5 ms·

Adding more wheels does not reduce stopping distance.

by coryrc 3y ago

itsoktocry 3y ago
Doesn't it though? More rolling resistance and braking surface area?
- ars 3y ago
  But then pressure (force) on each tire is lower now, so the friction is also lower. Effective friction is a function of weight and coefficient of friction between the materials. Interestingly the surface area of the contact is not part of the equation. On a flat surface a truck and a car using the same tire compound should have identical stopping distances. The issue comes when you are going downhill.
  - lpapez 3y ago
    That equation works for friction between idealized solids. Tires are not even close to an idealized solid, they deform as more weight is added to them, so the contact surface area increases as more pressure is added. That is why you see racing cars with big fat tires, because it's more grip and the relationship is not linear.
    - ars 3y ago
      The equation still holds. As the tire deforms to have more area the pressure on any particular part of the tire is lower. The total friction is unchanged. > That is why you see racing cars with big fat tires If you mean drag racers those tires are glued to the ground, and do not work off of friction. Other than that, what race cars have big fat tires? Tires are usually sized so that the ground pressure of the tire is in the 40 to 100 psi range (which is also the tire inflation pressure). That's simply to keep the ground from deforming and the tire from popping. It does not affect friction. This is also why off-road tires have lower pressure - the sand and mud they work on is not strong enough for a higher pressure. It is NOT for increased friction, it's because the surface they drive on can't handle it, so you spread out the weight, and reduce the pressure.
    - coryrc 3y ago
      They have the fat tires because they're purposefully melting tire rubber into the pavement for a bit more traction. Rubber tires have maximum grip at 7% slip; just about the only material where sliding friction is greater than static friction. Road-going tires won't get hot enough unless you're skidding far below 7% slip.
    - caf 3y ago
      That friction equation comes originally from empirical experiments, not from idealised models. The explanation in fact relies on deformable surfaces. If you imagine keeping the normal force constant and reducing the apparent contact area, the pressure on the surfaces will be increased which will cause the surfaces to deform more, resulting in more microscopic points of adhesion.
      - caf 3y ago
        (If you find this surprising and want to read a little more, try this: https://www.stevemunden.com/friction.html https://www.stevemunden.com/friction.html)
  - Dylan16807 3y ago
    > On a flat surface a truck and a car using the same tire compound should have identical stopping distances. Well according to the article they don't have identical stopping distances, so let's figure out what would fix it. > The issue comes when you are going downhill. That just changes the angle of gravity. Why would it affect trucks and cars differently? I thought the point was that going downhill exacerbates differences in stopping distance, not that is causes them.
    - ars 3y ago
      > so let's figure out what would fix it. For one thing an 18 wheeler has 10 brakes, not 18. So that's an obvious thing to fix. > Why would it affect trucks and cars differently? The weight of the truck acts as an acceleration force when going downhill. The tire friction of a car and truck is the same - but then the truck has extra force trying to accelerate the vehicle when it's downhill.
      - Dylan16807 3y ago
        > For one thing an 18 wheeler has 10 brakes, not 18. So that's an obvious thing to fix. That sounds like it would make a huge difference, yes. > The weight of the truck acts as an acceleration force when going downhill. The tire friction of a car and truck is the same - but then the truck has extra force trying to accelerate the vehicle when it's downhill. So does the weight of the car. I don't understand this logic. Every pound that's giving you acceleration is also giving you extra tire friction. A car and a truck should theoretically lose the same number of meters per second squared of stopping force when going down a hill. If the truck started with weaker brakes then it loses a bigger percentage of stopping force, exacerbating the already-existing problem, but if you find a truck and a car with equal stopping distance on flat ground then they should have equal stopping distance on a slope, shouldn't they?
- eeklipse 3y ago
  From a fundamentals of physics standpoint; it’s only about the coefficient of friction and the normal force, surface area doesn’t come into play. Realistically, surface area does come into play because things deform and heat up, but I can’t say how much.

