B Stead

Cars under $75,000 that have extraordinary chassis construction:

Alfa Romeo 4C, structural carbon fiber tub with an aluminum rear section
BMW i3, carbon fiber body on an aluminum chassis
Corvette, SMC body on an aluminum chassis
Lotus Evora, SMC body on an aluminum chassis

Of course the modern aluminum chassis is most likely to be that of rectangular tubes with the cross sections turned tall-wise. That method of construction increases stiffness and mitigates the effect of stress cycles on aluminum
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B Stead

Low production cars can be found with round tube or square tube construction but a chassis like that usually has both a lower and an upper layout.

A rectangular tube chassis usually has just has a lower layout but then a rectangular-tube cross-section that reaches up high.

Here's a link to the Lotus Evora chassis:

http://gglotus.org/blog/wp-content/u...a-chassis1.jpg


Lotus Evora complete car:

http://buyersguide.caranddriver.com/...model/6896.jpg


Here's are some links to the Alfa Romeo 4C chassis:

http://cnet1.cbsistatic.com/hub/i/r/...gallery-12.jpg

http://www.seriouswheels.com/pics-20...-1920x1200.jpg

The carbon fiber tub appears to mount the rear section and directly mount the front suspension.

Alfa Romeo complete car:

https://en.wikipedia.org/wiki/Alfa_R...o_4C_-_009.jpg


Here is the BMW i3 chassis:

https://upload.wikimedia.org/wikiped...833675545).jpg

BMW i3 complete car:

https://en.wikipedia.org/wiki/BMW_i3...:BMW_i3_01.jpg


Here are some links to the Corvette chassis:

http://image.vetteweb.com/f/features...hter-frame.jpg

http://www.corvetteonline.com/image/...e-history5.jpg

Note the tight bend of the rectangular tube at the front. That rectangular tube runs from front suspension to rear suspension in one piece because it is a hydroformed piece. But hydroformed bodywork of other cars shouldn't be confused with hydroformed rectangular tubes of an extraordinary chassis.

C7 Corvette complete car:

http://blogsdir.cms.rrcdn.com/91/fil...rvette-043.jpg
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dmaxx3500

some brit car had wood frames