Rolling Resistance - Physics

[User]

 

[Vapin' Joe]

My brain hurts after reading that.

 

[Spinney]

You are talking about the force to keep the moving vehicle moving, not to accelerate it.

If the load is 4000 kg, the weight is 40 000 N (approx, taking g = 10 N/kg for simplicity). So the force needed is 320 N.

Wikipedia tells me that rolling resistance is due to the tyre/wheel deforming as the load is applied to different parts of it. If that were the only force that needed to be overcome then the number of wheels should not matter. But there will also be a certain (small) amount of friction in bearings etc, which will increase as the number of wheels increases.

I think...

 

[vernon]

At a simplistic level, friction is independent of surface area of contact and by extension is also independent of the number of wheels bearing the load.

It's counter-intuitive but true.

 

[nickyboy]

You are talking about rolling resistance but thinking about all mechanical inefficiencies in the wheel/bearing/axle set up

If you completely forget about any resistance in the bearing friction etc, more wheels = less hysteresis in each tyre = less overall rolling resistance

But more wheels = more bearings etc = more frictional losses

So pure rolling resistance, more wheels gives less resistance. But there are other resistances increase as you have more wheels

 

[Spinney]

[QUOTE 4157006, member: 9609"]so the principle of my OP is correct then, pure rolling resistance would not go up if you added more wheels, but in the real world it may be a little more difficult due to other losses in bearings etc. So when a figure such as (0.0010 to 0.0024 for Railroad steel wheel on steel rail) it doesn't necessarily mean you have to count all the wheels. you would need 5 - 12 ton of force to move a 5000 ton train ?[/QUOTE]
To keep it moving, yes. You would need more than that to accelerate it up to speed.

 

[Accy cyclist]

This kind of stuff reminds me of school in the 1970's. 3 hours of physics on a Monday morning = skive off and head to the hills for air rifle practice instead!

 

[AndyRM]

42.

 

[Spinney]

There is the standard F = m x a force, which depends only on the inertia of the thing you want to accelerate.
But in the real world, as you say there is 'stiction' (static friction) - but I think this would be small with a vehicle where bearings etc are designed to minimise friction.
I think rolling resistance only applies once the thing is actually rolling.

This page gives some typical acceleration values, and says for a train it is in the order of 1 m/s2 (that is for a Swedish High Speed train, so may not the the same for the train you are talking about. So for your 5 000 000 kg train, the force needed to accelerate the mass is 5000 kN. From your figures above, using the rolling resistance co-efficient of 0.001, the force needed to maintain its speed would be 5 kN. But then once it is going you also have to take air resistance into account, which increases with speed squared.

 

[nickyboy]

[QUOTE 4157324, member: 9609"]from what I am reading there is a less obvious force needed than acceleration to initiate movement, seems that under normal coefficient of friction an extra bit of force is needed to get things moving from a standstill, but don't know if this applies to rolling resistance.[/QUOTE]

Static Friction and Rolling Resistance are two different things.

Static Friction is the thing that resists the initial force you apply, typically things like the viscosity in bearings, lubrication etc

Rolling Resistance is one of the resistances you experience when the body is moving. It means you have to keep applying a force to keep moving forward once you are moving (along with air resistance and mechanical inefficiencies)

 

[vernon]

[QUOTE 4157324, member: 9609"]from what I am reading there is a less obvious force needed than acceleration to initiate movement, seems that under normal coefficient of friction an extra bit of force is needed to get things moving from a standstill, but don't know if this applies to rolling resistance.[/QUOTE]

From direct experience of pushing railway wagons up an extremely shallow inclined siding by schoolboy power alone and jumping into them for the ride back down the incline, 'stiction' has to be overcome first. Once that has been overcome the effort of pushing the wagon decreases markedly. We found a pinch bar which, when applied between a wheel and the track, helped considerably to get the wagon moving. Once moving, the force applied to maintain motion remained constant as far as our shoulders and straining legs could tell.

At the sort of velocities that we obtained, air resistance was negligible. The wheel bearings were cast/forged steel axles running in whitemetal shell bearings lubricated by a thick oil/grease compound.

 

[vernon]

Is that you, Vern?

No. It's Fenton moving a truck single-handed.