Do you care about your spare? - Car and Driver

[Klaus]

The Steering Column
Do you care about your spare?
BY CSABA CSERE
May 2003


After we completed our BMW M3 long-term test (March 2003), our production editor, Juli Burke, and copy chief, Patti Maki, were driving the M3 on M-14, a local divided highway, when the car started to make a bad noise. They pulled over and found a large hunk of road debris embedded in the left rear tire.

They proceeded cautiously into the dark, cold night, and soon the debris dislodged with some commotion in the wheel well. The noise stopped. Then the low-tire-pressure dash light came on. Ready and able to change their flat tire, the ladies exited the highway and made it to a lighted convenience-store parking lot.

They popped the trunk, and to their surprise, instead of a spare tire they found something called the M Mobility System, a combination air compressor and tire-sealant dispenser designed to reinflate a flat tire for temporary use.

Following the directions in the M3's owner's manual, Maki and Burke achieved zero success with the M Mobility System. Perhaps the hole in the tire was larger than the 0.16 inch the system was designed to accommodate. Maybe the zero-degree weather curdled the tire sealant. Either way, they were stranded.

Not really, of course. They did what a lot of drivers would have done in the first place and called for help on a cell phone.

Still, had the M3 been equipped with a spare, our two damsels in distress could have changed the tire and continued on their way. These days, full-size spares are still common in pickups, vans, and SUVs, but they've become rare in high-performance rides.

Part of the problem is that tires have been growing faster than cars have. For example, the 255/40-18 rear tires on the BMW M3 are about two-and-a-half inches wider than the 7.75-15 tires that a '67 Corvette wore. Finding room for such fat rubber is difficult in a compact car.

An early solution to this space problem was the collapsible spare tire, introduced on the Pontiac Firebird in 1967. This spare tire was about the same width as the regular tire, but it was stored fully deflated and folded within its wheel.

I owned a 1974 Porsche 911 equipped with one of these tires, and it was about half the size of a full-size spare, allowing a nearly five-gallon increase in the size of the fuel tank. It wasn't much lighter, however, if you included the small electric air compressor required to inflate the tire. For the complete package, these collapsible spares actually cost more than conventional ones.

Next came the space-saver spare, a fully inflated, 60-psi tire that is much narrower and a little smaller in diameter than the standard tire, yielding a one-third reduction in volume and eliminating the need for an inflation device. Bill Hopkins, Goodyear's vice-president of product planning, estimates they're about 20 percent lighter than full-size spares. The tires are cheaper as well (on a BMW 325xi, for example, a full-size spare is a $250 option).

Both of these space-efficient spares have their downsides. They neither perform as well nor last as long as conventional tires. When using one, your owner's manual warns you to stay below 50 mph and get the tire replaced within 100 miles, although tests we've conducted on the high-pressure rubber suggest they are far more durable than that (August 1992).

There's also the problem of what to do with your flat full-size tire. Since it won't fit in the small space allotted to the minispare, you must load it in the trunk or the back seat—if it fits—or perhaps in your significant other's lap.

The next evolution in flat-tire solutions eliminates this problem because you never need to remove your flat tire. One version is that employed by the BMW M3, which left Burke and Maki stranded. Although it's not completely reliable, this approach saves most of the bulk and all of the weight (the compressor weighs as much as the no-longer-needed jack) of a spare, along with most of its expense.

A more reliable, spareless solution is the run-flat tire. You can continue driving for at least 50 miles even after it has lost all of its inflation pressure. Although run-flats eliminate the spare tire and jack, you don't save any weight because run-flat tires are substantially heavier than normal ones.

Sounds like the perfect solution except for one major compromise—ride. The current run-flat tires achieve their airless performance primarily through reinforced sidewalls that are much stiffer than those on regular tires. As a result, the tires don't flex readily to assist in absorbing small, sharp bumps. The more rigid sidewalls also transmit more road noise into the cabin.

Due to their perceived value, four run-flats are also a bit more expensive than five conventional tires and a jack. This cost includes a tire-pressure warning system, to save dim drivers who might fail to notice that one run-flat has lost the benefit of air pressure. This cost penalty will disappear when such warning systems become government mandated in 2004.

Most cars using run-flat tires have been designed to work with them from the get-go, but some of these collaborations are more successful than others. The C5 Corvette has achieved an excellent combination of ride, noise, and handling with Goodyear Eagle F1 EMT run-flats. That said, you could still reduce the Vette's highway noise and improve compliance by fitting conventional tires of similar quality.

Other cars are less happy with run-flats. In the Lexus SC430, run-flats yield a welcome increase in trunk space in exchange for a jarring ride that incites serious structural quivering—at least here in the Midwest. The Mini Cooper also incurs a ride penalty with run-flats. Goodyear's Hopkins predicts these tradeoffs will be reduced with further development.

He also has high hopes for the Michelin-developed PAX System, which Goodyear is also adopting. Instead of stiff sidewalls, this approach relies on a plastic inner donut to support the tire in the event of air loss, reducing the ride compromise. But the PAX System also requires unique rims with positive bead locking, which has never been an easy sell, as Michelin learned with its TRX tires 20 years ago.

Flat tires are so rare I'd rather not compromise the performance of any car I own just to avoid carrying a spare. But I don't ever want to be stranded, either, so for now, the space-saver spare strikes me as the best solution. But if you think otherwise, don't wait until a bitterly cold night to learn what's in your car's spare-tire well.

