E61 m-sport - ride quality / comparisons

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Jeff Strickland wrote:


That's not the entire story. A tire's load capacity is directly related to its cross-sectional area. Reduce the sidewhall height, and width should be increased to compensate. (This is something most ricers do not concern themselves with, I'm sure.)
You can also compensate for reduced cross-sectional area by increasing air pressure, but that, obviously, has repercussions also.
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wrote:

That's not true, Dizzy. You can decrease the sidewall height and increase the rim diameter, and keep the same tread width.
Within a few revolutions over an entire mile, a 225/55x15 and a 225/45x17 are identical. The tread is the same while the sidewall height is narrower, but the circumference is so close that over a mile, the tire turns within 10 rotations of the same for each. So, narrower sidewall does not require a wider tire. If one wanted to stay with the same rims, then you would be right, sorta. If you went to a wider tire, you would have to reduce the aspect ratio to keep the same overall diameter. The problem here is that wider tires often times do not fit on the same rims -- a rim that's 7 inches wide can only carry a tire that is relatively narrow in the first place, so if one was inclined to replace a 16x7JJ rim with a 16x8.5JJ, then why not get an 18x8.5JJ and get the tires to fit the rim properly and reduce the sidewall height in combination with the wider tire so the overall diameter remains within the specification for the car.
I just pulled a "standard" tire size out of my ass to illustrate the point, if you had a stock tire of 205/60x16, you could replace it with a 245/40x18, and have exactly the same -- for any practical purpose -- tire in terms of diameter. The 16 goes 785.209 revolutions per mile, the 18 goes 784.248. For any practical reason, these two tires are identical, except one is wider and should fit the bigger rim than a wider tire on the original rim. The 16 tire is 8 inches wide, the 18 inch tire is 9.5 inches wide. Both of these examples would fit nicely on the rims I have described.
I haven't explored the fender clearances and undercarriage obstructions, but I just wanted to illustrate that you can change the numbers around to get a tire that fits the general spec of the original fitment, and do it very easily.
If you wanted to stay with the same width of a rim, but change the diameter, then you would retain the original tire width and simply decrease the aspect ratio by 5 for every inch increase in rim diameter. If you had a 15x8JJ rim that took a 225/55x15, you could replace the rim with a 17X8JJ and fit it with a 225/45x17 tire, and nothing would be affected -- except in my case the speedometer error was corrected from an error of about 6% to an error of about 1% -- my car went from reading 80 and doing 75 to reading 80 and doing 79.
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Jeff Strickland wrote:

Everything I stated is indisputable fact.
My point was about how load capacity is affected directly by the cross-sectional area and the pressure, and how this should be kept in mind when juggling wheel/tire sizes and widths.
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Scott Dorsey wrote:

It's kind of obnoxious, though, to have to spend a bunch of money (and four nice wheels are tires will cost you more than that), just to "fix" something on what is already a pretty expoensiove car.
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wrote:

I agree with Dizzy to a point. But I think that the sport package is doing its job of reducing body roll and increasing road feel at the expense of a soft and squishy ride.
I'd replace the run-flats with actual tires on the same rims and wait for the lease to expire, then rethink plans to buy the sports package in the future.
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My take on this is that the Sport suspension is designed to BE pretty much what you are complaining about.
Luxo-boats are known for soft, supple rides. The sport package is intended to firm the ride and increase the road feel. I'm not sure what the run-flat tires do for the equation, but it could be worth your while to get away from them.
I had a 3 Series with the sport package that I also changed out the 225/55x15s for 225/45x17s, but I never thought the ride was "jarring." It was stiff to be sure, but I like that quality in a car much more than the wallowing-around-on-a-sack-of-pillows feeling. Granted, my 3 and your 5 are a comparison of apples and oranges, but the overall point is that the sport package is supposed to increase road feel and decrease body roll, which by definition shoulld make a firm ride.
Just a thought to bring some perspective.

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