[xDrive35i]

I have the 19" 309s with summer performance tires. They are staggered; F245/45/19 R275/40/19. The lowest PSI rating is 31F and 35R.

They are being replaced with the 19" 309s with all seasons. These are not staggered; F245/45/19 R245/45/19. The lowest PSI rating is 31F and 31R.

When replaced, will my ride comfort improve?

Driving in Chicago with these god-awful roads, I continue to feel "mechanical sympathy" for my car, and driving is becoming less enjoyable.

I also want to know why the 19" 309s can't be fitted with tires that have more sidewall. The run flats are just terrible, and I'd like some more cushion. The GLK is standard with 19s, but the tires have significantly more meat. The X3s wheel wells look as though more tire radius would fit.

I also want to know why BMW did not fit the front wheels with double-wishbone suspension. I know it's more expensive, but they put it on the X5. I know double-wishbone is more comfortable than MacPherson strut suspension, and it also allows for greater driving stability. What gives?

[fedinand]

I don't see your ride comfort improving...you still have the same amount of sidewall. Are you 309s a square set up? The rears are an inch wider otherwise...and looks like you are running 245s all the way around for the winter.

If you increase the sidewall on the tires on the 309s make sure it is the same front and rear and make sure you know how that will affect your speedometer.

[paul386]

Originally Posted by juddholland

I have the 19" 309s with summer performance tires. They are staggered; F245/45/19 R275/40/19. The lowest PSI rating is 31F and 35R.

They are being replaced with the 19" 309s with all seasons. These are not staggered; F245/45/19 R245/45/19. The lowest PSI rating is 31F and 31R.

When replaced, will my ride comfort improve?

Driving in Chicago with these god-awful roads, I continue to feel "mechanical sympathy" for my car, and driving is becoming less enjoyable.

I also want to know why the 19" 309s can't be fitted with tires that have more sidewall. The run flats are just terrible, and I'd like some more cushion. The GLK is standard with 19s, but the tires have significantly more meat. The X3s wheel wells look as though more tire radius would fit.

I also want to know why BMW did not fit the front wheels with double-wishbone suspension. I know it's more expensive, but they put it on the X5. I know double-wishbone is more comfortable than MacPherson strut suspension, and it also allows for greater driving stability. What gives?

BMW is all about feeling the road... I would consider that a plus of the car. Seems odd that you are trying to dampen the feature!

The best way to improve your road feel is to go with smaller diameter wheels, decrease the stiffness of the tires, and reduce the rolling mass of the wheel assembly. Run flat tires are very stiff and have a large mass so this contributes to the problem. Perhaps your best option is to get 17" or 18" tires that are less stiff.

Double-wishbone suspension generally has the fewest compromises of all the suspension designs and allows the designer to have the most freedom in his design. However, double-wishbone is also not very spatially efficient. To make more room for the engine components MacPherson struts are used. I'm not sure that is such a bad decision. Double-wishbone suspension can offer excellent camber stability as well as other advantages (structural advantages are probably immense). But a passenger car suspension has such a massive range of travel, usage, loads, ect that it seems extremely difficult to tailor the suspension to actually use these benefits. If the designers choose to use a double wishbone because of the perceived superiority to MacPherson struts but generates poor geometry to save space then they have not created a better geometry. I guess what I am getting at is... a bad double wishbone suspension is not better than a good MacPherson strut suspension.

[sfax]

Style over comfort Judd

[Le Chef]

I personally think it's a pretty soft setup. My preference is to switch to "Sport" where you get more road feel and less of the waterbed effect. I have the tire pressures set at 32F/35R.

[xDrive35i]

Originally Posted by fedinand

I don't see your ride comfort improving...you still have the same amount of sidewall. Are you 309s a square set up? The rears are an inch wider otherwise...and looks like you are running 245s all the way around for the winter.

If you increase the sidewall on the tires on the 309s make sure it is the same front and rear and make sure you know how that will affect your speedometer.

The factory screwed up and delivered my car with the staggered 309s with summer tires. I ordered the all-season 309s, which are squared. The dealer is switching them out free of charge. I will not be spending any money on the rims/wheels/tires or anything--aside from the insurance--so I won't be increasing sidewall. I only wonder if the rear sidewall increases slightly because the summer performance 309 rears are 275/40/19, and the all season rears are 245/45/19.

Originally Posted by paul386

BMW is all about feeling the road... I would consider that a plus of the car. Seems odd that you are trying to dampen the feature!

The best way to improve your road feel is to go with smaller diameter wheels, decrease the stiffness of the tires, and reduce the rolling mass of the wheel assembly. Run flat tires are very stiff and have a large mass so this contributes to the problem. Perhaps your best option is to get 17" or 18" tires that are less stiff.

Double-wishbone suspension generally has the fewest compromises of all the suspension designs and allows the designer to have the most freedom in his design. However, double-wishbone is also not very spatially efficient. To make more room for the engine components MacPherson struts are used. I'm not sure that is such a bad decision. Double-wishbone suspension can offer excellent camber stability as well as other advantages (structural advantages are probably immense). But a passenger car suspension has such a massive range of travel, usage, loads, ect that it seems extremely difficult to tailor the suspension to actually use these benefits. If the designers choose to use a double wishbone because of the perceived superiority to MacPherson struts but generates poor geometry to save space then they have not created a better geometry. I guess what I am getting at is... a bad double wishbone suspension is not better than a good MacPherson strut suspension.

