The engine braking effect
When the roads get slick, a vehicle’s drive system — front, rear, all or four — affects handling.
For example, an email from reader GC reminds drivers of the engine braking effect of a front-wheel drive system, warning, “I really enjoy your column but would like you to address the problem of front wheel drive autos on snow/ice/slick roads. When traveling at normal speeds in a front wheel drive auto in the aforementioned conditions and you let off on the accelerator, the front of the auto will decelerate due to engine compression. The rear of the auto continues at the same speed as before. At this point the rear of the auto will try to pass the front of the auto resulting in a skid.”
Drivers enjoy the extra traction that front-wheel drive autos offer over their rear-wheel drive counterparts when leaving a stop or climbing a grade in slippery conditions. That advantage is simply due to the extra weight (engine and transaxle) residing directly over the drive wheels.
Once the vehicle is rolling, however, one should be aware of the inherent handling characteristics of a weight-laden front end. There is a reason that race cars and sports cars are generally rear-wheel drive configuration: it helps achieve a desired 50/50 front/rear weight balance. Nearing that balance allows the most predictable vehicle behavior when cornering, accelerating and braking at the limits of adhesion. The same principles of tire adhesion loss at high speeds on dry pavement occur at lower speed on wet pavement, and lower yet on snowy/icy roads.
That’s why I stress the importance of gentle input to controls in the slipperiest conditions. As GC suggests, even backing off the accelerator too suddenly can cause a spin due to the engine braking effect on a front-wheel drive car. And actually, a spin can be initiated by backing off the gas of a rear-wheel drive car too, since the engine braking effect on the rear wheels will cause a rear tire adhesion loss initiating the rear to pass the front. Logic may infer that rear-only braking would make the rear “drag” stably behind the front, but breaking rear adhesion actually speeds up the rear, having the same effect described by GC. Motorcycles also go sideways if the rear brake is applied aggressively by itself.
Four-wheel and all-wheel drive cars are less prone to this effect due to a more favorable front/rear balance and extra weight over each wheel, but they can still spin with too-sharp deceleration on icy surfaces.
So, whether it’s going, stopping or cornering, a very gentle input is imperative to successful vehicle operation on wintry roads.
Due to engine braking, an effective driver technique for even, four-wheel braking in slick conditions, especially in emergencies, is to momentarily move the shift lever to neutral (or depress the clutch with manual transmission) while braking. This uncouples the engine from the drive wheels, negating the interfering effect of engine braking. This is an advance driving technique that should only be applied by drivers confident with their vehicle and how it operates and handles.
Also try to avoid tire spin. With a front-wheel drive car, the front will become unstable and move sideways and with rear-wheel drive tires, the rear will slide out. Tire spin can usually be avoided with four and all-wheel drive, and is being mitigated with the growing fleet of vehicles having traction control.
Readers may contact Bill Love via e-mail at precisiondriving@spokesman.com.
