Before Plus sizing Read this.

[UncleScottie]

The following is from Inside Line's review of the 2011 BMW Z4 sDrive35is full test.

http://www.insideline.com/bmw/z4/201...full-test.html


But Those 19s Are Gorgeous
There's plenty of freshly poured blacktop on a tourist route like Highway 1, so we don't give much thought to the 2011 BMW Z4 sDrive35is' ride quality. Once we're back on Southern California freeways, though, it's top of mind. The Z4 sDrive35is wears standard 19-inch wheels and 225/35R19 front and 255/30R19 rear Bridgestone Potenza RE050A run-flat summer tires. Our butts have filed a restraining order against these tires.

It has been documented that the Inside Line editors are wimps. And with this lower-profile rubber, the Z4 35is should grip better and handle better than the 2009 Z4 35i with 18s (225/40R18 front, 255/35R18 rear), and all will be forgiven, right? Sorry, no.

Apart from the tires, the suspensions on these two cars are pretty much identical. Our 2011 Z4 35is comes standard with the M suspension, which specifies adaptive dampers (and driver-selectable Normal, Sport and Sport+ modes) and a 0.4-inch-lower ride height. Our 2009 Z4 tester had this setup as an option.


Yet the Z4 sDrive35is is much slower than the 2009 Z4 through the slalom. Its 66.7-mph performance — versus the 2009 Z4's 68.9 mph — is closer to what we'd expect from a front-wheel-drive family sedan than a rear-drive sports car.

Blame goes to those very short, very stiff sidewalls, which don't have enough compliance to stay glued over bumps and ripples. There's a small dip at the third cone in our slalom that sends the car skittering sideways and makes our test-driver late for the next cone. The more aggressive damper maps in Sport and Sport+ modes only make the car more nervous, something we also notice when picking up the pace on local back roads.

Through it all, our tester's steering stands mute. BMW hasn't changed the steering ratio on the sDrive35is model, but the electric power steering assist map has been updated. The result is a leaden heft to the wheel in Sport and Sport+, but nothing you would describe as steering feel.

If this was a full-on M car, we'd expect BMW would have retuned the springs and dampers to make them work better with the 19-inch wheel/tire package, likely dumping the run-flat rubber in the process. The Motorsport-division engineers probably would have reverted to conventional hydraulic power steering assist, as they did on the earlier Z4 M.

Braking, at least, is unaffected by the 19-inch-tire package. Our 2011 Z4 has the same braking hardware as the 2009 Z4, and it stops from 60 mph in 106 feet — versus 105 feet for the earlier car. Skid pad performance is slightly better on the 2011 Z4 (0.90g versus 0.87g).


My comments:
Does BMW ever learn??? It is like they keep whacking their head against a brick wall and wondering why it hurts.

Let's hope those were still the old 2nd generation Bridgestone RFTs and not the new, better riding, 3rd gen Bridgetone run-flats.

Wow, more power and yet still 2.2 mph slower through the slalom! That is a lot of sting just for your car to have a little more bling.

[Nate2046]

Seems to me to have less to do with wheel size than shitty tire selection and and poor suspension tuning.

[UncleScottie]

Originally Posted by Nate2046

Seems to me to have less to do with wheel size than shitty tire selection and and poor suspension tuning.

Exactly You hit the nail on the head. Plus sizing without tuning the suspension for that wheel size is a bad idea.

As you read both cars had the same suspension and the same shitty Bridgestone RFT selection. The only difference was the wheel/tire size.

Its too bad BMW hit their thumb instead of the nail.

[dekaliber]

Runflats just suck all around. I think the M with the stiffer suspension actually rides nicer than the 3.0si with Sport Pkg and runflats.

19" is mostly for looks anyway, although I wonder why they spec-ed the all-business M3 CSL with 19"s vs. lightweight 18"s. The wheels on the Motorsport Z4 are 18" IIRC.

[pokeybritches]

Thank you for posting this. I don't know why, but huge wheels on a car that doesn't need them has really started to get at me. 19's plus a very low profile RFT = terrible ride, no matter what the vehicle. I'm not sure how you can tune it out without making the ride softer and handling suffer. Can someone with experience help?

