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Old 06-07-2018, 06:33 PM   #1
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How to drive a steep downgrade?

I have a 36' conventional bus with a Cummins 5.9 engine and an Allison 2000 transmission. The engine is governed at 2500 RPM and the speed is governed at 60 MPH. No retarder, jake brake, or magic magnetic driveshaft thingie.

I understand "stab" braking (coast until you hit your max desired speed, brake with gusto to get it down 5 MPH, rinse and repeat). I also understand poking my way downhill rather than letting the speed get up at all.

I have heard that downshifting is a good trick to know. I also think that the transmission will not cooperate if the downshift puts the RPM too high. But while I can grab the gearshift and click it down one more notch, I don't know how and when (or how much) to downshift.

Can someone give me specifics as to what to do so I don't need a runaway ramp? We'll be crossing the Rockies with the back loaded with dressers, piano, and a cow (well, in a freezer).

Thanks!
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Old 06-07-2018, 06:43 PM   #2
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Shifting down one gear will not allow the bus to pick up as much speed.
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Old 06-07-2018, 07:57 PM   #3
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Shifting down one gear will not allow the bus to pick up as much speed.
OK, so do that at the top of the hill, presumably, and try to never pick up excessive speed in the first place?

I have "OD, D, 2, 1" or some such. Just take it into Drive instead of Overdrive?
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Old 06-07-2018, 07:59 PM   #4
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OK, so do that at the top of the hill, presumably, and try to never pick up excessive speed in the first place?

I have "OD, D, 2, 1" or some such. Just take it into Drive instead of Overdrive?
Yes, maybe even 2nd if the situation calls for it.
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Old 06-07-2018, 08:00 PM   #5
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Put it in 2d gear, not Drive or Overdrive, before you start down hill. Then, stab braking.
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Old 06-07-2018, 08:02 PM   #6
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Put it in 2d gear, not Drive or Overdrive, before you start down hill. Then, stab braking.
And this is from someone who lives in the Rockies. Thanks, pal!
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Old 06-07-2018, 08:03 PM   #7
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It kind of sounds like you've got it, everything you spoke of is pretty much what I do the only thing I would add is that I would actually pull off the highway when space was available, and let the brakes cool down and give the engine a chance to rest I'm talking about on the big downstroke of the mountain. A few years back I went from Minneapolis up through Montana Idaho Utah Washington and Oregon... I cringed every time I saw a sign that said semi-truck chain up pullover area... That meant there is a mountain coming and there was probably 25 or more 3 to 5 Mile pulls up the mountain the only good thing is you got to come down the other side and I'll tell you a little trick you might be afraid to do this but there's a thing called Georgia Overdrive which means when you can see what's coming at the end of the road like a long straightaway put it in neutral and let it roll. My bus would over rev if I left it in gear at 75 miles an hour in neutral I could do 85... Passing all the semi trucks that passed me on the other side this sounds really dangerous and I'm sorry but it's what I did and it was a blast.
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Old 06-07-2018, 08:07 PM   #8
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AND if you're going downgrade and stab braking, PUT ON YOUR FLASHERS.
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Old 06-07-2018, 08:13 PM   #9
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They'll hear me screaming, too...
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Old 06-07-2018, 08:15 PM   #10
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Will they need a pry bar to get you out of your seat?
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Old 06-07-2018, 08:25 PM   #11
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I went down this 6% LONG grade in an untested bus I'd just picked up.
Went down in 3rd, it had a 643.
I'd try it in D if you have a 2000 unless maybe its more than a 5% or 6% grade. My allison 2000 has pretty smart downshifting in its programming.
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Old 06-08-2018, 08:53 PM   #12
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All the advice so far is excellent, the biggest thing for when you get into proper mountains with long grades or excessive grades. Set yourself up early, get out of the gas, downshift without braking if the situation permits (no point in generating heat in the flat) get to a lower speed than you think you'll need and let the drivetrain do the work. Keep doing this on hills until you get the feel for your rig.
x2 on the flashers both up and down hill, let the other heavies know what's up so they can plan accordingly.
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Old 06-09-2018, 12:17 AM   #13
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It is much easier to start out slow at the top and work your speed up than to start at the top fast and slow yourself down.

Ideally, with stab braking, the engine and transmission are supposed to be doing most of the work.

If after you have reached your target speed and get off the brakes, if you have to almost immediately need to get back onto your brakes you are one gear to high for your target speed. If it takes adding throttle to keep your target speed you are one gear to low. Most of the time you will discover your are always one gear to low or one gear to high.

Until you get a lot of confidence it is much better to be one gear to low than one gear to high.

The first time I ever drove a ski bus, on the way down the steep part at the top of the hill, I was in the same gear I used going up. By the end of the 9-week season I was comfortable using one gear higher going down the hill. It is all about getting comfortable with your equipment and knowing your limitations. It is much better to go slow and have no drama than to go faster and end up having some drama.
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Old 06-09-2018, 09:13 AM   #14
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the allisons in school busses dont detect grade braking.. (D) is gears 1-5, D is gears 1-4, then 2 then 1.. there is no way without installing TAP shifting to get into 3.. so run it in 4 and stab brake it.. if you have a manual control for your engine fan turn the fan on.. with the revs up thats good for a decent amount of resistance on the engine.

if you really want 3rd gear, overspeed it for second and the trans will never drop down to 2.. it will hold in 3... they are programmed to never over-rev the engine even if you downshift it..





dont just let the bus overspeed then stab it down.. chances are you will reach the summit of your hill at a nice slow 35-40 or so.. so maybe let the bus get up to 45 or so and plan to hold it there.. many of the curves are good for 55-60 so if you are running down the hill at 45 or so then you have some room to let it speed up a bit if you have to..



Map your downgrade.. look at it before you hit it to see where the "hot" spots are.. ie the sharpest curves.. so you know where you must be running at slower speeds..


the biggest time truckers and others get into trouble is in unknown territory... and always trying to run at the maximum safe speed... leaving themselves with no room to slow down if they get an unexpected sharp curve.. most roads are marked pretty good to help prevent this.. you are in a school bus.. no need to fly like a corvette.
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Old 06-13-2018, 04:56 PM   #15
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Air brakes? Google proper use of Air brakes. YouTube can save your life.
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Old 06-13-2018, 05:00 PM   #16
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Putting in neutral and letting it roll is illegal for a really good reason. Donít create a middle. Air brakes utilize engine rpm to pump sufficient volumes of air to safely stop the bus. Run low on air and your brakes will automatically come on... at 70mph itcould be tragic.
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Old 06-13-2018, 05:01 PM   #17
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Stab braking is wrong for air brakes.
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Old 06-13-2018, 05:03 PM   #18
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https://www.truckingtruth.com/trucke...-stab-breaking
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Old 06-13-2018, 05:04 PM   #19
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https://www.truckingtruth.com/cdl-training-program/page46/using-air-brakes-and-stopping-distance
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Old 06-13-2018, 05:05 PM   #20
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Not stab braking- thatís for emergencies

http://www.irv2.com/forums/f258/stab-braking-is-an-improper-term-for-slowing-descent-on-hills-241694-2.html
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