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Old 11-12-2021, 12:23 AM   #1
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Mountain Driving and Downhill Braking

Hi yall - new to the forum and RV driving. Have a lot paranoia related to mountain driving and brake glazing. We have an 38' RV converted from a school bus. We're about 26000lbs, Cummins ISB5.9 with an Allison 4 speed transmission, air brakes, but no jake or exhaust brakes. We're heading thru AZ later this month and will be driving on I17 to and from Camp Verde up to the Cottonwoods Campgrounds. The Mountain Directory says '6% grade for 7 miles". Do yall have suggestions for safe speeds/gear (2nd or 3rd) to maintain for that length of descent with my rig capabilities and any other strategies for navigating these and future downhills?

Any advice will be greatly appreciated. Thanks in advance.

-Anand
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Old 11-12-2021, 04:21 AM   #2
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Stay in a lower gear. Do not ride the brakes. Occasionally press down to slow momentum and release.
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Old 11-12-2021, 04:38 AM   #3
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You do not want to be the first to the top nor the first to the bottom. By that I mean drive slow and do not worry about other folks. Long time ago I was told that use one gear lower than the gear you used getting up the hill. Now that may not hold true all the time but it really makes sense.

You will be ok if you take it slow, you have to start slow instead of trying to catch up to slow. Its really a pro active thing. Down shift before you start the decline and as said do not ride the brakes. One more thing have you had your brakes checked for condition.

Good Luck safe travels
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Old 11-12-2021, 07:27 AM   #4
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All above gave you good advise. When I was in Army truck driving school, I was taught "Go down the hill in the same gear you went up." Good advise I used 50 years later and when in doubt I use a lower gear.
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Old 11-12-2021, 07:27 AM   #5
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Others made gear selection comments which are correct, when it comes to braking, let the Motorhome build up speed on the way down, when the RPMs get close to max, get on the brakes hard, slow down 15-20 mph, let the Motorhome build up speed again and repeat. You want to slow down hard and fast, give the brakes time to cool down between having to slow down 15-20. Biggest issue is people want to keep foot on brake and reduce speed 5 mph at a time. Brakes will get hot like that.

Brake hard and fast, then donít use until you get up to max rpmís again. Run your emergency flashers if you are under 45, which you most likely will be. If it is a windy road, brake before the corner, do not brake during the turn.

Donít get in a rush, only you know your comfort level and every mountain is different so you need to determine your required speed.
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Old 11-12-2021, 07:36 AM   #6
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I drive the same route for a quite a few years now with my motorhome pulling a trailer. My Allison transmission shifts down and I try to keep my speed to about 50 and don't allow the RPMS to go over 2700 RPMS and hit the brakes just in burst to keep within that range. The big thing is not to allow your MPH get to high as it's difficult to slow down once you get to high without overheating your brakes.
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Old 11-12-2021, 07:42 AM   #7
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Let's not forget the most important: Ensure the braking system is in good condition. Good pads, no leaks, air filters serviced, no water in the tanks.

When you see the grade sign, slow down before it starts to steepen. As others have said, downshift and let the motor drag slow you down.
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Old 11-12-2021, 07:43 AM   #8
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The "correct speed" on a grade is one that keeps your speed in EQUILIBRIUM-- that is you are neither speeding up nor slowing down without using the service brake (brake pedal).


That speed may be 15 or 50.


If you start speeding up, apply the service brake hard and long enough to allow you to drop to a lower gear.


Do NOT worry about going too slow. The loaded 18 wheelers will be slower-- just straight physics.
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Old 11-12-2021, 08:31 AM   #9
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Originally Posted by wolfe10 View Post
The "correct speed" on a grade is one that keeps your speed in EQUILIBRIUM-- that is you are neither speeding up nor slowing down without using the service brake (brake pedal).


That speed may be 15 or 50.


If you start speeding up, apply the service brake hard and long enough to allow you to drop to a lower gear.


Do NOT worry about going too slow. The loaded 18 wheelers will be slower-- just straight physics.


