The fact that they use steel wheels and rails where possible is evidence that the tyre / road friction is very relevant..Scott said:They are certainly not equal.
You want to reduce friction at the axle and increase it at the road surface.
The coefficient of friction for the tire on a dry road surface is about 0.7.
http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/frictire.html
The coefficient of friction between two lubricated hard steel surfaces is about 0.07.
http://www.school-for-champions.com/science/friction_sliding_lubricated.htm#.WcO6WrJ96po
I'm not sure I catch what you're saying. For a train, the coefficient of friction between the wheels and the rails are comparable to automobile tires on a road surface.sophiecentaur said:The fact that they use steel wheels and rails where possible is evidence that the tyre / road friction is very relevant.
Are you saying that the choice of rails is just based on load carrying capability?.Scott said:I'm not sure I catch what you're saying. For a train, the coefficient of friction between the wheels and the rails are comparable to automobile tires on a road surface.
But, of course, that has nothing to do with friction in the axle of the car where the components are lubricated.
One might glance at these numbers and think "That's a ratio of 10 to 1. So we can pull ten train cars on wheels with the same effort as dragging one over the tracks with no wheels". But that reasoning misses another factor of ten or so. The wheel bearings are only about 10% as far from the center of rotation as the wheel rims. Accordingly, the rolling resistance from the wheel bearings is only about 10% as much as if the wheels were locked up on greased rails. Which is, in turn, only 10% as much as sliding rubber tires on dry pavement..Scott said:They are certainly not equal.
You want to reduce friction at the axle and increase it at the road surface.
The coefficient of friction for the tire on a dry road surface is about 0.7.
http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/frictire.html
The coefficient of friction between two lubricated hard steel surfaces is about 0.07.
http://www.school-for-champions.com/science/friction_sliding_lubricated.htm#.WcO6WrJ96po
The question was about rolling friction. I read somewhere that steel wheels on rails are approx. 6 times as more efficient than tyres on a road and water transportation at low speeds is approx. 6 times more efficient than rail transport. This fact triggered the said question.sophiecentaur said:It would be useful to know whether the OP concerned braking or rolling. The conversation would take us in different directions, depending.
Ahhh. I was suspecting that there was a difference in how we interpreted the OP, and now I believe I have identified it.sophiecentaur said:Are you saying that the choice of rails is just based on load carrying capability?
I guess the rolling resistance would be a more relevant quantity. I believe tyres are much more lossy than steel wheels.
I would disagree about the terminology. This is not about friction. It is about rolling resistance. The two are different things..Scott said:So the road friction I have been talking about is what would limit your acceleration or braking. While the friction you are talking about would lower you mileage.
Note that the link talks about rolling resistance, not wheel bearings.sophiecentaur said:Reality check: wheel bearings get warm. According to this link (they did some sums) a car could have, say 180N of rolling resistance.
It was the OP who asked about "friction", not "resistance" or "rolling resistance". So if you're disagreeing about terminology, it isn't with me.jbriggs444 said:I would disagree about the terminology. This is not about friction. It is about rolling resistance. The two are different things.
If you lock up your tires and skid to a stop, that's "friction".
If you leave your brakes off but fill your tires with silly putty and coast to a stop, that's "rolling resistance". It is usually better characterized as a torque than as a force.
True; It was a mistake on my part; What I really wanted to say was "rolling resistance", not friction..Scott said:It was the OP who asked about "friction", not "resistance" or "rolling resistance". So if you're disagreeing about terminology, it isn't with me.
You mean this is about torque and overcoming the rolling resistance? Could well be but I was looking for friction on the axle.jbriggs444 said:Note that the link talks about rolling resistance, not wheel bearings.
I am suggesting that in an automobile tire there are two sources of retarding torque/rolling resistance. One is friction at the bearings. The other is tread squirm and the lack of complete elasticity in the tire material. The article you quoted seemed to be focused on the latter.sophiecentaur said:You mean this is about torque and overcoming the rolling resistance? Could well be but I was looking for friction on the axle.
Say a typical wheel diameter is 630mm. Radius 315, the torque needed to overcome rolling resistance will be 0.38 X 180 Nm
Yes I got that but it seemed that the OP wanted some kind of comparison between rolling resistance ("friction") and friction at the axle ("bearings"). It's a perennial problem when the OP is not put in a universally understandable way.jbriggs444 said:Note that the link talks about rolling resistance, not wheel bearings.
We will have to wait for further instructions from the OP.jbriggs444 said:I am suggesting that in an automobile tire there are two sources of retarding torque/rolling resistance. One is friction at the bearings. The other is tread squirm and the lack of complete elasticity in the tire material. The article you quoted seemed to be focused on the latter.
Friction at the axle refers to the resistance between the axle and its surrounding components, such as bearings and lubricants. Friction at the tyre, on the other hand, refers to the resistance between the tyre and the surface it is in contact with.
Friction at the axle can impact the overall performance of a vehicle by affecting its speed, handling, and fuel efficiency. Higher levels of friction can lead to decreased speed and maneuverability, and can also increase fuel consumption.
Yes, the materials used for the axle and tyre can have a significant impact on the level of friction. Different materials have varying levels of roughness and smoothness, which can affect how they interact with each other and the surface they are in contact with.
Friction at the axle and tyre can be reduced by using lubricants, such as oil or grease, to create a barrier between the surfaces. Additionally, using materials with lower coefficients of friction and maintaining proper tire pressure can also help reduce friction.
No, friction at the axle and tyre cannot be completely eliminated. Friction is a natural force that occurs whenever two surfaces are in contact with each other. However, it can be minimized through proper maintenance and use of lubricants.