Snow driving and handling

Dear off-road and nature enthusiast friends, we are together with another sharing of our Off-road Basic Technical Information article series. The information I will provide; These are concentrated, technical-based, lived experiences, and some of the details that will find a name in the off-road literature for the first time. With the hope that it will benefit the readers.

Our topic today is “Driving on snow and handling ” :

I would recommend you to read it in your quiet time. It’s long and a little confusing. At least read the “ results and recommendations ” section.

There is nowhere else you can find such in-depth analysis. In general, the subject is whether we will inflate or deflate the tire, and there is a math in this business. An original work. Inside; there are the basic laws of physics, soil mechanics, strength and statics. Mathematics exists only to express this information.

The basic rule of road holding is not to slide. If the vehicle can hold the road, it will walk, if it cannot hold the road, it cannot walk or be controlled. Whether offroad or onroad, this is the most basic rule. There must be a force between the tire and the surface to avoid slipping. This force appears in three ways. Friction, shear strength, passive thrust.

Traction is the basic force that allows the vehicle to walk between the tire and the road. Traction is essentially a medical term and not a definition that has anything to do with offroad. However, since it is a term expressing handling and traction in the unwritten offroad literature, I use the same expression. Only the surface is the case of force arising from the surface. The upper limit of this force, that is, the highest value it can take, is the “friction force” between the surfaces. The frictional force is equal to the product of the normal force (perpendicular force) between the surfaces and the coefficient of friction between the surfaces. If the force transferred from the tire of the vehicle to the road exceeds the friction force, skidding starts. It occurs on the surface where the outer side of the tread of the tire contacts snow or ice. Since the coefficient of friction between two surfaces is equal to the product of the total vertical load, whether the surface is wide or narrow is unimportant in terms of friction force as long as the coefficient of friction remains constant. However, tire width can affect the friction coefficient depending on the snow condition. The surface pressure is high on the narrow surface, and the surface pressure is low on the wide surface. As the surface pressure increases, the snow becomes more crushed and icy, as the icing increases, the friction coefficient will decrease and the friction force will decrease accordingly. This means; When evaluated in terms of the friction force between the solid part of the tire and the surface, the contribution of road holding will decrease due to the decreasing friction coefficient as the tire gets narrower. This is the first case. However, since the total horizontal friction force is only the parameter of the total vertical load and the coefficient of friction between the two surfaces, if the friction coefficient does not change, the friction force does not change either. So wide or narrow tire may not make a difference. This is the second case. Which of these two opposite situations will occur will be determined by the snow quality. In other words , the snow quality will give the answer to the question of whether it is a narrow tire or a wide tire.

The tire material is the parameter of the coefficient of friction. Winter tires coded as M+S give a higher coefficient of friction in cold weather, while tires with normal rubber provide a lower coefficient of friction.

The quality of the abdomen is the most important parameter in terms of the coefficient of friction. It is higher in dry snow, lower in wet snow, and close to zero in ice.

The coefficient of friction between the tire and normal snow is considered to be ~0.20. So this means that if the other road holding parameters of the tire, such as shear strength and passive thrust, do not occur, when you release the vehicle from a slope of 20% or in other words arctan(0.20)=11.31°, if there is no pile etc. to prevent it from moving, it will move in the direction of the slope. can slide with its weight.

There are two types, kinetic and static. The coefficient of kinetic friction is lower. This means that if the skid starts, the handling of the vehicle will become even weaker .

If the snow that enters between the tire treads and the crushed snow on the ground stick together and work as one piece, this compressed snow will provide as much grip as shear resistance (also known as shear or shear resistance). Here, whether the tire is wide or narrow is an important and contradictory parameter. The wider the tire presses on the ground, the higher the shear strength will be, as the amount of snow will be more intervening. This is the first case. Another factor that positively affects the shear strength is the compression of the abdomen. The greater the pressure on the snow, the more it will be compressed. This will only be possible if the tire is narrower. This is the second case. Which of these two situations will provide more positive handling will again be determined by the quality of the snow. In other words , the answer to the question of narrow tire or wide tire will once again be given by the characteristics of the snow.

