carl said:
I don't get all this stuff about inducing understeer. It's a strut brace, not an anti-roll bar. All it does is stop the chassis/body flexing. Are we saying the handling and compliance is better if it does flex? Seems wrong to me.
First of all, let me say I always like the look of strut-braces in the engine bay, and I've seen plenty of so called 'motorsport' companies selling strut-braces that don't even do what they're intended to due to having curves/hinges in them (which means the forces aren't transmitted to the other strut tower as they should be, and just end up being absorbed by the bar itself). These companies know it's a flawed product, yet continue to sell them as some owners like the 'thought' of having a strut brace.
The BMW one (and lots of others) is not like that, so it does do what it is intended to do. However what it is intended to do, and what people buy it to do are not necessarily the same thing.
Basically, the stiffer the car the less compliant/progressive it is.
Think of a 1ft cube made of concrete on a smooth surface, then gradually tip that surface to 45º and what happens? Nothing until the whole lot slides down the surface without warning.
Think of the same cube on the same surface, but made of jelly, and do the same test in your head and what happens? The jelly will start distorting as soon as you move the surface, and will keep tipping in line with the angle of the surface until it can't grip anymore and will gradually let go and slide down.
If you're on a track then stiffer is better (unless it's wet) as you'll get better feel through the steering, more direct response to your steering inputs, etc.
If you're on the UK roads then you need some softness/flex/compliance to stop the steering wheel being jerked out of your hands, to decrease tramlining, and to allow one side of the car to react without affecting the other side. For example, if you catch the edge of a pothole with your LHS tyre, then you'll feel it a bit of a shock through the car and the LHS suspension will take care of it. If you have strut brace on, that shock will be transmitted to the RHS shock tower and make the RHS wheel/tyre/suspension react too - even when it's on a perfectly smooth piece of tarmac itself.
The reason it tends to add to understeer is that in normal use the two front tyres/struts can act independently to differing road surfaces on different sides of the car to keep the tyre's contact patch at an optimum position in relation to the road surface. With the strut brace in place the two side are forced to act in parallel. In some cases that works fine, in others it doesn't.
The effect is different on different cars, and no amount of roll-bars/strut-braces would make a road-going e34 M5 into a track weapon...(thicker front & rear roll bars, front & rear strut braces, and still it rolled like it was in a Force 9 storm)...
bmw offered one and we fitted it.... Cant really hurt having one fitted so im happy with it. 
Liverpool