Silly question about roads and percent grade.

Originally posted by sonhouse
Ah, then you haven't heard about the new braking system invented recently: Explosive bolts that drive anchors into the road with ropes that play out and slow down the trucks in an emergency, all they need are two such devices, one on either side and the truck slows down evenly. On curves there is a computer that follows the curve and plays out the lines differentially so it handles those as well.

I think what he said was that it has nothing to do with emergencies, but rather the fact that trucks must brake continuously when going down a slope, and if they use the brakes rather than the engine to do so they will overheat the brakes (and probably wear them out faster).

Originally posted by leisurelysloth
The problem with trucks isn't so much about slipping as it is over-heating the brakes. On long grades they'll need to run slower and in a lower gear, so that their "braking" comes from engine compression rather than riding the brakes all the way down the grade.

I know nothing of truck driving, so I never would have thought of this.

Regardless though, the issue of slipping does into play if the hypothetical grade gets too
steep regardless of the means of braking (engine compression versus friction braking). I was
just curious what that grade was.

Nemesio

Originally posted by PBE6
Just tried working this out on a spreadsheet, and got a nice simplification (which I should have seen in the first place, and is probably the reason why civil engineers use grade % in the first place).

Since the maximum angle of elevation before disaster, theta(max), is Tan(-1)(mu), and the maximum grade % is just tan(theta(max)), we end up with:

maximum ...[text shortened]... nce, etc... but it seems that as long as the grade never goes about about 20% we should be fine.

Did you write that last sentence incorrectly? Didn't you mean a 40% grade (21.8 degree angle)
for wet roads and a 70% grade (35 degree angle) on dry ones?

Can the kind of tires you have increase the mu coefficient or is the road the only consideration?

Nemesio

Originally posted by Nemesio
Did you write that last sentence incorrectly? Didn't you mean a 40% grade (21.8 degree angle)
for wet roads and a 70% grade (35 degree angle) on dry ones?

Can the kind of tires you have increase the mu coefficient or is the road the only consideration?

Nemesio

Yes, tires have a huge effect on traction. For instance, our own civilian tires are advertised to last 50,000, 60,000 miles, etc. What is really going on is they have figured out how to make the tires harder and thus don't wear out as quickly but they pay for that longevity in reduced traction. When you see the traction rating, B, A, AA, etc., they trade off traction for road life, more traction =softer tires=shorter life. The other extreme is the drag racing tire. It lasts only a couple of races, maybe a mile or two on the track but the traction is thee times that of regular tires, allowing a 1500 pound dragster to achieve brief excursions to over 4 G's of acceleration. With regular tires that is impossible. The reason is the deformation of the tire in front of the spin which digs into the pavement but ablates like a spacecraft re-entry shield. It would be kind of like a pavement of grooves perpendicular to the travel of the dragster and the tires having grooves matching the grooves of the roadway, you could achieve the same effect that way and still have long lasting tires. I imagine that would make for a somewhat bumpy ride for civilians though. Talk about singing tires🙂

Originally posted by Nemesio
Did you write that last sentence incorrectly? Didn't you mean a 40% grade (21.8 degree angle)
for wet roads and a 70% grade (35 degree angle) on dry ones?

Can the kind of tires you have increase the mu coefficient or is the road the only consideration?

Nemesio

No, I really did mean 20% because of a good ol' engineering safety factor of 2 (in case the roads are really slick and the truck has bald tires, or whatever else might happen).

Yep, the coefficient of friction is evaluated for the interface between two surfaces, so a change to either surface can impact its magnitude.

EDIT: Oops! sonhouse already answered the second part...