a chassis like the Z4 with short wheel-base and a relatively stiff chassis, body-roll shouldn't be your first concern. If you need to dial out the inherent understeer built into these cars, there are two areas you need to address first.

First is the tires. Either go wider up front (say, go up to 245 up front) and get the tires to "square up", or put more air into the front tires (up to a certain point). Both of which are 10X more effective than using a swaybar to eliminate understeer, since going wider or finding the optimum pressure for the front means you're adding grip up front to cure understeer, while using a stiffer rear bar means you're taking away cornering grip to cure understeer.

Second, suspension geometry. Adding more negative camber up front will cure understeer. Again, you're adding "grip" to the end that needs it rather than taking away grip to the end that doesn't.

I would add swaybars to cure excessive body-roll in corners, but only after stiffer springs and wider tires aren't doing the job, or if the car's a daily drive and I need soft springs for driving comfort but still want that "cornering on rail" feeling in the turns.
 
The HACK said:
I have to question the "wisdom" of using swaybars to cure understeer/oversteer problems on a car as a first step, when there are a ton of simple, free changes that would go a lot more to cure which end will lose grip first.
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Nearly every purpose-built race chassis has driver adjustable swaybars. Why? So the over/under-steer of a car can be tweeked during the course of a race.

I agree that other factors should be considered when tuning your suspension, but to dismiss swaybars as one of the tuning tools shows a lack of understanding of vehicle dynamics.
 
mmakay said:
The HACK said:
I have to question the "wisdom" of using swaybars to cure understeer/oversteer problems on a car as a first step, when there are a ton of simple, free changes that would go a lot more to cure which end will lose grip first.
Click to expand...

Nearly every purpose-built race chassis has driver adjustable swaybars. Why? So the over/under-steer of a car can be tweeked during the course of a race.

I agree that other factors should be considered when tuning your suspension, but to dismiss swaybars as one of the tuning tools shows a lack of understanding of vehicle dynamics.
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These aren't purposefully built racecars. And no purpose-built race chassis, not even F1 cars, have on-the-fly adjustable sways. Even the PTG E46 M3 race car does not have driver adjustable sways. They have levers in the engine bay that allows sway-bar adjustment from up-top that slides the bracket in and out of the bar to tune for stiffness, but I have never seen any series or any race-cars with in-car adjustable sway-bars. This is a first for me. :?

When I work with club racing crews, they'll use the swaybar adjustment as the last adjustment to make in the car-setup adjustments. First thing they'll do, is tire pressure, then alignment/camber, then damper rate, then when all the other adjustment have yielded their optimum results, they'll adjust the sway-bar for that last tenth of available grip since all the other factors yield much more impact on the result.

And please read my post more carefully. What you want to do in terms of suspension tuning, is you want to ADD GRIP to the ends that are slipping first to cure your under-steer or over-steer problems. Hence tire pressure/size, suspension geometry, spring rate, damper rates all are easier to adjust to ADD grip first. Sway-bar stiffness may actually takes away overall grip (that's why when you stiffen up the front bars, the car will end up under-steering, or you stiffen up the rear bars the car will over-steer) if suspension geometry and spring rate isn't tuned properly to the sway-bars. Therefore, you should always maximize the grip with tires, camber/toe, springs, dampers, then adjust the sway-bars accordingly.

Makes no sense to me to add sway-bars first when a simple change in pressure will likely cure your under-steer issues AND give you more overall grip. The only time it makes sense to add sway-bars first is if you intend to keep the ride as soft and as stock as possible. Even then, adjusting tire pressure or using wider tires up front is more effective.

EDIT: Spoke too soon. I guess Porsche GT3 cup cars do have in-car adjustable sways. Certain series do allow cockpit adjustable sways. My bad.
 
I agree with just about everything your saying but things are more complicated than that. For example, these cars are primarily road cars. Excessive adjustment of such parameters as camber will have a negative effect on the braking efficiency due to reduced tyre surface contact in a straight line. This also applies to rear wheel traction under braking or accelerating. Although increased negative rear camber may give you better grip through the corner it will then lose you traction out of the corner etc. The same problems arrise when changing the tyre pressures too much beyond spec. Your average daily driver will just wear them excessively on the outer or inner surfaces while cruising down the motorway. The other problem is that these pressure adjustments will only come into effect when the tyres are pushed to the extremes and manage to reach that pressure. Damper settings are another great way of changing the handling charactristics but nobody's going to get out of the car and adjust them every time they see a nice twisty section of road just to adjust them back at the other end because road gets a bit bumpy. In a pure race car your quit right that roll bar adjustments should come last but for a dual purpose vehicle which will mostly be used on normal roads under normal driving conditions everything has to be a compromise. If you simply prefer a bit of oversteer to understeer and the easiest solution is to loose a bit of rear end grip then why not? Having said that it is worth recommending that you should remove the front dowel on the strut top mounts and move the struts inboard as far as they'll go. This gives you about 1.5 degrees of negative camber and a bit more bite on cornring without compomising any other factors too dramatically. Easy and free mod!
 
