New To Trucking How does a driver predetermine CONTROL SPEED for an upcoming downgrade?

Thomas Mixter

Well-Known Member
What I've learned so far about gear selection for a downgrade is that it should be the gear that puts the truck's engine just under governed speed at control speed. I suspect governed speed information as well as gear ratio information should be in the truck's operator guide.

Generally, a truck will be in the same gear going down the hill as it climbed up it if the truck is old or a gear or two lower than the climbing gear if the truck is modern.

However, there will times that road goes from a stretch of flat and level (no slope) to a downgrade and there is no uphill climb to determine your gear selection for going over the summit.

Let's say you are in a tractor-trailer with one trailer on a level highway at 80,000 pounds and this highway is a speed zone of 55 mph and you are cruising at this speed. Your transmission is at its normal gear range for this driving condition. You see a sign indicating there is a 5% downgrade ahead, road conditions are dry, it's broad daylight and weather is clear.

What should you now do? How will you calculate your correct CONTROL SPEED for this upcoming downhill run? Which gear will you select?


Lets' change 5% to 10% downgrade in the scenario above, what is your correct control speed now?

Keeping that 10% downgrade, let's change road conditions from dry to wet, what is your control speed now?

Now let's change road conditions from wet to packed snow, what is your control speed now?

Let's now change your fully-loaded truck to an empty truck, what is your control speed now?

Now let's even change your truck from a single to a double, now what should your control speed be?

What is the mathematical formula in mountain driving for determining control speed based upon:

  1. load
  2. transmission type and governed engine speed
  3. vehicle age and design
  4. number of trailers/vehicle composition
  5. visibility
  6. posted speeds
  7. grade
  8. road conditions
  9. traffic
  10. weather
There must be rules established for predetermining control speeds for many driving variables to be safe.

A driver will use the the snubbing 5-3 braking method on top of engine braking to maintain control speed downhill.

Engine braking will never be used under slippery road conditions to avoid skids.
 
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tommyh

Well-Known Member
a oldtimer told me to go down 2 gears than what you went up it in
for me,It depends on the grade and how long it is,and whether I am loaded or empty
 

mndriver

curmudgeon extraordinare
Supporter
With an aero truck should be 1-2 gears lower than you climbed. You don’t have the wind resistance from the brick out front. At least that’s what I was taught.
Not because of aero.

Because transmission and gearing have become more efficient, one lower than what you climbed it with.

With the new power being available, two gears down.

My century makes enough power, I had to drop 4.
 

mndriver

curmudgeon extraordinare
Supporter
The classic isn't much different..

Sure it's not the only trucks on the road capable of it either. Since Cummins has their Signature 600.
 

ironpony

Professional Pot-Stirrer
Supporter
What I've learned so far about gear selection for a downgrade is that it should be the gear that puts the truck's engine just under governed speed at control speed. I suspect governed speed information as well as gear ratio information should be in the truck's operator guide.

Generally, a truck will be in the same gear going down the hill as it climbed up it if the truck is old or a gear or two lower than the climbing gear if the truck is modern.

However, there will times that road goes from a stretch of flat and level (no slope) to a downgrade and there is no uphill climb to determine your gear selection for going over the summit.

Let's say you are in a tractor-trailer with one trailer on a level highway at 80,000 pounds and this highway is a speed zone of 55 mph and you are cruising at this speed. Your transmission is at its normal gear range for this driving condition. You see a sign indicating there is a 5% downgrade ahead, road conditions are dry, it's broad daylight and weather is clear.

What should you now do? How will you calculate your correct CONTROL SPEED for this upcoming downhill run? Which gear will you select?


Lets' change 5% to 10% downgrade in the scenario above, what is your correct control speed now?

Keeping that 10% downgrade, let's change road conditions from dry to wet, what is your control speed now?

Now let's change road conditions from wet to packed snow, what is your control speed now?

Let's now change your fully-loaded truck to an empty truck, what is your control speed now?

Now let's even change your truck from a single to a double, now what should your control speed be?

What is the mathematical formula in mountain driving for determining control speed based upon:

  1. load
  2. transmission type and governed engine speed
  3. vehicle age and design
  4. number of trailers/vehicle composition
  5. visibility
  6. posted speeds
  7. grade
  8. road conditions
  9. traffic
  10. weather
There must be rules established for predetermining control speeds for many driving variables to be safe.

A driver will use the the snubbing 5-3 braking method on top of engine braking to maintain control speed downhill.

