
As the 128i had been nursed back to full health and become fun to drive, I found myself driving it regularly, pitching it around with the enthusiasm of a dog off-leash at a park full of rabbits! Apparently, driving your sporty RWD coupe in an ahem "spirited" manner has a tendency to wear down a set of tires.
The car came with a set of Bridgestone Driveguard run-flat tires, and I noticed they were down to 3/32nds of tread, with winter steadily approaching. I decided to replace them with a set of OEM-sized Continental ExtremeContact DWS 06 Plus (Ultra High Performance all-season) tires.
I know what you're thinking: "I thought this was a blog about car modification and performance driving? All-season tires? Blasphemy!" Put down the pitchforks and torches. A set of aftermarket wheels with sticky summer rubber is coming.
Living in Colorado, we get a bit of snow in the winter, and running around on summer tires during January here is like poking a bear with a stick repeatedly, waiting for his response. I considered a set of dedicated snow tires, but I already have an AWD vehicle equipped with a winter setup for the heavier snow we might encounter. Having two vehicles with winter tires seemed like overkill, so I settled on a set of Ultra High Performance all-seasons. This way, I could continue to have as much fun as possible during the 5 months or so that the weather becomes less-than-ideal here.
I should note that the new Continental all-seasons are not run-flats. After installing them, the car felt a bit different. The steering felt lighter, which I wasn’t a big fan of. Compared to the Bridgestone run-flats, the car had a bit more grip in the corners, but the body roll became more pronounced. It seemed the slightly stickier tires accentuated the soft (worn) suspension and the less-than-ideal rear subframe bushings, making the car feel a bit wobbly once I pushed it past about 6/10ths.
On the run-flat Bridgestones, I have a theory that the stiffer sidewalls made the suspension feel tighter, or at least stiffer, than it actually was. Despite the increase in grip, the car now felt a bit more floppy and floaty. Not to worry, though—that will be addressed in a later post.
Now, part of the reason the car felt floatier initially might have been my fault. I think I might have set the pressures too high based on the car's GVWR.
Warning: Tire nerd math to follow! Read at your own risk (of severe boredom)!
Allow me to explain: Several posts I found over on 1Addicts show the 128i weighing in around 3100 lbs unoccupied. By contrast, the Gross Vehicle Weight Rating (GVWR) on the inside of the door reads as follows:
GVWR: 4134 lbs
Front: 2028 lbs
Rear: 2249 lbs
The GVWR is what the manufacturer rates the vehicle's maximum weight to be when fully loaded. The curb weight is the weight of the vehicle when it's unloaded.
When I initially set the tire pressures, I used the GVWR as the starting point for my tire pressure math, which was probably a bad idea in this case.
The tires in question (Continental ExtremeContact DWS 06 Plus), in stock sizes (205/50R17 front & 225/45R17 rear), are rated with a load index of 93W front and 91W rear, with a max pressure of 51psi. So, I did the following math:
Front: A 93 index means that tire can handle a total weight of 1433 lbs. I multiplied that number by 2, ending up with 2866 lbs, which I divided by 51 (psi) = 56.2.If I then divide the front GVWR (2028 lbs) by that number, it comes out to ~36psi (cold).
Rear: A 91 index means that tire can handle a total weight of 1356 lbs, which I multiplied by 2 for a total of 2712 lbs. I divided that number by 51 (psi) = 53.2.If I divide the rear GVWR (2249 lbs) by that number, it comes out to ~42psi (cold).
The problem is that the fully loaded GVWR is about 1000 lbs heavier than the car likely weighs when unloaded. With my wife and I in the car, the actual weight is closer to 3400 lbs. If I calculate for 3400 lbs, with a rough 50/50 weight distribution (1700 lbs on each end), math suggests front pressures should be closer to 30.25psi front and around 32psi in the rear, which is a drastic reduction from my GVWR calculations.
I should mention that these tire pressures are estimations for what they should be set to when cold, preferably in the morning if the tires are outside, before they're exposed to too much sunlight. Anyone who’s spent time at the track can tell you that sunlight affects tire pressures—the side of the car exposed to the sun will typically have higher pressures than the shaded side.
When I reset the tire pressures, the amount of floatiness decreased, and high-speed stability improved.
That said, I’m not totally enamored with these tires. As they broke in a bit, they felt less greasy and offered a bit more response, but they’re far from what I would consider a truly responsive tire. They lack some feedback, as they don’t load up as much as I’d like in the corners. However, as many reviews have pointed out, they’re very comfortable and quiet on the road. Overall, they’re not bad, but if I were shopping in this segment again, I’d probably lean toward Bridgestones or Michelins (if available) for a little more road feel and response. For an all-season, the grip level is actually pretty good, but on the street, I find more enjoyment in feel and response than I do overall grip.
Don’t worry, when spring comes, the E82 will get a much-needed dose of extra tire responsiveness.
Comentarios