Down hill braking for a motorhome?

[dbrown048]

I will soon be purchasing a 36' Journey SG (GVWR-28,000#) with a 300hp Cummings diesel engine. I'm wondering how it will handle the decent on long, steep grades (like a 7% grade for 7 miles) and how people "gear down" to keep the coach under control without heavy braking. I much prefer to let the engine/transmission do the braking so I don't have to "smoke" my brakes.

I have pulled an 8000# travel trailer in the mountains and simply put the Suburban in low gear, turn on my flashers and make the decent at a very slow speed. This I am comfortable with. I am not comfortable with going down steep grades at 50+ mph - I feel that I'm outta control.

How do you guys (gals?) handle the long, steep decents? In your response's, if you would, please tell me the hp of your deisel. As I said, I will have a 300 hp Cummings, and I assume that it will handle differently than a 330 Cat or surely a 350 Cat.

THANKS for your help

 

[dbrown048]

I will soon be purchasing a 36' Journey SG (GVWR-28,000#) with a 300hp Cummings diesel engine. I'm wondering how it will handle the decent on long, steep grades (like a 7% grade for 7 miles) and how people "gear down" to keep the coach under control without heavy braking. I much prefer to let the engine/transmission do the braking so I don't have to "smoke" my brakes.

I have pulled an 8000# travel trailer in the mountains and simply put the Suburban in low gear, turn on my flashers and make the decent at a very slow speed. This I am comfortable with. I am not comfortable with going down steep grades at 50+ mph - I feel that I'm outta control.

How do you guys (gals?) handle the long, steep decents? In your response's, if you would, please tell me the hp of your deisel. As I said, I will have a 300 hp Cummings, and I assume that it will handle differently than a 330 Cat or surely a 350 Cat.

THANKS for your help

 

[AdrianLee]

dbrown -

Welcome to the forum -

You do pretty much the samething as with your TT. Crest the hill top at a reasonable speed preparing for the down hill run. If I am on a 2 lane road and do not know what the grade is on the other side I will crest the hill at about 25 mph.

You can down shift and use your PacBrake. If you start picking up speed and are not comfortable, take a stab with your service breaks to get your speed down, then down shift again. You need to read your manual because thee is a peak RPM you will not want to be at for a long period of time.

Someone posted somewhere recently that if you hit your service brakes for 2 seconds, allow 4 seconds for them to cool down before hitting them again. That is a pretty good rule of thumb IMHO. Watch your air pressure. It should be OK but keep an eye on it.

Do not use your cruise control. It will prevent your PacBrake from engaging as long as it is engaged.

It is kind of hairy the first few times but you will get the hang of it. I'm a flat lander and I doubt I ever will get used to it

 

[Joe-K-WO]

Howdy and congratulations on the soon-to-be new motorhome.

We've got a 36' Journey with the 330 HP Cat, but if your motorhome with the 300 HP Cummins comes with the exhaust brake feature your downhill decent procedure will be the same.

As you know from previous experience with the travel trailer, gearing down helps with braking. An engine is basically an air compressor. With a gas engine, when you gear down, it causes the engine to speed up pumping more air. Since the throttle is closed, the engine has an air restriction on the air entering the engine which creates the braking force. A diesel engine is different in that it does not have a throttle plate, therefore for braking resistance an exhaust brake is used. The exhaust brake is simply a valve which closes over the engine exhaust outlet restricting the flow of exhaust gas out of the engine. Again, this restriction creats an effective braking force.

As you decend any hill, the key is to use the engine to control the speed and not rely on the service brakes. For long decents, you should apply the exhaust brake which will automatically cause the transmission to downshift, or alternatively, you can manually downshift and then apply the exhaust brake. Once the exhaust brake is applied, watch your speed. If the exhaust brake is able to maintain a speed that you are comfortable with decending the hill and you are not gaining speed, you can continue down the hill.

If however, you find that the exhaust brake is not holding you back adequately and the motorhome is gaining speed, it is important to get the motorhome into the next lower gear. To do that, use the footbrake to slow the motorhome enough to allow the engine to downshift and again let the exhaust brake to do the braking.

What your goal should be is to decend the hill at a speed that allows the transmission to downshift enough to permit the exhaust brake to provide all the braking force necessary to safely decend the long grade.

With some experience in learning how your rig performs, you quickly know which gear to use on what hill. As an example, you might find that 3rd gear lets you decend a 7 % grade at a safe speed without any use of the footbrake. Many folks use the rule of thumb that as a starting point for a decent, you can use the same gear as required when you went up the hill.

 

[Cruzer-WO]

Joe's info on exhaust braking and tranny gears is excellent. I might add that "IF" you do need to use the service brakes, use them intermittently. If you crest a hill and look down and feel that you are going to be in trouble later on, hit the brakes hard right away to scrub off that speed. It's easier to scrub it off now rather than later on when it's trying to run away like a freight train. Scrub enough enough at the start so that you can "let 'er drift" and allow the speed to build somewhat without getting into the danger zone. Then let the tranny and exhaust brake do their things. If it's not enough, then apply the service brakes firmly for a short time to scrub off speed, then get off the pedal to let them cool. Repeat as necessary, but intermittently. Riding the brakes continually will heat them up and they won't be of much good to you later on.

 

[SargeW]

I have a 32' Itasca Meridian with the 300 HP Cummins and a 6 speed Allison. The Variable Geometry Turbo acts as the exhaust brake, and is awesome. The turbo and the Allison work together perfectly, and will easily hold me back on a 6% to 7% percent grade with my toad behind me. Follow Joe's advice closley and use the exhaust brake and you will be safe and comfortable on any grade.

The VGT Turbo brake on my MH works WAY better than the Pac Brake I had on my 04 Dodge Dually.

 

[mopargo]

I drive a 2000 Journey, 275 hp Cummins with an Allison 4-speed, on a steep downgrade that I'm not familiar with I crest the hill at forty with the exhaust brake on, downshift to 3rd gear, and proceed down the hill at a comfortable (to me) speed, using either second or third gear. Quite often I don't have to use the service brake's at all.