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Author Topic: How steep a grade can I climb?  (Read 3848 times)
paul102a3
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« on: May 20, 2009, 02:24:00 PM »

Hello All,

I recently purchased a 1988 MCI 102a3 powered by a 400 HP 892T DDEC 2, running through an Allison trans. The bus is converted since new and the previous owner claims it weighs in at 45,000 pounds. Since this is my first bus, I have no frame of reference as to it's ability to climb steep grades.

I would like to be able to store the bus behind my barn in NC however, I don't know if the bus will make it up the driveway. To enter the driveway, I must start out from a dead stop. The first section is a curved tar one lane, 25% grade that is about 300 feet long. The drive then flattens to a gravel one lane that maintains a 16% grade for the next 3.5 tenths of a mile. To make things more interesting, there is a 170 degree switchback about halfway up (I have measured the radius of the switchback and the bus has a much tighter turning radius so I will fine in that department).

Assuming I don't drag the tail of bus at the transition from the main road or highside it at the transition from the 25% to 16% grades, does anybody think the bus will have enough HP/torque to make it up the hill?

I know an overloaded big box 26ft Uhaul has made it up the hill as have fully loaded semis with a 40 foot trailer. I have been told 53" trailers can't get up the hill but I don't know why.

One last comment, to exit the property, I would need to back down the way I came as there is not enough room to turn around.

Any thoughts, ideas, suggestions would be appreciated. Should I just forget it or should I sell tickets to the show?

Thanks,

Paul

 

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« Reply #1 on: May 20, 2009, 02:45:04 PM »

I wanna watch the first trip up but I'd put money on you making it.  Getting back down is gonna be a huge problem though.  I'm guessing that automatics don't lock up in reverse so its gonna be pretty hard on your brakes. 
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TomC
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« Reply #2 on: May 20, 2009, 02:51:39 PM »

Using what Freightliner uses to figure out startability, your 400hp engine puts out about 1200lb/ft torque. With 3.38 ratio and the Allison HT740 with your coach weighing 45,000lb, that works out to be 30.1% startability.  You should be able to make it.  Good Luck, TomC
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JimGnitecki
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« Reply #3 on: May 20, 2009, 03:05:57 PM »

Paul: I am inexperienced on buses right now (I am a newbie), but I do have an engineering degree, and I have worked on vehicle and machinery drivetrains in the past, so let me take a stab at it, and maybe other smarter, more experienced people can correct any errors I might make.

The bus weighs 45,000 pounds and the maximum grade is 25%.

A 25% grade converts 25% of the bus's weight to rearward force when climbing, so that means a rearward force of 25% of 45,000 pounds = 11,250 pounds.

This dwarfs the force required to accelerate the bus from zero mph to say 5 or 10 mph (anything faster would be unsafe on a steep hill!). So, basically the question is can the combination of engine and gearing produce more than 11,250 pounds force.

You don't say what model of Allison, but 1st gear is going to be at LEAST a 3.0 multiplier of torque. (1st gear in any Allison specs I have seen has always been greater than 3.0).

You don't say what rear axle ratio is on the bus, but let's assume the WORST probable case, which would be ,maybe 2.75 if someone REALLY misgeared it.

I don't have a dyno charty for an 8V92T in front of me, and the dyno chart would not go low enough in rpm anyway (they commonly stop at 1200 ro 1000 rpm), but any turbocharged 8V92 has to make at LEAST 1000 ft lb of torque even at low rpm (the torque converter will ensure that you have at elast SOME rpm to work with!). Do NOT assume you will have the PEAK (published) torque at low rpm. The torque curve falls off BOTH at higher and lower rpm than at the peak troque rpm.

So, I am seeing at LEAST 1000 ft lb x 3.0 transmission mutliplication x 2.75 axle multiplication = 8250 ft lb of torque at the wheels.

If you axle ratio is actually say 3.7 and your transmission 1st gear is actually 3.69 (like mine is), you might have as much as 1000 x 3.7 x 3.69 = 13,700 ft lb. of torque at the wheels.

You didn't give you wheel and tire size, but I'm going to assume just about the worst you might have: 11R24.5 with a diameter of 43 inches.

That means your 8250 to 13,700 ft lb of torque converts to somewhere between 4600 to 8100 pounds force at the intersection of the wheels with the driveway. (Froce available = torque divided by RADIUS of the tire in FEET (not inches).

Since you need 11,250 pounds of force, it looks on the surface like you will NOT be able to climb the 25% grade.

Now, you DO have a torque converter. And a torque converter is specifically designed to multiply torque from a dead start, and can mulitply it by up to about a factor of 3. So, theoretically, that torque converter can multiply your force at the wheel by up to 3 tims, which woudl give you 25,000 to 41,000 pounds of force - which IS enough to make the climb. Theoretically.