The basis of the opinion recommending that tires be inflated rather than lowered in snow is actually a recommendation related to shear strength. When the air of the tires is deflated, an inward collapse occurs in the tread. This, in the form of snow tires, causes the gaps between the treads of the tires to narrow and less snow to get in between. Less snow means less shear strength and will automatically weaken the grip. MT or XT tires with coarse treads will not behave like narrow tread tires, as they are already designed to serve at low air pressures.

In order for passive thrust to arise, the part entering the snow must be very low in proportion. Passive propulsion is almost nonexistent in soft snow. The snow should be crushed and hardened by the weight under the tire. May occur on medium hard ice and hard snow. It is difficult for the tires to interact to create passive resistance alone. Passive resistance is expected to occur in the use of studded tires and chains. The forces generated by passive resistance are much greater than the forces generated by friction and shear strength. That’s why studded tire or chain grip is generally better than any type of tire. An effect that can be considered as a passive effect can also be evaluated as the scratching effect of the scratches on the tire or the tread on the snow, which is hard enough to allow this behavior. This effect is more pronounced on snow tires or AT type tires rather than coarse tread tires. Therefore, it can be said that AT tires are more successful than MT tires under certain conditions. However , it is up to the wife to decide on this issue .

The friction coefficient is the ratio of the force pressing against each other to the frictional force that occurs between them. It is a scalar (unitless) value. It has two different values as static and kinetic friction coefficient. Its values are determined by the inclined plane test.

It is the force obtained by multiplying the normal (perpendicular) force on the contact surface between two objects by the coefficient of friction between the two objects. It is in the opposite direction of the motion in sliding objects, and in the same direction as the motion in rotating objects.

It is the case of not applying any force while at rest, but countering it with a force when a force is applied to it. It can be evaluated within the framework of Newton’s 3rd law (action-reaction principle). To illustrate with a clearer example, consider a paw on ice. After the claw enters the ice, a state of equilibrium is established and there is no mutual force. If you claw on the claw, the place where the claw enters exerts a force on the claw as much as the force on the claw. As long as the material strength in the place where the claw enters is not exceeded, the cling will continue.

The total resistance to the shear stress occurring in the plane parallel to the applied surface is called shear strength.

The rule of driving on a slippery surface is very clear, neither the accelerator nor the brake should be pressed too hard. If it is downhill, the vehicle is left with the flow of the road in low gear. If it is uphill, it is tried to walk in high gear at low speed without skidding.

If your wheel is not turning at a speed appropriate for the speed of the vehicle, you will either spin or the vehicle will tend to slow down and come to a stop. However, if the ground is icy and the friction coefficient is too low, the vehicle will slide in both conditions. If the car slides downhill, gravity makes the rules.

If the tires are spinning at a speed higher than the speed of the vehicle, it will be positive spin, and if the tires are spinning at a speed less than the speed of the vehicle, it will be negative spin. If there is skid, there is no grip.

The most successful tire on snow is the winter tire , especially if you are driving on a paved track. The Extreme tire is the most successful tire when making the first trail. AT tires, like winter tires, provide good grip when driving on open tracks. While MT tires are successful in opening the first track, unfortunately, they exhibit extremely poor handling performance, especially when walking on an opened track. This is not a valid determination for all MT tires.

When we examine the tread patterns of large MT tires, you will see that the treads are also large and the outer surface of the tread is flat. The outer surfaces of these teeth are so wide that when driving on a trail opened in snow, a few of them can carry the vehicle on hardened and icy ground without sinking into the hardened snow. As a result, the vehicle behaves as if you were trying to drive on ice with a zucchini tire . After all, you go wherever gravity takes you.

Let’s talk a little bit about MT tires. Although MT is called “Mud Terrain”, “mud ground”, it is not very successful in mud. The track where the MT tire is successful is dry loose ground and rocky ground. If you watch BF Goodrich commercials, you will see MT tires on DAKAR-style tracks rather than mud or snow tracks. I will examine the subject of tire types later as a separate topic.

When you bring the same tire with 235 tread width side by side with a 35”x12.5” Bf Goodrich MT tire, you will immediately notice that the patterns of the 235 tread tire look like an AT tire next to the 35”x12.5” tyre. If we take into account that AT tires are more successful on the opened track, small size MT tires can also act like AT and be more successful. While a 35” MT mounted rubicon will struggle, a 235 base MT suzuki will be able to walk away.

