Downshift or exhaust brake

[jwmaustin]

I have an exhaust brake as well and leaving it on does prevent me from forgetting to use it. But it clearly impacts fuel mileage.

[DMTTRANSPORT]

Originally Posted by Mr_D

With the present rig I haven't found a grade yet that required me to touch the brakes to keep the rig slowed down. But most don't have a three stage engine brake with up to 600hp of retarding either.
With the previous Dutch Star having just an exhaust brake I did have to "snub" brake to keep it slowed at times. WA CDL test manuals say not to ride the brakes as they can overheat.

Riding the brakes and using them to keep your speed in check are two different things, most brake shoes/pads are made to work better with heat (not hot) Stabbing creates a hot/cold situation causing the drum/rotor to create small fractures, use your brakes how ever you feel comfortable using them... I ran 547,000 on my Truck before replacing the shoes (they still had 40%) but I had it all apart to service the seals.

[Tony Lee]

Problem with riding the brakes is that inexperienced drivers don't know any better. Instead of being in a low enough gear to result in high enough revs that the exhaust brake can achieve anything, they just come over the top in 5 or 6 at 50 or 60 mph and then lift their foot off the throttle and wonder why little is happening. They look at the gear selection display and see "2" so shrug and then start easing down on the brake. Half way down the mountain they are more than easing on the brake because it has become ineffective due to heating and they would not be able to stop in an emergency. Selection of correct gear PLUS either snub braking or continuous braking is what works. Riding the brakes without understanding the whole dynamics is a big gamble. Best way to teach drivers would be to put them in my MC8 with drum brakes, 17 tons and a crash 4-speed gearbox and no engine or exhaust brake.

BTW, unlike gas engines, a diesel has virtuallyt no inherent compression braking within the engine. It does have friction and windage and other losses, but so does a gas engine and that is why they add the exhaust brake to do the same job as the valve on the inlet of a gas engine.

[DMTTRANSPORT]

The Diesel does have Compression Braking when the "Engine" brake is NOT used, just not as effective as the Engine brake....

[Ray,IN]

Originally Posted by 96 Wideglide

I've probably read this tidbit of advice a hundred times on this forum and always wondered, what if you've never climbed this grade !

They are referring to mountain driving, you go up one side, down the other. I agree with you though; sometimes you are driving on semi-level road and then you are descending a 4% grade. I 90 at Chamberlain, SD is like that.

Downhill speed can become dangerous. When using your exhaust brake, the Allison tranny downshifts. If you allow your engine RPM's to reach the tach redline, the ECM will tell the Allison TCM to upshift to prevent engine damage. If you allow that to happen you have reached the point where your service brakes are your only option to further reducing speed. Well there's the truck runaway lanes in mountainous areas for a last resort.

[96 Wideglide]

Yes they are. I drive in mountains all the time, and very few are perfectly shaped. One side you can climb 4% grade for miles, decend the other side at 8%.
Like I said, unless you've driven the highway both ways, no way to know what gear you climbed that hill in.

[palehorse89]

Originally Posted by DMTTRANSPORT

Really? I have been doing it wrong? All the signs that read "Trucks use lower gears" are misleading? A Diesel Engine does provide "Brake Horse Power" when down shifting (even with an Automatic) I read the comments on how to maintain down grade speed with a 40,000 lb Coach and get the feeling it's more of a chore to do than a Quarter Million pound Truck n Trailer with a Heavy piece of Equipment! That can be rolled down the Baker Grade and any other long/steep grade I go down without touching the brake pedal, "stabbing" the brakes would heat them up in a big hurry as opposed to holding a steady 5#'s of pedal pressure (basically resting your foot on the pedal) ..Exhaust Brake, correct your speed at 5mph over with light pedal pressure NOT STABBING! (Your comfortable down hill speed corrected @ 5 mph over) 50 + 5, at 55 lite brake pressure till 50, repeat......

X 2
I agree with the above due to a lot of miles driven in a 80K vehicle in all kinds of driving conditions. In my opinion, this "Stab Braking" is very hard on the brake system and its components!
This is based on real experiences of both methods on heavy vehicle's, not RV's, and not from watching video's on the internet. But, as posted, it is your equipment, use it as you see fit, and what ever works the best for you.

[mel s]

Originally Posted by Gary RVRoamer

Adding to the excellent answers so far...
Downshifting by itself does nothing on a diesel because there is no inherent engine resistance (braking), so your choice is really only "both", with the gear chosen controlling the amount of braking effect you get. Your coach will automatically downshift on a downgrade as long as the engine or exhaust brake is turned on. Just take your foot off the go-pedal. Mine targets 4th gear, but some are set up for 2nd instead. It won't actually achieve that gear until the speed is low enough to safely do so, but the tranny is working toward that. You will feel it shift as it does. Use the brake pedal as needed to help out.