X2 on this. Dont try for a "target speed" ... instead go slower and slower (using lower gears) until you find that equilibrium where you dont speed up. I barely use my brakes at all going down mountain grades
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Old 11-12-2021, 11:02 AM   #10
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You can use your brakes. Thats what they are for. If you have air brakes be sure they are set up properly. A quick check if you have an application gauge (if you don't, install one. Its cheap and easy) is ten mph and a ten pound application should stop you in ten feet. And a hard application in neutral should show a drop in pressure of two pounds per axle in the primary tankif they are correctly adjusted. If all that is fine, you go down the same speed you went up, and never use more than ten pounds of air application. If you find yourself needing more you need to drop a gear. In theory ten pounds applied steadily will shed heat as it builds so you can leave your foot on the brake, but that assumes everything in the brake system is in perfect shape, and they are usually not on older vehicles. So ten pounds intermittently is safer. If you have hydraulic brakes you should go slower and make dang sure they are in perfect condition because they are not nearly as effective.
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Old 11-12-2021, 12:06 PM   #11
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I'm not really sure why I do this from time to time but I do! There are lots of techniques that are safe to descend a grade. However, there are ways htat work and there are BETTER ways.

I think most of you will agree that Wyoming is a mountain state and has lots of steep grades that large vehicle navigate. This is a QUOTE from their CDL drivers manual:

"2.16.4 – Proper Braking Technique

Remember. The use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following are the proper braking techniques:

1. Apply the brakes just hard enough to feel a definite slowdown;

2. When your speed has been reduced to approximately five mph below your “safe” speed, release the brakes. (This brake application should last for about three seconds.);

3. When your speed has increased to your “safe” speed, repeat steps 1 and 2. For example, if your “safe” speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.
"

Notice that the CDL manual uses the word GRADUAL in reducing excess speed. Consider this, a vehicle starts downs a grade at 40 MPH. It is going 40 MPH at the bottom of the hill and the average speed is 40 MPH. Assuming that there is no braking except the friction brakes the amount of BTUs that the brakes must dissipate is the same regardless of the braking technique whether it be one long applications for the complete grade, a few HARD applications or a few moderate applications. As an engineer I know this to be true. I also know that HARD braking produces the highest peak temperatures in the brakes. Using the one long application may result in one wheel doing most of the braking especially in air brake systems. SOOO, the "proper" technique is to apply moderate brake applications that cause a gradual speed reduction as necessary.

I don't like the HARD braking technique. Hard anything has a downside. Hard braking is detrimental to your brake system, it adds stress to the suspension, it adds stress to the coach, the dishes, the dog on the couch not to mention the DW. Moderate braking is much easier on all of that and reduces the PEAK temperature of your brakes.

I'm not interested is changing anyone's mind if they already have a method that works for them. However, if someone is asking how...this is the BETTER way.

I live in Colorado, I grew up in Wyoming, driving mountain grades is a way of life for me.

Someone said in this thread that hydraulic brakes are not as effective as air brakes. I respectfully disagree with them. A fully loaded Boing 777 weighs ~750,000 lbs. It has hydraulic brakes and stops from landing speed of ~ 155 MPH with no problems. A Prevost motorcoach has hydraulic brakes. A properly designed and sized hydraulic brake system is every bit as effective as air brakes.

If your motorhome has brakes that are adequately designed and sized for your vehicle weight they both work well.
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Old 11-12-2021, 12:13 PM   #12
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We are saying the same thing except I quantified how hard to apply the brakes. My experience is over a million safe miles in class 8 trucks grossing over 140,000 pounds in British Columbia.
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Old 11-12-2021, 12:21 PM   #13
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We are saying the same thing except I quantified how hard to apply the brakes. My experience is over a million safe miles in class 8 trucks grossing over 140,000 pounds in British Columbia.

Guess I am puzzled. Not aware of many DP's with pressure gauge on brake circuit.


I still feel the best advice for beginners is to not count on the service brakes, but use engine braking/transmission gears to basically control rate of descent.


Use the service brakes if you need to get to a lower gear.
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Old 11-12-2021, 12:53 PM   #14
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We are saying the same thing except I quantified how hard to apply the brakes. My experience is over a million safe miles in class 8 trucks grossing over 140,000 pounds in British Columbia.

And there are lots and lots of mountains, some with very steep grades too!

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