Community Forums

Steve Ownby · 09-21-2013, 08:21 PM

Quote:

Originally Posted by EricGT

This is probably a stupid question, and will surely get answered if I take a course, but how do the air brakes work if stuck in stop and go traffic? How do you keep the pressure from bleeding out? A lot of my RV'ing involves stop and go traffic.

Not stupid at all. No one is born knowing about air brakes.

When you apply the brakes and hold them as at a traffic light, the air charges the brake chambers and holding the brake treadle down keeps the air pressurized in the brake chambers. This causes a drop in pressure when the brakes are applied but there is no further pressure drop as you hold the treadle unless you have a leak. When the brakes are released the air pressure is released from the brake chambers. During this time, if the pressure in the air supply system drops below the governor cut in pressure then the compressor kicks in and sends air pressure to the wet side of the front air tank which builds pressure in the system back to the fully charged pressure of 120-130 psi. Unless you have a leak or air supply malfunction the need for air will not exceed the ability of the system to provide air.

ShapeShifter · 09-22-2013, 07:52 AM

Steve has it right on.

To amplify it a bit...

Pumping air brakes is bad. Stop and go is not a problem. Both involve frequent brake applications, so what's the difference? Timing.

With hydraulic brakes, each time you press the brake pedal, some fluid is pumped from the master cylinder reservoir to the brake pistons. When you release the pedal, that fluid returns back to the reservoir. It's a closed system, and you never lose any fluid unless there is a leak or other failure.

With air brakes, each time you press the brake pedal, some air flows from the air tanks to the brake cans. When you release the pedal, that air is vented to the atmosphere. It's an open system, so you could theoretically run out of air. However, the engine driven air compressor will start pumping when the pressure gets low so that the tanks are replenished.

In stop and go, even though you are using the brakes a lot, the compressor has enough capacity to keep the tanks pressurized. But if you pump air brakes rapidly (as we used to be taught to do in our cars in slippery conditions) you could be releasing so much air so fast that the compressor can't keep up. The bad news is that if you use too much air and lose too much air pressure, there won't be enough air to work the service brakes. The good news is that if the air pressure gets too low, there is a dashboard warning light and audible alarm to warn you. The better news is that if you lose even more air pressure, the parking brake is automatically applied.

While the service brakes need air pressure to apply them, and releasing pressure releases them, the parking brake is just the opposite: apply air pressure to release them, release the air pressure to set them. Heavy springs are always trying to apply the brakes. Air pressure is needed to push against the springs to release the brakes. If there is no air pressure, the springs apply the parking brake.

There are some significant differences in the way that air and hydraulic brakes work. Air brakes are much more complicated, but also much more powerful. But for the most part, all of those differences are behind the scenes: With both systems you press the brake pedal to apply the brakes, and release the pedal to release them. The harder you press, the harder the brakes are applied. The feel is different, but how you use them is essentially the same.

EricGT · 09-23-2013, 06:21 AM

That really helps answer a lot. Thanks for the great post!

Medico · 09-23-2013, 02:47 PM

It's basically a matter of driving your rig under controlled conditions for a while to get accustomed to how they work.

Just realize, as has been eloquently stated previously, if you loose your air pressure, the parking brakes will be applied FULLY. This is what happens when you pull that little yellow knob. It also happens if you loose air pressure in the rear system (The parking brakes are on the rear axle). This brings up another point that probably has been covered. Air brake systems are actually 2 systems with 2 separate air tanks. The front and rear systems are independent of each other for the most part.

EricGT · 09-25-2013, 05:21 AM

Well, it appears to me if money were not an object (well within reason) that a diesel is a no brainer. There is nothing a gas unit does better than save $$$. And that appears to be negligible in most instances.
Are there any videos out there one could recommend to the basics of a dp? I have watched class a videos, but even so dp seems to have their nuances too.
It appears I will be waiting for a good deal on an early 2000's Winnebago Journey 36'. They seem to have the best floorplan for me.
Lastly, what is the minimum hp one would get on a 34-36' dp? I see some of the late 90's dp's have 250-275 hp. Is that enough? Or will the engine be maxed out when towing 4500# and 6 adults inside?

Steve Ownby · 09-25-2013, 05:35 AM

Quote:

Originally Posted by EricGT

Well, it appears to me if money were not an object (well within reason) that a diesel is a no brainer. There is nothing a gas unit does better than save $$$. And that appears to be negligible in most instances. Are there any videos out there one could recommend to the basics of a dp? I have watched class a videos, but even so dp seems to have their nuances too. It appears I will be waiting for a good deal on an early 2000's Winnebago Journey 36'. They seem to have the best floorplan for me. Lastly, what is the minimum hp one would get on a 34-36' dp? I see some of the late 90's dp's have 250-275 hp. Is that enough? Or will the engine be maxed out when towing 4500# and 6 adults inside?

Here are a series on videos you might find helpful:

http://betterrving.com/rv-classroom/drivers-confidence/

The benchmark for hp in a motorhome is 1 hp per 100 pounds of GVWR. If you stay above that threshold you should be fine. Journeys of that vintage were powered by Cummins ISBs and Cat 3126 or the later C7. In that time frame both engines should be rated at 275 hp or more. If you can find a coach with the Cummins ISC, that's a great motorhome engine. The Cummins will be the economy champ and the Cat will be more powerful. Hold out for the Allison 3000 transmission. The Cat will have that for sure but the ISBs were some times equipped with one of the smaller Allison's. The 3000 will be a 6 speed the others may not.

In that era you might find a nice Dutchstar or Monaco Diplomat.