[esqu1re]

I know what you mean w/ the large wheels on everything. I saw a venza with stock 20" and I'm like...WHY?

[UncleScottie]

Originally Posted by pokeybritches

Thank you for posting this. I don't know why, but huge wheels on a car that doesn't need them has really started to get at me. 19's plus a very low profile RFT = terrible ride, no matter what the vehicle. I'm not sure how you can tune it out without making the ride softer and handling suffer. Can someone with experience help?

Hmmm!!! You would think the makers of the "Ultimate Driving Machine" would have the experience and know how to solve such a suspension tuning issue. I guess Suspension Engineering jobs most not pay much if one expects to find greater expertise on forums like this than working at BMW.

Joking of course. I have no doubt that there are some very good Suspension Engineers working for BMW, but for some asinine reason BMW's priorities are elsewhere and these engineers are not able to fully apply their craft.

Anyhow, the basic point of my original post is that "plus sizing" does not always yeild positive results when all else is kept the same. And per the review can even have negative results.


P.S. I guess the pay for Suspension Engineers is a lot better over at that other German performance car company.

[silversprint]

BMWs are tuned for comfort. Most people who buy and drive them would not buy them if they were actually tuned for sport or tuning. BMWs priorities are in selling as many cars as they can

[UncleScottie]

Originally Posted by silversprint

BMWs are tuned for comfort. Most people who buy and drive them would not buy them if they were actually tuned for sport or tuning. BMWs priorities are in selling as many cars as they can

I'll agree that BMWs use to tuned for that "European" feel. That near perfect compromise between comfort and sport that driving enthusiest lusted after and other manufacturers tried to emulate. That hasn't been seen since the E39/E46 days. And there is absolutely no way you can convince me that putting 19" wheels with 2nd generation Bridgestone RE050A RFT tires on a car is tuning for comfort. Did you read the review? Does "Blame goes to those very short, very stiff sidewalls, which don't have enough compliance to stay glued over bumps and ripples. There's a small dip at the third cone in our slalom that sends the car skittering sideways and makes our test-driver late for the next cone." sound comfort oriented? Unfortunately the slalom performance indicates that some sportiness was lost as well.


If BMW's priority is to sell as many cars as they can, then it must be a very short term goal as they are not winning the long term loyalty they use to with the harshness many of their cars recently have had that come equipped with the rigid sidewall 2nd generation Bridgestone RE050A RFT tires. And per the review it was not just comfort that was lost. Some handling went as well.

In all the many automotive forums, those that complain of a poor ride quality are always told it is the sacrifice they have to make for a sportier drive. But it appears here only a sacrifice was made without any gain.

[fitchesbass]

RFTs suck no doubt.

19s are about the limit / high end of max wheel diameter on these types of cars imo. My 19" summer wheels are lighter than the stock 18" wheels so it all depends....

I agree with other comments on what the REAL issue was with the suspension and tire selection.

[bigdog16028]

just as unclescottie says. The auto makers tell us we have to put up with crappy rides to have a sporty car. When and how did it come to pass that big wheels with zero profile tires equals sporty good handling anyway?

Someone needs to inform the race community. As most race cars run 15-16" wheels. and those slow bad handling cars on the F1 circuit are running 13's. Just think how much faster they could be if they would emulate the Main street USA cruiser crowd and switch to 20's.

Cheers
Jerry

[UncleScottie]

Originally Posted by bigdog16028

... When and how did it come to pass that big wheels with zero profile tires equals sporty good handling anyway?....

Cheers
Jerry

What, you never seen an old Western? Those Stagecoaches could really handle with their big wheels with zero profile bands of iron wrapped around them.

Unfortunately two schmucks, Robert William Thomson and John Boyd Dunlop, came along and blew smoke up everybodies arse about the benefits of pneumatic tires. Some crap about them being able to enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground.

It has taken awhile, but BMW and Bridgestone are committed to set things right. To take us back to the early years of the wheel. Now if they can only figure out how to make them square.