Are large gas motor Rv's underpower?

[invmartyc]

At the risk of getting someone angry for me asking the same question they don't want to see again, that no doubt some people feel has been discussed enough.

Are gas Class A's underpowered to tow a toad in any area with mountains or large hills.

I have read people with gassers say that they are powerful enough for them to tow a toad anywhere. And I have read that no gas motor Class A has the power and that you need a diesel motor.

What is the truth, not opinion on this?

[Bumpyroad]

Empty Nest, Soon wrote:

JayGee wrote:
A five mile hill at 60 mph takes 5 minutes,
at 30 mph it takes 10 minutes.

No worries.

Exactly!

Are gas motorhomes underpowered? How do you define “underpowered”? From a power-to-weight standpoint, any motorhome is not the ride you want for grudge night at the drag strip.

How fast do you want to climb that hill? How much extra fuel do you want to burn to go faster up that hill? Any modern motorhome will get you to the top. And traveling 30 or 35 instead of 55 or 65 up those hills will not cost you very much time by the end of the day.

But the more you mash down on that peddle on the right, the more fuel you will burn.

Life is full of choices. This is one more.

Wayne

I don't plan on hitting that hill at 30 mph. probably speed up to 60 or more at the base.
bumpy

[Empty_Nest__Soo]

JayGee wrote:
A five mile hill at 60 mph takes 5 minutes,
at 30 mph it takes 10 minutes.

No worries.

Exactly!

Are gas motorhomes underpowered? How do you define “underpowered”? From a power-to-weight standpoint, any motorhome is not the ride you want for grudge night at the drag strip.

How fast do you want to climb that hill? How much extra fuel do you want to burn to go faster up that hill? Any modern motorhome will get you to the top. And traveling 30 or 35 instead of 55 or 65 up those hills will not cost you very much time by the end of the day.

But the more you mash down on that peddle on the right, the more fuel you will burn.

Life is full of choices. This is one more.

Wayne

[rgatijnet1]

Mr.Mark wrote:
Rgatijnet1, our coach should be at the paint factory as the converter should have received it at the end of May. It takes a month to paint the exterior than back to the converter to made into a motorhome.

So, the rear end is already installed. The coach was driven from Canada as an empty shell with the four slides installed at the Prevost factory.

Whatever Prevost put in I'll just leave it as is as I wouldn't want to void the chassis or Volvo/Allison warranty's.

Learned a lot about Torque, RPM's and HP... thanks everybody.

MM.

I'm sure that it will be great. Prevost is top of the line and from the one that I have driven a thousand miles or so, they are in a class of their own. I envy you. :B

[Mr_Mark1]

Rgatijnet1, our coach should be at the paint factory as the converter should have received it at the end of May. It takes a month to paint the exterior than back to the converter to made into a motorhome.

So, the rear end is already installed. The coach was driven from Canada as an empty shell with the four slides installed at the Prevost factory.

Whatever Prevost put in I'll just leave it as is as I wouldn't want to void the chassis or Volvo/Allison warranty's.

Learned a lot about Torque, RPM's and HP... thanks everybody.

MM.

[Daveinet]

valhalla360 wrote:
Of course in terms of a MH climbing a mountain, if you run out of traction, you have other issues to worry about other than can you maintain 60mph.

That's a problem I have in the rain. I can't floor it at freeway speeds without spinning the tires in the wet.

[valhalla360]

rgatijnet1 wrote:

valhalla360 wrote:

Mr.Mark wrote:
Valhallo360, I had changed my post slightly before yours came up. The John Deere was digging itself into a whole while the steam engine tractor was pulling it. It was good video!

MM.

In that case traction was definetly the issue. Once the wheels start spinning, you aren't able to harness either the HP or the torque.

The rear wheels on those old steam tractors probably literally weigh a ton each, let alone everything else built out of heavy solid metal pieces.

