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Originally Posted by awy
wife wanted me to get some fender protectors to protect against paint scratches on the new family hauler (xc90), so naturally this is an excuse to overinterpret the mandate and mission creep into aero tinkering.
This car is really gas guzzling at city driving (~18 mpg) but is fairly efficient on highways, so the aeromodding is pretty much just for fun/tinkering, rather than any real effort at saving gas. I think this SUV, despite having big frontal area, is fairly aerodynamic in the class at a 0.33 cd, but could be better. i'm thinking of going primarily in the vortex generator/airflow modification direction, rather than doing any huge and ugly shape modding. i'll just state the starting specs to serve as control baseline. I think modding for airflow separation has the additional benefit of reducing wind noise if it works out.
on the last road trip with new tires and about 5500 lb in gross weight, we had a highway mpg of about 30 at 70-75 mph. however, the mpg gets noticeably worse going over 80 mph. it also came with 275/40R21 pirellis which are pretty heavy and inefficient, but we can't change that for a while.
I'm thinking of experimenting with some vortex generators on the top and sides, front and side skirts, possibly vortex canards in front, and covering up the wheels with carbon fiber patterned wheel covers that mimic the new volvo EV wheel covers. It'll be limited to DIY level tinkering since part of the fun is tinkering on it myself.
I've been reading some CFD studies on the car to identify big gain areas will update this thread for specifics as I digest them.
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* after 4,000-miles on the new tires, the rolling resistance will fall to the minimum for that tread depth. As you continue to wear the tire tread down, the R-R will continue to drop, and reach minimum R-R when they're worn out.
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* wheel covers would be a 'GO'
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* skirts would be a 'GO.'
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* automotive VGs were introduced by aeronautical engineer, Gary Wheeler, which became 'Air Tabs.' Gary designed them for his notchback Honda Accord, for the same reason Mitsubishi would later incorporate them on their nasty notchback Lancer EVO, which otherwise would have had total flow separation beyond it's backlight header.
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* since your Volvo is a 'two-box' design, with 'nothing' behind it's separation line, there's nothing for the separated airflow to reattach to, rendering the VGs impotent as far as drag reduction.
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* a 'boat-tailed', receiver-hitch cargo-carrier, snugged up tight against the back of the XC90 would be about the only thing capable of meaningful drag reduction ( you could tell your wife that you're so lousy at fabrication that, you were incapable of creating 'straight' lines, and it was only incompetence which is responsible for the 'crooked' tail ).