I've had two X3s. The first one I ordered and took delivery in February, and it had the 18" 308s. That car was lemoned, and I ordered a new one and took delivery of it in August with the 19" 309s. I did not know how dramatic the difference in ride quality would me, but I just wanted my car to look a tad less.....stumpy than it did with the 18" 308s.

Originally Posted by sfax

Style over comfort Judd

Well I'll hardly be sacrificing style. The tires just won't be staggered, so the rear will be a little less brawny, but no matter. The side profiles are enhanced by the 309s whether or not they're staggered.

Originally Posted by Le Chef

I personally think it's a pretty soft setup. My preference is to switch to "Sport" where you get more road feel and less of the waterbed effect. I have the tire pressures set at 32F/35R.

You know it's funny how different we all think our cars ride. The roads here SUCK, and yes, the suspension is a bit wallowy normal mode, but the ride is too harsh in sport with these 19" 309s. Too many expansion joints, too many potholes, too many washboarded roads, too many lips, bumps, cracks, etc.

[paul386]

Originally Posted by juddholland

The factory screwed up and delivered my car with the staggered 309s with summer tires. I ordered the all-season 309s, which are squared. The dealer is switching them out free of charge. I will not be spending any money on the rims/wheels/tires or anything--aside from the insurance--so I won't be increasing sidewall. I only wonder if the rear sidewall increases slightly because the summer performance 309 rears are 275/40/19, and the all season rears are 245/45/19.



I've had two X3s. The first one I ordered and took delivery in February, and it had the 18" 308s. That car was lemoned, and I ordered a new one and took delivery of it in August with the 19" 309s. I did not know how dramatic the difference in ride quality would me, but I just wanted my car to look a tad less.....stumpy than it did with the 18" 308s.



Well I'll hardly be sacrificing style. The tires just won't be staggered, so the rear will be a little less brawny, but no matter. The side profiles are enhanced by the 309s whether or not they're staggered.



You know it's funny how different we all think our cars ride. The roads here SUCK, and yes, the suspension is a bit wallowy normal mode, but the ride is too harsh in sport with these 19" 309s. Too many expansion joints, too many potholes, too many washboarded roads, too many lips, bumps, cracks, etc.

Unsprung (Wheel / Tire / Spindle / Brakes / A portion of the control arms) plays a huge rolls in the ride of a vehicle. Reducing the mass will cause significant improvement. It is difficult for me to guess how 1" diameter affects the ride or to really explain its effects (it has been a few years since I took vibrations!)... but it does play a pretty significant role.

[fedinand]

Originally Posted by juddholland

The factory screwed up and delivered my car with the staggered 309s with summer tires. I ordered the all-season 309s, which are squared. The dealer is switching them out free of charge. I will not be spending any money on the rims/wheels/tires or anything--aside from the insurance--so I won't be increasing sidewall. I only wonder if the rear sidewall increases slightly because the summer performance 309 rears are 275/40/19, and the all season rears are 245/45/19.

The sidewalls will still be 4.3 inches...due to the narrower width, but I would think it wouldn't be as stiff so maybe you will get a bit of an improvement. This is a handy site for aftermarket wheel connoisseurs: http://www.miata.net/garage/tirecalc.html

[xDrive35i]

Thanks ferdinand

[LEDZEP]

By more sidewall, I assume you mean a higher aspect ratio tire. You can only achieve that by going to a smaller diameter wheel, like an 18". The diameter of the outside of the tire must remain constant (within a very low tolerance) or it can cause you some problems. Most importantly, it can cause problems with the drivetrain for AWD. You could check with a knowledgeable technician on that, in case I'm wrong. It will also throw your speedometer and odometer out of calibration.

[xDrive35i]

Originally Posted by LEDZEP

By more sidewall, I assume you mean a higher aspect ratio tire. You can only achieve that by going to a smaller diameter wheel, like an 18". The diameter of the outside of the tire must remain constant (within a very low tolerance) or it can cause you some problems. Most importantly, it can cause problems with the drivetrain for AWD. You could check with a knowledgeable technician on that, in case I'm wrong. It will also throw your speedometer and odometer out of calibration.

Ahh, so what do the 40 in 275/40/19 and the 45 in 245/45/19 actually mean? Does anyone know how it's calculated? I was under the impression that it is the measurement of the edge of the rim to where the tread pattern hits the ground. Is this not so?

[Hutt]

Originally Posted by juddholland

Ahh, so what do the 40 in 275/40/19 and the 45 in 245/45/19 actually mean? Does anyone know how it's calculated? I was under the impression that it is the measurement of the edge of the rim to where the tread pattern hits the ground. Is this not so?

I have always thought it was aspect ratio of height to width. So the 45 would mean the height of the sidewall is 45% of the width (110mm) in the example 245/45/19. I think.

[fedinand]

Tirerack to the rescue...from their site: Sidewall Aspect Ratio

Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.

P225/50R16 91S

The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.