March 13, 2013
Ford Engineer Tom Salmon. Click image to enlarge
Review and photos by Justin Pritchard
I failed calculus.
That’s why Ford Engineer Tom Salmon took me to a whiteboard to draw a diagram explaining what’s called the ‘Bike Model’ by vehicle dynamics experts. The Bike Model is a highly calculus- and physics-based bunch of brackets and lines and symbols and equations next to a simple overhead diagram of a bicycle riding around in a circle.
Salmon is an energetic middle-aged fella’ who gushes happily about traction control and AWD technology using terms like ‘split mu’, ‘yaw rate’ and ‘y-accel’. Or that Bike Model – which illustrates how a vehicle’s front and rear wheels follow each other around a circle in perfect situations. If nothing is wrong, the front and rear wheels follow the same path, and the complicated-looking mathematical formula is happy.
In real life, and especially on snow and ice, wheels don’t always follow the same path. This upsets the math gods and un-balances the Bike Model equation. Oversteer, and the rear wheel goes around the circle faster than the front. Understeer, and the front wheel plows out of the circle, dragging the rear one with it. In either of these cases, the equation behind the Bike Model is all out of whack, and the vehicle in question, and its occupants, are in trouble.
That unsettling of the equation in a newer Ford model (and may other brands, for that matter) instantly sets off a concert of reactions from various systems and modules that set off trying to balance the equation – thereby bringing the front and rear wheels back in line with one another via intervention through the electronic stability control (ESC) system. If the model has AWD, power delivery to the wheels can even be recalibrated to pitch in and help.
Ford AWD and Stability Control. Click image to enlarge
At the wheel of a Ford Explorer Sport Ecoboost with the traction controls turned down, you can feel it while skidding intentionally, hard on the throttle, around an icy skid pad. Only a moderate drift is allowed. Exceed a few degrees of slip, and you can feel the brakes pulsing and AWD system moving power about to try and keep the skid within preset limits. With the traction controls on fully, it behaves similarly – though with much tighter tolerances for slides.
Last time I saw calculus, it made me toss a high-school textbook across my room, stab a mechanical click pencil into my wall, consider harming myself, and cry loudly into a pillow for the better part of an hour. At my teacher’s, Mr. Fievoli’s, advice, I dropped the course a week later.
Fast forward a decade and a bit, and while watching Salmon and his team drift Fords at alarming speeds around a snow-covered track, I realized Mister Fievoli was right: calculus IS fun. Oh, and it makes vehicles that are safer, more confident and better handling.
Salmon even told me that Ford pays someone, with actual money, to drive a winter-tire equipped Shelby GT500 around a similar test track, to ensure it’ll work in the snow. You know, for those owners that’ll set off in a blizzard with a rear-drive super Stang packing more horses than a truckload of Whoppers.
But drifting and sliding around in the snow is juvenile business that doesn’t fall into the realm of concern for most folks about to shell out hard-earned cash on a new family ride. Drifting is fun, it looks pretty in photographs and makes for sometimes-entertaining YouTube videos. But snow drifting is, largely, a loss of control – and it’s mostly confidence that new car shoppers are after.
Traction. Grip. The sense that their ride has their back if they have a ‘whoopsie’ that’d otherwise jam them a metre and a half into a snowbank.