However, having the torwue converter multiply torque to get a bus GOING froma  dead stop to where 1st gear can take over on its own is one thing. To expect the troque converter to  work in that manner for almost 4/10 of a mile is going to generate a TERRIFIC amount of heat. I would be worried about the torque converter and transmission fluid overheating and failing. And, if it fails part way up a 25% or 16% hill, you are in serious trouble.

That's my first reason for advising against trying this.

Secondly, I don't think I would WANT to try to climb that steep a grade in a 45,000 pound bus even if I COULD. Way too dangerous, and a HUGE load on the entire drivetrain, let alone the torque converter.

Plus, once you get to the gravel, I doubt you can apply enough torque, even if you had it, to move the bus - the tires would likely spin instead, because that would be easier than "lifting" the bus.

The switchback on a hill that steep would also just be INVITING a tipover, and buses ARE pretty high.

Finally, I definitely would NOT want to back the bus down that hill. I regard that as very dangerous given the turns required.

I am assuming you are asking a serious question, and that this is not some sort of joke posting . . .

Jim G
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Nusa
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« Reply #4 on: May 20, 2009, 03:58:18 PM »

Just the description of what you want to try sounds dangerous, especially on a vehicle that's new to you.

If you do try it, I suggest going backwards going up rather than down. Depending on your exact transmissoin, reverse may be a better ratio than first. More importantly, if you make it up, you know you can get down again...forward is easier than backwards on steep grades with curves.
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edroelle
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« Reply #5 on: May 20, 2009, 04:52:31 PM »

You likely will have serious difficulties backing down, IF you make it up.

Rear axles do not like massive torque in reverse.  It could fail.  They are designed for more strength going forward.

Do you have other options?

Where are you located?

Ed Roelle
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« Reply #6 on: May 20, 2009, 05:30:19 PM »

          "Assuming I don't drag the tail of bus at the transition from the main road or highside it at the transition from the 25%
           to 16% grades,"

That may be a very optimistic assumption.  I have dragged the tail of a bus on a driveway that I am sure was less than a 25% grade.  For now, go to plan "B".  Are you sure it is an actual 25% grade?
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paul102a3
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« Reply #7 on: May 20, 2009, 06:19:05 PM »

Ed, the property is located in Burnsville, NC and yes I do have other options. I would prefer to keep the bus on my property as I have a very extensive workshop to rehab the interior.

If it is not feasible to get up the drive, I can leave the bus in Florida and do the work here.

Paul
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« Reply #8 on: May 20, 2009, 06:39:08 PM »

I think folks have already weighed in with an answer, but I'm left wondering whether the 40 foot semis also back down the hill?
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paul102a3
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« Reply #9 on: May 20, 2009, 06:54:41 PM »

Thanks for all the input.

To answer some questions, the steepest part of the entrance is a 24.7% grade. The drive starts fairly flat but builds very quickly over a 125 foot distance.

The semis all came up the hill in forward and then reversed back down. One 40 footer actually drove up the hill empty with a dusting of snow on the ground and then backed down fully loaded later in the day.

Paul
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« Reply #10 on: May 20, 2009, 07:16:31 PM »

With or without steps and handrails?  Ever seen the Haiku Stairs?  Wink

Oh, you mean you COACH.

There are two things to worry about -- not only the max grade, but also the max ANGLE.  Will your tail hang up where the grade starts?  Will you high-center at the top?

I think I'd find a local storage option which doesn't include fear of death with each trip.
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Greg Roberts
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« Reply #11 on: May 20, 2009, 07:36:29 PM »

I would hire a local farmer/construction company with a nice big tractor and hook it up for the first time up and first time down. Let the bus do the work if you like and see if it can actually do it. If not, you are safe. Of course you will want to use hefty chains/cables that are rated and angled and rigged properly to an appropriate attachment point on the bus.
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« Reply #12 on: May 20, 2009, 08:17:51 PM »

To me it sounds way too dangerous, the backing down part. Keep your bus at a friends place in the flat lands.
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HB of CJ
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« Reply #13 on: May 20, 2009, 08:45:25 PM »

My Old Crown Supercoach 40' 3 axle 10 wheeler tandem drive would have no problem.  Ten speed Roadranger.  I'm afraid you will do just fine until the Bus Conversion hits the gravel where it will just spin out.  No locking differencial(s)?

Hiring the tractor sounds like a good idea.  Then if you find you can't make it, just back off the throttle and let the pulling vehicle do some of the work. Also like already said, you might melt your Allision.  Be sure to run fast idle a long time afterwards.

No disrespect intended upon your fine Coach.  They were designed for hauling passengers for a profit, not for climbing hills like some logging truck.  25% is really steep: how did they build the house?  Ice in the winter?  Dunno.  HB of CJ
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Nusa
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« Reply #14 on: May 20, 2009, 09:51:03 PM »

Seasoned professional truck drivers develop some amazing backing-up skills. I've watched my trucker friend back a loaded flatbed up faster than I would in a car with no trailer....and put it exactly where he wanted it. Of course, he does have 27 years of practice.
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