Just a few ideas of a Stingray/Manta Ray inspired bmw.

http://kaiyuanar.deviantart.com/art/BMW-Stingray-Sketches-96557934

Comments and criticism please

Kaiyuan

The proportion on your sketches are kinda off...the cars looks like they have the proportion of X6, but the roof / the green house is chopped, and just look unnatural.
Drivers need to be able to look through the glass.

Your design language is too busy for the car.. lessen the details, and focus more on the proportion of the cars.

It looks like your sketches are too small, just go a little bigger so you can explore/put more details on your cars.

Do some side views as well, you can explore the general shapes of the car better that way.

Hope this helps.

Thanks!
But I'm still a bit confused about what you mean by the proportions.
Do you mean by like sedan/SUV/wagon proportions, something like that?

And I usually work with smaller to try and get used to drawing my lines longer and straighter, and I try to draw bigger each time.

Well...you should be able to answer to this.

What kind of car are you trying to do? Sedan, sports car, SUV, 2dr coupe..which one?

I can understand that you are inspired by sting ray/manta ray, but I can't tell from your sketches what kind of car you are trying to design.

Ohh, I see.
Sorry got a little confused
I don't usually aim for a type of car, I just draw whatever comes to mind and to experiment.
But I do know the sketches look pretty impractical so far haha.