Adding more wheels does not reduce stopping distance.

by coryrc 3y ago

Adding more wheels does not reduce stopping distance.

itsoktocry 3y ago

Doesn't it though? More rolling resistance and braking surface area?

ars 3y ago
But then pressure (force) on each tire is lower now, so the friction is also lower. Effective friction is a function of weight and coefficient of friction between the materials. Interestingly the surface area of the contact is not part of the equation. On a flat surface a truck and a car using the same tire compound should have identical stopping distances. The issue comes when you are going downhill.
- lpapez 3y ago
  That equation works for friction between idealized solids. Tires are not even close to an idealized solid, they deform as more weight is added to them, so the contact surface area increases as more pressure is added. That is why you see racing cars with big fat tires, because it's more grip and the relationship is not linear.
  - ars 3y ago
    The equation still holds. As the tire deforms to have more area the pressure on any particular part of the tire is lower. The total friction is unchanged. > That is why you see racing cars with big fat tires If you mean drag racers those tires are glued to the ground, and do not work off of friction. Other than that, what race cars have big fat tires? Tires are usually sized so that the ground pressure of the tire is in the 40 to 100 psi range (which is also the tire inflation pressure). That's simply to keep the ground from deforming and the tire from popping. It does not affect friction. This is also why off-road tires have lower pressure - the sand and mud they work on is not strong enough for a higher pressure. It is NOT for increased friction, it's because the surface they drive on can't handle it, so you spread out the weight, and reduce the pressure.
  - coryrc 3y ago
    They have the fat tires because they're purposefully melting tire rubber into the pavement for a bit more traction. Rubber tires have maximum grip at 7% slip; just about the only material where sliding friction is greater than static friction. Road-going tires won't get hot enough unless you're skidding far below 7% slip.
  - caf 3y ago
    That friction equation comes originally from empirical experiments, not from idealised models. The explanation in fact relies on deformable surfaces. If you imagine keeping the normal force constant and reducing the apparent contact area, the pressure on the surfaces will be increased which will cause the surfaces to deform more, resulting in more microscopic points of adhesion.
    - caf 3y ago
      (If you find this surprising and want to read a little more, try this: https://www.stevemunden.com/friction.html https://www.stevemunden.com/friction.html)
- Dylan16807 3y ago
  > On a flat surface a truck and a car using the same tire compound should have identical stopping distances. Well according to the article they don't have identical stopping distances, so let's figure out what would fix it. > The issue comes when you are going downhill. That just changes the angle of gravity. Why would it affect trucks and cars differently? I thought the point was that going downhill exacerbates differences in stopping distance, not that is causes them.
  - ars 3y ago
    > so let's figure out what would fix it. For one thing an 18 wheeler has 10 brakes, not 18. So that's an obvious thing to fix. > Why would it affect trucks and cars differently? The weight of the truck acts as an acceleration force when going downhill. The tire friction of a car and truck is the same - but then the truck has extra force trying to accelerate the vehicle when it's downhill.
    - Dylan16807 3y ago
      > For one thing an 18 wheeler has 10 brakes, not 18. So that's an obvious thing to fix. That sounds like it would make a huge difference, yes. > The weight of the truck acts as an acceleration force when going downhill. The tire friction of a car and truck is the same - but then the truck has extra force trying to accelerate the vehicle when it's downhill. So does the weight of the car. I don't understand this logic. Every pound that's giving you acceleration is also giving you extra tire friction. A car and a truck should theoretically lose the same number of meters per second squared of stopping force when going down a hill. If the truck started with weaker brakes then it loses a bigger percentage of stopping force, exacerbating the already-existing problem, but if you find a truck and a car with equal stopping distance on flat ground then they should have equal stopping distance on a slope, shouldn't they?
eeklipse 3y ago
From a fundamentals of physics standpoint; it’s only about the coefficient of friction and the normal force, surface area doesn’t come into play. Realistically, surface area does come into play because things deform and heat up, but I can’t say how much.