why wider tires on rear of x5?

[Ranger]

Originally Posted by X5rolls

I think you are 1/2 right.

I don't agree with your points that it's only for looks.

Performance vehicles have wider wheels and tires. Not every day vehicles or fleet vehicles.

Fleet vehicles priorities are about fuel consumption, not increased handling characteristics. Narrow tires and ones made to have a lower rolling resistance are used to lower fuel consumption. You won't find wider tires in the rear there for performance reasons. You might find dual wheels in the back, then again, it's not for performance but for a wider base to support the weight of the load.

In my opinion, front wheel drive cars are not performance vehicles from a handling perspective, other than the potential argument for straight line racing. But not for me in any case. Even all wheel drive vehicles that have roots as front wheel drives have a different bias towards the front and wouldn't benefit from wider tires - front or rear. In fact, wider tires in the front would be an issue.

Performance handling cars like a Porsche Carrera have the weight in the rear and certainly perform better with wider tires.

I have several vehicles with wider tires in the rear, they are all performance oriented. In one of them, it came stock with equal width tires and I switched to staggered wheels/tires and gained better handling.

Plus they look better. Much better.

Your fuel consumption arguement is reasonable as a factor, but it doesn't alter the validity of the point.

Referring to our truck as "performance oriented" from a tire handling perspective is optimistic. For all practical purposes, Tire's don't really "handle" until they start to slip. This assumes a quality tire and a suspension in good order. Max traction is acheived at 4-8deg of slip, depending on the tire. It's the interplay of suspension, weight distro, tire geometry and tire compound that determines the degree of slip on all 4 corners of the car, millisecond to millisecond. That varying degree of slip at all 4 corners is what tire "handling" is all about.

Note that I'm not talking about the car's handling, because that adds all the suspension components and car's static weight distro, into the equation. I'm only talking about the handling of the tire itself.

How often does the X5 owner take a corner so fast that the tires develop a slip angle? I see "performance oriented" trucks and SUVs on the track occasionally, but it's still pretty rare. We drive our X5's well within the limits of the tires, which is to say, no slip. So talking about the handling of our tires is almost a moot point. Not entirely so, because there are some handling characteristics that occur below the slip threshold, but they are pretty inconsequential.

Yes, performance vehicles do have wider wheels and tires. But why? On an ideal surface the force of friction is independent of surface area. And although roads aren't an ideal surface, it's not a strong arguement to say that fleet vehicles have accepted risk in reduced traction in order to optimize gas mileage. They have accepted very little risk because traction isn't reduced much under normal (meaning no slip angle) conditions.

And since we don't drive our trucks so hard that they acheive slip angle, "normal conditions" pretty much applies to us too.

Re. front wheel drive cars are not performance oriented. There's a lot of folks that would be surprised to hear that. There's numerous race series' devoted to front wheel drive car's. I can probably come up with a dozen just off of the top of my head. A couple Pro ones are Mini Challenge and Volkswagon TDI Challenge. NASA (North American SportsCar Assoc), the guys I race with have Honda Challenge 1-4, and numerous front wheel drive cars in GTS1-3.

Your Porsche example doesn't fly. 911 based cars are rear weight biased, and our X5's are front weight biased. I spent a couple yrs on the track in 911's and they are great cars. But they demand a lot from the rear tires and they wear quickly. In contrast, we don't demand much from our rear tires.


Re. You bought bigger rear tires for your car and it handled better. No offense, but whenever we spend money on our cars we're always convinced of the resulting improvement. But "feeling" it and having dyno or data logger records from the track are two different things.

[X5rolls]

Thanks, geography and history are a few of the subjects I enjoy. Did you get a chance to hear what the guys in the video are saying? I bet one of them is saying, "ok already, slow down!"

[lo_jack]

I would have to agree with Ranger as applies to X5s. Now, If you are trying to launch a RWD sports car at the track, and have the motor, going from a 245 to a 315 will make a huge difference, but you would only want to add as much wheel as it takes to accommodate the extra width without making the sidewall deflect beyond design. Ranger is right because X5s a)don't really go to the track that much and b)do not have the motor to really break the rears loose in a dry 'performance' environment, like a quartermile launch.