Malcom Elston is a certified Ontario Driving Instructor as well as a high performance driving instructor. He has 15 years experience and is very highly regarded in the driver training community.
What follows is his response, in point form, to questions from John Mahler about the use of winter tires on MFAVs for an article he was writing for Wheels.ca (Click here to read John's article in Wheels.ca)
In point form, here are some of Malcom Elston's observations:
. it is well-understood and documented that the traction capability of a summer-rated tire is adversely affected as ambient temperatures drop below +7°C. Just as it is well recognized (and provable) that winter tires can be engineered to deliver traction capability down to -40°C. Having said that, I cannot cite any knowledgeable source that recommends mixing summer-season temperature range tires with winter-season temperature range tires on the same vehicle, at the same time. And to do so with the winter tires providing the drive function (leaving the summer tires to deal with the most critical elements - 100% of the steering, and about 70% of the braking...) just seems completely illogical. To equip a child-carrying bus in such a manner is, in my opinion, misanthropic.
. vehicle dynamics are one of the least understood elements within the process of driving, but one of the most critical. Yes, most drivers can tell you what happens when they brake sharply (the nose of the vehicle 'drops'), and again, most drivers know that their vehicle will 'lean' to the outside when cornering. But very few drivers truly understand what these motions mean, or how they effect traction at the interface of the road and the tires.
To understand basic vehicle dynamics, first you must know a bit about the architecture of the vehicle you are driving: where are the engine and the transmission located? Is the vehicle front-wheel-drive, rear-wheel-drive, or all-wheel-drive? The answers to those questions will give the driver a good indication if their vehicle is front-heavy or not. If you think that total vehicle weight (load) is evenly divided amongst the four tires, you are sadly mistaken... very, very few vehicles can claim that balance. Most front-engined, rear-wheel-drive (and virtually all front-engined, front-wheel-drive models) have significantly more weight on the front axle than on the rear.
When a driver brakes hard, and the front of their vehicle 'drops', effectively they have just experienced dynamic weight transfer. Although nothing in the vehicle has moved, the energy transfer has effectively moved weight towards the front, compressing the springs and shocks and 'dropping' the front end. This can be a good thing because typically, the front brakes do about 70% of the braking work, and getting that dynamic weight transfer helps push the front tires onto the road, thus increasing grip. This works very well - so long as the front wheels are pointing straight ahead!
If a driver brakes hard whilst in the middle of a turn, or suddenly swerves to make an avoidance manoever while braking hard, vehicle dynamics can be a key contributor to the driver giving up control of his vehicle, and spinning or crashing it. Here's why.
A sudden transfer of weight longitudinally (along the length of the vehicle) occurs under hard braking, as noted. Put another way, that means there is now less weight on the rear tires. A sudden steering input while braking hard will dynamically transfer weight from side-to-side, greatly increasing the probability that the rear tires will slide sideways.
Vehicle dynamics are far more convoluted than the simple examples I've given above. Factors such as the amount of gas in the tank (65 litres weighs more than 100lbs) and the number of passengers (4 teens = 550lbs+) are obvious variables, but putting six sets of skis into roof-mounted box not only adds weight, it also changes the centre of gravity for the vehicle - and greatly increases the sail-area (the side-profile) which makes the vehicle more susceptible to cross-winds.
. obviously the impact of vehicle dynamics vary from vehicle to vehicle, but it's hard to think of a much more extreme example than a bus. Driven empty, the handling and dynamics would be extremely different than when the bus is full. Empty, it is a box of air. Full, it has a passenger payload that could easily exceed 3,000lbs... and that's not including luggage! Put another way, a school bus would leave its home base with just the driver on-board, and the load (and therefore, vehicle dynamics) would change every time another student climbed on.