Downhill Speed Control - Proper Gear Selection vs Retarder

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Downhill Speed Control - Proper Gear Selection vs Retarder

May 03, 2018, 09:59:03 pm

Since we will have many significant up and down hill grades during our six month trip out west I want to clarify something. I have read extensively about using the transmission to both climb and the descend hills, i.e., use the same gear downhill that you used uphill. Obviously, I've also heard extensive comment on the use of the retarder to manage downhill speed. However, there seems to be a heat related result from extensive use of the retarder.

Thus my question... If one can control downhill speed with the transmission, how is use of the retarder integrated from a best practices point of view?

Thanks

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #1 – May 03, 2018, 10:09:55 pm

Good question... I'm all 👂 this one as we are going to Colorado at the end of July

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #2 – May 03, 2018, 10:14:15 pm

Probably a lot of different opinions on this one. I believe the rule of same gear downhill as uphill was before the engines had the horsepower they do today. If you have the VMSps you can watch trans. temps better than the gauge. The gauge seems to be slow to detect change. My way is to use the temps. as my guide, if they start to get too high I go to a lower gear. I travel over and back the Bishop grade 4 times each year and this seems to work for me.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #3 – May 03, 2018, 10:26:43 pm

Using the retarder properly does entail keeping an eye on transmission temperature. As transmission temperature starts to rise, back off on amount of retarder and down arrow to select a lower gear. That will give a little more engine braking and much more importantly, turn the water pump faster and therefore increase cooling.

As you know, even with Transynd, fluid life is materially affected by temperature.

To me it is a balancing act.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #4 – May 03, 2018, 10:27:54 pm

I was on a couple pretty steep hills with curves and a 25 mph limit for trucks. I would down shift to third and or second me use the retarder. The combination would keep,the speed well in control and not overheat the transmission.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #5 – May 03, 2018, 10:36:45 pm

You'll find that the trans fluid will heat faster in the higher gears. For example, if you are going down a 6% grade in 5th and use the retarder hard continually, it will heat up quickly. If you drop down into 4th you'll see it cool right down. If you want to do the whole hill in 5th just alternate between the retarder and brake. Just stab the brake to get down a bit lower than your desired speed.
jor

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #6 – May 03, 2018, 10:50:15 pm

Quote from: jor – May 03, 2018, 10:36:45 pm

Just stab the brake to get down a bit lower than your desired speed.
jor

A little more clarification for those not familiar with mountains, heavy vehicles and air brakes.

Service brakes (brake pedal) is not used for maintaining speed of descent. That is the job of the engine, transmission and retarder/engine brake.

If you do use the service brakes because you are accelerating on a descent, apply them firmly and long enough to drop to a lower gear. Allow them to cool some. Do it again, until you reach an equilibrium speed based on "non-service brakes".

Do not PUMP air brakes. A big no no, as it can quickly deplete air supply and offers no advantage to the brakes life or in speed control.

Why all the discussion about use of brakes. Easy-- brakes do one thing for a living. They turn momentum into HEAT. While a motorhome weighs many, many times as much as a car, brake area is not that substantially larger. The brakes still have to fit inside the wheels! This applies to all heavy vehicles-- again basic physics.

From a practical standpoint, your "equilibrium speed" on a descent (where you are holding speed constant and safe without use of the service brakes) is FASTER than a loaded 18 wheeler and SLOWER than an empty one. That should give you a good starting point as you get into mountains.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #7 – May 04, 2018, 12:09:42 am

I monitor our trans and engine temps via our VMSPC while going up and down grades and change gears, use the retarder and service brakes as appropriate to control those temps. AS Brett said: A Balancing Act.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #8 – May 04, 2018, 12:54:28 am

Quote from: floridarandy – May 03, 2018, 09:59:03 pm

... Thus my question... If one can control downhill speed with the transmission, how is use of the retarder integrated from a best practices point of view?

A diesel engine doesn't compression engine break the same way a gas engine does. Downshifting a diesel engine will not check or hold the speed going down a hill the same way a gas engine does. Diesels coast more. With a diesel downshifting may help reduce the speed some, but mostly the RPMs will go up.

Transmission retarders and Jake Brakes are added to heavy diesels to provide that extra resistance needed to slow down, and hold speed on descents, without relying on the service brakes.

Rather than thinking of downshifting and upshifting as controlling the coach's speed, consider thinking of downshifting and upshifting as allowing the driver to keep the RPMs in an optimal range so that:

the coolant is pumping at a rate needed to keep the engine cool

the transmission temperature is kept in check

the engine does not over-rev

The retarded is added in to the mix to keep the speed in check without having to use the service brakes.

As Brett said earlier, it's a balancing act.

One thing that helped me was to go through the Foretravel manual for our coach and engine and find these three items for our transmission and engine:

normal operating temperature range for the transmission

normal operating temperature range for the engine

maximum engine RPM

With this information you know when your setup is running within the specs, or when it is reaching an upper limit and you need to change something to bring things back into the specs. There really is no guessing or wondering.