A similar analogy would be to adapt the engine out of a big diesel pusher and put it into a smart car and put it up against a 1 ton pickup with the base gas engine. I can guarantee the pickup would hardly notice the smart car back there even though smart car would easily have more HP and torque.

I think that you will find is that with a steam engine, the power is in the pressure in the boiler, which is already built up to the maximum, before the drive train is put in to motion. It is not like a gasoline or diesel engine that has to develop RPM's before it can start delivering power. As soon as the steam pressure is released to the piston, all of the power is there, even at 1 RPM. The same goes for certain electric motors, which is why they are used in heavy locomotives. The diesel engine in a train powers the generator which powers the electric drive motor that makes the train move. The full power of the electric motor is there to apply to the drive wheels, right from a standing start. It, like the steam engine, does not have to develop any RPM's to have the power to get a huge train in motion.

That may or may not be the case. Steam engines also benefit from momentum which is why they often have large flywheels. In fact if the engine is only 1 or 2 cyclinder, at very low RPM, it may have almost no torque while the piston rod is in line with the crank shaft (or close to it).

So it is similar to IC engines though to a lesser degree but once the JD tires start spinning, it doesn't matter how much HP or torque the engine can supply at the wheel, there is no traction so it can't harness the power and you can't determine which engine/transmission would win.

Of course in terms of a MH climbing a mountain, if you run out of traction, you have other issues to worry about other than can you maintain 60mph.

[bobman]

JayGee wrote:
A five mile hill at 60 mph takes 5 minutes,
at 30 mph it takes 10 minutes.

No worries.

Most sensible post in the thread

[JayGee]

A five mile hill at 60 mph takes 5 minutes,
at 30 mph it takes 10 minutes.

No worries.

[rgatijnet1]

valhalla360 wrote:

Mr.Mark wrote:
Valhallo360, I had changed my post slightly before yours came up. The John Deere was digging itself into a whole while the steam engine tractor was pulling it. It was good video!

MM.

In that case traction was definetly the issue. Once the wheels start spinning, you aren't able to harness either the HP or the torque.

The rear wheels on those old steam tractors probably literally weigh a ton each, let alone everything else built out of heavy solid metal pieces.

A similar analogy would be to adapt the engine out of a big diesel pusher and put it into a smart car and put it up against a 1 ton pickup with the base gas engine. I can guarantee the pickup would hardly notice the smart car back there even though smart car would easily have more HP and torque.

I think that you will find is that with a steam engine, the power is in the pressure in the boiler, which is already built up to the maximum, before the drive train is put in to motion. It is not like a gasoline or diesel engine that has to develop RPM's before it can start delivering power. As soon as the steam pressure is released to the piston, all of the power is there, even at 1 RPM. The same goes for certain electric motors, which is why they are used in heavy locomotives. The diesel engine in a train powers the generator which powers the electric drive motor that makes the train move. The full power of the electric motor is there to apply to the drive wheels, right from a standing start. It, like the steam engine, does not have to develop any RPM's to have the power to get a huge train in motion.

[valhalla360]

Mr.Mark wrote:
Valhallo360, I had changed my post slightly before yours came up. The John Deere was digging itself into a whole while the steam engine tractor was pulling it. It was good video!

MM.

In that case traction was definetly the issue. Once the wheels start spinning, you aren't able to harness either the HP or the torque.

The rear wheels on those old steam tractors probably literally weigh a ton each, let alone everything else built out of heavy solid metal pieces.

A similar analogy would be to adapt the engine out of a big diesel pusher and put it into a smart car and put it up against a 1 ton pickup with the base gas engine. I can guarantee the pickup would hardly notice the smart car back there even though smart car would easily have more HP and torque.

[rgatijnet1]

Mr.Mark wrote:



Rgatijnet1, yes, I understand what you are saying about the gear ratio. I've never seen it as an option on any coach, but I could be wrong.

