4 ms·

I've seen trucks (tractor-trailers) with flames coming from the brakes on 6% grades downhill... and the uphill sides of those the trucks are in low gear at high

I've seen trucks (tractor-trailers) with flames coming from the brakes on 6% grades downhill... and the uphill sides of those the trucks are in low gear at high RPM doing 15-20mph MAX, running at 3-4x the rpm they'd run doing 75 on the highway... people underestimate grade and the effect it has on heavy vehicles The maximum commercially viable (due to weight restriction) grade for freight railroads is 2.2% ...

jcranmer 10y ago
If you want some hard numbers on why high grade is so problematic: When you're pulling something along a road or a railtrack on level ground, the primary force you need to overcome is rolling friction, which is proportional to µ×N, where µ is in the range of 0.001 (rail) to 0.01 (road) and N is the normal force against the surface. For a 10 kN car (that's about 1000 kilos), this means that you need a force of about 10 N for a railcar. When you go up an incline, the rolling friction decreases according to the cosine of the angle, but now you need to overcome gravity--and this amount goes up by the sine of the angle. A measly 2% grade requires pulling a force of about 210 N, a factor of 21×. Basically, grade really kills because you actually have to counteract gravity as opposed to friction.
- jmcdiesel 10y ago
  First, thanks! I genuinely appreciate that knowledge, I'd have never looked it up on my own.. Second... why's gravity gotta be such a dick? :)
PhantomGremlin 10y ago
The maximum commercially viable (due to weight restriction) grade for freight railroads is 2.2% ... A 2.2% grade seems insignificant to the average person, but to a freight train it is no laughing matter. Here's a serious derailment that happened in California: https://en.wikipedia.org/wiki/San_Bernardino_train_disaster#Train_derailment https://en.wikipedia.org/wiki/San_Bernardino_train_disaster#... At that grade it's not sufficient to use brakes in the individual rail cars, it's also necessary to use dynamic braking by turning electric traction motors into generators, then dissipating energy in giant resistive grids.

jcranmer 10y ago

If you want some hard numbers on why high grade is so problematic: When you're pulling something along a road or a railtrack on level ground, the primary force you need to overcome is rolling friction, which is proportional to µ×N, where µ is in the range of 0.001 (rail) to 0.01 (road) and N is the normal force against the surface. For a 10 kN car (that's about 1000 kilos), this means that you need a force of about 10 N for a railcar. When you go up an incline, the rolling friction decreases according to the cosine of the angle, but now you need to overcome gravity--and this amount goes up by the sine of the angle. A measly 2% grade requires pulling a force of about 210 N, a factor of 21×. Basically, grade really kills because you actually have to counteract gravity as opposed to friction.

jmcdiesel 10y ago
First, thanks! I genuinely appreciate that knowledge, I'd have never looked it up on my own.. Second... why's gravity gotta be such a dick? :)

PhantomGremlin 10y ago

The maximum commercially viable (due to weight restriction) grade for freight railroads is 2.2% ... A 2.2% grade seems insignificant to the average person, but to a freight train it is no laughing matter. Here's a serious derailment that happened in California: https://en.wikipedia.org/wiki/San_Bernardino_train_disaster#Train_derailment https://en.wikipedia.org/wiki/San_Bernardino_train_disaster#... At that grade it's not sufficient to use brakes in the individual rail cars, it's also necessary to use dynamic braking by turning electric traction motors into generators, then dissipating energy in giant resistive grids.