Since I'm sure that everyone will look at my 6 tables first, I'll explain those and then fill in the rest of my story for what I believe is the safest way to descend mountain grades. The tables give the HP required to maintain a constant MPH when descending various %Grades. For combinations of MPH vs %Grade that are on or above the yellow line, the throttle is at idle, and a negative braking HP is required, and below the line a positive HP requiring various amounts of throttle is necessary.

The first set of 3 tables is for my 20,500 lb GCW truck+trailer, and the second set of 3 tables, in the second post, is for my empty truck of 7,500 lb GCW. For each set of 3 tables, the first table is for 4th=0.71, the second table is for 3rd=1.0, and the third table is for 2nd=1.54 gear ratios, where the gear ratios are indicated in the lower left hand cell. The HP only depends on MPH, but the MPH vs RPM rows are for my 4R100 + 4.10 Diff + stock 265 75R 16 tires that measure 31.6" in diameter. Those with a different setup can still use the HP indicated for a given MPH and %Grade, but their RPM for estimating engine breaking will be a little different than the ones shown in the tables.

As an example of how to use the tables, look at the first one (20,500 lb GCW truck+trailer) at MPH=64.6, and you'll see that you can coast down about a 2.5%Grade at a constant 64.6 MPH without applying throttle or brake. A 2%Grade requires +22 HP of throttle, and a 3%Grade needs -14 HP of braking to maintain the constant MPH=64.6.

Now, -14 HP to descend a 3%Grade might not sound like a big deal, but look at it this way. Consider a standard 1,500 Watt heater which is the maximum that can be plugged into a 15 amp outlet. I use one of these to heat my trailer when I'm stupid enough to be staying somewhere that gets cold enough to use it. That heater will keep me warm down to about a 20 F ambient temp, and then I've got to turn on the propane furnace. My point is, that heater is only putting out 2 HP of heat. Applying my service brakes to hold back my truck + trailer on a 3%Grade is almost like having two of those heaters on each wheel, and that's a lot of heat for those brakes to dissipate on a continuous basis.

Now, look up the column for MPH=64.6 to a 4%Grade, and you get HP=-49, then -84, then -120 at a 6%Grade! Descending a 6%Grade at 64.6 MPH using only your service brakes is like having 15 of these 1.5 kW heaters attached to each wheel! Now you understand why my first mod over 8 years ago was a US Gear exhaust brake to avoid burning up my truck brakes, and my second mod shortly thereafter, was a BD TCC=Torque Converter Clutch lockup device to avoid burning up the tranny when using my exhaust brake.

As I stated in Posts # 1 and #16 here, https://www.ford-trucks.com/forums/629052-torque-converters-and-diesel-engine-breaking.html it's not necessary to have the TCC locked to get engine breaking, and you even get better braking with the TCC unlocked, but you'll burn up the tranny if you're not careful. As I point out here, the key for good engine braking is to select a lower gear to get the RPM up to 2500 RPM or more, and that's why I included the 3rd and 2nd gear tables so you can better see what gear is required at a given MPH. In the example discussed at 64.6 MPH in 4th, the RPM is only 2000 which is too low. If you look at the 3rd gear table for MPH=64.2, you're at 2800 RPM which is a much better choice of gear at that MPH. Look at the table for 2nd gear, and you'll see my recommendation for descending a 6%Grade is to use 2nd at 2800 to 3000 RPM where your MPH is down to a little less than 45 MPH. Keep reading to see why this is the best choice!

In these Posts # 34, 36, 37, 39, 40, and 42 https://www.ford-trucks.com/forums/628367-using-engine-to-slow-down-when-towing-not-an-exhaust-brake-question-3.html#post4897266 I posted pics from the official Ford shop manual regarding engine breaking for a PSD with an auto tranny, and the key is to make sure your CC=Coast Clutch is in good working order. The TCC is not needed for engine braking, but it's useful for controlling tranny heat on a long descent. However, if the CC doesn't engage it's like shifting to neutral!