ShapeShifter · 09-25-2013, 05:48 AM

Minimum horsepower? That's a tough one. So much depends on your expectation and driving style.

If you drive a subcompact economy car with a tiny engine, your horsepower needs will be less than if your daily driver is a muscle car with a supercharger and nitrous. A rule of thumb I've heard is around 1 horsepower for every 100 pounds. It seems like a reasonable starting point to me. Some will be very happy with less, others will want more. You'll get all sorts of opinions.

I have 400 HP on my 40 foot rig, which when towing my heavy truck is right about 40,000 pounds - right at the rule of thumb number of 1 HP per 100 pounds. While no speed demon, it's quite drivable. On level ground it does well, on the mild hills on interstates I have no trouble maintaining speed (but can't really accelerate) and can do better on the hills than most semi trucks (but most cars blow right past me.) On the steeper hills, especially on the back roads around me, my recurring thought is "I think I can, I think I can" as my speed drops right down to the 30 to 40 range. I'm sure I'd be thrilled with 500 HP, until I get to the fuel pump! I'm happy with the 400.

When not towing, I'm closer to 1 HP per 85 pounds, and while no sports car, it is more than responsive and driveable. If I never towed, I'm sure I would be more than happy with less power.

EricGT · 09-25-2013, 09:38 AM

Quote:

Originally Posted by Steve Ownby

The benchmark for hp in a motorhome is 1 hp per 100 pounds of GVWR. If you stay above that threshold you should be fine. Journeys of that vintage were powered by Cummins ISBs and Cat 3126 or the later C7. In that time frame both engines should be rated at 275 hp or more. If you can find a coach with the Cummins ISC, that's a great motorhome engine. The Cummins will be the economy champ and the Cat will be more powerful. Hold out for the Allison 3000 transmission. The Cat will have that for sure but the ISBs were some times equipped with one of the smaller Allison's. The 3000 will be a 6 speed the others may not.

In that era you might find a nice Dutchstar or Monaco Diplomat.

I just printed this out for future reference.

EricGT · 09-25-2013, 09:39 AM

Quote:

Originally Posted by ShapeShifter

Minimum horsepower? That's a tough one. So much depends on your expectation and driving style.

If you drive a subcompact economy car with a tiny engine, your horsepower needs will be less than if your daily driver is a muscle car with a supercharger and nitrous.

My current car is over 700 engine hp, blown.

ShapeShifter · 09-25-2013, 09:48 AM

Quote:

Originally Posted by EricGT

My current car is over 700 engine hp, blown.

I don't think you're gonna be happy with ANY motorhome!

JFXG · 09-25-2013, 09:49 AM

Quote:

Originally Posted by EricGT

........It appears I will be waiting for a good deal on an early 2000's Winnebago Journey 36'. They seem to have the best floorplan for me.
Lastly, what is the minimum hp one would get on a 34-36' dp? I see some of the late 90's dp's have 250-275 hp. Is that enough? Or will the engine be maxed out when towing 4500# and 6 adults inside?

A) Journey 36': In that size range, check the wheelbase as you shop. You may have a longer, heavier chassis with a wheelbase in the 270-280 inch range. Or, your MH could be built on a smaller chassis, with lesser weight capacity and a shorter wheelbase in the 240 inch range. Go with the bigger chassis if at all possible. I would even advise to bump up your target size to 38-40 ft in order to get it. It will make a huge difference to you in terms of carrying capacity, and in terms of ride quality and stability.

B) My Dutch Star weighs about two large orders of fries short of 31000lbs,

and I tow about 3500lbs. My Cat 3126 is rated at 350hp, so I'm right at the 1hp/100lb point. Generally I'm fine with it and have no complaints. As Clint famously said: "A man's gotta know his limitations." The only times my HP is even noticeable are circumstances that require a lot of excess HP, like accelerating on an upslope highway on ramp, or climbing long, steep grades. Neither of these is a large or dangerous problem when properly handled. On long grades you learn to live life at 40, and keep your RPMs at optimal for good cooling.

ShapeShifter · 09-25-2013, 09:53 AM

Just keep in mind that HP is only part of the equation; the other half is torque, which is the low-end grunt what gets you going. Compared to a gasser, I think you'll find that any diesel has lots more torque.

My 400 horse Cummins has 1200 ftlbs torque - pulling away from a stop light is never an issue. (But trying to speed up to pass a truck on the highway is a whole 'nother matter...)

ARTWERKE · 09-25-2013, 03:11 PM

Eric I wouldn't let air brakes worry me, the difference in stopping isn't much different than going from non power hydraulics to power brake hydraulics, but you may be to young to have made that switch. The driveability difference from a gasser to diesel pusher is amazing.Journeys are nice, but like Steve said you might find a Dutch Star,they all have side radiators, if you like to work on things yourself ,that's worth tons. Try a diesel you'll have the hook set in 10 miles.

EricGT · 09-26-2013, 05:26 AM

Quote:

Originally Posted by Steve Ownby

In that era you might find a nice Dutchstar or Monaco Diplomat.

These are both very nice motorhomes! I really like the Newmar and am finding very nice examples for $40-50K. The Newmar reminds me of back when I was shopping for my class c and the reputation of the Lazy Daze. It also appears that most Newmar Dutch Stars have been kept in nice condition.

Quote:

Originally Posted by ARTWERKE

The driveability difference from a gasser to diesel pusher is amazing....Try a diesel you'll have the hook set in 10 miles.

This is what I am thinking. It is not as if I need the extra towing capablity. At most, I wil be towing a Wrangler. However the ride, lower noise levels, air brakes, and air suspension are the major reasons to choosing a diesel.