In terms of best practices... in general, before you descend a mountain make sure the coach's speed is already at the correct speed and that you've selected the right gear. It easier to start a descent too slow and be able to speed up, than to start too fast and try to reign in speeds, temps, and RPMs. There's always that driver who pulls around you and cuts back into your lane without a thought about how heavy you are.

I've found the transmission retarder nice to use in traffic jams. It allows the coach to crawl forward at a more constant and consistent pace (like 18-wheelers do) and not be stepping on the service brakes all the time.

When we get into crowded cities there are times I've activated the retarder and kept it at setting one or two incase someone stops suddenly in front of us. This way when you stomp on the service brakes the retarder also kicks in and provides extra stopping power. It's helped several times, but you also have to be aware of what the drivers behind you are like.

Chris

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #9 – May 04, 2018, 09:27:45 am

I think on many coaches, once you turn the retarder switch on, it is automatically connected to pressure switches in your braking system. The more pressure on the brake foot pedal, the more retarder effect available.
If you have the multi position toggle switch on the side panel, it will apply retarding, according to position, any time your foot is not on the accelerator.

EDIT just remember, on most coaches, brake lights are not attached to retarder switch on side panel, so if you use it and not the foot brake and a big truck is following you closely..................

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #10 – May 04, 2018, 09:33:31 am

Quote from: jcus – May 04, 2018, 09:27:45 am

If you have the multi position toggle switch on the side panel, it will apply retarding, according to position, any time your foot is not on the accelerator.

Ya, and that defeats my favorite "gear": COASTING!

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #11 – May 04, 2018, 10:51:55 am

Quote

I think on many coaches, once you turn the retarder switch on, it is automatically connected to pressure switches in your braking system. The more pressure on the brake foot pedal, the more retarder effect available.

I think this feature is sometimes not clear to drivers unfamiliar with the typical joystick setup. If you leave the retarder rocker switch on at all times, the retarder will activate every time you hit the brake. According to the the Allison user manual it applies retarding power in three increments depending how hard you are pressing the pedal. So, if you have the rocker switch on and the retarder joystick in position zero (all the way forward) the retarder is assisting your braking when you hit the pedal and you have full retarder in an emergency stop situation. You can just ignore the joystick.
jor

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #12 – May 04, 2018, 10:58:28 am

One of our fire trucks was a grant from the Offices of Emergency Services in California. It was a big devil and the only one with a 6 speed automatic and retarder to slow down the 10,000 lb load of water and fuel. Heading down a steep grade, we would shift down until the speed was about right for the grade. Then, if the speed started to build we would step on the retarder pedal until it slowed, release it and then used it again as the speed built until the yellow warning light came on. The engine and retarder were mostly used so we could keep the service brakes in reserve and be able to stop and cool it down if necessary.

With the U300, the Interstates are not usually that steep that I have to worry. Going down Donner Summit over I-80 west to east, I drop a gear to third, have the Jake switched on and occasionally use the service brakes to slow the coach about 2-300 RPM, release the brakes and then repeat as the speed builds. I think I only have to use the service brakes 2 or 3 times before getting down to Truckee. (I keep the Jake switched on all the time) Most of the other grades, I let aerodynamic braking plus the Jake in high gear to keep the speed to 65 or so.

With steeper highways like the 10% decent into Jackson, Wy, I generally pull over at the top and shift into the gear I think will do for the whole descent. Usually, second gear. This is so I know if I am in second gear or second direct with the transmission locked up with no surprises as we start down. I then descend, letting the RPM build to about 2000, apply the service brakes to drop that to 1700 or so, release the brakes and repeat as the speed builds. I have never overheated the brakes this way and if I were to get a hint they were getting hot, I would pull over and cool everything down. Make sure to check your air pressure before you start down and every thirty seconds on the way down.

A retarder will work about the same way and like the Jake, being more efficient at higher engine RPMs. Just watch the transmission temps as they build. Most Foretravels have transmission lockup so torque converter slippage and heating is not a factor once you get into second gear lock up (or third in some early models). It's just the retarder that will heat the fluid.

If on a two lane road during your decent, never let traffic behind dictate your speed. They can wait. Nothing wrong to use a pull out to stop and check things.

Be extra conservative as you first encounter steep descents. There is a learning curve. Once the brakes get really hot, you are in trouble.

I used to RV in a ex Greyhound with a 8V-71 Detroit, manual 4 speed and drum brakes. No Jake or retarder. The Sierras and Rockies were no big deal just using the engine and transmission with the proper gear selected and the service brakes used pretty conservatively.

Heading up any grade, downshift to keep the RPM high for efficient water pump circulation and to keep the temps in check. If the speed drops, shift down to a lower gear. Summer may have surprises with higher engine temps than your normally have in the other seasons. I don't let our water temp get much above 200 degrees ever and start shifting down well before that.

Pierce

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #13 – May 04, 2018, 11:01:39 am

We just have the switch,from what I have expiereinced and read here the "switch only" system seems to have less overheating.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #14 – May 04, 2018, 11:17:44 am

Quote from: John44 – May 04, 2018, 11:01:39 am

We just have the switch,from what I have expiereinced and read here the "switch only" system seems to have less overheating.

