WINTER DRIVING 101: It’s all about controlling weight transfer
By Mark Cox
An overnight dusting of snow crunched under the tires and left distinct tracks as the driver of a new 4×4 crew cab pickup rolled slowly through the suburban Colorado neighborhood and made a turn towards the main highway a few blocks down the hill.
The light at the “T”-intersection flipped to red a half-block before the highway and the driver instinctively slipped his foot to the brake pedal and started braking.
A second later the driver realized there was a big problem: A combination of a bit too much speed and too little traction on the icy road surface locked up the front tires and kicked the tail out.
There wasn’t any stopping. There wasn’t any steering. He was just along for the ride as the truck slid sideways into the busy intersection.
Welcome to the realities of vehicle weight transfer.
There’s no doubt any contractor or business owner who lives or works in the Snow Belt has had the unpleasant experience of losing vehicle control in the winter.
For some it may have been only a momentary happening that didn’t result in anything more than the skip of a few heartbeats. For others, well, lesson learned the hard way.
Being a good winter driver is predicated around one simple precept: controlling vehicle weight transfer.
TIMING AND WEIGHT TRANSFER
Typically, the more slippery the road, the higher your speed, or the heavier your vehicle, the longer it takes for driver steering, acceleration and braking inputs to affect the vehicle.
A driver’s winter driving skill is the result of timing and understanding the basics of vehicle weight transfer.
Timing is critical on two fronts: 1) the point at which you begin a maneuver, and 2) the rate at which you perform the maneuver.
When you turn the steering wheel quickly on dry pavement, the vehicle responds almost instantly. Make the same quick turn on ice or any slippery surface the vehicle’s reaction can be considerably slower as the tires fight for grip.
So driving on slippery road surfaces requires you to make vehicle control inputs far earlier and much slower and smoother than you would driving on dry pavement.
It’s all related to weight transfer. As you travel down a level road at a constant speed, your truck’s weight is “neutral,” or balanced evenly front to back.
However, as soon as you begin to decelerate, that weight balance shifts forward. This puts more weight on the front tires, giving them more grip, helping both braking and steering.
At the same time the rear tires have less weight on them and, consequently, less traction.
Conversely, if you accelerate, more weight shifts to the rear tires, which gives them more grip – and the front tires less grip.
Using this weight transfer effectively is the most important tool in controlling your truck in every driving situation. None more so than on slippery driving surfaces.
BASIC SNOW DRIVING
In simple terms, when negotiating a turn on a low-grip surface like a snow-packed road, always slow down slowly in a straight line so you are utilizing your truck’s available grip at all four corners for deceleration.
Then begin to steer into the turn after you have finished decelerating.
Doing so allows the vehicle’s weight transfer to provide braking to the front tires and, you gain steering effectiveness through the corner because you are using 100% of the grip to steer.
As you exit the turn, then slowly accelerate, using the weight transfer to the rear tires for added grip.
Remember: Brake. Steer. Accelerate. But never overlap those inputs.
At this point, someone always makes the comment “but my truck has four-wheel drive, so it’s okay,” or something similar.
Sorry to burst your bubble: Four-wheel drive can never overcome the simple laws of physics and weight transfer. Four-wheel drive does allow a slightly larger margin for error during acceleration, but it cannot magically create better braking or steering.
That’s where so many 4×4 owners make their biggest driving mistakes. In fact, 4×4 trucks are more difficult to brake and steer than cars because they are heavier – and more mass is harder to control.
TIMING IS EVERYTHING
Now let’s consider the effect of timing on weight transfer.
If you decelerate or brake suddenly or harshly, weight transfer will occur in a corresponding manner. Decelerate or brake smoothly and progressively, weight transfer will follow suit.
Keeping vehicle control in winter drivng conditions requires smooth, progressive transitions. (Drive like you literally have raw eggs under your feet on slippery surfaces.)
For example, consider approaching a curve. If you slow down abruptly, the weight will abruptly shift to the front tires. Not good. The front tires can grip, lose all grip, or a combination thereof while that abrupt weight transfer will unload the rear wheels.
The result is very likely to be a snap “oversteer” situation, where the rear of the truck suddenly swings out. (This effect can be further aggravated by limited-slip of locking rear differentials.)
Now you are forced to control your truck with a quick correction. If your correction isn’t precise (too much, too late or both), a rebound oversteer will occur.
The result can easily become a series of failed recovery attemps that, although entertaining to watch from afar, result in a total loss of vehicle control and tragic results.
PLAN AHEAD, WAY AHEAD
So, what’s the alternative? By focusing far enough ahead and reading the upcoming curve, you can begin to decelerate earlier and over a longer period of time.
If you do this, your vehicle’s weight transfer and speed adjustment will happen more slowly and thus be more precise and predictable.
Although the grip will still increase at the front wheels and decrease at the rear wheels, it will happen in a lineal and predictable fashion, almost as if the weight transfer is occurring in slow motion.
Timing and controlling weight transfer are also critical when you exit a curve, especially on low grip surfaces. One of the most common problems affecting weight transfer in cornering occurs when you accelerate too soon.
By reading a curve correctly and adjusting your speed accordingly, you can decelerate, complete the turn, and then accelerate smoothly out without causing any vehicle control issues.
But if the speed of the truck is even close to the grip limit, the slightest amount of early acceleration will unload the front wheels.
This will increase the possibility of losing grip on the front wheels and will severely reduce your steering effectiveness. This is the dreaded “understeer.”
(To get back steering control in an understeer situation, turn the steering wheel back towards center – not in the direction you want to really go. Turning sharper only aggravates the problem.)
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It’s really quite simple: if the wheels are still turned, you aren’t finished steering through the turn. So, don’t unweight the front wheels by accelerating too soon at the exit of a turn.
The concepts of weight transfer and timing are quite simple. But applying them in the real world can take years to perfect—or hours practicing in a big, open snow-covered parking lot. – Pro
ABOUT THE AUTHOR: Mark Cox is the director of The Center for Driving Sciences in Steamboat Springs, Co.., and a professional Rally driver. He also runs the Bridgestone Winter Driving School; 800-WHY-SKID. See Video.