Friction at the axle vs at the Tyre

[Frenemy90210]

Typically how do the two frictions, namely
1. at the axle of a car wheel vs
2. friction experienced by tyre at the road surface
compare with each other? are they some what equal ?.. I am interested in approximate ratio of the two values . Thx.

 

[.Scott]

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

 

[sophiecentaur]

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 fact that they use steel wheels and rails where possible is evidence that the tyre / road friction is very relevant.

 

[.Scott]

The fact that they use steel wheels and rails where possible is evidence that the tyre / road friction is very relevant.

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.

 

[sophiecentaur]

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.

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.

 

[jbriggs444]

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

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.

 

[sophiecentaur]

It would be useful to know whether the OP concerned braking or rolling. The conversation would take us in different directions, depending.

 

[Frenemy90210]

It would be useful to know whether the OP concerned braking or rolling. The conversation would take us in different directions, depending.

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.

 

[.Scott]

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.

Ahhh. I was suspecting that there was a difference in how we interpreted the OP, and now I believe I have identified it.
You interpreted the OP to be referring to sources of drag. I took "friction" literally - and since the load is roughly the same at the axle as it is in contact with the road, I equated it to coefficient of friction.

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.

 

[jbriggs444]

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.

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.

 

[sophiecentaur]

I guess the back of a fag packet answer would be that , if steel rails are "six times more efficient" (i.e. the friction force is 1/6) and the bearings are at 10% of the distance (I would have thought more, actually), we can sort of assume the same friction force from the wheel bearings as for steel rails then the work done against the bearings would would be something like 0.12 X 1/6 of work done against the tyre's rolling resistance. Thats 2%.
Reality check: wheel bearings get warm. According to this link (they did some sums) a car could have, say 180N of rolling resistance. At 30m/s, that would mean 6.4kW of power, dissipated in the tyres. That would imply 128W (2%) dissipated in the bearings. 70 W each bearing. That would keep them a bit warm but it's less than I would have thought. The initial figures are not total nonsense, though.

 

[jbriggs444]

Reality check: wheel bearings get warm. According to this link (they did some sums) a car could have, say 180N of rolling resistance.

Note that the link talks about rolling resistance, not wheel bearings.

 

[.Scott]

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.

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.

 

[Frenemy90210]

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.

True; It was a mistake on my part; What I really wanted to say was "rolling resistance", not friction.

 

[sophiecentaur]

Note that the link talks about rolling resistance, not wheel bearings.

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

 

[jbriggs444]

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

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]

Note that the link talks about rolling resistance, not wheel bearings.

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.
Now we've decided it means the torque to overcome rolling resistance (yes?).

 

[sophiecentaur]

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.

We will have to wait for further instructions from the OP.