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Originally Posted by jimepting
Phil, I'm a little confused by this description, mostly because I don't know what the word "tumblehome" means. I don't find the term in Hucho, so I am at a loss, that being my only substantial reference.
Also, thinking the "side" problem through a bit, I think the sides of most street cars may already be badly compromised in terms of the ultimate aero shape. For example, the "max camber" of the sides is not a well defined point and may extend over several feet. The problems seems to manifest itself quickly when one considers a home built boat tail. The max camber point of the roof and the sides is going to be at significantly different points, keeping one well away from anything resembling a shape of rotation. 3-Wheeler resolved this problem on his boat tail by pretty much maintaining the initial rear convergence angles on his Insight. We haven't seen any tuft tests, so it is a bit difficult to know how that approach worked out. Your comments?
JimE
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Hi Jim.Tumblehome is used by automakers to describe inward leaning of the cars greenhouse sides,beginning at the beltline (bottom of greenhouse).
If you stand behind today's HONDA Fit and look at the A,B,C,and D-pillars,you notice that each consecutive pillar is leaning in towards the center of the car a bit steeper.This would be the 'morphing',as the sides of the car are integrated into more of a body of revolution type of architecture.
If done correctly,the velocity of airstreams from top and sides will meet at similar velocity,preventing the formation of high-drag longitudinal vortices.
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And yes,up until the EV-1,passenger cars were aerodynamically compromised for the sake of interior volume.AeroVironment knew they would have to pull out all the stops to meet battery range issues,so the Impact side max camber point was at the center of the door,with generous plan taper,necessitating the narrower rear wheel track.
HONDA went part way with Insight in 2000,tapering the upper 2/3rds more than all as in Impact.Impact had a virtually perfect windshield and almost completely flat belly,besting Insight in the 'tunnel.
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When I did the VW bus,I just followed VWs lead,mimicking the sides and top,wrapping them around to a 20-degree slope with compound curve edge integration.
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On the CRX,I did allow the sides of the boat-tail to morph from HONDA's faceted sheetmetal body side into a more organic highly radiussed integration as it rolled into the top.
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On the DODGE pickup I dispensed with the original bed altogether which eliminates the upright end rails at the tailgate,This has allowed me to keep increasing the tumblehome such that at the tailgate there is an elliptical body cross-section now,very much like a body of revolution.The wake is reduced significantly do to this alone and there's little chance for any vorticity.I maintain 'room' for a 48" load and narrow tailgate.The spare tire has its own 'closet' at the rear,of which its floor creates the diffuser of the bellypan.Eventually,I'll ditch the T-100 bed and do likewise,allowing a more 'ideal' tail,morphing right into the boat hull.
Of course,this is easier with a truck compared to uni-body.We've got to work with what we have.
With a long straight-edge,with ends equidistant from the center of the vehicle,the side max camber point can be found easily as well as a 'look' at existing side camber.From there we can see what we can do,and it will of course be different for each vehicle.
Had Alfred P.Sloan Jr.and Harley Earl never existed,we might have rational vehicle design today,making it much easier to to reach 'ideal' flow.But things are what they are so we'll have to act accordingly.