Car & Tire for Canadian Winter

Near Golden, BC, on Trans Canada Highway

I finally got to drive Calgary-Vancouver (click for map) in winter 2010. I saw at least 10 accidents along Trans-Canada Highway and most of them were roll over accidents, suggesting tire slip as the primary cause. I have experienced bad snow storms while driving, although luckily not during the Calgary-Vancouver trip.

It started interesting between Lake Louise and Revestoke, which is about 230 km stretch. The divided highway from Calgary is no more and there are a lot of trailer trucks as the Trans-Canada Highway is the only major land route to Vancouver. A steep 6% downhill grade stretch at the Kicking Horse Pass greeted us. Reaching Golden, the drive got even more challenging as we approached Rogers Pass which is one of the world's most avalanche prone areas. The grade continued around 6% mostly uphill for about 30 minutes until we reached Rogers Pass. There are 7 avalanche-protection tunnels in this area. I felt the remoteness of this area especially when driving at night. Between Revelstoke and Kamloops, the road levels off somewhat but it meanders wildly around Shuswap Lake. We experienced the steep grade (6-8%) again along the Coquihalla Highway between Kamloops and Hope. Once you reach Hope, it is another 150 km, mostly flat, to Vancouver.

Winter tire is a must for Canadian winter driving

There are several lessons I learned from this experience that I want to share with you.


Do I need a four wheel drive or all wheel drive car for the Calgary-Vancouver winter drive? No. I had been wanting to buy an all wheel drive car for such winter drive, but I found out that it is not necessary. The main reason a front wheel drive is sufficient is that the Trans-Canada Highway is relatively well-maintained and I did not encounter a 100% off-road condition. I won't recommend a rear wheel drive car, however, since it drifts more easily and you may not have enough time to correct it when moving at a high speed on a highway.

Do I need to use winter tires for the Calgary-Vancouver winter drive? Yes. There are in fact several warning signs along the Highway that you need winter tires during winter months (October through May for Canadian Rockies, thank you very much). There is a physics to this. The minimum coefficient of friction between tire and road to maintain a no-slip rolling tire is approximately equal to a third of the road grade. The factor 1/3 comes from the fact that the tire both rolls and its center of mass moves.

A 9% road grade thus requires a minimum static coefficient of friction of 0.03 in order for a rolling tire not to slip. The coefficient of friction between rubber and a bare asphalt - occuring mostly in summer - is 0.25 or larger. When snow covers the asphalt road, the coefficient of friction can dip to 0.05-0.1. It is thus clear that winter tires are required.

Winter tires are also required to protect from slip when the car accelerates or decelerates. Without hitting a gas pedal or brakes, the maximum downhill no-slip acceleration is almost linearly proportional to the vehicle's mass. This maximum downhill acceleration, however, also depends inversely proportional to the vehicle's tire radius. Still, a truck or an SUV can accelerate without slip faster than a subcompact car since the mass ratio of a truck v. a subcompact car is larger than their tire radius ratio.

I drive a compact car, so I know I won't be able to accelerate as fast. I thus have to apply brakes when going downhill to prevent slipping. Applying brakes can induce slip on its own if too sudden, so braking must be gentle. There is nothing wrong with the car. It is just physics.

When you hit the gas pedal or the brake pedal, you apply torque to the tires. The vehicle dynamics changes a lot. Tire design becomes important. (In tech speak, tire's moment of inertia becomes important.) When going downhill, applying brakes increases the minimum coefficient of friction to maintain a no-slip condition, thus destabilizes the vehicle. In other words, decelerating a vehicle too suddenly can cause slip to occur. This is I think the most common mistake a lot of people make when driving in winter. They hit the brake too suddenly and slip ensues.

My math shows that accelerating a bit while going downhill stabilizes the vehicle, i.e., reduces the minimum coefficient of friction. But, accelerating too much can cause slip, so I won't recommend accelerating on downhill. For me this practically means I do not need to panic when going downhill by applying brakes. This requires me to get to the appropriate speed before the downhill stretch begins.

When going uphill, the converse is true. Braking a bit will stabilize the vehicle, but of course I do not want to lose the vehicle's momentum. Hitting a gas pedal destabilizes the vehicle.

The destabilizing effect of accelerating while going uphill and generally downhill shows that it is important to mantain a constant speed on a snow covered road. In fact, a simple strategy I use for winter driving is to maintain car's momentum. It means I do not make sudden swerves, turns, and brakes.

Which other car attributes do I have to pay attention to? Bright head lights and bring enough winter windshield wiper fluid. Bright head lights are essential when you drive at night. It is likely you will drive more than 12 hours to cover the Calgary-Vancouver distance in winter. One keyword here: visibility. You want to be seen clearly by other vehicles and you want to see the road very clearly when the weather is really bad. I also brought blankets, down parkas, snow shovel, snack, drinking water, matches, and even camping stoves and fuel in case of emergency on the road.

Do I still need an all wheel drive car for a winter drive? Only if I want to venture out into a secondary road or lower, that receives no or little snow plowing. For me the main advantage an AWD has over an FWD is a lower risk of traction loss. When I drive over a well maintained road this risk is small. The lower the vehicle speed is the higher risk the traction loss has. Quantitatively, the risk is proportional to the ratio between the wheelbase and the distance between snow covered areas along the road.