Gary RVRoamer
That is correct

Originally Posted by DMTTRANSPORT

Really?? I have been doing it wrong All the signs that read "Trucks use lower gears" are misleading? A Diesel Engine does provide "Brake Horse Power" when down shifting (even with an Automatic) I read the comments on how to maintain down grade speed with a 40,000 lb Coach and get the feeling it's more of a chore to do than a Quarter Million pound Truck n Trailer with a Heavy piece of Equipment! That can be rolled down the Baker Grade and any other long/steep grade I go down without touching the brake pedal, "stabbing" the brakes would heat them up in a big hurry as opposed to holding a steady 5#'s of pedal pressure (basically resting your foot on the pedal) ..Exhaust Brake, correct your speed at 5mph over with light pedal pressure NOT STABBING! (Your comfortable down hill speed corrected @ 5 mph over) 50 + 5, at 55 lite brake pressure till 50, repeat......

DMTTRANSPORT
Apparently you have been doing it wrong?

When descending a grade "Stab Braking", (as described in an earlier post), will HELP PREVENT the service brakes from overheating...
Whereas "holding a steady 5#'s of pedal pressure (basically resting your foot on the pedal)" when descending a grade can cause the service brakes to overheat, (and possibly fade/fail).
---------------------------------------------------------------------------

BTW the signs that read "Trucks Use Lower Gears" are for trucks with a STANDARD transmission... (NOT for MHs or trucks with the Allison 6 speed automatic transmission).

If/when a lower gear is "SELECTED", (whether manually with the shift pad... or automatically by PacBrake activation), to protect itself, (and the diesel engine), the Allison will not "ACHIEVE" the "selected" gear until the coach slows to a speed low enough to safely do so...(as Gary said).

Mel
'96 Safari

[Tony Lee]

Originally Posted by 96 Wideglide

Yes they are. I drive in mountains all the time, and very few are perfectly shaped. One side you can climb 4% grade for miles, decend the other side at 8%.
Like I said, unless you've driven the highway both ways, no way to know what gear you climbed that hill in.

It is a matter of being aware what your vehicle is doing. Never seen a sign telling me what gear to downshift to. If the snub braking is having to be done too often or for too long to keep the rig from racing away, then you have to act decisively by making a harder brake application to get the speed down low enough for the vehicle to downshift (assuming your auxiliary brake is commanding a downshift that couldn't be achieved) or if no aux brake then you have to downshift.
The longer you leave it before reacting, the more difficult it will be to regain control. The self preservation upshift the transmission does will be so smooth you won't even notice it until it is too late.

BTW the term is SNUB Braking, not STAB braking and the concept and the effect are quite different. Look it up. Plenty of reading to be done and then some practice needed.

[Travelwrench]

Ok Lots of info, so from what I'm reading is if I have exhaust brake on and crest hill start at say 45 the tranny and brake should do its job.

[Travelwrench]

My route is going to take me from Phoenix up through flagstaff on 17. I drove it last month in car and seems alot of up hill but definitely some down. Once I get past flagstaff pretty much smooth sailing to WI.

[Dr4Film]

Originally Posted by Travelwrench

Ok Lots of info, so from what I'm reading is if I have exhaust brake on and crest hill start at say 45 the tranny and brake should do its job.

Depends on your combined weight, coach and tow.

I am about 48,000 lb's combined with a PRXB PacBrake so in many steep grades the coach always creeps up in speed which then has to be brought back down before it up-shifts into 5th. On very steep grades I try for 3rd gear at about 30-35 mph but I still need to snub it back down once the speed increases BEFORE it up-shifts.

Again, depends on your weight.

If I had a two stage engine brake chances are I would not have to use my service brakes at all or very little when descending any grade.

Dr4Film ----- Richard

[mel s]

DMTTRANSPORT
Apparently you have been doing it wrong?

When descending a grade "SNUB Braking", (as described in an earlier post), will HELP PREVENT the service brakes from overheating...
Whereas "holding a steady 5#'s of pedal pressure (basically resting your foot on the pedal)" when descending a grade can cause the service brakes to overheat, (and possibly fade/fail).
---------------------------------------------------------------------------

BTW the signs that read "Trucks Use Lower Gears" are for trucks with a STANDARD transmission... (NOT for MHs or trucks with the Allison 6 speed automatic transmission).

If/when a lower gear is "SELECTED", (whether manually with the shift pad... or automatically by PacBrake activation), to protect itself, (and the diesel engine), the Allison will not "ACHIEVE" the "selected" gear until the coach slows to a speed low enough to safely do so...(as Gary said).

Mel
'96 Safari[/QUOTE]

Originally Posted by Tony Lee

BTW the term is SNUB Braking, not STAB braking and the concept and the effect are quite different. Look it up. Plenty of reading to be done and then some practice needed.

OOPS!
I meant "SNUB braking", (not "Stab braking").
Sorry
Mel

[MSHappyCampers]

Very interesting thread!