I saw a video of a new John Deere (?) diesel farm tractor on a pull-off contest with an old steam engine farm tractor. The steam engine tractor pulled the four wheel drive John Deere with the JD going at full throttle while it dug itself into the ground with the big-treaded tires (they were attached back to back). The torque on the steam engine was unbeatable. The guy said the steam engine gear ratio had to be set for pulling OR speed.

MM.

I remember seeing that video also and it was sort of neat to watch the steam powered tractor.
Since your new coach is being built on a Prevost chassis, I assume that you can pick the rear axle ratio. Most Prevosts are built as busses and the rear axle ratio is determined by the route/use that they are to be used. For instance a cross country bus will have a different final gear ratio than a city use bus. The same goes for a bus to be used in an Eastern flat route versus a bus to be used on a Western mountain route. The bus companies are looking for a compromise that will give them adequate performance with maximum fuel mileage. If it is not too late, you may want to ask about what gear ratio is being put in your coach. That is why I was saying that what you heard about the performance about the different engines, and torque, since they were so close, is really dependent on the axle ratio as to which one will get you to the top of the hill the quickest. The final axle ratio on my gas coach is so high(5.38:1) that I can easily pass most coaches going up a hill but I am limited to about 80 MPH top speed, by the ECM. If you have watched some of the Greyhound/Trailways buses out West, on the open highway, you will see that 80 MPH is just loafing along for them.

[Mr_Mark1]

Valhallo360, I had changed my post slightly before yours came up. The John Deere was digging itself into a whole while the steam engine tractor was pulling it. It was good video!

MM.

[DanTheRVMan]

Daveinet wrote:

DanTheRVMan wrote:
How fast you accelerate to that max velocity up hill is a function of torque and weight.

I'm not sure I agree with this statement, because it makes an assumption about RPM. All the torque in a world is nothing without RPM. Back in the day, I could leg press over 1000 lbs, but give me a bicycle pedals and hook me up to your drive shaft, and I'm not going to accelerate your 30K lbs motorhome. Leg pressing 1000 lbs was very slow, so I would be limited by RPM. Since the RPM is limited, there would be no RPM room for torque multiplication to increase the torque to the drive wheels.

Basically it comes down to the fact that one can not quantify any performance without both torque and RPM. Torque alone does not quantify the actual output. That is why the quantification of HP exists as it quantifies total output.

I probably should have said torque at the wheel, but from a practical view point you can get an isb and isl engine with an allison 3000 transmission which means they have similar gearing depending on other variations in the drive line. The higher torque will get it accelerating quicker which is all Mr. Mark really wants to know.


If you want to accelerate thru several gears you keep shifting back down to low rpm at each shift point and that is where low end engine torque helps.

Everybody is going to go up the hill and most of the time maintaining speed not accelerating.

Most DP benefits IMO are about air ride, exhaust brake, fueling in easy to maneuver truck lanes, and quieter ride. Not the power that is constantly debated. Most MHs out west are gas that go up every mountain just like the DP.

[valhalla360]

Mr.Mark wrote:

rgatijnet1 wrote:

Mr.Mark wrote:

rgatijnet1 wrote:

Mr.Mark wrote:

the silverback wrote:
I designed engines for 35 years. It is about horsepower and weight of the Motorhome. NOT TORQUE!! Except the more torque means more horsepower. So if a gas MH has a good horsepower to weight ratio it will do just fine.

Mr. Silverback, I'm certainly not questioning your advice because you probably have forgotten more than I'll ever know about engines.

But, you are the first person I've seen that says 'not torque' is not a major component to power an engine. All the diesel converstaion I've read about engines is torque, torque, torque.

For instance, on the new motorhome that we have ordered, the engine has been replaced from a Detroit with 515 hp and 1,650 lb. ft. of torque to a Volvo 500 hp with 1,750 lb. ft. of torque. The Prevost guys say that I should be able to tell a difference on the increase ability to climb steep hills. That's what they are telling me if I understood them correctly.

MM.