[Klaus]

The Steering Column
Do you care about your spare?
BY CSABA CSERE
May 2003


After we completed our BMW M3 long-term test (March 2003), our production editor, Juli Burke, and copy chief, Patti Maki, were driving the M3 on M-14, a local divided highway, when the car started to make a bad noise. They pulled over and found a large hunk of road debris embedded in the left rear tire.

They proceeded cautiously into the dark, cold night, and soon the debris dislodged with some commotion in the wheel well. The noise stopped. Then the low-tire-pressure dash light came on. Ready and able to change their flat tire, the ladies exited the highway and made it to a lighted convenience-store parking lot.

They popped the trunk, and to their surprise, instead of a spare tire they found something called the M Mobility System, a combination air compressor and tire-sealant dispenser designed to reinflate a flat tire for temporary use.

Following the directions in the M3's owner's manual, Maki and Burke achieved zero success with the M Mobility System. Perhaps the hole in the tire was larger than the 0.16 inch the system was designed to accommodate. Maybe the zero-degree weather curdled the tire sealant. Either way, they were stranded.

Not really, of course. They did what a lot of drivers would have done in the first place and called for help on a cell phone.

Still, had the M3 been equipped with a spare, our two damsels in distress could have changed the tire and continued on their way. These days, full-size spares are still common in pickups, vans, and SUVs, but they've become rare in high-performance rides.

Part of the problem is that tires have been growing faster than cars have. For example, the 255/40-18 rear tires on the BMW M3 are about two-and-a-half inches wider than the 7.75-15 tires that a '67 Corvette wore. Finding room for such fat rubber is difficult in a compact car.

An early solution to this space problem was the collapsible spare tire, introduced on the Pontiac Firebird in 1967. This spare tire was about the same width as the regular tire, but it was stored fully deflated and folded within its wheel.

I owned a 1974 Porsche 911 equipped with one of these tires, and it was about half the size of a full-size spare, allowing a nearly five-gallon increase in the size of the fuel tank. It wasn't much lighter, however, if you included the small electric air compressor required to inflate the tire. For the complete package, these collapsible spares actually cost more than conventional ones.

Next came the space-saver spare, a fully inflated, 60-psi tire that is much narrower and a little smaller in diameter than the standard tire, yielding a one-third reduction in volume and eliminating the need for an inflation device. Bill Hopkins, Goodyear's vice-president of product planning, estimates they're about 20 percent lighter than full-size spares. The tires are cheaper as well (on a BMW 325xi, for example, a full-size spare is a $250 option).

Both of these space-efficient spares have their downsides. They neither perform as well nor last as long as conventional tires. When using one, your owner's manual warns you to stay below 50 mph and get the tire replaced within 100 miles, although tests we've conducted on the high-pressure rubber suggest they are far more durable than that (August 1992).

There's also the problem of what to do with your flat full-size tire. Since it won't fit in the small space allotted to the minispare, you must load it in the trunk or the back seat—if it fits—or perhaps in your significant other's lap.

The next evolution in flat-tire solutions eliminates this problem because you never need to remove your flat tire. One version is that employed by the BMW M3, which left Burke and Maki stranded. Although it's not completely reliable, this approach saves most of the bulk and all of the weight (the compressor weighs as much as the no-longer-needed jack) of a spare, along with most of its expense.

A more reliable, spareless solution is the run-flat tire. You can continue driving for at least 50 miles even after it has lost all of its inflation pressure. Although run-flats eliminate the spare tire and jack, you don't save any weight because run-flat tires are substantially heavier than normal ones.

Sounds like the perfect solution except for one major compromise—ride. The current run-flat tires achieve their airless performance primarily through reinforced sidewalls that are much stiffer than those on regular tires. As a result, the tires don't flex readily to assist in absorbing small, sharp bumps. The more rigid sidewalls also transmit more road noise into the cabin.

Due to their perceived value, four run-flats are also a bit more expensive than five conventional tires and a jack. This cost includes a tire-pressure warning system, to save dim drivers who might fail to notice that one run-flat has lost the benefit of air pressure. This cost penalty will disappear when such warning systems become government mandated in 2004.

Most cars using run-flat tires have been designed to work with them from the get-go, but some of these collaborations are more successful than others. The C5 Corvette has achieved an excellent combination of ride, noise, and handling with Goodyear Eagle F1 EMT run-flats. That said, you could still reduce the Vette's highway noise and improve compliance by fitting conventional tires of similar quality.

Other cars are less happy with run-flats. In the Lexus SC430, run-flats yield a welcome increase in trunk space in exchange for a jarring ride that incites serious structural quivering—at least here in the Midwest. The Mini Cooper also incurs a ride penalty with run-flats. Goodyear's Hopkins predicts these tradeoffs will be reduced with further development.

He also has high hopes for the Michelin-developed PAX System, which Goodyear is also adopting. Instead of stiff sidewalls, this approach relies on a plastic inner donut to support the tire in the event of air loss, reducing the ride compromise. But the PAX System also requires unique rims with positive bead locking, which has never been an easy sell, as Michelin learned with its TRX tires 20 years ago.

Flat tires are so rare I'd rather not compromise the performance of any car I own just to avoid carrying a spare. But I don't ever want to be stranded, either, so for now, the space-saver spare strikes me as the best solution. But if you think otherwise, don't wait until a bitterly cold night to learn what's in your car's spare-tire well.

[Dan]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Klaus:
This cost penalty will disappear when such warning systems become government mandated in 2004.<HR></BLOCKQUOTE>

Interesting...

- Dan

---------------------------------
SUT on order - 13 months to go!
---------------------------------

[RasterMaster]

The CTIS adapted for the H2 would be nice too.

Check out the OkC Chapter Page!