1 day ago in Project Cars
We pick up our new M235i straight from BMW.
Written by Per Schroeder
From the Feb. 2008 issue
Posted in Tires & Wheels
Dunkin’ Donuts Munchkins aren’t really made from the dough cut out of the center of each doughnut. If they were, you’d end up with a really funny-looking doughnut, one reduced to a thin ribbon of dough. That wouldn’t do a good job of filling you up (and out) in the morning. As we all know, the best doughnuts are those with the smallest holes in their centers.
However, what’s true for doughnuts isn’t always best for performance tires. The general consensus is that a low-profile ring of rubber delivers the sharpest, crispest performance, passenger comfort be damned. Car came with 15-inch tires? Then 16s or even 17s will make it faster. The manufacturer fit 17s from the factory? Then you’ll need 18s to really fly, the pundits will say.
This concept of plus-sizing is pretty easy to grasp: Increasing the wheel diameter while reducing the tire’s sidewall will keep the overall outside diameter constant—thus not changing the gearing—yet replaces the flexible rubber sidewall with unforgiving metal to improve lateral performance. These low-profile tires generally don’t look half bad, either. In theory larger wheels also provide more room for bigger brakes. Plus-size tires do have their downsides, though. Bigger wheels can add more weight to each corner of the car, and those short sidewalls mean a reduction in the tire’s inherent spring rate, leading to a rougher ride over pavement irregularities.
No matter what the perceived downsides, larger wheels are currently ruling the street scene for their cosmetic effect, and the concept has been taken to the extreme. Think huge rims wrapped with thin rubber bands. Sure, those big, honking wheels can look neat, but do they really offer any increase in performance? Many manufacturers are now offering 17-, 18- and even 19-inch options on street cars; do they know something that we don’t? Or is this all just about style?
What’s certain is that this trend has made it very hard to find good street tires for our older sporty cars. Our beloved 15-inch performance tires are becoming obsolete, while requesting a high-performance 13- or 14-inch tire will usually earn a laugh from the local tire dealer.
The concept of running upsized wheels is nothing new. Two decades ago, in fact, we published our first plus-size tire test. It ran in the January 1987 issue, and we used a then-new Pontiac Trans-Am and a bunch of Firestone Firehawk tires at the University of Texas in Arlington.
Our test data showed that bigger was better as the larger tires performed best in every category measured. The Plus Two 245/50R16 combo was faster than the 215/65R15 Plus One and 205/70R14 Plus Zero setups. (And for a little terminology, a Plus One setup features a wheel diameter that is one inch larger than stock; Plus Two would be two inches bigger, and so forth. The Plus Zero term is used when fatter than stock tires are paired with the original wheel diameter; these fatter tires feature a lower aspect ratio so the outside tire diameter doesn’t change.)
We ran another plus-size tire test in the January/February 1999 issue of GRM, this time using a 1998 Honda Civic EX coupe owned by Koni’s Lee Grimes. That test was done at The Tire Rack’s old headquarters in South Bend, Ind.
Despite a totally different car plus another decade in tire technology, the results were the same. The Plus Three 215/40R17 Dunlop SP Sport tires were faster than Plus Two, Plus One and Plus Zero setups. Slight penalties in ride comfort, weight and acceleration were offset by improvements in grip and transient response.
Fast forward to another decade and we are still bench racing about the pluses and minuses of plus-sizing. Time to order up another dozen funny-looking doughnuts so we could stage another plus-size tire test.
This study’s subject is a late-model BMW 325i, and the lab is the new Tire Rack test track. The giant tire retailer is still in South Bend, albeit now on the other side of town.
To remove one potential variable, we used the same make and model of tire for each step in this test. We chose the new Yokohama S.drive, the replacement for their popular, sporty AVS ES100. The S.drive, however, has better wet and dry grip than its predecessor while still maintaining great comfort and affordability in a summer tire.
We tested three sets of tires and wheels starting with the Plus Zero upgrade of 16x7.5-inch wheels and 205/55R16 tires. We then jumped to 225/45R17 tires on 17x8-inch wheels followed by 225/40R18 tires on 18x8-inch wheels. (To witness the effects of a Plus Zero upgrade, check out our October cover story; going from all-season 215/45R17 Bridgestone Potenza RE92 tires to some very sporty 225/45ZR17 Bridgestone Potenza RE-01Rs on our Subaru Impreza WRX cut autocross lap times from 32.871 seconds to 31.882.) Our drivers for the test were The Tire Rack’s Woody Rogers and our own Per Schroeder, both old hands at The Rack’s proving grounds. In other words, no time would be wasted learning the course. The testing order would go from smallest wheel to largest, with the first set retested at the end of the sequence to ensure consistency.