Engine braking will never be used under slippery road conditions to avoid skids.
There's not a defined number. A lot of it depends on how your truck is set up, the rest is external conditions.

My Cascadia has a strong jake brake, but the engine speed needs to be up around 1500 RPM for it to be at its most effective. I have drum brakes that are subject to brake fade - the newest model years are being equipped with disc brakes that don't have that problem.

Brake fade is caused by heating of the drum. As the brake absorbs energy, the drum expands away from the brake pads. Worst case, a poorly adjusted brake won't be able to contact the drum when its hot. You don't want to ride your brakes down a long steep grade on a hot day. If you have a brake failure (due to fade, compressor failure, severe air leak, etc) on a steep grade when you're heavy and you're not geared down, you really don't have any options left.

When your truck is heavily loaded, you want to come down in a lower gear with your jake engaged, holding the truck back. In bad weather, you may not be able to safely use your jakes, so you'll have to gear down much lower to start with. Ideally with the proper gear and jakes, you won't need to use your brakes much. If you do use your brakes, there is considerable controversy over whether its better to use light continuous pressure or stab braking.

Whatever you do, you want to maintain a "safe speed" throughout the descent. That speed may be set by a maximum truck speed limit, curve speeds, or what's comfortable to you given road conditions. As a newb, you do not want to try to downshift once the descent has started, so slow down and downshift before the descent starts, especially in bad weather. If you get out of gear and can't get the engine speed high enough to match your transmission speed, you're screwed. Between 70 and 80,000 lbs on a 5 or 6% grade with good road conitions, I'll usually choose 8th gear. Steeper grades, I'll go to 7th - that keeps me in a good RPM range for maximum jake brake efficiency - on my truck. That also puts me at 35 to 45 mph, 7th is 25 to 35 mph - depending on the steepness of the grade, gross weight, etc. Your mileage may vary. It also depends at what's at the bottom - if there's a "deadman's curve" at the bottom, a hairpin curve that requires a lower truck speed, a dead stop or a lower town speed limit, you want to be going down slower at the git go.

9% and 10% grades are relatively uncommon, but they do exist, especially in the eastern part of the country on US and state highways; that also means an even lower gear. You don't always go down the same percent grade as you went up, or even go up on an uphill side. Emigrant Pass on I84 in Oregon is a good example. OTOH, there's long 5%, 6% grades with straightaway at the bottom where its quite safe to go down at 70 mph in a high gear - there's a couple in PA that everyone takes fast in good weather - if you have a strong enough jake brake. If you're light, on a dry road, you can certainly go down in high gear - but stay below your governed top speed. If for some reason your trailer wants to come around in a jackknife, your only way out is to go faster.

Experience will have to be your guide. Its better to be conservative at first until you get your sea legs underneath you. Going slower than necessary isn't a bad thing until you get it figured out. Just make sure you have your fourways on, and stay in the granny lane.
 
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Hillbilly Canuck

Well-Known Member
It comes with time and experience. You are SERIOUSLY overthinking this.

Rough rule of thumb is same gear you pulled the hill up, is the one you go down in
I hate people regurgitating that with every fibre. There are so many downgrades where there wasn't an uphill or it's not the same or you can pull up way more than you can handle down. ironpony ironpony explained it best I reckon.
How many 7% grades you have in Minniesoota?
None that I've seen but I have seen plenty of those "uncommon" 9 and 10 percent grades in B.C.
 

Ontario Outlaw

Hozer Witta Hood
Supporter
It comes with time and experience. You are SERIOUSLY overthinking this.


I hate people regurgitating that with every fibre. There are so many downgrades where there wasn't an uphill or it's not the same or you can pull up way more than you can handle down. ironpony ironpony explained it best I reckon.
None that I've seen but I have seen plenty of those "uncommon" 9 and 10 percent grades in B.C.
Fair enough, that’s a valid point. Usually on steep grades there are posted signs, and on a lot of them there is a truck maximum speed limit. Some even have those brake check areas, with the billboard map.

And yes, I’ve seen a truck get pulled over by a LEO for exceeding the grades speed limit.
 

dchawk81

Well-Known Member
Supporter
I think the grades by me across rt30 are something like 14% in spots. So basically you learn where your trucks RPMs are at a given speed and crawl down in the desired gear to be at the top end of the rev band for maximum jakes.

No harm if you have to creep down too slow. Lots of harm if you blast down too fast.

People can get pissed all they want afaic.
 

