Winter Tires – A Gripping Tale!

roarengineering Accident Reconstruction, Articles, mechanical & biomechanical, Uncategorized

Written by:

Alan Morris, PhD., P.Eng.

Accident Reconstruction Engineer/ Biomechanical Engineer

Introduction

As our Canadian winter is upon us, we are often told that we should be getting winter/snow tires for our cars and trucks.  In Quebec, it’s the law.  Should the rest of us buy a set?  When should we changeover back to summer tires?  Let’s take a deep dive for the evidence for winter tires.

The internet is full of various sites/videos reviewing winter tire performance in a qualitative manner, discussing such characteristics as handling, braking and road noise.  While winter/snow tires are great for the tire business, the most important question for insurers and the general public is whether they provide an increased level of safety.

As forensic engineers, we like to put numbers to things – in this case looking at the braking rate differences (in g-force) between tires.  We identified a number of articles [1] [2] [3] [4] that put numbers to the performance of tires in terms of braking distance.  Friction between tires and ground (the grip!) will depend on tire design characteristics (tread patterns, tread depth/width, sidewall construction, rubber composition, etc.) and road characteristics (temperature, dryness/wetness, roughness, and the amount of snow/slush/ice on the ground).  There is significant science behind tire construction but it doesn’t necessarily help us answer our questions.

Track Testing of Tires

A few years back, Edmunds.com2 evaluated three sets of Michelin tires (winter, all-season, and summer) on snow proving ground in Minnesota, on a wet testing track in Arizona, and on a dry testing track in California put onto a 2009 Honda Civic Si – not a high performance car.  The goal was to measure the braking distance of the car initial travelling at 60 mph with three different sets of tires in three different road conditions.

Michelin Pilot HX MXM4 – All-Season tire

Michelin Exalto PE2 – Summer Tire

Michelin Primacy Alpin PA3 – Winter/Snow Tire

Table 1 shows the braking performance between these three example sets of tires.  Shorter braking distance equated to better braking.  Summer tires offered the best braking in dry conditions (8% better than all-seasons and 22% better than winter tires).  Summer tires offered the best braking in wet conditions (13% better than snow tires and 26% better than all-season tires).  Snow tires offered the best braking in snow conditions (15% better than all-season tires and 55% better than summer tires).   This test is an example and performance will vary depending on tire brands and models.

 

Braking Condition

Tire Braking [g-force]
Summer

Michelin Exalto PE2

All-Season

Michelin Pilot HXMXM4

Winter/Snow

Michelin Primacy Alpin PA3

Dry 1.00 0.92 0.78
Wet 0.76 0.56 0.66
Snow 0.15 0.29 0.34

Table 1:  Braking friction [g] vs. tire type and braking conditions (Edmunds.com, 2009)

So what do these results mean?  One conclusion is that braking performance on snow is generally much worse than in wet and dry conditions.  Another conclusion is that all-season tires are a compromise for all road conditions and are not really great in wet conditions.  Another conclusion is that one should not keep using their snow tires once roads are dry in the spring as it compromises a cars ability to brake on dry roads.

Real-World Scenario

Now what does the 15% better braking performance by winter tires than all-season tires actual mean?  Let’s consider a typical scenario (Figure 1) – the driver of a lead car, travelling at 60 km/h with winter tires on a snowy road, jams on the brakes in the middle of a block to avoid a pedestrian running across the road.  A car following 4 car lengths (approximately 16 metres) behind, also travelling at 60 km/h, has to avoid hitting the car ahead.  In this situation, it typically takes a trailing driver about 1 second to perceive the car ahead braking and hit the brakes.  For a trailing car with winter tires, the trailing car would impact the lead car at about 7 km/h faster than the lead car.  For a trailing car with all-season tires, the trailing car would impact the lead car at about 18 km/h faster than the lead car.  Aside from the need for all drivers to increase their following distance in winter, winter tires can significantly reduce the severity of collisions and any injuries that result.

Figure 1:  Sudden Braking Scenario with Trailing Vehicle

Aside from the improved winter handling of tires, when it comes to braking in snow, winter tires make enough of a difference in braking that they should be necessary equipment to safely navigate our Canadian winters.  Get a grip and buy a set.

[1] Eddie, C.R. (1994)  Ice, ABS, and Temperature.  Society of Automotive Engineers 940724.

[2] Edmunds, D. (2009)  Tire Test:  All-Season vs. Snow vs. Summer.  http://www.edmunds.com

[3] Leiss, P.J., Becker, S., and Derian, G. (2013)  Tire Friction Comparison of Three Tire Types.  Society of Automotive Engineers 2013-01-0783.

[4] Malmivuo, M., and Luoma, J. (2017)  Effect of Winter Tyre Type on Roughness and Polishing of Road Surfaces Covered with Ice and Compact Snow.  European Transportation Research Review  9: 2