Torque at the rear wheels, which is all that really matters, is a function of gearing, which is why a hand held 1" air ratchet can produce 2000+ foot pounds of torque in a truck shop. Plenty of torque with the air ratchet but it won't get you up a hill very fast. :B
Then you talked to a salesman and there is the old adage about if his mouth was moving he was lying.

Honestly, I don't know what that means about the air ratchet compared to a diesel engine. The info I got is from the Prevost sight where all the gear-heads hang out, they are not salesman but very intelligent folks (apparently, they have me fooled) LOL!

MM.

What it means is to ask about the gear ratio on the different rigs. Same engine and same amount of engine torque can climb hills much differently depending on the rear axle ratio. If one coach is geared for mountains and the other is geared for top speed, the engine can be the same and the one that is geared for the mountains will beat the other to the top every time. One axle ratio can give you much better mileage and another gear ratio can give you much better hill climbing ability, all with the same engine horsepower and torque. The axle ratio changes the amount of power that is available at the rear wheels.

Rgatijnet1, yes, I understand what you are saying about the gear ratio. I've never seen it as an option on any coach, but I could be wrong.

I saw a video of a new John Deere (?) diesel farm tractor on a pull-off contest with an old steam engine farm tractor. The steam engine tractor pulled the four wheel drive John Deere with the JD going at full throttle (they were attached back to back). The torque on the steam engine was unbeatable. The guy said the steam engine gear ratio had to be set for pulling OR speed.

MM.

Your example fits perfectly with my statement of:
- torque determines if you can get up the hill.
- HP determines how fast.

Most likely, the steam tractor simply had more torque at the wheels via lower gears but it probably never attained any significant speed.

There also may have been secondary issues. Old steam tractors are typically very heavy and it probably had excellent traction even if only 2wheel drive. If the JD's wheels started spinning, it may have lost due to traction not because it didn't have more pulling power.

[valhalla360]

DanTheRVMan wrote:

Mr.Mark wrote:

d1725829 wrote:
My opinion, does it really matter how fast you get there as long as you get there ? I go for the thrill, I am in no hurry !

What my concern has been is that when we get stuck behind a very slow moving semi and we fall to their speed of 35 mph going up a steep hill, will I have enough 'umpth' to pass him without causing a major slow-down in the passing lane.

I'm not wanting to race anybody, I just want the ability to pass within reason instead of being stuck behind a very slow truck.

MM.

You should read http://prevostcommunity.com/PDF/Motor%20Home%20Fuel%20economy.pdf

max speed up hill is determined by weight/hp

torque is not a factor for steady state velocity up hill

GETTING to max velocity is determined by how fast you accelerate

For the same mass the more Torque the faster you will accelerate (F=MA)

So torque will get you to accelerate faster around a slow moving truck.

But the max velocity up hill is still only governed only by weight/hp as you are not accelerating so torque is not an issue.

How fast you accelerate to that max velocity up hill is a function of torque and weight.

I am sure you will love your new toy.

How exactly does torque figure into the F=MA formula?

Torque is neither mass nor acceleration by itself.

To convert Torque to force, you have to divide by the length of the arm (or factor in the gear ratios).

Just to keep the math simple, lets assume:
- Diesel with 600ft-lbs torque at 1500rpm
- Gas Engine with 300-ft-lbs torque at 3000rpm.
- The gas engine is run at half (or is it twice) the gear ratio of the diesel, so the steady state speed is the same.
- Assume the engine torque for the diesel directly translates to the torque at the wheel and the wheel has a radius of 1 ft (no it's not realistic gearing but keeps the math simple)

The diesel has 600ft-lbs of torque at the wheel and when you accound for the length of the moment arm (1ft), it provides 600lbs of force pushing the rig forward.

The gas engine is only putting out 300ft-lbs of torque but the gearing doubles that at the wheels, resulting in 600ft-lbs at the wheel, which provides exactly the same 600lbs of force pushing the rig forward.

So for a given HP output (torque X rpm), F=MA turns out to be the same for both engines.