:thumbup:

So, over which side of the fence are you leaning, V?
The part in the Tire Rack quote about less rotational mass allowing the suspension to work more effectively does make theoretical sense - but I can't imagine a few pounds per wheel/tire making enough of a difference to see enhanced suspension performance on an X5... (just my guess though, so I won't be offended if anyone challenges this notion ;) )
And the rest of the quote really just focuses on accel/braking, which I think we can all concede would certainly be improved with less rotational M. (Although how much I couldn't even guess... )

Hayaku, where are you - we need more info?? :)

I'm with Hayaku. Reduced weight whether at the wheel or the suspension makes for better response. BMW for instance uses aluminum extensively for the suspension components for this reason. If you reduce weight on individual components, as a system the effect is going to be great.

The advantage with large wheels is that there is less sidewall deflection. Remember the Lambo Countach? It only had 15" wheels. These days supercars have 17" and 18" wheels, because materials science has yielded lightweight materials at lower cost.

relax guys... bench racing and keyboard engineering may be fun, but lets keep it fun...

look at the open wheel cars... extremely stiff sidewalls on 15-16" rims with overal tire diameters at 25-28 inches... F1, IRL, Champ, Formual Maxda, Formula Atlantic, etc etc etc... sidewalls so stiff that on a 2006 F1 race, i forgot whose car, the rear right tire blew and tore away all the tread leaving just the 2 sidewalls.. the driver DROVE back to the pit lane and the car was still at its normal ride hieght... the sidewall was strong enough to hold up the weight of the car.. and this is a tire that supports the engine weight...

its the flex in both the sidewall and the distortion of the tread blocks that makes tuning and cornering possible. too stiff and its irrelevent (like a train on the street, if it can stand on the ground and not roll over, it'll slide in every corner)... too soft and it rolls like bread dough...

i guess i should have qualified my statement earlier... the 17" wheel would need to be of custom width and offset.. then the proper search for a great performance tire would be required and most likely lead us to a shorter tire (not oem spec diameter) and would change some of the acceleration/braking characteristics... as you know, the larger the wheel, the less choices you in tires and we haven't been present with a good high performance tire yet.. all street tires only...

I see that they make a 325/30/21 in the Michelin Diamaris. It has almost the same ratio as the stock 315/35 (.04% difference). I wonder if that tire would fit on the 21 x 10.5 BBS RX wheels? You then could use the 285/35/21 up front, which Michelin also makes.

Guys guys we're driving a heavy truck with a relatively high center of gravity, not a sport car. So get whatever wheels you need. Honestly this conversation about weight savings on a vehicle this heavy doesn't really make any sense. Now if you're driving an M6 then yes let's talk wheel weight. JMO.

I personally wouldn't go with a 21" wheel as it's an "odd" size and tires have a more limited selection and are more expensive. :dunno:

I was thinking the same thing.

m6 aint a light weight either... its a fast street pig of a car... ;)

you're right tho.. no one here is going to turn their x5 into a race car of any form.. these cars will be street cars only and will never even hit their fullest potential on the street, so to that end, it won't matter what size wheel or tires you put on...

I agree... that was my intended point in this semantic discussion, really.
That's why I don't think that smaller diameter-than-stock 17" wheels/tires on an X5 are a viable consideration for someone who just wants to extract better street handling, but not try to turn their rig into some quasi-track monster. I can't imagine that a few extra lbs of additional rotational mass per wheel/tire would result in a handling detraction on an X5.

As for whether 21's or 22's would be 'better' handling on the street than 20's... don't know about that one. I DO know 20's feel much more crisp than 18's, but maybe 20's are the sweet spot. (I personally wouldn't want to go any lower profile than 20's regardless due to road hazard damage concerns... )
Has anyone driven an X with OEM diameter 21's or 22's that can compare them to 20's?