A convertible version of the apple-cheeked Mini? What could go wrong? Whatever unit of measurement one might use to index adorability - calories, girlish squeals per square inch - a ragtop version of the Mini would certainly have more of it than the hardtop. This is a can't-lose, positive- integer function, like giving a Ferrari more horsepower or Paris Hilton less dress.

Isn't it?

The Mini is an amazing automobile. Let us count the ways. First, it is a pitch-perfect revival of the British automotive icon - a car that sold millions over four decades and whose front-drive, transverse-engine-mounted layout transformed car design. The Mini - built at Oxford, England, to the great relief of the unions there - combines the superior engineering and craftsmanship of German giant BMW, which owns the brand, with the perverse charm of a noodling British runabout.

The Mini made small cool again. When this car came out three years ago, gas was $1.25 a gallon. SUVs were getting a bad reputation for crushing the cars they hit - oops, sor-ree! The Mini overcame all that on the strength of its styling alone and created the U.S. market for small premium cars, into which now Mercedes-Benz's Smart brand and others are rushing. Mini ought to get a medal for that.

There is a protean quality to Mini - which has sold more than 400,000 copies worldwide - that, I think, accounts for its wide-ranging success. The company offers a dizzying array of wheels, paint schemes, personalization and high-end options. In red, with white top and white wheels, the base model Mini (115 horsepower) is so cute it can send you to Candy Land rehab. But the top-of-the-line Cooper S, in purple metallic, with the 200-hp John Cooper Works-edition supercharger under the bonnet and 18-inch hunks of meat in the wheel wells, looks small and deeply malevolent, like a piranha zeroing in on one's tender fleshy bits.

So what's not to love about the Mini convertible? Well, let me get my eyeballs to stop rattling and I'll tell you.

Because of the shortage of real estate, the Mini doesn't have room for a spare tire - no room, even, for one of those space-saving rubber doughnuts. The car uses run-flat tires. Now, run-flats - tires that have extra-stiff sidewalls that allow drivers to carry on for 50 miles or so even after a puncture - are a wonderful invention, and lots of cars use them.

But the Mini Cooper S's run-flat sport tires (205/45R17) ride stiff and tend to transmit sharp, percussive impacts from imperfect road surfaces (the 10 Freeway or, say, Mars) to the steering column and cabin. The car weighs more - because of extra steel compensating for the loss of the roof structure, the weight of the one-touch top mechanism itself, and the extensive safety equipment for the open-top car, including the twin-hoop rollover bar, which looks like a piece of playground equipment.

The heavier car is, structurally, not as solid - this is almost always the case with drop-top versions. All these factors taken together, and spread over a 97-inch wheelbase, give the Mini convertible a harsh, rather shaky ride.

Fred Flintstone called. He wants his suspension back.

Deal breaker? Not necessarily. As I said when I first reviewed this car, I'm a great believer in imperfection. Perfection often imposes a loss of personality, a Xanax-like flattening of affect. The hardtop Mini doesn't exactly ride like a cloud, either, so if you love the Mini, you will probably love the convertible version, though perhaps for not so many miles at a sitting.

The car is still feisty. I took our test car to Irwindale Speedway and the car turned a very respectable 10 seconds/70 mph in the one-eighth mile. The supercharged inline 1.6-liter puts out 168 hp, channeled through a close-ratio six-speed manual transmission. The Mini engineers have changed the car's gearing for quicker acceleration, but on the 605 back from the speedway I noticed that it is, now, rather short-geared. At freeway speeds the Mini Cooper whines like an overburdened food processor.

The car is full of interesting sounds. The exhaust system is designed to snarl and burble on overrun (when the engine slows down from high rpm). It sounds more like breakfast cereal.

On the road the car retains its go-kart eagerness and quick reflexes, though it wants to skitter sideways a bit more in cornering. Hardtop or soft box, the Mini remains silly fun to toss around.

The interior is striking for its new polished aluminum-finish bezels on the switches and instruments, as well as the ball shifter and hand brake. With its orange ambient light, world-of-tomorrow design and stainless-steel pedals, the interior is as atmospheric as a rave club at 6 a.m. Our test car had the automatic climate control panel, with its rheostat central knob, and a Harman/Kardon stereo - plenty loud to hear KCRW's Metropolis show at 80 mph, even above the breakfast cereal.

The top is brilliant. The one-button operation goes in two stages: In the first, the forward canvas pleats rearward to create a sunroof, while the outer rails remain attached to the windshield header. Hold the button down, and the rails detach and the entire assembly retreats behind the roll hoop. Here comes the sun.

But, again, the lack of real estate causes problems. Between the narrow rear window, the imposing roll hoops and the headrests, visibility out the rear with the top up is as limited as it is in a Lamborghini Gallardo - where else, I ask you, will you see these two cars compared? The Mini boys have thoughtfully included a parking-assist sensor so you won't crunch the car's new rear-bumper fog light.

Southern California has the perfect climate for the Mini convertible. As for the roads, well, in the words of Bette Davis as Margo Channing: "Fasten your seatbelts, it's going to be a bumpy night."

*

2005 Mini Cooper S Convertible

Base price: $24,950 ($550 destination charge)

Price, as tested: $31,308

Powertrain: Supercharged, intercooled 1.6-liter OHC, 16-valve inline four-cylinder, six-speed manual transmission, front-wheel drive

Horsepower: 168 hp @ 6,000 rpm

Torque: 162 pound-feet @ 4,000 rpm

Curb weight: 2,844 pounds

0-60 mph: 7 seconds

Wheelbase: 97.1 inches

Overall length: 143.1 inches

EPA mileage: 25 miles per gallon city/32 highway

Competitors: PT Cruiser convertible, VW New Beetle Turbo convertible

Final thoughts: Oxfordshire sleigh ride

Automotive critic Dan Neil can be reached at dan.neil@latimes.com.