When evaluated in terms of friction force depending on the snow quality, the narrowing of the tire will adversely affect the road holding.

When evaluated in terms of shear strength depending on the snow quality, narrow or wide tire can either increase or decrease road holding. Nothing definite can be said, the qualifications of the abdomen are decisive.

A studded tire nail or chain that penetrates hard snow or medium-hard ice by inserting a fingernail adds passive resistance to the tires as the tire tries to turn. This resistance provides grip to the vehicle. If the ice has become too hard due to extreme cold, the nail or chain cannot penetrate the ice, so it serves only with the grip provided by the friction between the two materials, not the claw effect. So in extremely cold and very hard ice, chain or studded tires are not very efficient either.

In narrow-toothed tires like snow tires, reducing the air of the tire and providing contact with a wider base will create a negative effect, not a positive one, for the reason we explained above. In MT and XT tires, on the other hand, providing a wider contact surface by deflating your tires may or may not have a positive effect depending on the snow conditions.

If the snow is deep; The passive thrust created by the snow touching the chassis, axles, bumper, body, etc., the snow piling up in front of the tire and the slope are the major effects that prevent the vehicle from walking.

If there is a slope, the effect of the component of the weight parallel to the road. If there is a slope in the direction of movement, it has a positive effect, and in the opposite direction, it has a negative effect.

Other than that, even the wind is an effect, but it won’t be a big one.

If the snow is deep, the narrow tire will cause more sinking and more pile resistance against the vehicle.

Nails and chains are inefficient on soft snow, more efficient on medium-hard snow and normal-hard ice, and useless on very hard ice.

In the case of going uphill, a restraining force occurs due to the slope.
If we write the parameters collectively;
– tummy tuck
– snow depth (depending on vehicle height)
– the slope of the road. (This is just a topic on its own)
– the dough of the tire,
– tread pattern of the tire
– tread depth of the tire
– diameter and width of the tire
– the mechanical capability of the vehicle (presence of mid-rear-front lock)
– piloting

`
Let’s examine the depth of the snow a little.
In deep snow, my observation is that solid axle vehicles with a differential watermelon in front are more successful. If you examine the trace it leaves behind, you will see the trace left by the differential in the middle of the trace swept by the axle sleeves. In other words, solid front axle vehicles have a tendency to sweep, not rise above the snow.
If IFS (independent front suspension) vehicles are equipped with a flat-plate skid plate to protect the front wheels and the engine, when you drive in deep snow, the snow will tend to rise over it rather than sweep it. In this case, the grip of the front tires weakens, and sometimes they can be completely cut off the ground. Especially if you come back on your track to get the snow and speed up and move on the clean snow, you can hang. This situation may give different results depending on the characteristics of the abdomen.

A straight road is the most ideal situation, but it is the least achievable situation in the field. If the surface is slippery, the vehicle will tend to slide in whichever direction the slope is greater. That’s why on snow trips, cars often slip and fall on the right or left bank.

If it is uphill, it is not possible to proceed. The most dangerous, in my opinion, is the descent. On the descent, gravity takes control. If your tire does not hold the road, you will go uncontrollably wherever gravity takes you. If possible, enter this state at low speed. Do not apply the brakes, try to ensure that the vehicle moves on foot rather than sliding. The brake will cause you to lose control completely.

Don’t get into pointless negotiations about narrow tire – wide tire, increase tire pressure – decrease. If you catch a bird with your mouth, there will be no more than what the snow quality allows.

If the ice sheet has formed and the tires cannot hold the road, it will not be possible to advance by piloting. If the tread layer is thin and there is no deep snow, the studded Tire, if the snow is deep, there is no alternative other than chains. (I know snow pallets too. We are talking about normal road conditions)
The laws of physics always apply. Even tracked construction machines are stranded under very severe conditions.

There is no exact arithmetic equation for this job. The quality and depth of the abdomen is the most important. Sometimes you can go with normal tires on snow cover, sometimes you can’t go with winter tires, sometimes you can’t go with 4×4 and MT+chain.

The main factor that causes slip is the icing of the surface that the tire is in contact with.
This situation sometimes occurs when the vehicle is opening its own trail, sometimes 2. or 3. It can happen when the vehicle is passing. Sometimes it does not happen at all due to the nature of the abdomen.

Regards, Love

09/03/2020