Grade Climb & Descent Considerations

Steep grades are serious business which will test both the vehicle and the driver. The following information should be considered before attempting a posted grade. You must establish your own decisions based upon the circumstances at hand. These issues are provided for your consideration only, and do not constitute the only issues of consideration required for a successful mountain or grade crossing.

All of the information provided in this topic should be considered and then you should make your own choices about how to get across that grade. You are the only one who knows and understands your particular situation. This topic is not a step by step guide to keep you out of harms way. Only you can make the necessary judgement calls to keep yourself out of harms way.

A couple of well known grades in the United States:

Grade Nickname Grade Location Grade % & Length
Sherman Hill East of Laramie, WY 5% for 5 miles
The Sisters PA MM30 WB,MM6 EB Not posted
Parley Summit I-80 UT WB to Salt Lake 3-6% for 10 miles
Mount Eagle I-24, TN EB 6% for 4 miles
Sandstone I-64, VA EB 7% for 5 miles
Cajon Pass I-15, South of Victorville,CA 6% for 12 miles/scale
The Grapevine I-5, CA 6% for 14 miles
Donner Pass I-80 WB Reno/Sacramento 3-6% for 30 miles
Tehachapi US-58 4-6% for 14 miles
Black Mountain I-40, NC 6% for 5 miles

 

Climbing the Grade

Before climbing that grade, you should try to determine the weather at the top and on the other side of the grade if a mountain is involved. Road surface conditions have a profound effect upon the safety of your transit. How many and how tight the curves are is also another factor. If chain requirements are posted, then this is a good indication that things are not rosey for this grade climb. More importantly, slippery road surfaces could stall you on the climb if you don't have chain or cable traction devices for your drive wheels.

Remember, it takes torque to get a load moving, but it takes power to accelerate the load to speed. During the climb, your upshifts will have to be precisely executed. A missed upshift will cause a severe loss of momentum, and could even result in a stall on the grade, or at least a lost gear down instead of a new gear up.

If you don't have your act together, you will fail to match the next gear before your RPMs have gotten to low, and then you are in serious trouble. Some drivers report that using the jake brake while double-clutching on a grade, forces faster engine drops while double-clutching, which allows an easier gear match. None of us have tried this, but this sounds reasonable since the truck speed is falling faster than normal, therefore it would be good to have the engine RPMs also falling faster than normal.

Upshifting on a grade is entirely different than normal shifts. You absolutely must use the clutch while upshifting on a grade, no matter how good of a driver you are. On level ground you can unload engine torque and match the next gear without the clutch (if you are good). On the grade, this no clutch approach is very risky. Truck speed is falling off rapidly, and if you are using the jake brake retarder, then the engine RPMs are also falling rapidly. Chances are against your engaging that gear with a perfect torque and power match. This mismatch will result in a severe shock to the transmission or drive train at the least, and broken parts in the worst case. Use of the clutch will allow the clutch and clutch disc torsional springs to absorb some of this mismatch, with much less chance of transmission and drive line damage.

While building speed in the beginning of the climb, you will reach a gear in which your engine very slowly approaches the governor RPMs. This will be your top gear for this climb. As long as each up shift results is rapid RPM building, then you can pick up another gear when you reach the governors. Now that you have reached your top gear for this climb, the engineers would say to ease off the power and return to the sweet spot of engine efficiency which is midway between your engine's maximum torque RPM rating and maximum power RPM rating. This sweet spot might work on a dyno, but on a grade, don't you believe it.

Momentum is your friend, the more you have , the better off you are. Grade slopes are seldom consistent during a climb. Some sections of the climb will be steeper than other sections. By building all the momentum that you can, you have room for more pull down when the grade gets steeper. So you want to keep the engine up against the governors during the climb, and you do not want to cut back to that sweet spot RPM that you would look for on level road.

If you enter the grade on a fast roll, then the vehicle speed will fall off as the climb increases. Because torque keeps the load moving, let the engine RPMs fall down to the max torque range to see if the speed will hold. If the speed continues to fall, then drop a gear and repeat the process.

Of course, hot weather climbs will be more dependent upon an adequate cooling system. If you see your temperature gauge approaching the boiling point for your coolant (225 degrees), then you should drop a gear to ease the thermal stress. If your transmission or drive axle(s) are getting up around 300 degrees, then dropping a gear is required to reduce those temperatures also. Dropping a gear will result in the truck moving slower for the same engine RPMs. This means that less work is being done by the engine per unit of time, which places less work demand upon the engine, transmission, and drive axle(s). This gear drop may be enough to limit the over temperature condition for the remainder of the grade climb. If not, drop another gear.

Descending the Grade

Ok, we got up there, and now we must get back down. What goes up, must come down. The important thing is to remain in control while it comes down. This is one place where you do not want to be out of control. It is important to set up things before you start that down grade roll.

