Karma S or Sunset convertible in action at 2009 NAIAS

Fabulist said:

if it follows the usual pattern, the convertible is going to be heavier than the hard top version because of all the reinforcement needed to keep the chassis stiffness.
-- Fab.

Click to expand...

Ah, but that is just the point, Fab, it doesn't follow the usual pattern. Fisker's aluminum space-frame design provides the structural rigidity. In a normal monocoque designed car, the body panels themselves provide part of the structural rigidity, but with a space-frame, the body panels are primarily "draped" on the frame. That is why cars like the Karma, Audi R8, Ferraris, etc. can have lightweight body panels or virtually unlimited shapes made of non-steel material. It is also why the Sunset doesn't have to be heavier (except for the convertible top mechanics and hardware). On another note, I do not believe there was ever any consideration given to including solar on the sunset.

The timing of the convertible Sunset would more likely follow other models simply because it is a smaller market. At the higher price, it is a niche car. Given the same amount of cost to set up and prepare for production, it probably makes more sense to move into larger-volume market segments when making capital spending decisions.

Dave_Car_Guy said:

Fabulist said:

if it follows the usual pattern, the convertible is going to be heavier than the hard top version because of all the reinforcement needed to keep the chassis stiffness.
-- Fab.

Click to expand...

Ah, but that is just the point, Fab, it doesn't follow the usual pattern. Fisker's aluminum space-frame design provides the structural rigidity. In a normal monocoque designed car, the body panels themselves provide part of the structural rigidity, but with a space-frame, the body panels are primarily "draped" on the frame. That is why cars like the Karma, Audi R8, Ferraris, etc. can have lightweight body panels or virtually unlimited shapes made of non-steel material. It is also why the Sunset doesn't have to be heavier (except for the convertible top mechanics and hardware). On another note, I do not believe there was ever any consideration given to including solar on the sunset.

The timing of the convertible Sunset would more likely follow other models simply because it is a smaller market. At the higher price, it is a niche car. Given the same amount of cost to set up and prepare for production, it probably makes more sense to move into larger-volume market segments when making capital spending decisions.

Click to expand...

Even with a space frame, the convertible version of a car is almost always heavier. The Aston Martin DB9 Volante's Curb weight is 50 KG higher than the DB9 Coupe. For the Jaguar XK, the difference is closer to 100 KG and in both cases the convertible loses a big chunk of torsional stiffness in the process to keep the weight gain down. If they wanted to keep the same level of stiffness, the weight gain would have been a lot more significant. So you are basically getting a much softer car that behaves differently in the corners in exchange for a drop top plus more weight. When the car starts out pretty light, like the Jag, for example, adding another 100 KG is not a big deal, but the Karma is already pretty heavy and any added weight would negatively impact performance.

You are also right that the convertible Fisker will most likely NOT have solar panels on the roof and that should save some weight but a Karma that carries another 100 Kilos on top of what it is lugging around now would not really be very desirable from the performance perspective. So before they go down that road, it would make sense to save some weight in the drive train and even the body and also improve the power output of the car to keep the performance at least where it is now.

-- Fab.