Here in Post # 1 here https://www.ford-trucks.com/forums/617062-how-good-are-your-brakes.html , I calculated stopping distance at 60 MPH for an empty truck with a GCW=7,500 lb, and showed that a 150 ft stopping distance on a 0%Grade required 475 HP of braking power, but only for 3.5 seconds. Those brakes need to cool after that stop, and that's running EMPTY ON THE FLAT! God help us if we ever have to make an EMERGENCY STOP from 60 MPH while descending a 6%GRADE with a TRAILER, and I'll post some numbers for doing it later to show why divine intervention is required in addition to good service brakes, and an exhaust brake, and possibly dragging your foot as well. In the meantime, it's all about WATCHING YOUR SPEED, and I'll repeat the mantra I recite when descending grades, you can go down a steep grade too SLOWLY as many times as you want, but you can only go down it too FAST once!

For those interested in the details, the RWHPR=Rear Wheel Horse Power Required values discussed above are given by the following equations. The first term in each equation accounts for the HP needed to overcome rolling friction (tires, bearings, etc..), the second term is the HP for overcoming aerodynamic drag (and includes a reduction for my wing when towing), and the third is for %Grade, and gives the HP needed to lift a GCW up a given % incline at a given rate. If you're pulling up a grade, all three terms are positive, but if you're descending down a grade, the first two terms are helping to retard your descent, and it's the third term that gives the effect of gravity pushing you down the grade that needs to be overcome with additional breaking.

RWHPR = (1.0)*(MPH)^0.5+[(MPH)^3]/(3200)-[(GCW)*(%Grade)*(MPH)]/(375), Truck+Trailer


RWHPR = (0.5)*(MPH)^0.5+[(MPH)^3]/(4800)-[(GCW)*(%Grade)*(MPH)]/(375), Truck Empty


The first set of 3 tables for my 20,500 lb GCW truck+trailer is below.

 

The second set of 3 tables for my empty truck of 7,500 lb GCW is below.

 

I don't go down a hill without my exhaust brake on either.
I had the "previledge" of being a passenger one time with someone who had no understanding of big hills and red hot brakes. The wheels were smoking by the time the ground leveled out.
I told the supervisor when we got back about it and they did nothing. So I moved on to a different job soon after that.

You don't actually fill in all those boxes do you?
Excel does it for you?

 

Or put another way...There are old truck drivers and there are bold truck drivers, but there aren't any old AND bold truck drivers!

I taught many drivers how to drive class 8 trucks and my mantra was "whatever gear you were in that got you to the top of the grade, that is the gear you start down the other side in." The problem is most folks speed up on the crest of the hill and start down the grade at too high a speed...thus they have run out of gears AND they might just run out of brakes too!

Note to Gene: you know that mathematical modeling is very addictive, don't you?

 

Yes Excel does it for him Kwik, but he writes the equations that are driving Excel!

 

Must be handy when designing things.
I struggled to get a D in algebra and flunked geometry in high school. I figured that was enough for me and never tried again.

 

I was hoping that someone else might raise this question, but since they haven't I figure I'd better raise it and answer it before someone looks at my tables, and gets the wrong idea, and crashes big time. Go to the first table, for example, and look across a row like for a 6%Grade. As MPH increases from 19 to 110, the required braking HP first increases, reaches a maximum at 58 MPH, then starts decreasing until at about 100 MPH you can just coast down the grade without using your brakes at all. So why not use my tables to see how FAST you need to go for a given %Grade to save the wear and tear on your brakes? Except for maybe a 1% or 2% Grade, PLEASE DON'T USE THE TABLES THIS WAY!

The proper interpretation of this effect is that looking across a row for a given %Grade gives your terminal MPH if your brakes overheat and you completely lose all braking capability. Then your speed will increase until aerodynamic drag limits it just like a skydiver in freefall. This is the speed you'll be going if you're lucky enough to come to an emergency run-off ramp before crashing! Besides, you should always be concerned about needing to come to a stop before reaching the bottom of the grade.