Another important factor is your experience with the down hill grade. Have you been down it before? If you haven't, don't make assumptions. Don't assume that since it was a straight climb up that it will also be a straight descent going back down. It may be curvy on the way down. A little over-speed run-away on a straight down hill is not so deadly, but the same over-speed on a curvy down hill could terminate your trucking career early.

Another important factor for your down hill run is the surface conditions of the road. If there are slippery spots, then the jake brake will have to be terminated to prevent jackknife. This could mess up your entire plan for keeping in control of your down hill speed.

If the down hill grade is similar to the up hill grade, then a general rule of thumb is to drop two gears below your top gear up. This will typically allow for a safe and conservative pass down the grade. To be a little riskier, drop one gear lower than the top gear up. So, if the climb top gear was 5th, then it is safe to use 3rd gear going down, and less safe using 4th gear going down. This is if you do not have a retarder such as a jake brake. If you choose to let it all hang out on the retarder, then select one gear higher than your top gear going up. The problem with this approach is that should the jake brake fail, or if the road surface becomes slippery, then your speed and gearing is to high to regain control using just your service brakes. You are in deep trouble at this point in time.

Once again the down hill grade will not be the same all the way down. Do not be fooled into going up one gear, only to find out later that you had not reached the maximum grade yet. Once you realize that the grade has steepened, you are already pushing to many RPMs for a down shift back to where you should have stayed.

Another false sense of security is when you see other rigs passing you by. Take a careful look at the trailer bounce factor. If the trailer is bouncing, then they are empty. Do you think your loaded rig will be as easy to control as an empty rig? Of course not; so pay attention and do not get fooled into over-extending yourself with false signals from passing rigs.

What would you do if you lost your retarder during a down grade? What you do is your business, but what I would do is to use short term heavy service braking, and then with reduced speed, drop to the appropriate lower gear to help hold my speed for the rest of the down grade. If you don't get into lower gears, then things are only going to get worse. Of course your service brakes may not be able to reduce your speed sufficiently to regain control. Do not under any circumstances maintain light service brake application for extended periods of time. This will over heat your brake drums, resulting in brake fade, which terminates the effectiveness of your service brakes. Some run-away drivers have found soft shoulders which would not tip the rig over and have regained control that way. Some claim to have ground against the side of the rocks to loose speed (and gain severe damage to the rig) and save their lives. We are not saying you should try any of these things. We are just passing on information which has been spoken by others.

Engine RPM determines the hold back capability of the engine with or without a retarder. The retarder improves this hold back capacity beyond engine RPM alone. The spinning engine without a retarder still has friction losses which provides hold back energy dissipation. The trick is to match your gears where the spinning friction energy requirement is enough to hold back the vehicle. Because this friction energy is slight, higher gears will not do the job. You must get your rig slowed down enough so that the lower gears will provide adequate hold back energy dissipation.

It is not recommended to change gears during your down grade. A missed shift could result in a neutral gear, wild ride all the way to the bottom. Never, ever, purposefully place your truck in neutral while transiting a down grade. It is also bad business to hold the clutch released during the down grade, even for a short period of time. A released clutch can allow over RPM of the transmission input shaft (for that gear), which will over RPM the clutch disc(s) which can explode the clutch disk(s) resulting in no engine hold back for the rest of your exciting ride down the grade.

You must set up your gear choices to meet your requirements before you start the down grade. The gear selections referenced above would result in maximum spinning friction of the engine, and maximum braking action of your retarder (retarders mounted to the engine, that is). If you have an automatic fan clutch, manual activation of the fan will add a slight increase to the hold back power of the engine. The air compressor on your engine will also increase the engine hold back power when the compressor is engaged. Some drivers feel that in down hill runaway situations where your speed is building slowly, you might want to consider wasting enough air pressure to keep the compressor engaged. This could be just enough to regain control, but don't count on it.

In closing, you must always remember .... never use extended light service brake action. Braking causes heat, and prolonged light braking gets the brake drums real hot. Hot brake drums and brake linings have far less braking effectiveness. Brake drums dissipate heat to the outside ambient air temperature. The greater the difference between the brake drum and the ambient air temperature, the greater heat transfer from the drum to the air. Short heavy braking causes spiked brake drum temperatures, but these spiked temperatures dissipate heat more rapidly. Continued light braking keeps adding heat to the brake drums, so they never get a chance to recover through dissipation of the heat.

All of the information provided in this topic should be considered and then you should make your own choices about how to get across that grade. You are the only one who knows and understands your particular situation. This topic is not a step by step guide to keep you out of harms way. Only you can make the necessary judgement calls to keep yourself out of harms way. Good luck and keep in control on that down hill.

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