While test driving a BMW 5-series, i noticed that their official specs indicate different front and back tires. I have also found similar specs on some Acura TLs that i've seen on the road here in atlanta. I am sure it has something to do with a combination of performance and better road grip, however, unable to understand exactly why. Care to explain?

 

Boy, I used to hear that all the time when I drove the 7-Series... It really has to do first of all with rear wheel drive cars and under steer. It's supposed to control or decrease it significantly, which is debatable, but true nonetheless. It really serves no purpose on front wheel drive cars due to the inherent over steer. I don't think it is a good thing for our RLs due to the SH-AWD system/electronics, ABS, and who knows what else. It does look good but hurt quite a bit when the time came to change a set of tires, if you get what I mean. I was running 18s all around, the fronts were 18x8.5, 255/45 and the rears were 18x10, 285/40.

Just one man's thoughts...

Later,

 

thanks a lot Roy..

so even though you did have the same 18", you did use different tires (255/45 and 285/40). Any particular reason?

 

First, because it looked awesome/agressive and gave me a 3" lip in the rear. Second, becasue I was told by some folks that know about cars a gazillion more times than me, that it would help cornering response, stability and even steering. I did notice all of those improved when I went to the staggered setup. Not only that, but BMW offered it as an option, or was standard in all their "M" cars... As you probably know those are/were not bad cars at all....

 

A tire only offers so much traction. Traction is required to corner, brake, or accelerate... it takes more traction to do two of these things at once than just one.

A rear wheel drive car puts more demands on it's rear tires because they both do the acceleration AND have to hold the line of the car through a corner. Put wider tires back there, you can potentially corner harder under power. It works for pretty much the same reason putting a rear spoiler on a front drive car really doesn't do anything except making wax jobs take a little longer

Why not put what you want on the back and match it on the front? I'm not sure but I can guess... by going narrower in front you have less effort to steer, less rolling resistance, less weight... plus it looks "Hot Rod". Meanwhile the narrower tire still has no trouble doing it's work, since it's only responsible for steering, not putting power to the ground. Think about the difference in front and rear tires on a top fuel dragster... this is an extreme example of what I'm talking about. Most any motorcycle (front steering rear drive) has staggered tires as well.

On a front drive car, I can't think of a single performance reason to stagger tire sizes.

Incidentally, in my hemisphere at least (heh), front drive cars understeer and rear drive cars oversteer. Here's a crude explanation but it will have to do. Understeer, or "push" happens because in a front drive car you can begin to lose traction by using up steering AND putting power to the ground with the same tire. The car begins to go straight despite your steering input, it just can't do both. Rear drive, since the front wheels don't have to put power to the ground, they have more traction than the rear tires thus more influence over the direction of the car than the rear tires. The rear end basically starts to skid under power and the car changes direction more than you intended when you turned the wheel.

AWD cars work so well because front and rear tires share more equally in both the cornering and power transfer to the ground.

Rick S.
99 3.5RL White