This wouldn’t simply be a tale of lap times, however. Before hitting the test track, each driver sampled the tires on The Tire Rack’s street loop. This test loop goes from weather-pocked concrete to smooth asphalt, all at varying speeds. The end result is a nice picture of real-world ride and comfort.
Our performance track was the same configuration of The Tire Rack’s test facility that we use for our tire tests. It’s a nice combination of a five-cone slalom, quite a few offsets, a long skidpad section and a few high-speed sweepers.
Okay, time to don the white coats and start the testing.
Wheel size: 16x7.5 inches
Tire size: 205/55R16
Mean lap time: 43.353 sec.
As expected, the 16-inch tire package was very comfortable and mild-mannered on the road. Our drivers felt minimal road harshness, while small to medium bumps seemed somewhat distant. Larger impacts were apparent but not objectionable, and the steering was responsive, more so than an O.E. touring tire but not laser-sharp. We wouldn’t mind selling this combination to our mom. On the track, the the car felt free, not locked down. Steering response was linear, just not prompt. This Plus Zero combination required some steering input lead time when snaking through the slalom and negotiating the tighter 90-degree corners. Waiting too long to initiate a turn forced us to hack through the maneuver, grinding the front tires or sliding the rears. The free, tossable feeling of the tall sidewalls was fun to drive. It encouraged us to chuck the car around, but that’s not the way to quick lap times.
Wheel size: 17x8 inches
Tire size: 225/50R17
Mean lap time: 43.155 sec.
On our road loop, the 17-inch package delivered upgraded steering feel while still being tolerable to our (tolerant) butts. More high-frequency noises snuck into the cockpit from the pavement, but small and medium bumps were still well-damped. Larger bumps would occasionally crash through, but the ride wasn’t unpleasant. We might not put our mom on these tires, but we certainly would recommend them to a performance-oriented friend.
Once let loose at speed, there was a big improvement in steering response compared to the 16-inch combination. Interestingly, the increased responsiveness was not matched by a similar improvement in steady-state or mid-corner grip. We’ll gladly take the improved lap times, however.
Despite the tire’s added width, the 17-inch combo did not seem to stop quite as well as the smaller setup. This could possibly be due to the increase in tire and wheel mass.
Overall, the 17-inch combination felt the best of the three. This package had a good balance of responsiveness, stability, cornering traction and braking capability.
Wheel size: 18x8 inches
Tire size: 225/50R18
Mean lap time: 43.339 sec.
Here’s a shocker for those hailing from the bigger is better camp: The 18s didn’t win this little comparo.
For one, the 18-inch wheel and tire package yielded a real degradation in ride quality on our road loop. We heard and felt more high-frequency road noise as interior panels vibrated while crossing rougher sections of pavement. Medium and large bumps became objectionable when encountered at the same speeds as before. We’d steer away from a setup this radical on a daily-driven car.
Between the cones we saw another improvement in responsiveness, although not as big a change as going from 16- to 17-inch wheels. Interestingly, the 18-inch combination felt more stable around the steady-state skidpad than either of the other combinations. The short, short sidewalls could have been the reason, as there’s simply less vertical rubber to roll over when facing high g-loads.
There was a noticeable reduction in braking performance, unfortunately, as we could feel the extra mass fighting the car. Slowing the car for the tighter turns required more pedal pressure plus an earlier braking point. The extended stopping distances hurt lap times, as the 18-inch wheel and tire package was only slightly faster than the 16s and several tenths behind the 17s.
Whether talking about doughnuts or tires, there’s always a good middle ground. Some are too sweet, while others are bland and boring. The ones in the middle of the box can often be the keepers.
For us, our middle-of-the-road 17-inch Plus One setup yielded the best combination of ride comfort and track performance. There was enough edginess to feel sporty without intruding upon conversations when encountering less than perfect roads. We’ll gladly forego that extra inch of bling and settle on solid performance.
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