The "switch only system" system only works when you use the foot brake. The lever on the side dash will activate the retarder anytime your are not using the accelerator. Believe this the reason why. If coming down a hill, on, will retard constantly and build up heat.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #15 – May 04, 2018, 11:20:16 am

Quote from: jcus – May 04, 2018, 09:27:45 am

EDIT just remember, on most coaches, brake lights are not attached to retarder switch on side panel, so if you use it and not the foot brake and a big truck is following you closely..................

I think that is a GOOD THING! I've followed SOB coaches down steep grades and their brake lights were on all the time, so I never knew exactly what they were doing. A light tap on the brakes will give drivers behind plenty of warning.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #16 – May 04, 2018, 11:22:20 am

That's exactly what I meant.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #17 – May 04, 2018, 11:54:39 am

My brake lights come on whenever the retarder is active. If I forget to push the joystick forward when I get on the highway and activate the cruise control, I'll be reminded on the next downhill stretch, as the cruise backs off the throttle, the retarder activates, and the brake lights shut off the cruise control. Instead of switching from cruising to coasting, the retarder causes noticeable "braking".

On surface streets or in traffic, I keep the joystick in position two or three, works great. And of course, it kicks in to various degrees with the service brakes.

On the highway, I automatically reach for the joystick first whenever I need to slow. Works great.

The only overheating concern comes on long descents. Here is where gear selection comes into play.

If coming down a grade with the retarder and the speed drops below some set point, either from the retatder or service brake, the transmission will automatically downshift, and the retardind becomes greatly enhanced. At this point I push the joystick all the way forward to keep from going too slow.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #18 – May 04, 2018, 12:05:44 pm

Tom, so you leave your stick mode retarder on all the time in the city? Does not seem very efficient, every time you take your foot of the accelerator, the retarder applies and slows you down faster. Personally I like Bretts approach and "coast", rather than the accelerate, retard/slow, accelerate, mode of driving.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #19 – May 04, 2018, 12:55:09 pm

Quote from: jcus – May 04, 2018, 12:05:44 pm

Tom, so you leave your stick mode retarder on all the time in the city? Does not seem very efficient, every time you take your foot of the accelerator, the retarder applies and slows you down faster. Personally I like Bretts approach and "coast", rather than the accelerate, retard/slow, accelerate, mode of driving.

I find coasting in city traffic is often too fast for me. If one notch of retard is slowing me too much, I can always push the joystick forward. It's dynamic.

I adjust the joystick as needed. Usually in city traffic, I can easily modulate the retarding with the gas pedal if Im getting too much retard. No hard and fast rule as to where to keep the joystick, sometimes in the first position, occasionally all the way forward (but with my hand in the ready. Third notch is my favorite in stop and go traffic.

I feel the retarder gives me a jump on braking faster than I can get my foot on the brakes.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #20 – May 04, 2018, 01:16:50 pm

Maybe I'm the only one who has a 'Pacbrake' retarder. Brett modified the location of the Allison touch pad with the retarder rocker below, my left hand fits perfect on the rocker while city driving or on 'drops'. I drive as far ahead as I can, when traffic starts to slow, foot off the gas, coast, hit the rocker and the only brake use is to a easy stop.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #21 – May 04, 2018, 02:11:20 pm

Lots of good advise above. Should be required reading! Quick and dirty definition of use of brakes, retarder and transmission is:

Retarder/correct gear to maintain optimal speed on a decent.
Brakes only to check speed. They are only to slow or stop not maintain speed. Keep them cool so that they can stop you in an emergency.

Keith

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #22 – May 04, 2018, 10:01:46 pm

When Foretravel first used the retarder in their coaches the brake lights did not come on when the retarder was activated with the lever, but they came out with a bulletin to install relays that turns on the brake lights when the retarder is activated with the lever.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #23 – May 04, 2018, 10:13:26 pm

Quote from: red tractor – May 04, 2018, 10:01:46 pm

When Foretravel first used the retarder in their coaches the brake lights did not come on when the retarder was activated with the lever, but they came out with a bulletin to install relays that turns on the brake lights when the retarder is activated with the lever.

Must have missed all the bulletins. Have had 4 Foretravels and have not had those relays.
Would not want them, if going down a hill with retarder on, and you had to really use your brakes for emergency, people behind you would not know, because your brake lights would have already be on. In Tom"s case where he drives with retarder lever on all the time in the city, even a worst case, people would never know when he is braking.
JMHO.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #24 – May 04, 2018, 11:12:09 pm

Quote from: jcus – May 04, 2018, 10:13:26 pm

Must have missed all the bulletins. Have had 4 Foretravels and have not had those relays.
Would not want them, if going down a hill with retarder on, and you had to really use your brakes for emergency, people behind you would not know, because your brake lights would have already be on. In Tom"s case where he drives with retarder lever on all the time in the city, even a worst case, people would never know when he is braking.
JMHO.

I'm content with the present setup.

Whenever I take my foot of the throttle with the retarder active, the retarder does apply braking force, slowing the rig down more than merely coasting would, hence the need for brake lights.If I don't want or need that amount of braking, I can push the joystick forward or apply slight throttle.

Anyone with a 2003 should have the same setup as I do.

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