Cooling your brakes
Being from the east coast I've yet to encounter my brakes heating up enough to require pulling over to let them cool down, at least not that I know of. This summer/fall we're doing our first trip out ...
Forum Discussion
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The secret to going down any downhill and saving brakes is to go slow. It takes less braking effort to control the speed if the speed is less to start with. This is true of both heavy road vehicles and rolling railroad stock.
We can try to apply logic to the problem of stab vs steady braking.
The reason for going slow is that it does take the same energy transfer to go up a hill as it does to go down. In other words, the same amount of total heat generated by brakes can also be achieved by burning the hill-climb volume of fuel in a campfire. Stab vs steady add up to the same total btu's.
Fact: One can climb a hill using less fuel if one goes slower. I know this because when I railroaded, I could climb the same monotonous hill at a slower speed by dropping the throttle down from a max of #8, to #7 or even #6. The locomotive throttle position controls engine speed (and power) via fuel setting. Usually the only reason I did this was because of slippery track, overheating traction motors or knuckle breaking limits. Of course less throttle burns less fuel, but it also burns it for a longer period of time. But this "matching" longer period of time is also what we are looking for while braking. The same heat at a lower rate, therefore cooler brakes.
With modern brake pads, the gentlemen that subscribe to steady braking are not wrong unless they are doing it at a high speed. Ideally the brake discs will reach a steady equilibrium and hold at that temperature, and that temperature will not overpower (boil) the brake pad resin, creating gassing-off that "lubricates" the contact surface and shows up as "fade". But the same speed restriction applies for the "stabbers". And the extra heat from stabbing may begin to periodically cook the resin out of most normal pads since they do get periodically hotter.
Someone had mentioned that the brakes cool faster when stabbed. This faster rate is true for the short time it takes the brakes to drop and reach the same temp as steady state braking, next the cooling rate is momentarily the same, and finally the brakes cool slower as the temp drops below steady state. The end result is that the brakes cool at the same average rate either way, stab or steady. Note, if the brakes are not allowed to cool for some time at the slower rate below steady state, that the average stabbing temperature must logically be higher than the steady average-braking scenario.
TRIVIA:
Brake shoe materials vary in composition depending on what type service they are intended to be used in. Full roadracing brake pads are made with boil resistant resin. So are railroad brake shoes. Either have good characteristics under extended red-hot braking. And both have a lousy friction coefficent when cold. Racing pads don't work well for general consumer use since they must be warmed up before they work reasonably well. This takes about 15 seconds on a train, too long for an emergency automotive stop. Modern friction materials have extended the range, so some premium brake pads offer good cold friction as well as better heat resistance.
Almost all extended rail downhills have a permanent speed restriction below that of normal track speed to save brake shoes. The speeds were apparently selected very carefully, because I could often hold speed by the use of dynamic braking only... unless I didn't start soon enough and speed began to increase in a snowball fashion. Once I exceeded a moderate threshold, I would be forced to apply air brakes to correct my speed. FYI, dynamic braking employs the use of converting the traction motors to generators and wasting the generated electric current into large "toaster" grids that are fan cooled out the top.
Wes
...
The secret to going down any downhill and saving brakes is to go slow. It takes less braking effort to control the speed if the speed is less to start with. This is true of both heavy road vehicles and rolling railroad stock.
We can try to apply logic to the problem of stab vs steady braking.
The reason for going slow is that it does take the same energy transfer to go up a hill as it does to go down. In other words, the same amount of total heat generated by brakes can also be achieved by burning the hill-climb volume of fuel in a campfire. Stab vs steady add up to the same total btu's.
Fact: One can climb a hill using less fuel if one goes slower. I know this because when I railroaded, I could climb the same monotonous hill at a slower speed by dropping the throttle down from a max of #8, to #7 or even #6. The locomotive throttle position controls engine speed (and power) via fuel setting. Usually the only reason I did this was because of slippery track, overheating traction motors or knuckle breaking limits. Of course less throttle burns less fuel, but it also burns it for a longer period of time. But this "matching" longer period of time is also what we are looking for while braking. The same heat at a lower rate, therefore cooler brakes.
With modern brake pads, the gentlemen that subscribe to steady braking are not wrong unless they are doing it at a high speed. Ideally the brake discs will reach a steady equilibrium and hold at that temperature, and that temperature will not overpower (boil) the brake pad resin, creating gassing-off that "lubricates" the contact surface and shows up as "fade". But the same speed restriction applies for the "stabbers". And the extra heat from stabbing may begin to periodically cook the resin out of most normal pads since they do get periodically hotter.
Someone had mentioned that the brakes cool faster when stabbed. This faster rate is true for the short time it takes the brakes to drop and reach the same temp as steady state braking, next the cooling rate is momentarily the same, and finally the brakes cool slower as the temp drops below steady state. The end result is that the brakes cool at the same average rate either way, stab or steady. Note, if the brakes are not allowed to cool for some time at the slower rate below steady state, that the average stabbing temperature must logically be higher than the steady average-braking scenario.
TRIVIA:
Brake shoe materials vary in composition depending on what type service they are intended to be used in. Full roadracing brake pads are made with boil resistant resin. So are railroad brake shoes. Either have good characteristics under extended red-hot braking. And both have a lousy friction coefficent when cold. Racing pads don't work well for general consumer use since they must be warmed up before they work reasonably well. This takes about 15 seconds on a train, too long for an emergency automotive stop. Modern friction materials have extended the range, so some premium brake pads offer good cold friction as well as better heat resistance.
Almost all extended rail downhills have a permanent speed restriction below that of normal track speed to save brake shoes. The speeds were apparently selected very carefully, because I could often hold speed by the use of dynamic braking only... unless I didn't start soon enough and speed began to increase in a snowball fashion. Once I exceeded a moderate threshold, I would be forced to apply air brakes to correct my speed. FYI, dynamic braking employs the use of converting the traction motors to generators and wasting the generated electric current into large "toaster" grids that are fan cooled out the top.
Wes
...
