Questions about wheel diameter

[David1005]

The 28" diameter wheel, used on some auto racks, is rated at 55 tons even though that it uses 6 X 11 bearings, which are usually rated at 70 tons.  The wheel, axle, and bearing combinations, and their capacity, is all established by the AAR. 

[Erik_Mag]

Seems to me that for a given contact pressure (force/area), the larger wheel diameter would give a longer contact patch and thus support more weight. This is opposed to keeping diameter constant and relying on increased pressure to handle the extra weight. The problem with the latter is that fatigue life decreases with increasing pressure (stress).

[BaltACD]

SD60MAC9500

So here's my basic understanding.. Bogies rated for heavier axle loads use larger diameter wheels to increase loading capacity on the wheel. A 70T truck may use 28-33", 100T 36", 125T 38".

Overmod, Dave Goding, and others intimate with wheel/axle loading can provide greater context once they see your post.

I may be mistaken, but I think as wheel diameters increase axle diameters also increase and of course the size of the bearing for the axles increase which in turn allow for increased load carrying ability.

[SD60MAC9500]

So here's my basic understanding.. Bogies rated for heavier axle loads use larger diameter wheels to increase loading capacity on the wheel. A 70T truck may use 28-33", 100T 36", 125T 38".

Overmod, Dave Goding, and others intimate with wheel/axle loading can provide greater context once they see your post.

[timz]

Perry Babin

Or is the friction so limited between the wheel and track that that's the limited factor in braking?

You've heard how locomotive drivers slip when they try to exert a force greater than 25% of the weight on them. Maybe that means a freight-car wheel won't slip unless it's exerting 25+% of its weight on the rail -- which would mean it wouldn't slip until it was decelerating at more than 5 miles/hour per second, which trains never do.

Guess freights never decelerate at more than 2 mph/sec? So probably wheel-rail friction is never the limiting factor on halfway-clean rail. (We all know it is limiting on leafy rail, of course.)

[Backshop]

Intermodal.

[PJS1]

tree68

 I've always been of the impression that smaller wheels were generally employed to reduce the overall height of the car.  That they are generally used for IM cars, which will be lighter in most cases, is almost coincidental. 

The wheels on the auto carriers that I see rolling through central Texas are smaller than those on most other cars.  I too understand it is for height clearance.  

BTW, what is an IM car?  

[tree68]

Perry Babin

For brakes that work against the outer rim of the wheel, does the larger diameter have greater braking power? Or is the friction so limited between the wheel and track that that's the limited factor in braking?

I'm pretty sure the brake shoes are the same for each.  What the larger wheel will offer is some level of increased heat dissipation.

I've always been of the impression that smaller wheels were generally employed to reduce the overall height of the car.  That they are generally used for IM cars, which will be lighter in most cases, is almost coincidental.

I could be wrong.

[Perry Babin]

Is it true that bogies rated for heavier loads use larger diameter wheels than lower rated bogies?

If so, why?

Does the wheel/track deform enough to form a contact patch more or less with larger/smaller diameter wheels?

If so, does that determine the chosen diameter of the wheels?

For brakes that work against the outer rim of the wheel, does the larger diameter have greater braking power? Or is the friction so limited between the wheel and track that that's the limited factor in braking?