Calling susp geometry experts; Toying with front spacers. Ruin optimal scrub radius?
 

I added H&R 25mm DRA spacers to the rear of my '02 X last year. Awesome look. Adding rear spacers and doing nothing to the fronts will add some oversteer (not desirable). In the case of the X, while this rule may be true, it never the less made the car handle flatter (more fun).

Now I'm considering moving the 25mm spacers to the front and buying 30mm DRA spacers for the rear. 25mm sounds like a lot, but I have 19" wheels (10.5 inches wide). Looks wise, I think the X can easily handle that. The 30mm on the rear, I know will look dynamite.

Here's the potential issue though; Increased positive Scrub Radius. Read this H&R article.

Scrub Radius | Technical || H&R Special Springs, LP

I sort of instinctively knew what adding spacers to the front would do, but this article really explains it well. Given the size of our wheels, 25mm at the hub could translate to a lot of added positive scrub radius. This could potentially make the steering sluggish and hard and 'dull' the immediate responsiveness 'turn in'. It kind of depends on one thing, and this is where I need your help. I don't know where the factory OEM scrub radius is at?? The article says that mcpherson strut sytesms (most all BMWs) really like negative SAI (scrub radius). Pushing the wheels out will be going in the direction they don't like. But if maybe BMW designed the cars with substantial negative SAI to start with. Then adding some wheel spacers might take the tires to neutral SAI, and the steering response and tire wear would still be good.

Besides looks, the upside to front spacers is way less understeer and the car will handle even flatter.

I know the easy answer.. "BMW got it right the first time. Leave it be". But, there are things BMW did for "average joe" reasons, and can be tweaked with great performance results. Maybe this is one of them.

Does anyone know what our scrub radius number is?

Anybody had any experience with this mode and an assessment of the effects on steering feel and responsiveness?

Thanks!

i am thinkin about doing the same, why not just take them off and put them up there to see...

10.5 x 19 up front with the correct offset for the X will stick WAY out with 25mm spacers. but you already have them so swap em and see if they even fit.

I talked with H&R today. They're saying that the article is fairly old and dates from a time when spacers were not being used much by anyone. They're feeling is that pushing the wheel out will not increase the scrub radius. I'm thinkin' that doesn't make sense and it will.

I just put the snow wheels on last night (before the post). I think I'll wait until spring when the 19's go back on to try the 25's up front.

H&R mentioned that they have folks with X5's putting the 25mm DRA spacers on the front and 30mm on the back.

We'll just have to give it a shot!

Yes, but what size tire/wheel package is H&R referring to? I understand that you can probably use the 25mm/30mm combo for the 17x7.5 OEM setup, but does it also apply to the 18x8.5 or 19x9/19x10 setups? The OEM staggered 19" setup is 19x9 front and 19x10 rear. Are you using non-OEM wheels?

I'm using Star Spoke 132 19x9 for both front and rear. I didn't go with staggered because I like to be able to rotate the tires (though that is a minor thing, as tires are a wear-item no matter what you do), but mostly because I have a 3.0i engine and didn't want the extra weight of the 10" rears.

We should all keep in mind when we all talk about wheel widths that the wheel designer splits the difference when adding width in order to keep the tire center at the same scrub radius. So for example, going from an 18x8.5 to a 19x9 you're not adding a half inch to the outside. It only adds a quarter inch, as, the other quarter inch goes towards the inside.

From standing there studying the car, my opinion is that the 25mm spacer will be a tad too wide with the 19x9 .. but is worth a try. I have 'overly wide' spacers on my Z3 Coupe and I absolutely love the look. My only concern was would this increase the scrub radius to the point where it made the steering feel and turn in sluggish. That would suck.

Ahhh...it makes more sense now as your original post indicated that you have 19x10.5 wheels, which I was assuming to be aftermarket since OEM wheels for the X doesn't come in that size. From your OP, my thought was that a 25mm spacer would stick the 10.5" wide wheel past the rear quarter panel...so, trying a 30mm on it would make it look even more odd. But if you have 19x9, I think the 30mm would work fine.

Oops.. sorry about that. Yeah, that would make a heck of a difference! haha..