My point is, why use swaybars as the first adjustment to cure understeer? Tire pressure has a much larger effect. Camber has a much larger effect. And both cure "understeer" by adding grip, not subtracting grip.

As for "camber" settings up front affecting braking performance, even on the track, I find that to be minimal, if none-existing. Even with camber up front, when you brake the weight of the vehicle shifts to the front increasing the contact patch, despite camber settings. In fact, I find tire pressure changes have a bigger effect on braking performance than up to 1.5 degrees of camber change. Even with 0 degree camber at standing still, you're still going to get up to 1 degree of negative camber upon threshold braking simply from the suspension compressing.

And I've stated clearly, that sway-bars have their place in road going cars if you want to keep the suspension nice and soft or OEM. But there are TWO very simple, free adjustments you can make sans the sway-bar that will give you 100X the return in terms of curing understeer, so why buy a swaybar just to cure understeer? You should always adjust pressure on the end that is slipping first, or add camber to that end second. Much better return for your investment.

And no offense to those of you claiming to want to dial out understeer for street driving (which means you're about to be offended. I warned you)...There's no such thing as "understeering" for street conditions. If you're understeering at street speed, no matter if you're canyon carving or just driving aggressively, you're not driving the car right, and no amount of playing with your swaybar will fix that. These cars are set-up only to understeer when you do something seriously stupid or don't know how to drive. Typically if you enter a turn too "hot" and didn't take the right line (early apexing), the car's design to understeer to save your @ss. I've done about 12-15 days at the local track with the MZ4 Coupe, and all the rags complain about it understeering upon entry and oversteering on exit? Yeah, right. These cars (the Z4 chassis in particular) are about as neutral as it gets. Take the right line, slow-down enough, apply throttle as you unwind, do all the basics well and "understeer" will NEVER rear its ugly head. Understeer in a BMW means MISTAKE, period.

Frankly the best way to cure understeer in a BMW is to take some HPDEs and learn how to cure it from the driver's perspective.
 
I think it's all going to depend on what individuals are going to use the car for and as you say driver ability and knowledge of whats going on with the car underneath them and why is a big factor. Most people on this forum including myself will probably admit we're not the best drivers in the world and yes, this is why BMW built understeer into the car to keep us safe. The point is that some people have obviously reached that point and don't like the car to react that way and prefer oversteer. Whether it's their fault or whether they can handle that oversteer is obviously a different matter. The best way to learn is to leave the DSC on while you get used to the feel of it. I still think that relying on tyre pressures is only feasable when the tyre is constantly running at a consistant temperature ie under race conditions. In stop start motoring the tyre temperature and therefore pressure will be constantly fluctuating. We could talk all day about perfect race set ups and what happens on the track but that's completely different to what the majority of people are talking about here.
 
The HACK said:
mmakay said:
The HACK said:
And no purpose-built race chassis, not even F1 cars, have on-the-fly adjustable sways.
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Sorry, but you are 100% wrong. F1, Formula Atlantic, Champ, Indy, SCCA GT and many others have had or still have ON THE FLY, DRIVER ADJUSTABLE SWAY BARS. I have seen and touched them.
 
good pts by all. but in the end it all depends on what we as the owners/drivers of the car are looking for

if you want more oversteer and don't care about max grip/cornering speed, then y not, just stiffen up the back as much as possible and fun doing your donuts all day :dizzy:

as i and most have said, sway bars are only 1 of the factors that go into the tuning of the car's suspension

in my experience, by far the most important mod ever are your tires!!! for example, take a set of all season tires vs. R-compound tires, you will improve braking, handling, accleration all at once!! way more effective than any 1 mod. so if you are still on the stock tires, i would HIGHLY recommend going with a high grip tire as that will pay waaaaaaaaaaaaay more dividends than any sway bar, afterall, they are the only 4 parts of the car that are actually touching the ground!! :thumbsup:
 
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