Hammer166

Instigateur №166™
Supporter
Decending any hill, the Jake should hold you 1600-1750 rpm with little or no brake input.
How many 7% grades you have in Minniesoota?
Run plenty out west and British Columbia. So what's your point.

It's still a valid guideline
I tend to run a bit higher on the revs, but same idea. If I have to hit the brakes more than once or twice, I'll drag the anchor a bit so I can drop a gear. A lot of times the engine fan will provide enough extra drag to hang onto a gear that's a touch to tall for the Jake alone.

And unless you're absolutely crawling, turn off the dang 4 ways! All it does is make it very easy to miss a turn signal. I'm sure the idiots @ JJ Keller probably started this trend, but it's not how it's supposed to be done. (They have also encouraged high beam usage on the interstate because "the median is wide enough")

And it's really humorous to see someone who spends their time on the short, steep east coast hills talking smack to someone who runs out west.
 

ironpony

Professional Pot-Stirrer
Supporter
Run plenty out west and British Columbia. So what's your point.

It's still a valid guideline
RPM-wise, yes. Being able to hold without brakes sounds like the advise of a 90-day rookie, whose never seen a 7% grade at 80,000 lbs. That's why.

I tend to run a bit higher on the revs, but same idea. If I have to hit the brakes more than once or twice, I'll drag the anchor a bit so I can drop a gear. A lot of times the engine fan will provide enough extra drag to hang onto a gear that's a touch to tall for the Jake alone.

And unless you're absolutely crawling, turn off the dang 4 ways! All it does is make it very easy to miss a turn signal. I'm sure the idiots @ JJ Keller probably started this trend, but it's not how it's supposed to be done. (They have also encouraged high beam usage on the interstate because "the median is wide enough")
Plenty of morons out there... especially high beam morons, and idjits with their heads buried in their smartphones and driving way too fast - both four-wheelers and truck drivers. The four ways stay on if I'm geared down with the jakes on. I'm not worried about you... its the rest of the idjits and their personal injury attorneys.
 

Mike

Well-Known Member
Staff member
My jake sucks. Doesn't matter how slow I go, I still have to use the brakes, so I tend to go down the hills much slower than everybody else.

I would get this fixed, but I only think about it when going down long grades.
 

r3gulator3

Friendly Neighborhood Former Technician
Staff member
Supporter
What the **** does this mean?

And it's really humorous to see someone who spends their time on the short, steep east coast hills talking smack to someone who runs out west.
I ran into another TMC driver who runs mostly south east. Who told me that Monteagle was such a treacherous pass. I smirked. He was like what’s funny. I said go out on a two week run and tell em you want to go to Washington. Make sure you get over Fourth of July pass Look out pass and snoqualmie pass. Then get a reload out of kettle falls. (30 miles from Great Falls BC) make sure it’s winter and ask for the cascade load going to southern Utah. He was like what? I said that long 6% that’s almost straight will seem like flat highway after you make those runs.

He called me a dick and said something about my mother. I asked how long he’s been driving. He said 10 years. My jaw hit the ground. I said how long with TMC. He said a few months. I says ok. Well I didn’t mean to be a dick but, I’m just saying there’s far worse. Who did you work for before. He mentions some smaller carrier that runs the south east. I says, damn man you live a sheltered life. Walked back to my truck.

Maybe I have the wrong approach with people. Maybe I am a dick. 🤷🏻‍♂️
 

ironpony

Professional Pot-Stirrer
Supporter
What the **** does this mean?



I ran into another TMC driver who runs mostly south east. Who told me that Monteagle was such a treacherous pass. I smirked. He was like what’s funny. I said go out on a two week run and tell em you want to go to Washington. Make sure you get over Fourth of July pass Look out pass and snoqualmie pass. Then get a reload out of kettle falls. (30 miles from Great Falls BC) make sure it’s winter and ask for the cascade load going to southern Utah. He was like what? I said that long 6% that’s almost straight will seem like flat highway after you make those runs.

He called me a dick and said something about my mother. I asked how long he’s been driving. He said 10 years. My jaw hit the ground. I said how long with TMC. He said a few months. I says ok. Well I didn’t mean to be a dick but, I’m just saying there’s far worse. Who did you work for before. He mentions some smaller carrier that runs the south east. I says, damn man you live a sheltered life. Walked back to my truck.

Maybe I have the wrong approach with people. Maybe I am a dick. 🤷🏻‍♂️
Just tell 'em to do Wolf Creek during the winter...
 
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